Expedient Synthesis of (+)-Lycopalhine A

N N H

OH

O

H H

H

Benjamin M. Williams and Dirk Trauner

Angew. Chem. Int. Ed., 2016, 128, 2231-2234

1 Step McCabe @ Wipf Group Page 1 of 15 6/19/2016

Lycopodium alkaloids

2

obscurinineO

H

N

H

N

H

N

OH O

HH

Fawcettