The chemistry and biology of the bryostatin antitumour macrolides

Karl J. Hale,* Marc G. Hummersone, Soraya Manaviazar and Mark Frigerio

The Christopher Ingold Laboratories, Department of Chemistry, University College London,20 Gordon Street, London, UK WC1H 0AJ

Received (in Cambridge, UK) 11th February 2002, First published as an Advance Article on the web 14th June 2002

Covering 1982–2001

This review summarises the main developments that have occurred in bryostatin chemistry over the period 1982 to2001 and has 117 references.

Karl Hale was born in Liverpool, England, in 1961. He obtained his BSc (Hons) degree in Chemistry from Queen Elizabeth CollegeLondon in 1982, and then went on to do a PhD in synthetic carbohydrate chemistry under the supervision of Professors Leslie Hough andAnthony C. Richardson at King’s College London. Soon after he graduated in 1985, he joined the group of Professor Amos B.

Smith at the University of Pennsylvania in Philadelphia, where he worked on a range of total synthesisprojects that included the novel immunosuppressant, (�)-FK506. In 1989, he left Penn to take up aSenior Scientist position in the Medicinal Chemistry Department of F. Hoffmann-La Roche in Nutley,New Jersey, where he worked on the development of novel anti-inflammatory drugs. One year later, hewas invited to join the faculty of University College London as a lecturer in organic chemistry. In 1995,he was promoted to Senior Lecturer, and three years later to Professor of Chemistry. In 1997, hebecame a visiting associate professor at L’Université de Louis Pasteur in Strasbourg, where he nowteaches annually. His research interests are in the total synthesis of complex, pharmacologically-active,natural products and in the design of new asymmetric organic reactions. His research activities alsoencompass medicinal chemistry and chemical biology. He has co-authored 49 organic chemistrypublications to date which include two books: Organic Synthesis with Carbohydrates (with Geert-JanBoons), and The Chemical Synthesis of Natural Products, both of which appeared in 2000. He was arecipient of the 1997 Zeneca Research Award in Organic Chemistry and of the 1998 Pfizer AcademicAward for Chemistry. He currently serves on the Editorial Advisory Board of Organic Letters.

Marc Hummersone is a native of Hertford, England. He graduated with a BSc (Hons) in Chemistryand Analytical Chemistry in 1996 from Leeds University before embarking on a PhD in syntheticorganic chemistry at UCL under the supervision of Professor Hale. For his PhD, Marc completed afully stereocontrolled synthesis of the bryostatin B-ring using a novel C2-symmetry breaking tactic forstereospecific olefin formation. He presented this work at the 2000 Pfizer Poster Symposium and wasawarded a runner-up prize. After doing postdoctoral work in Professor Hale’s group on the bryostatins,he accepted a post as a Senior Scientist at ChemOvation, Horsham, England.

Soraya Manaviazar attended Concord College, Shrewsbury, prior to beginning her University studies.After graduating with a BSc (Hons) in Chemistry and Computing from the University of Brighton in1990, she moved to University College London where she obtained an MSc in Chemical Research (withDistinction) in 1991. Subsequently, she went on to do a PhD in Synthetic Organic Chemistry inProfessor Hale’s group at UCL, obtaining the degree in 1995. During her time in his group, she wasawarded a runner-up prize in the 1993 Pfizer Organic Chemistry Poster Symposium for a postercovering her PhD work. Upon leaving UCL, she took up a position as a Synthetic Organic Chemist inthe Medicinal Chemistry Department of Oxford Glycosciences in Abingdon, where she remained for3 years. In 1998, she returned to UCL Chemistry Department to occupy the position of PrincipalResearch Fellow in Organic Chemistry. Her research interests are mainly in natural product totalsynthesis and medicinal chemistry, and over her career she has worked on the synthesis of manybiologically important molecules, that have include inter alia the antitumour agents A83586C,bryostatin 1 and more recently halichomycin.

DOI: 10.1039/b009211h Nat. Prod. Rep., 2002, 19, 413–453 413

This journal is © The Royal Society of Chemistry 2002

Born in 1977 in Kent, England, Mark Frigerio graduated from University College London with anMSci (Hons) in Chemistry in 1999. His great passion for synthetic organic chemistry and totalsynthesis led him to staying on at UCL to work for a PhD in Professor Hale’s group. His doctoral workis concerned with the design and synthesis of novel analogue structures of the bryostatin antitumourmacrolides. He is already the co-author of a lengthy review on cyclodepsipeptide synthesis, in additionto two Organic Letters papers on bryostatin 1 and halichomycin synthesis respectively, and he expectsto submit his PhD thesis later this year.

1.0 Introduction2.0 Isolation and structure elucidation of bryostatins

1–183.0 Biosynthesis of bryostatin 14.0 Bryostatin biology 3,4

4.1 The antitumour profile of bryostatin 14.2 Mechanisms of antitumour action4.3 Can bryostatin 1 accelerate the growth of some

tumours and act as a tumour promoter?5.0 Bryostatin total synthesis 34

5.1 Masamune’s enantioselective total synthesis ofbryostatin 7 (1990)

a Masamune’s retrosynthetic strategy for bryostatin7

b Synthesis of the C(3)–C(16) segmentc Synthesis of the C(17)–C(27) segment 4d The Masamune bryostatin 7 endgame5.2 The Evans enantioselective total synthesis of

bryostatin 2 (1999) 36

a Model work by Evans (1990)b Evans’ retrosynthetic analysis of bryostatin 2

(1999)c Asymmetric synthesis of the C(1)–C(9)

glycosylsulfone 63d Evans’ asymmetric route to C(10)–C(16)-B-ring

fragment 65e Synthesis of the C-ring sector and elaboration into the

BC-segment 64f Completion of the bryostatin 2 synthetic venture5.3 The Nishiyama–Yamamura total synthesis of

bryostatin 3 (2000) 37

a The Nishiyama–Yamamura retrosynthetic plan forbryostatin 3

b Preparation of the enantiopure AB-aldehydeintermediate 114

c Synthesis of the C-ring sulfone 115d Union of the AB- and C-ring intermediates 114 and

115 and completion of the bryostatin 3 syntheticventure

6.0 Synthetic studies on the bryostatins6.1 Thomas’ synthetic studies on bryostatin 11 72,73

a Thomas’ retrosynthetic analysis of bryostatin 11b Thomas’ racemic free radical routes to the bryostatin

B-ring (1989 and 2000) 72

c Thomas’ asymmetric synthesis of the C-ring sulfone163 (2000) 73

6.2 Vandewalle’s synthetic studies on bryostatin 11 78

a Attempted construction of the C(1)–C(9) backboneof the bryostatins via a dithiane coupling strategy(1991) 78a

b Synthesis of the C(1)–C(9)-segment of the bryostatins(1991)

c Model studies on the bryostatin B-ring(1991) 78a

d Vandewalle’s synthetic strategy for the C-Ringregion of bryostatins 1 and 11 (1994) 78b,c

e Vandewalle’s use of (R)-carvone for a newsynthesis of the Masamune C(27)–C(34)-alkynefragment (1997) 78d

6.3 R. W. Hoffmann’s racemic route to the C(1)–C(9)segment 267 (1995) 87

6.4 The Kalesse enantioselective route to the C(1)–C(9)-segment 257 (1996) 89

6.5 The Kiyooka reagent-controlled asymmetric aldolroute to the C(1)–C(9) segment of the bryostatins(1997) 90

6.6 Roy’s synthetic studies on bryostatin 1 (1989,1990) 91

a Roy’s synthetic path to the C(1)–C(9) fragment ofbryostatin 1 (1990) 91b

b Roy’s enantiospecific route to a C(21)–C(27)-synthon 307 (1989) 91a

6.7 The H.M.R. Hoffmann route to the C(1)–C(16) ABSegment 308 (2000) 95

a H. M. R. Hoffmann’s retrosynthetic analysis of theC(1)–C(16) fragment 308 95

b H. M. R. Hoffmann’s asymmetric synthesis of theC(1)–C(9) dithiane 310

c H. M. R. Hoffmann’s asymmetric construction ofthe B-ring triflate 309 and elaboration into theAB-system 308

6.8 Hale’s synthetic work on the bryostatins (1995, 2000,2001) 101

a Hale’s retrosynthetic planning for bryostatin 1b Hale’s C2-symmetry breaking olefination tactic for

the totally stereocontrolled asymmetricconstruction of the bryostatin B-Ring (2000) 101a

c Hale’s first attempt at synthesising the “SouthernHemisphere” of bryostatin 1 (1995 and 2000) 101b,c

d Hale’s fully stereocontrolled asymmetric synthesisof the fully-elaborated bryostatin 1 “SouthernHemisphere” intermediate 346 (2001) 101d

6.9 Janda’s polymer-supported synthesis of the C(21)–C(27)-sector of bryostatin 1 (2000) 113

6.10 Yadav’s synthesis of the bryostatin “NorthernHemisphere” (2001) 114

a Yadav’s asymmetric synthesis of theketophosphonate 423

b Completion of the AB-intermediate 4207.0 Wender’s analogue work (1998, 2000) 3,116

8.0 Mendola’s aquaculture solution to the bryostatin 1supply problem (2000) 117

9 Acknowledgements10 References

414 Nat. Prod. Rep., 2002, 19, 413–453

1.0 Introduction

In this review, we will attempt to summarise the main develop-ments that have occurred in bryostatin chemistry and biologyover the period 1982–2001.

2.0 Isolation and structure elucidation of bryostatins 1–18

The bryostatins are a structurally novel family of marine

macrolides first encountered by Pettit and co-workers duringtheir search for new anticancer drugs from the bryozoan inverte-brates Bugula neritina Linnaeus and Amathia convulata(Scheme 1).1 Eighteen bryostatins have so far been isolated fromthese two organisms, which are indigenous to the Gulfs ofMexico, California, and Sagami off Japan. The structures ofthe bryostatins were deduced by detailed spectroscopic and/orX-ray-crystallographic analysis. A number of interesting

Scheme 1 The bryostatin family of antitumour macrolides.

Nat. Prod. Rep., 2002, 19, 413–453 415

patterns have emerged. All bryostatins possess a 20-memberedmacrolactone in which there are three remotely-functionalisedpyran rings interconnected by an (E )-disubstituted alkene anda methylene bridge; all family members also contain a pair ofgeminal dimethyls at C(8) and C(18); each bryostatin has afour-carbon side-chain emanating from its A and C-rings, andvirtually all have an exocyclic methyl enoate in their B and Crings. In the majority of cases, the only difference resides in thesubstituents at C(7) and C(20). Saying this, there are some bryo-statins that do not conform to this basic structural blue-print. For example, bryostatin 3 has a butenolide appended tothe C-ring (as opposed to an exocyclic methyl enoate), and bry-ostatins 16 and 17 both have a glycal in place of the C(19)- andC(20)-hydroxys. Further differences can be found in bryostatins17 and 18, where there is opposite methyl enoate geometry inthe C-ring.

3.0 Biosynthesis of bryostatin 1

Elegant radiolabelling and incubation studies by Kerr andcoworkers 2 have shown that acetate, S-adenosylmethionine(SAM) and glycerol are all key building-blocks involved in bryo-statin 1 production. Propionate, n-butyrate, isobutyrate andsuccinate are not required for bryostatin 1 biosynthesis. Kerrhas suggested that the geminal dimethyl groups of thebryostatins originate from a series of SAM methylations onincorporated acetate units, as was found for the biosynthesisof lankacidin and aplasmomycin. As for the exocyclic olefinunits, these probably derive from the addition of acetate to apolyketide chain followed by dehydration. Kerr is currentlyattempting to pinpoint the sites where the labelled precursorsare incorporated, and clearly, his results are awaited with greatinterest.

4.0 Bryostatin biology 3,4

4.1 The antitumour profile of bryostatin 1

Bryostatin 1 shows remarkable in vitro and in vivo anticancereffects against a range of mouse tumours that include P388lymphocytic leukaemia,1f ovarian sarcoma,5 B16 melanoma,6,7

and M5076 reticulum cell sarcoma.7 Combinations of bryo-statin 1–auristatin PE, and bryostatin 1/dolastatin 10, havesuccessfully cured five out of five, and two out of five, SCIDmice with human chronic lymphocytic leukaemia xenograftsrespectively.8 Bryostatin 1 has recently completed severalanticancer trials in man,9,10 where its most significant side-effect 10d,11 was mylagia. The trials clearly demonstrated thatbryostatin 1 has considerable potential for the treatment ofovarian and relapsed low-grade non-Hodgkin’s lymphoma, itbeing effective when given alone, or in combination with otheranticancer drugs. To date, only one patient has been completelycured by bryostatin 1,10b but many partial remissions have beenseen.9,10 The total cure was observed in a 41-year old womanwho had stage 4 follicular small-cell-cleaved non-Hodgkin’slymphoma, that had recurred at multiple sites five yearsafter she had been brought into remission with alkylator drugtherapy. Eight fortnightly cycles of bryostatin 1 elicited thecure, each cycle consisting of a 72 h intravenous infusion at adose of 120 µg m�2.10b

4.2 Mechanisms of antitumour action

The antitumour effects of bryostatin 1 have been linked to itsability to selectively modulate the functioning of various indi-vidual protein kinase C (PKC) isozymes within cells.12 PKCs areserine and threonine kinases that catalyse the O-phosphoryl-ation of proteins involved in cell-signal transduction.13,14 Theyare thought to play a critical role in the control of cell division,their over- or under-expression having been linked with thetransition of some tissues into malignancy. In this regard,Mushinski and coworkers have observed that when nPKC-ε is

overexpressed in NIH3T3 cells, it makes them highly neoplasticand tumourigenic towards nude mice.15 Elevated n-PKC-εlevels in rat 16 fibroblasts also make them malignant andtumourigenic, whilst PKC-β overexpression renders such cellshighly susceptible to transformation with the Ha-ras oncogene.The cotransfection of human small-cell lung carcinoma cellswith the c-myc and Ha-ras oncogenes serves to increase PKC-βII levels, and significantly, this has been correlated with theirconversion into the much more malignant large-cell phenotype.It has been reported that cPKC-α levels are considerably higherin human A549 lung cancer cells than in other nearby healthylung tissue,17 suggesting that up-regulated PKC-α activity mightbe a significant contributor to the onset of this particulartumour. PKC activity is also significantly elevated in humanbreast cancer tissue compared with other nearby normal breasttissue. Taken together, these findings very strongly implicatederegulated PKC-signalling in the development of a range oftumours.

Because the growth inhibition of human A549 lung andMCF-7 breast cancer cells by bryostatin 1 correlates veryclosely with its down-regulation of cPKC-α,17 it is logical tosuppose that the down-regulation of individual PKC isozymesmight be a general mechanism through which bryostatin 1exerts its antitumour effects. However, this cannot be true forall bryostatin 1-responsive cancers, for bryostatin 1 canactually protect some “tumour-suppressing” PKC-δs fromundergoing down-regulation in certain cells. PKC-δs can playa decisive role in cancer cell growth, their degree of expres-sion frequently determining whether a cell will undergogrowth-arrest or proliferation. For example, PKC-δ over-expression in mouse keratinocytes results in apoptosis,18

which is the hallmark of efficient tumour suppression. PKC-δwill also overwhelm the effects of an over-expressed c-src-proto-oncogene in 3Y1 fibroblasts, preventing such cells fromundergoing transformation.19 Deliberate over-expression ofPKC-δ in NIH 3T3 cells likewise halts their proliferation.20 Byway of contrast, its enforced under-expression in some cells canresult in a substantially increased rate of growth.19,21 The factthat bryostatin 1 can prevent PKC-δ from undergoing down-regulation in mouse keratinocytes and human fibroblasts,22

appears to support the notion that bryostatin 1 might beinhibiting the growth of some tumours via a PKC-δ-protectiveor -stabilising mechanism.22,23

Given that bryostatin 1 acts upon PKC isozymes in a highlytissue-specific manner, this greatly complicates the positionwith regard to precisely explaining many of its observed anti-tumour effects.

Bryostatin 1 competitively binds to the phorbol ester–diacylglycerol binding sites of PKC isozymes at two highlyconserved regions known as the cysteine-rich domains 1 and 2(PKC CRDs 1 and 2). Considerable effort has gone into eluci-dating the three-dimensional structures of several CRDs, withthe result that an NMR solution structure has recently beendeduced for a murine PKC-α CRD2 construct,24 and a 2.2 Åresolution X-ray crystal structure has been solved for a murinePKC-δ CRD2 complexed to phorbol 13-acetate.25 The latterwork unambiguously showed that tumour-promoting phorbolesters sit in a polar groove that exists between two openedβ-strands at the tip of the CRD. It also revealed that complex-ation does not induce a significant conformational change inthe PKC activator-binding domain. In essence, the phorbolester sits over the polar-inside of the groove to create a longcontinuous hydrophobic surface that extends over roughlyone-third of the complexed protein. It has been postulated thatthe greatly increased hydrophobicity of the phosphorylatedPKC-δ-phorbol ester complex promotes its insertion into theplasma membrane, from where it can engage in tumouri-genic signalling.25 It is likely that when bryostatin 1 binds tothe CRDs of PKCs, similar increases in phosphorylatedPKC hydrophobicity occur. However, for bryostatin 1, the

416 Nat. Prod. Rep., 2002, 19, 413–453

complexation probably elicits different conformational changesin the PKCs than does phorbol 13-acetate. It is quite reasonableto suppose that the protective action of bryostatin 1 upon somePKC-δs might be the result of it inducing a “stabilising” con-formational change in these enzymes, preventing them frominserting into the plasma membrane and/or being degraded.For phosphorylated PKCs that are down-regulated by bryo-statin 1, the complexation more than likely induces a conform-ational change that favours plasma membrane insertion anddegradation.

With regard to the latter proposal, remarkable insightshave recently been gained into the way in which bryostatin 1down-modulates PKC-α in renal epithelial cells, and PKC-αand PKC-ε in human fibroblasts. Through a detailed set of 32Plabelling studies, Bingham Smith and coworkers 26 have shownthat for these two cell lines, the down-modulation of PKC-αand -ε proceeds through the ubiquitin-proteasome pathway

(Scheme 2). They demonstrated that soon after a phosphoryl-ated PKC-α- or -ε-bryostatin 1 complex is formed, auto-phosphorylation occurs, with the result that the drug–proteincomplex is translocated from the cytosol to the plasma mem-brane where it then becomes embedded. Once there, the com-plex is apparently rendered susceptible to dephosphorylation bymembrane-bound alkaline phosphatases. Dephosphorylationyields a catalytically-inactive form of the PKC protein thatpredisposes it to ubiquitinylation by Ub-activating (E1),-conjugating (E2), and -ligating (-E3) enzymes found within thecytosol. As the ubiquitin ∼26 proteasome organelle resides bothin the cytoplasm and in the nucleus, it is likely that the cyto-plasmic variant is responsible for degrading the membrane-bound ubiquinylated PKC-α and -ε proteins to bring abouttheir down-regulation (Scheme 2).

This ability of bryostatin 1 to bind selectively to differentPKC isoforms and render them susceptible or non-susceptible

Scheme 2 Hypothetical pathway of PKC synthesis and downregulation by bryostatin 1.

Nat. Prod. Rep., 2002, 19, 413–453 417

to undergoing degradation within cells is really quite fascinat-ing, and is probably the key to many of the observed anti-tumour effects. It is quite easy to imagine that, for somecancers, the selective down-regulation of a particular up-regulated PKC pathway by bryostatin 1 might be the criticalevent required for correcting the aberrant mitogenic state, whilefor others, it could be a bryostatin-mediated PKC-up-regulationthat is required for switching off the cell’s proliferativemachinery.

Whilst the bulk of biological research on bryostatin 1 hasattempted to document its interactions with various PKC iso-zymes, a substantial body of evidence has accumulated whichindicates that bryostatin 1 can function as a powerful immuno-stimulant. In fact, some researchers have suggested that thismight be the primary mechanism of its antitumour action insome patients.9b,27 Bryostatin 1 readily activates resting humanT cells 27 and neutrophils 28 both in vitro and in vivo, 29 and it hasbeen shown to raise the levels of tumour-necrosis factor-α(TNF-α) in a number of patients receiving therapy.9a TNF-α isa powerful tumouricide produced by the body after immuno-stimulation has occurred. Bryostatin 1 can also induce the rapidrelease of TNF-α from MONO-MAC-6 cells.30 Significantly, ina murine macrophage ANA-1 cell line, bryostatin 1 significantlyincreased TNF-α mRNA expression and production, and itsynergised with IFN-γ in the production of [NO2]

� and in theexpression of the inducible nitric oxide synthase (i-NOS) gene.NOS catalyses the in vivo production of NO from -arginine; inturn, NO confers powerful tumouricidal effects upon murinemacrophages. NO also induces apoptosis in tumour cells.31

Such observations clearly support the idea that bryostatin 1might be exerting some of its antitumour effects through animmunostimulatory mechanism.

While bryostatins 1, 3, 8, and 9 are all capable of activatingneutrophil chemiluminescence and the cytotoxic killing ofK562 cells, identical actions have not so far been detected forbryostatin 13; a member of the 20-deoxy class of bryostatins,which are claimed to be even more potent as antitumour drugs.Bryostatin 13 is also incapable of stimulating colony formationfrom bone marrow progenitor cells, notwithstanding the factthat bryostatins 1, 3, 8 and 9 can all perform this task veryeffectively.32 Such data appear to suggest that while an immuno-stimulatory antitumour mechanism might be significant for theC(20)-O-acyl bryostatins in some patients, a similar mode ofaction is unlikely for molecules of the 20-deoxy class.

The divergent actions of the bryostatins on PKCs fromdifferent sources, coupled with the capacity of certain familymembers to function as immunostimulants, make it improbablethat a single, all-encompassing, mechanism of antitumouraction will ever be proposed for this class. Current biologicaldata suggest that the antitumour properties of bryostatin 1almost certainly vary from patient to patient, the precise anti-tumour response seen being contingent both on the cancer celltype and on the overall PKC content of the recipient’s cells.Clearly a great deal more biological research will have to bedone before a more accurate mechanistic picture is assembledof the ways in which the various members of this class arefunctioning.

4.3 Can bryostatin 1 accelerate the growth of some tumoursand act as a tumour promoter…

Recently it has been demonstrated that PKC-β isozyme levelscan markedly affect whether some cells rest or proliferate. Thisis significant, as bryostatin 1 can selectively target PKC-βIIs inboth human erythroleukaemia (K562) and promyelocytic(HL60) cells. Bryostatin 1 complexation leads to these PKCsselectively translocating to the nuclear membrane, where theyphosphorylate lamin B at its Ser-395 and Ser-405 residues.33

Lamin phosphorylation is associated with the disassembly andincreased-solubilisation of the nuclear lamina network during

mitosis, it causing a breakdown of the nuclear envelope.Importantly, these actions of bryostatin 1 correlate closely withenhanced proliferation for these cells. Data of this sort appearto suggest that bryostatin 1 might actually worsen sometumours, which points to the need for performing detailedin vitro tests on individual patients before therapy is com-menced. Preliminary in vitro screening could at least establishwhether bryostatin 1 will have a realistic prospect of arrestingor worsening a particular patient’s tumour before any treatmentis started.

5.0 Bryostatin total synthesis 34

The remarkable molecular structures of the bryostatins,coupled with their excellent antitumour properties and scarcityin nature, have served to educe considerable synthetic interest inthis class over the past two decades. So far, only three bryo-statins have succumbed to total chemical synthesis. Bryostatin 7was the first family member to be synthesised by Masamune in1990.35 Eight further years elapsed before Evans and coworkerscrowned this monumental achievement with an equally impres-sive total synthesis of bryostatin 2.36 Two years later in 2000,Nishiyama and Yamamura reported their asymmetric pathwayto the most structurally-elaborate member of this class, bryo-statin 3.37 In the coming sections, we will discuss each of thesesyntheses in detail.

5.1 Masamune’s enantioselective total synthesis of bryostatin 7(1990)

a Masamune’s retrosynthetic strategy for bryostatin 7

The strategic planning used by Masamune and coworkers fortheir total synthesis of bryostatin 7 is summarised in Scheme 3.35

Key elements of their proposed approach were the regio-selective macrolactonisation of seco-acid 1 to assemble the20-membered macrolide ring; the use of a reagent-controlledacetate aldol reaction to set the C(3)-hydroxy and install theC(1) and C(2) carbons of the polyketide backbone; and theimplementation of a Julia olefination 38 (involving 3 and 4) tofashion the sterically encumbered C(16)–C(17)-trans-alkeneand connect the three pyran segments together. A chemo-selective oxymercuration 39 of the less hindered olefin in diene 5was envisioned for B-ring assembly, while a borolane-mediatedasymmetric aldol reaction was proposed for uniting the A- andB-segments. Sulfone 4 looked accessible from the protectedketone 8 which, itself, seemed derivable from the addition of anappropriate vinylmetal to the α-alkoxy aldehyde 10, if followedby additional functional group interconversions. The presenceof a dimethoxybenzyl (DMBO) group 40 at the α-position ofaldehyde 10 could be expected to promote chelation control,which would clearly favour emergence of the desired alcoholstereochemistry at C(20).

b Synthesis of the C(3)–C(16) segment

For methyl ketone 6 (Schemes 3 and 4), efficient installation ofthe protected 1,3,5-triol motif was the primary concern. Someyears earlier, Masamune and Sharpless had already formulateda powerful solution to this general synthetic problem duringtheir work on the antifungal agent amphotericin B.41 Theirapproach centred around the creation of an appropriate chiral2,3,4,5-bis-epoxy alcohol via the asymmetric epoxidation (AE)reaction,42 and a tandem regioselective ring-opening of the bis-epoxide at C(2) and C(4) with REDAL. The epoxide substrateneeded for A-ring assembly was compound 14; it was preparedin nine steps from 2,2-dimethylpropane-1,3-diol 11 as detailedin Scheme 4. Diol 11 was converted to aldehyde 12 by selectiveO-benzylation and oxidation. Wadsworth–Horner–Emmons(WHE) reaction, DIBAL reduction and Sharpless AE sub-sequently procured the 2,3-epoxy alcohol 13 in 92% ee, whileSwern oxidation secured the epoxy aldehyde. After Wittig

418 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 3 Masamune’s retrosynthetic planning for bryostatin 7.

formylation and borohydride reduction, the second allylic alco-hol substrate was obtained; the AE increased the de of theproduct 14 to 99%. As expected, 14 underwent regiospecificalkoxide-directed ring-opening with REDAL to furnish com-pound 15 as the sole reaction product. Selective O-silylationand acetalation yielded 16. Birch O-debenzylation followed bySwern oxidation, methyllithium addition, and a second Swernoxidation finalised the sequence to methyl ketone 6.

Treatment of methyl ketone 6 with the (R,R)-borolane tri-flate† 18 and Hunig’s base produced the chiral boron enolate19 which added readily to the dienyl aldehyde 7. The desiredβ-hydroxy ketone 20 was obtained with 8 : 1 selectivity. Fischerglycosidation of 20 with MeOH, PPTS and trimethylorthofor-mate subsequently unmasked the C(5) and C(7)-hydroxys, andinstigated formation of the α-methyl glycoside 5. The B-ringwas elaborated through chemoselective intramolecular alkoxy-mercuration, O-acetylation, and oxidative demercuration underfree radical conditions. The final result was a 1 : 1 mixture ofthe alcohols 23, which were epimeric at C(15). This adversestereochemical outcome was soon rectified by oxidation to a1 : 1 mixture of the aldehyde epimers 24, and equilibration viaenolisation and protonation under mildly-basic conditions. A 9: 1 mixture of products ultimately emerged enriched in thedesired equatorial component 3.

Important steps in the route (Scheme 5) to aldehyde 7 werethe Corey trisubstituted olefin synthesis 43 of iodo allylic alco-hol 27, the protection of its primary OH with TBDPSCl,and the copper-catalysed cross-coupling reaction with allyl-magnesium bromide. The THP group was removed from 28 bytreatment with PPTS in ethanol; finally, a Collins oxidationsecured 7.

c Synthesis of the C(17)–C(27) segment 4

A two-pronged strategy was used to access 4, and fragments 9and 10 featured as prominent intermediates (see Schemes 6 and7). Vinyl iodide 9 was fashioned from -threonine via theroute shown in Scheme 6. The sequence commenced with aminediazotisation, O-esterification, and O-isopropylidenation foracquisition of methyl ester 31.44 The latter was then reducedto the aldehyde with DIBAL and this, in turn, subjectedto a Wittig olefination with methylenetriphenylphosphorane.

† The IUPAC name for triflate is trifluoromethanesulfonate.

Hydroboration and oxidation converted alkene 32 into alcohol33 which was further oxidised to aldehyde 34. Allenylzinc brom-ide was now added to 34 to fashion the desired acetylene neededfor elaboration of the exocyclic olefin. Chelation-control helpedensure that the C(23)-hydroxy stereocentre was set with 8 : 1selectivity. The occurrence of high selectivity in such a system ismost noteworthy, as β-alkoxy aldehydes are generally not per-ceived as good substrates for chelation-controlled nucleophilicadditions of organometallics. After protection of 35 as a PMBether, the alkyne terminus of 36 was homologated with t-BuLiand methyl chloroformate to access the alkynyl ester 37. Astereospecific Piers 45 stannylcupration next deposited the vinyl-stannane group within 38 in high yield. DIBAL reduction ofthe ester and protection converted 38 into 39; the latter wasthen subjected to a stereospecific metal–halogen exchange withiodine to obtain 9. The lithiation of 9 afforded a vinyllithiumintermediate that combined readily with aldehyde 10 to providea mixture of alcohols enriched in the chelation-controlled addi-tion product 40. Protecting group manipulation and oxidationsubsequently afforded ketone 41. The sulfone unit was fash-ioned by sulfide oxidation with MoOPH; these were conditionsthat left the trisubstituted double bond undisturbed. A DDQ-mediated removal of the PMB group now followed; the mixtureof hemiketals, so generated, was finally subjected to a Fischerglycosidation with Me3SiOMe and Me3SiOTf 46 to secure 4.

Compound 10 (the aldehyde coupling partner for vinyl iodide9) was prepared by the pathway shown in Scheme 7. Significantsteps were the Sharpless AE and the intramolecular urethane–epoxide ring-opening used to position the requisite stereo-chemistry at C(19).

d The Masamune bryostatin 7 endgame

Not only did the Julia olefination 38 between 3 and 4 efficientlyconnect the AB- and C-rings together (Scheme 8), it also fash-ioned the C(16)–C(17) (E )-disubstituted olefin geometry withgood stereocontrol (E : Z = 6.2 : 1). Selective O-acetylation ofthe C(20)- and C(7)-hydroxys was the next objective. For this,compound 2 had to be globally O-desilylated, and the threeliberated primary hydroxys selectively reblocked with TBSgroups prior to commencing the O-acetylation with aceticanhydride–pyridine. A second global O-desilylation with TBAFset the stage for allylic alcohol oxidation with MnO2 in THF. Byadhering to Corey’s recommended protocol for oxidising enals

Nat. Prod. Rep., 2002, 19, 413–453 419

Scheme 4 Masamune’s synthesis of the C(3)–C(16) AB fragment 24.

to methyl enoates (MnO2–HCN–MeOH),47 the desired bis-(methyl enoate) was obtained in excellent yield. Swernoxidation of the remaining OH procured aldehyde 47. An aldoladdition between 47 and 48 next allowed the C(1)–C(2) seg-ment to be elaborated with 3 : 1 selectivity in favour of thedesired alcohol product. Transketalisation of this intermediatewith MeOH–CSA unmasked the C(25)–C(26)-diol motif, whichwas O-triethylsilylated, along with the C(3)-OH. This tempor-ary protecting group device enabled the C(1)-thioester to behydrolysed successfully. After O-desilylation, compound 1 was

Scheme 5 Masamune’s synthesis of the C(11)–C(16) B-ring fragment7.

obtained. Seco-acid 1 was coaxed into undergoing a highlyregioselective macrolactonisation reaction with the reagentcombination of DCC and PPTS in pyridine–1,2-dichloro-ethane;48 a recipe that also brought about glycoside hydrolysisat C(9), but not at C(19). The electron-withdrawing O-acetategroup at C(20) undoubtedly conferred added acid-stability onthe C(19) glycoside by lowering its basicity. The fact thatundesired selectivity was observed in this cleavage was mostunfortunate as hydrolysis of the C(9)- and C(19)-methyl glyco-sides had been the next intended step in the originally-conceived synthesis. Notwithstanding extensive experimentaleffort by the MIT team, conditions were never identified forhydrolysing the C(19)-OMe that also did not inflict damage onthe remainder of the molecule. Masamune therefore elected tocleave the C(20)- and the C(7)-O-acetates with catalytic KOMeso as to render the offending glycoside more acid-labile. Thefact that the macrolactone ring of 50 could withstand thisselective transesterification reaction is most noteworthy. Whilstthis artifice did ultimately allow the C(19)-OMe group to behydrolysed under much less-forcing acid conditions, it still leftthe problem of restoring the acetate groups at O(7) and O(20).Fortunately, this was readily accomplished by selective silyl-ation of the C(26)-OH with TBSCl, and selective O-acetylationof 51. O-Desilylation of 52 with aqueous HF finalised thiselegant, high quality, total synthesis of bryostatin 7.

420 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 6 Masamune’s route the bryostatin 7 “Southern Hemisphere”.

Scheme 7 Synthetic pathway to aldehyde 10.

5.2 The Evans enantioselective total synthesis of bryostatin 2(1999) 36

a Model work by Evans (1990)

Evans’ 36c early synthetic incursions in the bryostatin area ledhim to evaluate the use of pyran-tethered phosphonoacetatesfor stereoselective fashioning of the exocyclic enoates, via theintramolecular WHE macrocyclisation process.49 For settingthe B-ring enoate, a six-carbon tether was deemed optimal, asMM2 calculations had indicated that a free-energy difference ofmore than 10 kcal mol�1 existed between both possible alkeneproducts, with the desired 14-membered macrocycle having thelower energy. It transpired that Evans and Carreira’s high-dilution ring-closure of tethered phosphonoacetate 54 (Scheme9), using 33 equiv. of lithium chloride and 30 equiv. of triethyl-amine in acetonitrile, afforded the desired enoate 53 as the solereaction product in 60% yield.36c While related model studiesindicated that such tethers could be cleaved successfully bytransesterification with K2CO3–MeOH, without competingolefin isomerisation, such a cleavage was not reported for 53itself. Although the tethered-phosphonoacetate technology wasnot eventually used by Evans for his subsequent bryostatin 2

total synthesis, it remains a strategy of note for the futurestereospecific assembly of related alkenic arrays.

b Evans’ retrosynthetic analysis of bryostatin 2 (1999)

Having grappled with the bryostatin problem for quite sometime, and having learnt much about the various synthetic pit-falls that lie en route to these molecules, Evans decided to post-pone exocyclic olefination until late-on in his projected totalsynthesis of bryostatin 2.36a,b Macrocyclic diketone 58 wasselected as his advanced sub-target, and a Fuji asymmetricWHE reaction 50 with 59 was proposed for stereoselective instal-lation of the B-ring enoate (Scheme 10). Evans believed that theC(21)-enoate would probably be best elaborated through analdol–dehydration sequence involving methyl glyoxalate andan appropriate C(20)-ketone intermediate. The C(20)-alcoholstereochemistry would be set through a stereoselective reduc-tion of the resulting keto-enoate. Diketone 58 appeared access-ible from the seco-acid 61 which, itself, looked derivable fromthe glycal 62. It was reasoned that the more electron-rich C(19)–C(20) enol ether double bond in 62 might undergo a regio-selective epoxidation with a limited quantity of oxidant; asubsequent glycosidation with methanol could then be expected

Nat. Prod. Rep., 2002, 19, 413–453 421

Scheme 8 Masamune’s synthetic route to bryostatin 7.

to position a methyl glycoside at C(19) and anchor the requisitehydroxy at C(20) in the forward route to 58. The forward path-way would also have to contend with the issue of C(1)-anilideconversion to a carboxylic acid. For linking the three pyransegments together, a Beau–Sinay alkylative C-glycosidation 51

was envisaged between 63 and 64. The BC-fragment 64 woulditself be prepared by a Julia olefination between 65 and 66.

Scheme 9 The tethered phosphonoacetate strategy for exocyclic olefinconstruction in the bryostatins.

c Asymmetric synthesis of the C(1)–C(9) glycosylsulfone 63

The synthesis of 63 started from methallyl chloride 67; it wastransformed into aldehyde 72 in five straightforward steps, oneof which was the acid-induced cyclopropylmethanol rearrange-ment of 71 (Scheme 11).52 Aldehyde 72 partnered oxazol-idinone 73 in an Evans asymmetric aldol reaction 53 that led tothe syn-aldol adduct 74. A zinc-mediated dehalogenation wasnext effected, and the acetate aldol 75 reduced to the diol 76with lithium borohydride. These combined tactics sculpted theC(9)–C(5) sector of the A-ring sulfone and simultaneously con-trolled the C(7)-hydroxy stereochemistry with 9 : 1 selectivity inthe desired direction. The route continued with the selectivep-methoxybenzylation of (OH)7;40 this was accomplished by atwo step procedure involving O-benzylidenation and regio-selective acetal reduction 54 with DIBAL. Aldehyde 77 (derivedfrom this primary alcohol by Swern oxidation) coupled to 78 ina Lewis acid mediated acetoacetate aldol condensation,55 whichcompleted the C(1)–C(9)-carbon backbone. Correct solventchoice proved critical for attaining high diastereoselectivity inthis reaction, toluene resulting in 94 : 6 selectivity in favour of79, and dichloromethane lowering the level of selectivity to6 : 1. An Evans hydroxy-directed reduction 56 was next imple-mented on the β-hydroxyketoester 79 to finalise installation ofthe asymmetric centres needed in this sector. The desired anti-1,3-diol 80 was isolated in good yield and with high stereo-control (ca. 10 : 1 selectivity). The C(3)- and C(5)-hydroxys werenow differentially protected via a PPTS-induced lactonisationand an O-silylation, and the lactone ring of 81 opened withthe aluminate anion obtained from aniline hydrochloride and

422 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 10 Evans’ retrosynthetic analysis of bryostatin 2.

trimethylaluminium. Ozonolytic cleavage of 82 provided a1.5 : 1 mixture of β-α-hemiacetals (72%) along with 20% of theC(9)-epoxide. The latter was thought to arise from an end-onapproach of ozone to the alkene, a pathway known to be facilewith sterically-demanding alkenes.57 The desired hemiacetalswere converted to the O-acetate mixture 83 uneventfully. AHanessian–Guindon thioglycosidation reaction 58 and a buff-ered m-CPBA epoxidation concluded this excellent pathway tothe α-phenylsulfone 63.

d Evans’ asymmetric route to C(10)–C(16)-B-ring fragment65

In 1996, Evans, Murry and Kozlowski 59 described a powerfulnew method for effecting catalytic asymmetric acetoacetatealdol reactions in high yield and excellent ee. Their chemistryappeared tailor-made for exploitation in the bryostatin area, asit readily permitted 1,3-diol arrays to be constructed efficientlywhen used alongside existing technology for the stereoselectivereduction of β-hydroxy ketones.56 Evans decided to enlist thispowerful new reaction combination for his assembly of the

B-ring synthon 65 (Scheme 12). The key asymmetric aldolreaction that sparked this sequence combined 84 with 85 andutilised the C2-symmetric copper() complex ([Cu(R,R)-Ph-pybox)] (SbF6)2) 86 as the chiral catalyst. Importantly, the pro-cess worked exceedingly well, it providing 87 in 99% ee and 85%yield on 10 g scale. After reduction with Me4NBH(OAc)3,

56 theanti-diol 88 predominated in the 94 : 6 mixture that resulted.Lactonisation and protection subsequently furnished lactone89 in 77% yield for the two steps. The latter reacted withp-methoxybenzyloxymethyllithium 60 to afford a mixture ofhemiketals that was then subjected to ionic reduction.61 Signifi-cantly, hydride delivery proceeded efficiently and with excellentstereocontrol, it furnished the desired product 90 in good yieldand with 94 : 6 diastereoselectivity. Standard protecting groupmanipulation and oxidation put the finishing touches to thisremarkably brief synthesis of 65.

e Synthesis of the C-ring sector and elaboration into theBC-segment 64

Evans’ route to the C-ring glycal 66 set off with the four-step

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Scheme 11 Evans’ synthesis of the A-ring glycosyl phenylsulfone 63.

Scheme 12 Evans’ B-ring synthesis.

preparation of 95 from 2,2-dimethylpropane-1,3-diol 11(Scheme 13). Aldehyde 95 combined with pentenylmagnesiumbromide to create 97 after oxidation, while dihydroxylation andoxidative cleavage converted 97 into aldehyde 99. A Brown–Paterson DIP-Cl-mediated asymmetric aldol reaction 62 with 93next provided 100 with 93 : 7 diastereoselectivity in the desireddirection. Ketone 93 had itself been procured through theSharpless kinetic resolution 42 shown in Scheme 14. Tischenkoreduction 63 of 100 with SmI2 not only set the C(25)-hydroxy,but also introduced the p-nitrobenzoate group at O(23) in 101.Protection and saponification followed subsequently. Mild acidtreatment finally afforded the desired phenylsulfonyl glycal 66.The Julia olefination 38 between 66 and 65 proceeded with high

geometric control (E : Z = 95 : 5), it affording 103 in 64% over-all yield. Triflate ester 64 was obtained by selective O-desilyl-ation of the primary TBS group and O-triflation with triflicanhydride and 2,6-lutidine.

f Completion of the bryostatin 2 synthetic venture

The fulcrum of Evans’ synthetic strategy for bryostatin 2 washis use of a Beau–Sinay glycosylsulfone C-alkylation tactic 51

for joining the A- and BC-segments together (Scheme 15). Thereaction between 63 and 64 initially afforded a mixture ofglycosyl sulfones 104, which subsequently underwent hydrolysisat C(9) when exposed to silica gel. The excellent yield observed

424 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 13 Evans’ synthesis of the bryostatin 2 BC-ring synthon 64.

in this alkylation is especially noteworthy given the high degreeof steric hindrance around the glycosylsulfone anionic centre,and the β-oxygen functionality that is present in the triflatecomponent. The synthesis proceeded with conversion of theC(1)-acyl anilide into a benzyl ester. For this, ring-opening ofthe hemiketal 62 with Et3SiCl proved necessary, followed byinstallation of a Boc group on the anilide nitrogen. The Bocunit greatly facilitated the amide cleavage with lithium benzyl-oxide. Selective epoxidation of the C(19)–C(20)-glycal in 106was now attempted. Fortunately, this proceeded with completeregiocontrol, it setting up the desired acid-catalysed epoxidering-opening reaction with methanol for the acquisition ofmethyl glycoside 107. Dess–Martin oxidation 64 of 107 yielded108 which underwent simultaneous O-desilylation and Fischer

Scheme 14 Asymmetric synthesis of methyl ketone 93.

glycosidation with HF–MeOH in THF buffered with pyridineto give 109. The latter was selectively O-triethylsilylated atOH(3) and OH(13). Transfer hydrogenation 65 with cyclohexa-1,4-diene over a palladium on carbon catalyst detached thebenzyl ester from this molecule to provide 61. The survival ofthe C(16)–C(17)-alkene through this hydrogenation step is par-ticularly noteworthy. A Yamaguchi macrolactonisation 66 nowsecured 60, which was selectively O-desilylated at O(13) andoxidised. Olefination of ketone 58 with the Fuji chiralphosphonoacetate 59 50 produced a 5.5 : 1 mixture of enoatesenriched in the desired alkene component 57. Significantly, theC(20)-ketone did not participate in this addition due tosteric hindrance from the two C(19)-substituents. The aldol-dehydration tactic was now implemented on 57 to obtain 56,and a CBS reduction was called upon for installation of theC(20)-hydroxy. It was generally found beneficial to trap theproduct alcohol as the O-methoxyacetate ester 111 to facilitateproduct isolation.

With compound 111 in hand, the O(3)-TES and the C(9)-methyl glycoside were selectively hydrolysed with PPTS inTHF–H2O. As observed by Masamune and coworkers in theirbryostatin 7 synthesis,35 the presence of a C(20)-O-acyl sub-stituent made hydrolysis of the C(19)-methyl glycoside diffi-cult to achieve under mildly acidic conditions. To overcomethis problem, the Masamune O-deacylation tactic was againmustered, whereafter the desired glycoside hydrolysis wasaccomplished readily with p-TsOH in aq. acetonitrile; condi-tions that did not cause product decomposition. The finalstages of this very beautiful synthesis of bryostatin 2 involvedselective esterification of the C(20)-OH with octadienoic acid(in the presence of the C(3)-OH), and DDQ deprotection of thetwo PMB groups.40

Nat. Prod. Rep., 2002, 19, 413–453 425

Scheme 15 Unification of the A- and BC-ring systems and the final stages of Evans’ bryostatin 2 synthesis.

426 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 16 The Nishiyama–Yamamura retrosynthetic analysis of bryostatin 3.

5.3 The Nishiyama–Yamamura total synthesis of bryostatin 3(2000) 37

a The Nishiyama–Yamamura retrosynthetic plan for bryostatin3

Structurally, bryostatin 3 is the most complex member of thisnatural product family by virtue of its additional C(22)-stereocentre which lies buried within the pyran-butenolideframework. The flanking functionality that is present on eitherside of the C-ring alkene conspires to make its stereocontrolledconstruction an even more challenging task than that for otherfamily members. Nishiyama and Yamamura hoped to forgethis domain through the addition of a vinyllithium inter-mediate (derived from 118) to the α-alkoxy aldehyde 119, underconditions of chelation control (Scheme 16). Such a sequencecould be expected to furnish an alcohol with the correct stereo-chemistry at C(22), and this could then be transformed intothe phenylsulfone 115. As in the Masamune 35 and Evans 36

synthetic ventures, a Julia olefination 38 was planned forestablishing the C(16)–C(17) (E )-disubstituted alkene, while aYamaguchi seco-acid macrolactonisation 66 would be harnessedfor constructing the 20-membered macrolide ring. Like theEvans group, the Keio workers thought it prudent to deferB-ring enoate installation until the final stages of their syn-thesis, and they too envisaged the use of a Fuji chiral phos-phonoacetate 50 for the stereoselective olefination process.However, their WHE substrate looked considerably moredelicate than that of Evans, which clearly added extra risk(if not spice!) to the final outcome.

b Preparation of the enantiopure AB-aldehyde intermediate114

The Keio synthesis of the advanced B-ring intermediate 116 setoff from the “chiral pool” starting material, -mannitol 120(Scheme 17).37e It was converted to -glyceraldehyde acetonide121 by standard literature procedures,67 and this subjected toa Danishefsky hetero-Diels–Alder (HDA) reaction 68 with 122under zinc chloride catalysis. The HDA reaction, which con-formed to the predictions of Cram’s rule, furnished 123 as thesole product in 72% yield. The latter readily engaged in acopper-mediated conjugate addition with vinylmagnesiumbromide to give 124 with total stereocontrol. Ketone 124 wasprotected as a dimethyl acetal with dimethoxypropane, a tacticwhich successfully preserved the O-isopropylidene group.Ozonolysis of the double bond subsequently furnished an axialaldehyde that readily underwent epimerisation to the desiredequatorial configuration via protonation of the aldehyde enol-ate. Aldehyde 125 was then reduced to the alcohol, an O-benzylether installed, and the O-isopropylidene group detached; the1,2-diol unit was then oxidatively cleaved and the resultingaldehyde reduced. Finally, product 126 was O-tosylated anddisplaced with iodide to access 116.

The A-ring was developed from the B-ring iodide by a seriesof reactions that began with alkylation of dithiane 117. Alde-hyde 128, obtained from 127 after this union, was thenemployed for an aldol addition reaction that yielded 129.Transesterification with trimethylsilylethanol converted 129into the β-keto-ester 130 which readily underwent Evans reduc-tion 56 in good yield to give the 1,3-anti-diol 131 with 24 : 1selectivity. A three step sequence was now effected to con-vert 131 into 132, and an O-trimethylsilylethyl to O-allyltransposition used to access 133. O-Debenzylation and TPAPoxidation concluded the synthesis of aldehyde 114.

Dithiane 117 was prepared in ten steps from 11 (Scheme 18),by modifying some of the chemistry originally developed byMasamune for his A-ring synthesis. The most notable step inthe pathway to 117 was 1,3-dithiane formation with propane-1,3-dithiol and magnesium bromide–diethyl ether, a reactionwhich proceeded in good yield without disruption of thepotentially labile OTBS groups.

c Synthesis of the C-ring sulfone 115

A carbohydrate starting material again featured for the prepar-ation of aldehyde 119 (Scheme 19); its carbon backbone equatedwith the C(27)–C(22)-sector of bryostatin 3. Commerciallyavailable diacetone -glucose 135 was tosylated at OH(3) andan E2-elimination performed to access glycal 136.69 A stereo-specific hydrogenation on the less-hindered β-face of 136inverted the stereochemistry at C(4) to thereby establish the syn-relationship needed between the C(25)- and C(26)-hydroxys inthe target. However, before the C(25)–C(27) region could befully tailored, it was necessary to selectively deprotect the exo-O-isopropylidene group from the hydrogenation product, anddeoxygenate the primary alcohol to obtain 138.69 Cleavage ofthe remaining O-isopropylidene group, and hemiacetal ring-opening (both accomplished with propane-1,3-dithiol) thenprovided 139, which was transformed into aldehyde 119 byBOM-protection and thioketal hydrolysis.

The coupling partner for aldehyde 119 was vinyl iodide 118.It was efficiently prepared from 11 via the route shown inScheme 20. A five-step sequence secured aldehyde 141, whichwas olefinated and dihydroxylated with potassium osmatein the presence of the Sharpless DHQ-PHN ligand.70 Theresulting diol 142 was O-isopropylidenated to obtain 143, itsester selectively reduced to the aldehyde with DIBAL, and aCorey–Fuchs olefination 71 used to acquire dibromoolefin 144.Lithiation transformed 144 into the lithium acetylide, whichreacted readily with paraformaldehyde to produce the expectedpropargylic alcohol. The latter willingly participated in ahydroxy-directed hydroalumination–iodination reaction whichfurnished the iodoalkene 145 after O-isopropylidene cleavage.Further protecting group adjustments posted a TBDPS groupon the allylic OH, and a PMB group upon O(19), to complete118.

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Scheme 17 Nishiyama–Yamamura synthetic strategy for the AB-aldehyde synthon 114.

Scheme 18 Route to dithiane 117.

The critical union of 118 with 119 (Scheme 20) was accom-plished by lithiating 118 with MeLi and t-BuLi, and addingaldehyde 119 to the resulting lithiodianion at �90 �C; 3 : 1selectivity resulted in favour of the desired alcohol, which wasprotected as a TBS ether. Selective oxidation of the thiophenylether to the phenylsulfone was achieved with m-CPBA. Notehow the potentially-sensitive alkene unit survived this reactionunscathed. The PMB ether was detached from this product,and the resulting alcohol oxidised to the β,γ-unsaturated ketone148 under Dess–Martin conditions.64 In yet another strikingtransformation, the BOM ether was cleaved selectively from 148

by catalytic hydrogenation without saturation of the nearbytrisubstituted double bond. The selectivity observed presum-ably arises from the bulky allylic OTBDPS group preventingcomplexation of the catalyst with the double bond. Somewhatsurprisingly, these conditions also partially cleaved the O-iso-propylidene group, which was restored by treatment with PPTS,dimethoxypropane and acetone. Fortunately, this reprotectiondid not move the trisubstituted alkene into conjugation with thenearby ketone, presumably, due to a protective effect beingexerted by the bulky C(20)-OTBS group. Formation of methylglycoside 149 was accomplished by reacting the product ketone

428 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 19 Synthesis of the C(22)–C(27) aldehyde segment 119.

Scheme 20 The Nishiyama–Yamamura synthetic pathway to the bryostatin 3 C-ring synthon 115.

with TBSOTf, TMSOMe 46 and dimethoxypropane in dichloro-methane. A protecting group interchange (OTES for OTBS)finally delivered 115.

d Union of the AB- and C-ring intermediates 114 and 115 andcompletion of the bryostatin 3 synthetic venture

A Julia olefination 38 again proved successful for joining thefragments 114 and 115. A single alkene product 151 emerged in52% yield (Scheme 21). The TES groups at O(22) and at theO-allylic position of the C-ring were now detached selectivelywith TBAF–AcOH (1 : 1) to access diol 152 in nearly quanti-tative yield. Selective allylic oxidation of 152 with TPAP–NMOmoulded the desired butenolide system. A second treatmentwith TBAF–AcOH subsequently deprotected the C(20)-OTESgroup to provide the key substrate 154 needed for attachingthe (E,E )-octadienoic acid unit; a task accomplished by theYamaguchi mixed anhydride esterification protocol.66 Transke-

talisation of 155 with CSA–MeOH unmasked the C(25)-and C(26)-hydroxys without damaging other potentiallysensitive functionality such as the C(1)–C(3) β-hydroxy allylicester or the (O)-7 TBS group. Triol 156 was now selectivelyO-silylated with TESCl at OH(26) and OH(3). A palladium()mediated O-deallylation reaction provided the seco-acid 113.The Yamaguchi methodology again proved its worth for accom-plishing the desired macrolactonisation, ring-closure now beingachieved in a spectacular 93% yield. The product was O-desilyl-ated with HF in aq. MeCN; conditions that concurrentlybrought about ketal hydrolysis at C(13) and C(9) to furnish theketo-dilactone 157. In a most risky, but quite breathtaking,master-stroke, an asymmetric WHE reaction 50 was performedon 157 with the chiral phosphonoacetate 95, having leftthe C(26)-, the C(9)-, the C(7)- and the C(3)-hydroxys allunprotected. Remarkably, this tactic substantially improved thestereoselectivity of olefination (relative to the Evans synthesis)to ca. 9 : 1 in favour of the desired alkene isomer. As found by

Nat. Prod. Rep., 2002, 19, 413–453 429

Scheme 21 Union of the AB- and C-ring intermediates 114 and 115 and completion of the bryostatin 3 synthetic venture.

Masamune and Evans in their synthetic endeavours, acidhydrolysis of the C(19)-methyl glycoside again proved trouble-some when a C(20)-ester group was present. However, unlikethese former two ventures, conditions were finally devised foraccomplishing this reaction in high yield without the needfor cleaving the C(20)-ester grouping. Specifically, the C(19)-glycoside was hydrolysed with aqueous trifluoroacetic acid atroom temperature for 1 h. Having successfully negotiated thisparticular hurdle, there now remained the critical issue of C(7)-O-acetate installation. This was achieved by selectively silyl-ating the C(26)-OH with TESCl to obtain 158, and selectivelyO-acetylating the C(7)-hydroxy. The observed regioselectivityundoubtedly arose from the C(3)- and C(19)-hydroxys bothparticipating in a very stable hydrogen bonding network whichpositions the C(3)-OH inside the 20-membered macrolactonecavity; this makes it considerably more hindered than the C(7)-and C(26)-OH groups. The last step of the synthesis wasO-desilylation with aq. HF in MeCN, which was accomplished

in high yield. So far, Nishiyama and Yamamura have been ableto prepare 25 mg of synthetic bryostatin 3 via this route. Thisis a testament to the great practical utility of this quite out-standing total synthesis; it suggests that a future commercialsynthesis might soon be possible.

6.0 Synthetic studies on the bryostatins

Alongside these three mammoth synthetic enterprises, a signifi-cant number of other groups have also been active in the bryo-statin area, with some having reported syntheses of severaladvanced synthetic intermediates en route to bryostatins 1 and11. The activities of these groups will now be discussed.

6.1 Thomas’ synthetic studies on bryostatin 11 72,73

Thomas’ work on bryostatin 11 has so far resulted in thepreparation of two fragments 159 and 163 which correspondto the B- and C-rings respectively. Both are equipped with

430 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 22 Thomas’ retrosynthetic gameplan for the BC-sector of bryostatin 11.

functionality appropriate for a future foray towards the naturalproduct. Their retrosynthetic planning for these segments ispresented in Scheme 22.

a Thomas’ retrosynthetic analysis of bryostatin 11

For the B-ring synthon 159, a very bold 6-endo-trig vinyl radicalcyclisation was proposed, involving the free radical intermedi-ate 162 (Scheme 22). Thomas reasoned that such a radicalwould conjugately add to the tethered enol enoate to generate acaptodatively-stabilised tertiary tetrahydropyranyl radical 161,which could then go on to abstract hydrogen from tributyl-stannane from the α-face of the pyran, opposite to the bulkyC(15)-CH2OTBS group. A key feature of their stratagemwas an assumption that the (Z )-vinyl radical would lie inequilibrium with its (E )-counterpart (as a result of rapidvinyl radical inversion), and that the former would cyclisemore rapidly due to this transition state being less stericallyencumbered than its isomeric alternative, which would place theester methoxy close to the enol enoate. Provided that the rate of(Z )-vinyl radical cyclisation was substantially faster that of the(E )-isomer, a rapidly equilibrating mixture of vinyl radicalscould easily be envisaged to channel towards the desired alkeneproduct with good selectivity. It was further anticipated thatboth vinyl radicals could be derived from a common vinyliodide by attack of a tributylstannyl radical; such an iodidecould, in turn, be potentially synthesised from (S )-glycidol.

For the C-ring phenylsulfone 163, a novel palladium()catalysed cross-coupling was envisioned for uniting the stereo-defined vinyl bromide 165 with the tributylstannyl enolate 164(Scheme 22). The fact that very few examples of such cross-couplings had previously been documented in the naturalproduct field undoubtedly made this a particularly appealingdisconnection for further evaluation.

b Thomas’ racemic free radical routes to the bryostatin B-ring(1989 and 2000) 72

To rapidly assess the viability of their planning, Thomas andcoworkers carried out the synthesis of 159 with racemic glycidolrather than with optically pure (S )-glycidol. Thus (RS)S )-glycidol was O-silylated with TBSCl, and the resulting epoxide166 ring-opened with the lithio-anion of methyl propiolate inthe presence of Lewis acid (Scheme 23). Alcohol 167 was thenconverted to the alkoxymalonate 169 by carbenoid insertionwith 168 and rhodium acetate.74 Alkylation of this malonatewith Me2N=CH2

� I� 75 and decarboxylative elimination sub-sequently provided the enol enoate 171. A highly chemo-selective stannylcupration 45 was now effected on alkynyl ester171, despite the presence of the enol enoate, which also couldhave functioned as a possible Michael acceptor. Ipso-substi-tution of the vinylstannane in 172 with iodine again left the

pendant enol enoate undisturbed. Vinyl iodide 173 was the keysubstrate needed for testing the aforementioned vinyl radicalcyclisation. In the event, when 173 was exposed to tributyltinhydride and AIBN in benzene at reflux for 45 min, a 4 : 1mixture of exocyclic alkene isomers was isolated, enriched inthe desired product 174. Borohydride reduction of the moreelectrophilic ester group in 174–175, and separation of the twoisomeric components, completed this very novel racemic routeto the bryostatin B-ring.

Thomas and coworkers have also prepared the racemic PMB-protected vinyl iodide 178 and investigated its radical inducedring-closure (Scheme 24). The latter cyclised with a 4 : 1 level of(E–Z) stereocontrol, which was comparable to that attainedwith 173. The selectivity observed in the installation ofthe C(11)-stereocentre was significantly diminished however.Thomas attributed the formation of 179 to competitive internal1,7-hydrogen-atom abstraction of the benzylic hydrogen fromthe PMB group, which would guarantee syn-delivery of thehydrogen to the more hindered β-face of the tetrahydropyranylradical. The formation of 179 can equally well be explained bythe reduced steric size of C(15)-CH2OPMB group making thisunit much less effective at directing the stereochemical courseof H-atom abstraction from the stannane. Undoubtedly thisfiner mechanistic point could be resolved by labelling-workwith Bu3SnD.

c Thomas’ asymmetric synthesis of the C-ring sulfone 163(2000) 73

The cardinal intermediate needed for Thomas’ Pd() catalysedenolate cross-coupling strategy was vinyl bromide 186; it wasprepared by the pathway shown in Scheme 25. β,γ-Unsaturatedester 182 was reduced to the homoallylic alcohol 183 withDIBAL, and alcohol 183 was protected as a TBS ether. Com-pound 184 was then subjected to a Sharpless AD reaction withAD-mix-β to obtain diol 185 in 85% ee (±5%).76 Thomas trans-formed diol 185 into the Masamune aldehyde 34 35a in a furtherthree steps: O-isopropylidenation, O-desilylation and Swernoxidation. The next six reactions were identical to those pub-lished by Masamune in his route to 39 (see section 5.2, Scheme6); the only difference noted was in the diastereoselectivity ofthe propargylation reaction with aldehyde 34, where Thomasrecorded 4 : 1 selectivity in favour of 35, which was substan-tially less than the 8 : 1 selectivity level reported by Masamune,who used aldehyde of 100% ee.35a Stannane 39 underwent anN-bromosuccinimide mediated halogen–metal exchange withretention of olefin geometry to produce 186 which proved awilling partner in the Pd()-mediated cross-coupling process.77

The desired β,γ-unsaturated ketone was isolated in 73% yield.It was converted to the target sulfone 163 by chemoselectiveoxidation with m-CPBA which, again, left the trisubstitutedalkene untouched.

Nat. Prod. Rep., 2002, 19, 413–453 431

Scheme 23 Thomas’ free radical cyclisation technology for controlling bryostatin B-ring olefin geometry.

6.2 Vandewalle’s synthetic studies on bryostatin 11 78

Vandewalle and his group have also been active in the bryo-statin arena since the early 1990s having disclosed the results ofseveral model studies, and the synthesis of various advancedintermediates. Their work will now be reviewed.

a Attempted construction of the C(1)–C(9) backbone of thebryostatins via a dithiane coupling strategy (1991) 78a

Early effort in the Vandewalle laboratory focussed on the useof a dithiane–epoxide coupling tactic for assembling theC(1)–C(9)-backbone, involving epoxides 194 and 197, and 1,3-dithiane as a C(5)-linchpin (Scheme 26). Epoxide 194 was pre-pared in five steps from (R)-pantolactone by the method ofLavallee.79 The route involved reduction of 191 to the ring-opened triol with lithium aluminium hydride, selective prepar-

Scheme 24 Outcome of free-radical cyclisation with a C(16)-OPMBprotecting group.

ation of the dioxolane acetal 192 with pentan-3-one, protectionof the remaining primary OH as a PMB-ether, and hydrolysisof the acetal to obtain the 1,2-diol 193. The latter was con-verted to the terminal epoxide 194 by treatment with sodiumhydride and N-tosylimidazole. Epoxide 197 was synthesisedfrom (S )-malic acid.80

Vandewalle noted that while epoxide 194 reacted efficientlywith 2-lithio-1,3-dithiane to give 196 after O-benzylation, thesubsequent alkylation between 196 and 197 proceeded poorly,even when conducted in the presence of TMEDA and DMPU.At best, alcohol 198 was obtained in a rather meagre 13% yield,along with 4% of the elimination product 199 and recoveredstarting materials. In light of this setback, Vandewalle alteredhis synthetic approach in favour of the one shown below (seesection 6.2 b, Scheme 27).

b Synthesis of the C(1)–C(9)-segment of the bryostatins (1991)

Epoxide 194 was again enlisted in the new route, which had nowidentified compound 210 as the target intermediate. Epoxide194 reacted with the Lipschutz HO vinylcyanocuprate reagent 81

to yield the homoallylic alcohol 202 which readily underwentdihydroxylation and oxidative cleavage. Aldehyde 206 was thenexploited for a substrate-controlled asymmetric aldol additionreaction with the lithium enolate of 207. Surprisingly, excellentstereocontrol manifested itself in this process; the correct alco-hol stereochemistry emerging at C(5). Some nineteen differentreducing reagents were evaluated before success was attained inthe hydroxy-directed anti-reduction of 208 and, somewhatunexpectedly, the normally reliable Evans reagent Me4NBH-(OAc)4

56 showed no selectivity in this system. After muchexperimental effort, the combination of LiI and lithium tri-tert-butoxyaluminium hydride 82 secured the desired result, namely,a 17.6 : 1 ratio of products in favour of the desired 1,3-diol209, which was converted to 210 by acetal exchange. Clearly,the orthogonal arrangement of protecting groups in this

432 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 25 Thomas’ palladium()-catalysed cross coupling strategy for the bryostatin 11 C-ring.

Scheme 26 Vandewalle’s attempt at implementing a dithiane linchpin strategy for assembly of the C(1)–C(9)-sector.

intermediate make it attractive for further manipulation into anatural bryostatin, but no additional results have yet beenreported in this direction.

c Model studies on the bryostatin B-ring (1991) 78a

Vandewalle’s gameplan for constructing the bryostatin B-ring is

predicted on the use of a stereoselective Michael addition reac-tion for setting the C(11)-stereocentre, and on the assembly ofan appropriate enone precursor from hemiacetal 217. His pre-liminary testing of these concepts (Scheme 28) demonstratedthat total stereocontrol could be attained by following such anapproach. Compound 220 was formed as a single product whena model tandem WHE olefination–Michael addition sequence

Nat. Prod. Rep., 2002, 19, 413–453 433

Scheme 27 Vandewalle’s stereocontrolled route to the C(1)–C(9) segment of the bryostatins.

Scheme 28 Vandewalle’s model studies on the bryostatin B-ring.

was applied to hemiacetal 217. Very recently, Yadav and co-workers 114 have further developed these ideas and reported aneven more elaborate WHE strategy which connects two highlydecorated bryostatin A and B-ring synthons together (seesection 6.10), emphasising the soundness of such an approachfor constructing this domain. Other work along these lines isawaited with interest.

d Vandewalle’s synthetic strategy for the C-Ring region ofbryostatins 1 and 11 (1994) 78b,c

Vandewalle has devised a unified pathway for obtaining theadvanced C-ring intermediates 235 and 237 which again looksapplicable to a future bryostatin synthesis. The main threads ofhis route are presented in Schemes 29 and 30. Commerciallyavailable -isobutyl lactate 221 was deemed an ideal startingmaterial for the C(21)–C(27) sector as a chirality match existedbetween its lone stereocentre and the C(26)-hydroxy of thebryostatins. Protection and reduction of 221 yielded 222 whichunderwent Swern oxidation and Keck allylation 83 to provide223 with high stereocontrol (97% de) (Scheme 29). p-Methoxy-benzylation 40 of this alcohol and two-stage oxidative cleavageof the double bond subsequently furnished aldehyde 224 whichparticipated in a second, highly diastereoselective, Keck allyl-ation 83 with allyltributylstannane (97% de). Dimethoxy-benzylation of 225 and oxidative degradation of the alkene

appropriated aldehyde 227 which was alkynylated with the Sey-ferth–Gilbert reagent 228 84 before homologation with n-butyl-lithium and methylchloroformate. Tactically the remainder ofthe route to 235 mirrored Masamune’s C-ring synthesis 35 (seeScheme 6) except for the use of a TBS-protecting group in alde-hyde 234; Vandewalle’s intermediate also had opposite absolutestereochemistry to the aldehyde used by Masamune (see com-pound 10 in Scheme 6). The primary effect of making these twochanges, however, was to erode the stereoselectivity observed inthe vinyl anion addition step quite substantially. It will berecalled that Masamune attained 8 : 1 selectivity in hischelation-controlled vinyllithium addition, whereas Vandewalleobserves only 2 : 1 stereoselectivity in his system.

It should also be noted that Vandewalle has investigated theopening of epoxide 236 with the HO vinylthienylcupratederived from iodide 233. Unfortunately the latter reactionproceeds in only 29% yield, and delivered the ketone 237after oxidation. The recently reported Thomas enolate cross-coupling strategy to 163 therefore represents a significantimprovement over this previous art for constructing the C(20)–C(21) bond in bryostatin 11 (Scheme 30).

e Vandewalle’s use of (R)-carvone for a new synthesis of theMasamune C(27)–C(34)-alkyne fragment (1997) 78d

Before departing completely from Vandewalle’s synthetic

434 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 29 The bryostatin C-ring synthesis of Vandewalle and coworkers.

Scheme 30 Vandewalle’s route to aldehyde 234 and epoxide 236.

efforts on the bryostatin C-ring, it is pertinent to discuss hisalternative route to the Masamune C(27)–C(34) alkynefragment 36 (Scheme 31), and his pathway to the alkyne 253,both of which proceed from (R)-carvone (Schemes 31 and 32).(R)-Carvone was stereospecifically epoxidised using basichydrogen peroxide,85 and an organoselenium-mediated reduc-tive ring-opening 86 used to regioselectively deoxygenate α- tothe ketone. A 4 : 1 level of selectivity in favour of 246 wasrecorded for this reduction, which served as the preamble toprotection and oxidative cleavage for accessing 247. Applicationof a double Baeyer–Villiger oxidation to 247 readily installedthe C(26) masked hydroxy with stereochemistry appropriatefor the target segment; the C(25)-hydroxy had previously beenintroduced at the epoxidation stage. The seven-memberedβ-acetoxy lactone 248 was then reduced to 249 and this selec-tively protected to obtain 250. O-Desilylation and oxidation of

250 furnished an aldehyde that willingly participated in aSeyferth–Gilbert alkynylation reaction. Although Vandewalle’ssynthesis of this fragment is four steps longer than that ofMasamune, his route does nevertheless commence from a chiralstarting material that is totally devoid of hydroxy stereocentres.Extra length is therefore to be somewhat expected (Scheme 32).

6.3 R. W. Hoffmann’s racemic route to the C(1)–C(9) segment267 (1995) 87

A diastereocontrolled, racemic, route to an advanced A-ringsynthon 267 has been disclosed by R. W. Hoffmann andStiasny 87 which capitalises on Matteson haloboronate dis-placement technology 88 for backbone assembly (Scheme 33).The central intermediate in this pathway to 267 is thegeminal dibromide 258; it is prepared in two steps through

Nat. Prod. Rep., 2002, 19, 413–453 435

Scheme 31 Vandewalle’s alternative route to the Masamune bryostatin 7 alkyne intermediate 36 from (R)-carvone.

Scheme 32 Vandewalle’s synthesis of the differently protected alkyne intermediate 253 from (R)-carvone.

Scheme 33 R. W. Hoffmann’s pathway to a racemic C(1)–C(9) bryostatin fragment.

the nucleophilic displacement of cyclic sulfate 257 with 1,1-dibromomethyllithium, and protection with TBSCl. Treatmentof 258 with n-BuLi in the Trapp-solvent mixture at �110 �Cproduced a mixture of carbenoids 259 and 260 in which theformer predominated with 3 : 1 selectivity. While carbenoid 260cyclised spontaneously to 261 in the absence of electrophiles,carbenoid 259 showed little tendency to do this, providedthe temperature was kept at �110 �C. As a consequence, itproved possible to intercept it with the cyclic boronate 262 withreasonable efficiency. The so-formed α-bromo-boronate 263was then coupled with the dienolate 264 and the product 265subjected to trimethylamine-N-oxide oxidation. The overallyield of alcohol 266 from 258 was 35%. Evans reduction 56 of

266 and protection completed the protected intermediate 267 inwhat was a novel and highly original route.

6.4 The Kalesse enantioselective route to the C(1)–C(9)-segment 257 (1996) 89

Kalesse has reported a C(1)–C(9) segment synthesis (Scheme34) in which a biotransformation is employed for the keyasymmetry-inducing step. His route to 275 commenced with theacetoacetate aldol reaction between 268 and 269 to obtain 270,which was protected and C-desilylated to obtain the racemicketo-alkyne 271. A stereoselective reduction–kinetic resolutionwas performed on 271 with Baker’s yeast in the presence of

436 Nat. Prod. Rep., 2002, 19, 413–453

water and sucrose to access alcohol 272 in 84% ee and 82% de;this was taken forward to the alkene 273 as shown. Hydro-boration and oxidation converted 273 into an aldehyde thatreadily partnered prenyltrimethylsilane 274 in a Lewis acidmediated Sakurai reaction; the final-result was an inseparable 6: 1 mixture of alcohols enriched in 275.

6.5 The Kiyooka reagent-controlled asymmetric aldol route tothe C(1)–C(9) segment of the bryostatins (1997) 90

In a rather stunning display of reagent-controlled asymmetric

Scheme 34 Kalesse’s chemoenzymatic approach to C(1)–C(9) sectorassembly for the bryostatins.

aldol technology, Kiyooka 90 has reported the application of hisvaline-derived sulfonamido-borane reagents 278 and 281 forthe efficient construction of the C(1)–C(9) intermediate 285.The strategy, which is outlined in full in Scheme 35, is basedupon the iterative use of the mixed silylketene thioacetal 276 asa nucleophile in a series of boron-mediated aldol reactions withaldehydes 277 and 280 respectively. A series of nickel-boride-induced desulfurisations were used to liberate the desiredacetate aldol products on each occasion. The final aldoladdition between 283 and 284 generated 285 as a single isomer.The Kiyooka synthesis is really quite superb as it proceeds inonly nine steps, and it operates with excellent levels of stereo-control. Further work from this laboratory is awaited with greatinterest.

6.6 Roy’s synthetic studies on bryostatin 1 (1989, 1990) 91

Roy has reported synthetic pathways to the A- and C-ring syn-thons 296, 298 and 307 which are attractive for their brevity(Schemes 36 and 37). His unified route to 296 and 298 reliesupon a biotransformation for asymmetric induction (Scheme36), while that to 307 exploits the sugar starting material,-galactono-1,4-lactone 299 (Scheme 37).

a Roy’s synthetic path to the C(1)–C(9) fragment of bryostatin1 (1990) 91b

Reduction of commercially-available dimethyl 3-ketoglutarate286 with sodium borohydride produced an alcohol thatwas protected as the MOM ether 287 (Scheme 36). Anenzymatic hydrolysis of the pro-S methyl ester in 287 withα-chymotrypsin was then used to secure the acid 288 in 94%ee;92 the latter was reduced to the primary alcohol 289 via amixed anhydride. After oxidation with PCC, the product alde-hyde was used for a substrate-controlled Mukaiyama aldoladdition reaction 93 with the enol ether 291. Considerable effortwas expended on modifying the stereochemical outcome of thisprocess to favour the desired anti-addition product 293. Aftermuch experimentation, boron trifluoride–diethyl ether wasidentified as the best Lewis acid for this purpose, it affording a1.9 : 1 mixture of epimers at C(5) enriched in 293. An almosttotally stereoselective Evans reduction followed for introducingthe C(7)-hydroxy. Separation of the resulting mixture was besteffected at the O-isopropylidene stage. Whilst one can readilyenvisage using 297 for a future bryostatin synthesis, Evans’ laterwork suggests that lactone 296 (obtained from 295) might beequally valuable for a future synthetic expedition in thisdirection.

Scheme 35 Kiyooka’s iterative asymmetric aldol approach to the C(1)–C(9) segment of the bryostatins.

Nat. Prod. Rep., 2002, 19, 413–453 437

Scheme 36 Roy’s chemoenzymatic route to the bryostatin A-ring.

Scheme 37 Roy’s synthesis of a C(21)–C(27)-dithiane intermediate for the bryostatins.

b Roy’s enantiospecific route to a C(21)–C(27)-synthon 307(1989) 91a

Starting from the inexpensive but rarely used -galactono-1,4-lactone 299, Roy has prepared the dithiane intermediate 307 inonly nine steps (Scheme 37). He equated the six-carbon back-bone of this hexose with the C(21)–C(27) domain of bryostatin1, having stereochemically matched the C(25)- and C(26)-hydroxys of 307 with the C(4)- and C(5)-hydroxys of 299.Clearly deoxygenation would be a prerequisite at C(3)and C(6) for success in this approach, but tactically, this didnot appear too problematical at the outset. Accordingly, 299was brominatively O-acetylated under strongly acidic condi-tions and the product 300 hydrogenated in the presence ofEt3N. The latter promoted β-elimination of the C(3)-acetate

to give the enol acetate 301 which then underwent alkenereduction from the less-hindered β-side; besides reductivedehalogenation. Thus, in only two steps the requisite carbonframework of the target intermediate had been hewed withcomplete stereocontrol. The remainder of the synthesis con-centrated upon selectively differentiating the hydroxys intetraol 303,94 which had been derived from 302 by lithiumborohydride reduction. For this, bis-acetalation was attemptedfollowed by regioselective hydrolysis of the less hinderedacetal. The latter was accomplished with either p-TsOH–MeOH or 1% iodine in MeOH. Selective O-tosylation of 305was next attempted, and the product tosylate converted toepoxide 306 with mild base. Opening of 306 with 2-lithio-1,3-dithiane and protection finally afforded the C(21)–C(27)segment 307.

438 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 38 H. M. R. Hoffmann’s retrosynthetic planning for the bryostatin northern hemisphere.

6.7 The H.M.R. Hoffmann route to the C(1)–C(16) ABSegment 308 (2000) 95

a H. M. R. Hoffmann’s retrosynthetic analysis of theC(1)–C(16) fragment 308 95

The 1,3-dithiane that is present at C(9) in 308 serves toprime the adjacent C(9)–C(10)-bond for retrosynthetic disjunc-tion and helped guide Hoffmann’s choice of triflate 309 anddithiane 310 as precursors of 308 (Scheme 38). Hoffmann wasattracted to the idea of controlling B-ring olefin geometry by aprotecting group-directed WHE reaction 49 between 312 and anappropriately protected phosphonoalkanoate. He envisionedthat the C(11)-trityloxymethyl group of 312 would effectivelyshield the upper-face of the ketone to favour stereospecificphosphine oxide formation and elimination. Accordingly 313was selected as the precursor of 312, and ozonolysis and reduc-tion were proposed as key steps in the forward sequence. Withregard to the important issue of asymmetric induction, Hoff-mann appreciated that a pathway that commenced from 313would create a meso-diol, and he intended to access optically-pure material by O-acylation and enzymatic desymmetrisationwith an appropriate lipase. Further retrosynthetic reason-ing suggested that pyran 313 could derive from the cationic[4�3]-cycloaddition of 315 with furan, a reaction invented byHoffmann in the early 1970s,96 and later refined by the groupsof Noyori 97 and Fohlisch.98 A similar cycloaddition between314 and 323 was envisaged for creating the racemic pyranones321 and 322, one of which could potentially be developed intothe A-ring dithiane 310. Here the issue was to identify a suitableasymmetric reaction that could effect an efficient “kinetic reso-lution” of 321 and 322. Hoffmann believed that a forwardsequence involving ketone reduction, alcohol protection, and

Brown asymmetric hydroboration 99 might suffice in thiscapacity. He reasoned that the latter process might proceedunder electronic control, which would lead to the moreelectron-rich alkene carbon bonding to the boron of the hydro-borating agent. A double oxidation could then be used to intro-duce a ketone on the carbon proximal to the geminal dimethylgroup, which would pave the way for use of a Baeyer-Villigeroxidation to access 320. With this as background, we will nowdescribe the Hoffmann synthesis of 308 in more detail.

b H. M. R. Hoffmann’s asymmetric synthesis of theC(1)–C(9) dithiane 310

Following the improved experimental procedure of Fohlisch,98

which was published in 1982, Hoffmann and coworkers reactedthe α-chloromethylketone 324 with furan and lithium per-chlorate in a mixture of ether and triethylamine (Scheme 39).These conditions generated an oxyallyl cation which readilyengaged in a [4�3]-cycloaddition with the furan 96 to provide aracemic mixture of the ketones 321 and 322. The latter werethen reduced stereoselectively with -selectride and the alcoholsprotected as O-benzyl ethers. Asymmetric hydroboration of 325and 326 with (�)-Ipc2BH 99 was not regioselective, but thiswas of little real consequence, for oxidative work-up andsubsequent oxidation with PDC afforded a mixture of tworegioisomeric ketones which underwent regioselective Baeyer–Villiger oxidation to give 320 and 327. Being diastereoisomers,the separation of these intermediates was feasible through flashchromatography. Transesterification converted 320 into a mix-ture of the hemiacetals 319 which were driven towards dithiane317. Compound 317 was homologated by Claisen condensationwith an excess of the enolate derived from tert-butyl acetate.

Nat. Prod. Rep., 2002, 19, 413–453 439

Scheme 39 H. M. R. Hoffmann’s asymmetric synthesis of the C(1)–C(9)-sector of the bryostatins.

This procured the β-keto ester 316, having the requisite C(1)–C(9)-backbone; the latter underwent stereospecific reduction tothe anti-1,3-diol 328 with the Evans reagent. Protection andester reduction were now used to obtain 329. The next deprotec-tion was particularly noteworthy, as it entailed reductivelyremoving the C(7)-O-benzyl group from 329 in the presence ofthe potentially labile 1,3-dithiane; significantly, the reactionwith lithium di-tert-butylbiphenyl 100 proceeded in excellentyield (98%). A series of silylations on diol 330 concluded thissynthesis of 310.

c H. M. R. Hoffmann’s asymmetric construction of the B-ringtriflate 309 and elaboration into the AB-system 308

The best method for preparing meso-ketone 313 is that ofNoyori and coworkers.97 They observed that heating a mixtureof tetrabromoacetone, diiron nonacarbonyl and furan at refluxfor two days brought about a reasonably facile [4�3]-cyclo-addition reaction between the intermediary dibromoallyloxycation and the furan to produce a 9 : 1 mixture of the twoisomeric bromopyranones 332 and 333, which could be reduc-tively dehalogenated with zinc–copper couple to give 313 in63% overall yield (Scheme 40). Anticipating problems in thesodium borohydride reduction step, if ozonolysis and reductionwere to be attempted on 313, Hoffmann elected to protect itscarbonyl as an acetal. Ozonolysis of 334 and reduction withNaBH4 now proceeded smoothly to generate 335 in high overallyield after O-acetylation. This was the crucial intermediateneeded for the enzymatic-desymmetrisation step, which was

accomplished with lipase PS at pH 7; this provided 336 in96% yield and > 98% ee. The carbonyl group was now regener-ated under Pd() catalysis, the primary OH O-tritylated, and anO-deacetylation performed to access the partially-protectedketone 338. A range of phosphonates were investigated for theWHE reaction on ketone 338; the best results were attainedwith sterically-demanding phosphonoacetates at low reactiontemperatures over prolonged reaction times. The optimal con-ditions employed isopropyl diisopropoxyphosphonoacetate intoluene at �8 �C for 7 days, and provided 339 in 99% yield andwith 49 : 1 (E–Z) selectivity. O-Silylation of 339 and enoatereduction with DIBAL transformed it into 340 which wasprotected as a triphenylsilyl ether. Another very noteworthydeprotection was now accomplished, namely, a zinc bromide-mediated cleavage of the O-trityl ether in the presence ofthe potentially labile allylic TPS ether. As can be seen, thisdeprotection proceeded cleanly, delivering 342 in 94% yield.

The requisite triflate 309 needed for coupling to 310 was bestprepared with triflic anhydride and the very hindered base, 2,6-di-tert-butyl pyridine; the latter prevented quaternisation of theresulting primary O-triflate ester. The latter is often problem-atical when other less hindered bases such as pyridine are usedfor triflation. Compound 309 coupled smoothly to the lithio-anion derived from 310 to furnish 343 in 63% yield. Hoffmanndrew attention to the fact that different protecting groups onO(7) quite dramatically affected the coupling yield; the bestresults were obtained when a TBS protecting group resided atthis position. The final two steps of the route to 308 involvedselective manipulation of the silyl protecting groups.

440 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 40 H. M. R. Hoffmann’s stereocontrolled bryostatin B-ring synthesis and completion of a fully elaborated “Northern Hemisphere”.

Hoffmann’s route to 308 is clearly very good, and shouldpermit a future bryostatin total synthesis; further results areawaited eagerly.

6.8 Hale’s synthetic work on the bryostatins (1995, 2000,2001) 101

a Hale’s retrosynthetic planning for bryostatin 1

Hale’s most recent retrosynthetic analysis of bryostatin 1 issummarised in Schemes 41 and 42. The biogenetically-modelledmacrolactonisation of 344 is currently envisaged for 20-membered macrolide assembly. A Julia olefination is alsoplanned for connecting the AB- and C-ring segments (345 and346) together (Scheme 41). The past collective precedents ofMasamune,35 Evans 36 and Nishiyama,37 all suggest that good(E )-selectivity will be attainable in this reaction. A Julia olefin-ation 38 is also proposed for linking 347 with 348 (Scheme 42). Ifsuccessful, the latter should afford an exocyclic glycal capableof being converted to a methyl glycoside under mildly acidicconditions. After selective removal of the primary PMB group,alcohol oxidation should create aldehyde 345.

Our retrosynthetic planning for aldehyde 347 commencedwith the selection of alcohol 349 as a possible precursor(Scheme 43). The primary effect of retrosynthetically position-ing a hydroxymethyl group directly adjacent to the pyranylether bond in 349 was to actuate the ring for retrosyntheticdisassembly via the Williamson etherification transform,102

which yielded epoxy alcohol 350 as a possible cyclisation sub-strate. Further evaluation of 350 suggested that it might poten-tially be obtainable from the enoate 351, and disconnection ofits double bond in turn yielded the C2-symmetric ketone 353.Conceptually, a Wittig or Peterson olefination reaction on aC2-symmetrical substrate such as 353 could allow for completestereochemical control of the exocyclic olefin geometry in thebryostatin B-ring, which was most attractive. Ketone 353 itselflooked readily accessible from a Smith–Tietze TBS-dithianecoupling tactic.103

For the C-ring phenylsulfone 346, bicyclic lactone 356 lookedan appropriate precursor, and a stereospecific Wittig olefinationwas planned for fashioning its exocyclic enoate (Scheme 44).Here the flanking functionality at C(20) could be expected tobeneficially direct the stereochemical course of olefination.

Nat. Prod. Rep., 2002, 19, 413–453 441

Further unravelling of the bicyclic array in 357 allowed theβ-keto ester 357 to be identified as an intermediate; its mostlogical site for retrosynthetic disjunction was across its C(18)–C(19)-bond via retro-Claisen condensation. The latter oper-ation yielded ester 359 as a sub-target structure. Due analysis of359 revealed that all of its stereocentres could potentially be

Scheme 41 Hale’s retrosynthetic analysis of bryostatin 1.

Scheme 42 Hale’s retrosynthetic planning for the “NorthernHemisphere” of bryostatin 1.

Scheme 43 Hale’s retrosynthetic plan for completely controlling theB-ring olefin geometry of the bryostatins.

installed by Sharpless catalytic AD reactions 70 on appropriatealkene precursors. With this as background, we will nowdescribe our efforts at implementing these pathways.

b Hale’s C2-symmetry breaking olefination tactic for the totallystereocontrolled asymmetric construction of the bryostatinB-Ring (2000) 101a

The route we have now developed to the bryostatin B-ring ispresented in Scheme 45.101a It commences with a Smith–Tietzebis-alkylation reaction 103 between 2-lithio-2-TBS-dithiane 354and the homochiral epoxide 355,104 which is available in onestep from commercially available (S )-glycidol. This reactiongenerates an intermediary alkoxide that gives the dithiane 360in 87% yield after in situ trapping with TBSCl. The thioketalunit of 360 was best detached with mercuric perchlorate in aq.THF, which yielded ketone 353. To our great dismay, ketone353 proved a most unwilling participant in all relevant Wittigand Peterson olefination processes that we examined. Giventhese difficulties, alternative methods for the olefination of 353were investigated. After much effort, a satisfactory solution tothis problem was eventually found. Ketone 353 was reactedwith allylmagnesium bromide to obtain 361, and its doublebond oxidatively cleaved. The resulting aldol 362 was thensubjected to a β-elimination reaction to obtain 363; our C2-symmetry-breaking olefination tactic had finally worked! 1,2-Reduction of 363 with DIBAL, and O-desilylation nextafforded the triol 364, which was one of the key intermediateswe had originally hoped to prepare in our first-conceived routeto 349.

We were now confronted with the challenging prospect ofhaving to differentiate between the two partially maskedterminal 1,2-diol units in 364, both of which were in a fairlysimilar environment. Fortunately this was accomplished fairlyreadily by chemoselective oxidation of the allylic alcohol withMnO2. Initially this delivered an enal which underwent spon-taneous hemiacetalisation followed by further oxidation to theα,β-unsaturated lactone in excellent yield. The diol differen-tiation process was completed by detaching the PMB groupsfrom the product with TFA–anisole,105 and blocking thehydroxy groups of 365 with cyclohexylidene and PMB groupsto obtain 367. Reduction of the lactone unit in 367 under Lucheconditions 106 procured diol 368. Selective introduction of aTBDPS ether into 368 then served as the prelude to chemo-selective removal of the cyclohexylidene grouping from 369with propane-1,3-dithiol and catalytic BF3�Et2O at low tem-perature.107 As long as the course of this reaction was carefullymonitored by TLC, and due care was paid to keeping the tem-perature below �10 �C, high yields of the desired triol 370could be isolated routinely. Regioselective O-mesylation of 370was most satisfactorily achieved with the mesyl chloride–collidine system of O’Donnell and Burke.108 We had anticipatedthat the dihydroxymesylate might be directly converted to 349

Scheme 44 Hale’s retrosynthetic tactics for the C-ring of bryostatin 1.

442 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 45 Hale’s fully stereocontrolled asymmetric synthesis of the bryostatin B-ring via a C2-symmetry-breaking tactic.

by treatment with 2 equiv. of NaH. However, when we exam-ined this reaction, we found that only the δ-hydroxy epoxide350 was formed in 80% yield. The desired pyran ring-closurewas accomplished by treating 350 with cat. camphorsulfonicacid in CH2Cl2;

102 it furnished pyran 349 in 87% yield as the solereaction product, completing this totally stereocontrolled routeto the bryostatin B-ring in 6.34% overall yield from 355.

c Hale’s first attempt at synthesising the “SouthernHemisphere” of bryostatin 1 (1995 and 2000) 101b,c

Turning attention now to the C-ring chemistry that was initiallydeveloped.16b At first we set out to introduce the C(26), theC(25)- and the C(23)-hydroxy stereocentres of the C-ring sub-sector through a Sharpless AD reaction 70,109,110 between(E )-hexa-1,4-diene 371 and excess AD-mix-β (Scheme 46).Whilst this reaction did successfully install the C(25) and C(26)-hydroxys with high stereocontrol, the second AD process on the

terminal alkene was non-selective, it affording an inseparable1 : 1 mixture of epimers at the C(23)-position. We thereforefocussed our attention upon the selective dihydroxylation of371 with a limited quantity of AD-mix-β (0.6 equiv.), asdescribed by Sharpless et al.110 Typically this provided thevolatile diol 374 in 58–59% yield and 94% ee. We now envisagedinstalling the C(23)-stereocentre through a Sharpless AE reac-tion.42 After blocking the hydroxys of 374 with TBSCl, thedouble bond of 375 was oxidatively degraded with cat. OsO4

and NaIO4, and the product aldehyde 376 pushed towards therequisite allylic alcohol 378 by Wittig reaction and DIBALreduction. The Sharpless AE on 378 proceeded smoothly, andenhanced the optical purity of the product to 96% ee. Regio-selective deoxygenation of 379 was now effected withREDAL;41 it led to the 1,3-diol 380 in good yield. Next, we hadto selectively position a PMB group on the C(23)-OH, oxidisethe C(21)-hydroxy to the aldehyde, and effect a Wittig reactionon 383 with Ph3P��CHCO2Et. All of these reactions proceeded

Nat. Prod. Rep., 2002, 19, 413–453 443

Scheme 46 Hale’s synthetic studies on the C-ring of bryostatin 1.

efficiently, producing the desired enoate 384 as a single geo-metrical isomer in 89% overall yield from 382. A Sharpless ADreaction on 384 with AD-mix-β was now used to introduce theC(20)-hydroxy stereocentre; it furnished diol 385 as essentially asingle diastereoisomer. To successfully implement the afore-mentioned Claisen condensation, this diol now had to be pro-tected. An O-isopropylidene group performed well in thiscapacity, it allowing the desired β-keto ester 357 to be obtainedin 89% yield from 359. To continue the synthesis we now had toremove the O-isopropylidene group with acidic methanol, andeffect an in situ butyrolactonisation with the C(20)-hydroxy.

This would allow the C(20)- and C(21)-hydroxy groups to bereadily differentiated, and would set the stage for a tandemFischer glycosidation 111 at C(19). Reasoning that introductionof an electron-withdrawing ester group at C(20) would almostcertainly necessitate us having to use fairly forcing acid condi-tions to bring about the desired glycosidation, we accepted thatwe would have to replace both TBS groups in 357 with muchmore acid-stable O-pivaloyl esters. With this duly done, thePMB ether was detached from 387 with DDQ to access 388.Treatment of 388 with methanolic HCl led to the expectedmethyl glycoside 389 in 58% yield. A Sharpless oxidation 112

444 Nat. Prod. Rep., 2002, 19, 413–453

with RuCl3–NaIO4 subsequently afforded ketone 390, whichreacted with Ph3P=CHCO2Me to furnish a 1 : 1 mixture ofexocyclic alkenes, that could only be readily separated by pre-parative TLC. A WHE reaction with MeO2CCH2P(O)(OMe)2

gave similar results. Clearly, the butyrolactone unit was insuffi-ciently bulky to direct the stereochemical course of olefinationin favour of the desired product. Given this impasse, we decidedto investigate the effect of increasing the size of the ester com-ponent in our phosphorus ylide. To our delight, changing theester alkoxy from OMe to OBu-t significantly improved thestereoselectivity of olefination to 3 : 1 in favour of the desiredisomer 356 (Scheme 46). However, the two olefin isomers againproved inseparable by conventional flash chromatography.Given that we were later unsuccessful in converting this mix-ture into the desired target phenylsulfone 346, we decided toradically rethink our synthesis.

d Hale’s fully stereocontrolled asymmetric synthesis of thefully-elaborated bryostatin 1 “Southern Hemisphere”intermediate 346 (2001) 101d

Our new proposal for arriving at phenylsulfone 346 called forthe implementation of a combined aldol–dehydration sequenceon aldehyde 396 and ketone 395 to obtain enone 393 (Scheme47). The C(20)-alkoxy group would be introduced by stereo-

controlled 1,2-ketone reduction and protection. Ketone 395would itself be derived from the glycal 398 which, in turn,would be assembled from the β-ketophosphonate 394 andaldehyde 383.

The β-ketophosphonate 394 was prepared in five steps from11 as detailed in Scheme 48.101d 2,2-Dimethylpropane-1,3-diol11 was selectively thioetherified with tributylphosphine andphenyldisulfide in DMF for 3 h at 70 �C, and the product

Scheme 47 Hale’s revised retrosynthetic planning for the C-ring ofbryostatin 1.

thioether 399 oxidised with oxone to access the phenylsulfone400. Its primary alcohol group was then oxidised with in situ-generated ruthenium tetraoxide, and acid 401 esterified withpotassium carbonate and iodomethane in DMF. Methyl ester402 condensed readily with an excess of the lithio-anion ofmethyl dimethylphosphonate at low temperature to give thedesired β-ketophosphonate 394 in 37% overall yield. The latterwillingly participated in a Roush–Masamune coupling reactionwith aldehyde 383 to produce enone 403 as essentially one geo-metrical isomer in 61–78% yield. While compound 403 could beconverted directly into alcohol 405 by catalytic hydrogenationover Pd(OH)2–C, it was generally found superior to selectivelyhydrogenate the double bond and then deprotect the PMBgroup with DDQ. By adhering to this regime, δ-hydroxyketone405 could typically be isolated in higher overall yield (79%) forthe two steps. Ring-closure to the glycal 398 was best accom-plished with camphorsulfonic acid in benzene at reflux underDean–Stark conditions.

A three-step sequence was needed to arrive at ketone 395 andglycal epoxidation was at its heart (Scheme 48). The most satis-factory protocol for forming the labile glycal epoxide 406reacted 398 with redistilled dimethyldioxirane in acetone andanhydrous methanol in the presence of 4 Å molecular sieves at0 �C for 12 minutes. Under these conditions a very cleanand almost totally stereospecific epoxidation took place on theα-face of the alkene to provide 406. After a catalytic quantity ofPPTS was added to the reaction mixture, trans-diaxial epoxidering-opening was driven to completion, and 407 was isolated asessentially a single product. By contrast, when epoxidation wasconducted with redistilled dimethyldioxirane in acetoneand dry CH2Cl2, a much less satisfactory outcome typicallyresulted. The trace amounts of water that were always presentin our redistilled DMDO–acetone solutions, invariably causeda significant amount of epoxide ring-opening to give anunusable hemiketal mixture, even when 4 Å sieves were added.Alcohol 407 was then oxidised with pyridinium dichromate inDMF, and ketone 395 isolated in 59% overall yield from 398. Arange of conditions were evaluated for effecting the key aldoladdition–dehydration step. The most efficient protocol enolisedketone 395 with n-butyllithium in THF at �78 �C, and trappedthe resulting enolate with the Marshall aldehyde 396. Afterwarming the reaction mixture to rt for 20 min, the tandemsequence of aldol addition and dehydration occurred succes-sively in the same pot to deliver 393 as a single geometricisomer in 73–79% yield. The C(20)-hydroxy was introduced byLuche reduction with sodium borohydride and cerium tri-chloride at low temperature. Complete stereocontrol wasobserved in favour of 391. The final step in the sequence to 346was O-triethylsilylation with TES-triflate and 2,6-lutidine whichproceeded very rapidly and cleanly to deliver the desired prod-uct 346 in good yield along with the elimination product 408.This is the shortest route so far developed to a fully-elaborated,appropriately protected, bryostatin 1 “Southern Hemisphere”.Importantly, it proceeds in 2.57% overall yield for 18 steps from(E )-hexa-1,4-diene, and 3.04% overall yield by the 19-steppathway.101d

Before departing completely from this discussion, it is appro-priate to mention that our application of the Evans–Wendermethyl glyoxalate aldol addition–dehydration tactic to ketone395 did indeed result in the formation of keto enoate 394(Scheme 47). We were also able to reduce ketone 394 under theLuche conditions to obtain the γ-hydroxy-enoate 392. However,when we attempted to selectively reduce the ester grouping in392, to obtain the primary allylic alcohol, we found that C(20)-hydroxy-directed 1,4-reduction occurred preferentially with allthe reducing agents we examined. We also encountered difficul-ties in the protection of alcohol 392 with a range of silylatingagents. In light of this, we concluded that the methyl glyoxalatepathway to 346 was fundamentally flawed. The successful unionof 395 and 396 has overcome these problems, and has actually

Nat. Prod. Rep., 2002, 19, 413–453 445

Scheme 48 Hale’s 18/19-step asymmetric synthesis of the bryostatin 1 “Southern Hemisphere”.

led to a much shorter synthesis of 346. The great brevity of ourroute suggests that a future 30 step (longest linear sequence)or less total synthesis of bryostatin 1 might soon be on thehorizon.

6.9 Janda’s polymer-supported synthesis of the C(21)–C(27)-sector of bryostatin 1 (2000) 113

Janda has recently reported a solution-phase polymer-supported pathway to the orthogonally protected alcohol 419

(Scheme 49) in which an intermolecular nitrile oxide cyclo-addition reaction between 410 and methyl vinyl ketone formsa key step. A racemic product mixture 411 was generatedwhich underwent stereoselective reduction to 412 and 413with L-Selectride. This mixture of enantiomers was thenenzymatically resolved via an acetylation reaction with thepolymer-supported enzyme Novozyme 435. While only a 40%conversion was achieved, the ee of the desired product 414 washigh (99%). Separation of the mixture was accomplished afterthe products (415 and 416) had been cleaved from the support

446 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 49 Janda’s soluble-supported polymer pathway to a bryostatin C-ring synthon.

with aq. HF. The primary alcohol in 416 was selectively O-silyl-ated with TBSCl, and 417 was converted to the β-hydroxy-ketone by treatment with Raney nickel. An Evans reductionnext afforded diol 418 with 89% diastereoselectivity. The morehindered, but also more nucleophilic, hydroxy in 418 was thenselectively triethylsilylated. While it is difficult to envision usingfragment 419 in an actual bryostatin synthesis, with dueprotecting group adjustment, it might prove possible to exploitthis chemistry in the future.

6.10 Yadav’s synthesis of the bryostatin “NorthernHemisphere” (2001) 114

Building upon Vandewalle’s earlier ideas for connectingappropriately tailored A- and B-ring segments via a WHE–intramolecular Michael addition sequence, Yadav and co-workers set out to examine the retrosynthetic strategy shownin Scheme 50 for assembling the bryostatin “NorthernHemisphere” 420.114 In this refined Michael approach to theAB-system, β-ketophosphonate 423 would be condensed withaldehyde 422 to access enone 421, and the B-ring pyran wouldbe constructed through O-desilylation with fluoride ion. AFischer glycosidation would then bring about closure of theA-ring to provide 420.

a Yadav’s asymmetric synthesis of the ketophosphonate423

A Jacobsen Salen-mediated kinetic hydrolytic resolution 115 fur-nished the key chiral epoxide 426 needed for the synthesis of423 (Scheme 51). Epoxide 426 was converted into the propar-gylic alcohol 427 by standard literature protocols, and thisreduced to the (E )-allylic alcohol to allow a Sharpless AE to beused for setting the C(5)-stereocentre with high diastereo-control. Given that the latter reaction positioned a surplusoxygen atom within the carbon-chain of 428, a reductiveepoxide ring-opening sequence was now required to jettisonthis functionality. The key deoxygenation was accomplished by

iodination and zinc-mediated dehalogenative elimination, andyielded 429 after silylation with TIPSOTf. Hydroboration andoxidation converted the terminal alkene of 429 into a primaryalcohol which was oxidised to the aldehyde. An aldol additionwith the anion of methyl isobutyrate finalised this route tothe C(1)–C(9) backbone. Unfortunately, this reaction was notparticularly diastereoselective, it delivering a separable 2 : 3mixture of alcohols enriched in 432. O-Desilylation and O-iso-propylidenation were next performed, along with a phosphon-ate anion condensation to obtain the key β-ketophosphonate423.

b Completion of the AB-intermediate 420

A fifteen-step sequence was devised by Jadav and coworkersfor assembling aldehyde 422 (Scheme 52). Its lone asym-metric centre (which corresponded to O(15) in bryostatin 1) was

Scheme 50 Yadav’s retrosynthetic planning for the bryostatin“Northern Hemisphere”.

Nat. Prod. Rep., 2002, 19, 413–453 447

Scheme 51 Yadav’s synthesis of the C(1)–C(10)-ketophosphonate 423.

Scheme 52 Yadav’s synthetic route to the bryostatin 1 “Northern Hemisphere”.

successfully installed through a combined Sharpless AE–reductive dehalogenation tactic. The key union between 422with 423 was implemented with lithium diisopropylamide as

base and proceeded with complete stereocontrol, as would beexpected from Vandewalle’s earlier model studies (see Scheme28).78a The critical intramolecular Michael addition of 421 also

448 Nat. Prod. Rep., 2002, 19, 413–453

delivered the B-ring pyran with excellent stereocontrol. Fischerglycosidation of 440 with PPTS and methanol finalised thisvery interesting route to the advanced AB-system 420, whichlooks appropriately protected for future elaboration into anaturally-occurring bryostatin.

7.0 Wender’s analogue work (1998, 2000) 3,116

The encouraging clinical trials and great natural scarcity ofbryostatin 1 have collectively served to ignite substantial inter-est in the synthesis of simplified analogue structures. The iden-tification of a suitably simplified analogue with an enhancedantitumour profile remains a primary research goal of manyteams active in this area. Realistically, such an analogue willprobably need to be synthesised in less than thirty steps if thereis going to be a genuine prospect of it being prepared on anindustrial scale. Notwithstanding the vast amount of syntheticeffort that has been expended on the bryostatin synthetic prob-lem over the past two decades, only Wender’s group has madesignificant progress on the analogue front, and in the comingparagraphs we will detail their efforts in this regard.

Early PKC binding studies on the bryostatins by Pettit,Blumberg and Wender indicated that structural changes to theA- and B-rings were often not that detrimental to binding activ-ity, whereas changes to the C-ring usually eroded affinity for theenzyme quite substantially. As a result, Wender decided toretain as much of the C(19)–C(27) domain as was possible inhis target analogue structures, confining most of his simplifyingchanges to the A- and B-regions. Molecular modelling provedespecially valuable for guiding the design process, it beingused to ensure that there was significant structural congruencebetween prospective target structures and the natural productsprior to any synthetic work being performed. By adhering tothis design strategy, Wender and associates were able to makesome really quite spectacular simplifying changes to the bryo-statin structure without causing a significant loss in PKC-binding or antitumour activity. Indeed, some of the structuresthey have prepared have a PKC binding affinity in the nano-molar range. Some also have powerful in vitro antitumour activ-ity against several human cancer cell lines. The most promisingresults are shown in Scheme 53.

Several interesting facts have so far emerged from allWender’s analogue studies: (1) a twenty-membered macro-lactone framework incorporating the C-ring is a requirementfor good PKC binding activity; (2) the C(3)-hydroxy is anotherkey feature needed for effective PKC binding; (3) (R)-stereo-chemistry for the C(3)-OH is important for potent binding to

PKC; (4) a free hydroxy at C(26) is essential for an effectiveinteraction with the PKC enzyme, and; (5) the B-ring exocyclicolefin and the A-ring can be completely dispensed with, withoutseriously reducing the affinity of analogues for the enzyme.3,116

The core fragment that has been used in most of Wender’sanalogue work is the α,β-unsaturated aldehyde 465,116b which herefers to as the “recognition domain”. Its use in the synthesis ofanalogue 442 is shown in Schemes 54 and 55; this is the mostbiologically-potent simplified bryostatin analogue so far pre-pared. Wender’s route to 465 sets off with the Ag()-promotedO-benzylation of methyl -lactate (Scheme 54); compound 445was then partially reduced with DIBAL and aldehyde 446 sub-jected to a Sakurai reaction. After protection with PMBCl,ozonolysis of alkene 447 furnished aldehyde 448 which con-densed non-stereoselectively with the dienolate derived from452; a separable 1 : 1 mixture of the pyranones 453 resultedafter acid-catalysed cyclodehydration. The enone with correctstereochemistry at C(23) was stereoselectively reduced from itsless hindered β-face with the Luche reagent (NaBH4–CeCl3),

106

and the resulting allylic alcohol used to control the stereo-chemical course of glycal epoxidation with m-CPBA in basicmethanol. Under these conditions, the intermediary glycalepoxide selectively underwent ring-opening at the anomericposition to produce the methyl glycoside 454 in 71% yieldfor the two steps. Selective benzoylation and Dess–Martinoxidation transformed 454 into the C(20)-ketone 455, whoseα-benzoyloxy group was reductively deoxygenated with samar-ium diiodide. A pioneering tactic for exocyclic olefin construc-tion in the C-ring was now implemented by Wender; ketone 456was converted to its lithium enolate and this reacted withmethyl glyoxalate to obtain a mixture of aldols that smoothlydehydrated when O-mesylated and treated with DBU.Keto-enoate 457, the sole product of this reaction, was thenreduced stereoselectively to the desired C(20)-axial alcohol 458under Luche conditions. The latter was unstable and so wasprotected as the O-octanoate ester via Yamaguchi method-ology.66 A TBS deprotection–alcohol oxidation sequence nowsecured aldehyde 460 which was allylborated and acetyl-ated. The terminal alkene in 461 was oxidatively degraded andβ-elimination induced by base. The end-result was enal 463; itsPMB group was removed with DDQ prior to hydrolysis of theC(19) methyl glycoside. Significantly this could be accom-plished with aq. HF in acetonitrile even though there was anelectron-withdrawing ester group at C(20). Compound 465 notonly served as the precursor for 442, but all the other analoguesshown in Scheme 53. The conversion of 465 into 442 required afurther 4 steps. Yamaguchi esterification with 466 yielded 467,

Scheme 53 Some examples of Wender’s PKC-binding bryostatin analogues and their antitumour properties.

Nat. Prod. Rep., 2002, 19, 413–453 449

Scheme 54 Wender’s asymmetric synthesis of the simplified bryostatin analogue 442.

while O-desilylation unmasked the C(3)-OH; intramolecularacetal exchange provided 468 in which the 20-memberedmacrocycle was established. The final step to 442 was a highlychemoselective hydrogenolysis of the O-benzyl group at O(26)in the presence of the exocyclic enoate, which proceeded in highyield.

Key transformations in the pathway devised to acid 466 werethe asymmetric HDA reaction used to fabricate ring-A, theClaisen rearrangement to set the C(5) stereocentre, and

the Brown asymmetric allylboration to control the C(3)-OH(Scheme 55).

The synthesis of 442 requires 43 steps in total, and has alongest linear sequence of 31 steps. While the route to 442 isconsiderably less arduous than the synthesis of a naturalbryostatin, it may still prove too lengthy to make this orrelated molecules viable candidates for commercialisation.Undoubtedly future work will focus on making even moresweeping changes to the bryostatin framework, given that the

450 Nat. Prod. Rep., 2002, 19, 413–453

Scheme 55 Wender’s synthesis of the “Space Domain” component of analogue 442.

essential structural domains needed for PKC binding have nowbeen clearly identified. Clearly, Wender’s efforts in the bryo-statin analogue area have highlighted what potentially can beachieved in the area of simplified analogue design, and hope-fully his and other groups’ work in the future will yield a goodantitumour drug that is both selective and effective in man.

8.0 Mendola’s aquaculture solution to the bryostatin 1 supplyproblem (2000) 117

Mendola and his colleagues at CalBioMarine Technologiesin California have recently developed 117 an efficient “in-sea”aquaculture method for producing Bugula neritina that is nowcapable of providing 100–200 g of bryostatin 1 annually. Theirfirst efforts at “in-sea” bryostatin 1 production led to a 7-mprototype submergable scaffold being constructed that couldaccommodate 20 perforated PVC panels of 0.5 m2 dimension.Fifteen of these panels were settled with tens of thousands ofbryozoan larvae in a shore-based hatchery, and these were thengrown for 3–6 weeks prior to being transferred to the underseaplatform by a team of divers. After four months in the sea, 6 ofthe panels were harvested, and all had an average density of2.88 kg m�2 of Bugula neritina deposited upon them (on a wet-weight basis). Significantly, the quantities of bryostatin 1 pro-duced by the aquacultured Bugula were about the same as thoseobtained from natural colonies. Calculations have since shownthat approximately 200 72-panel, commercial, scaffolds wouldbe needed to produce the 100–200 g of bryostatin 1 that will berequired annually. Of even greater significance is the fact thatthis quantity will be obtainable at only a fraction of the cost ofa fully synthetic route ($700000).

It is worthwhile noting that Mendola’s early efforts at pro-ducing bryostatin 1 through B. neritina aquaculture in 5000-litre sea-water tanks were not very successful. Tank-culturedBugula colonies generally have a much weaker appearance andcolour when compared with natural healthy colonies. Most ofthe problems arise from the need to feed the developing Bugulacolonies on large quantities of cultured plankton, which isitself rather difficult to produce on large-scale. Other artificialfoodstuffs were not very well received by tank-grown colonies.As a consequence, Mendola’s group quickly abandoned thetank-harvesting method for obtaining bryostatin 1.

The truly remarkable work of Mendola’s group on bryostatin1 has highlighted what can potentially be achieved by marineaquaculture when there is the appropriate will to succeed. Itonly remains for the world pharmaceutical industry to embracethis powerful new technology he has developed to provide uswith the next generation of Nature’s exciting miracle-drugs.

9 Acknowledgements

We thank the EPSRC, the BBSRC, the Royal Society, Pfizer,and Ultrafine for support of our work in this area. We alsothank Professors Nishiyama and Yamamura (Keio University,Japan) for very kindly supplying us with additional information

regarding their bryostatin 3 synthesis. We are most gratefulto Professor John Mann for valuable discussions on theaquaculture of bryostatin 1.

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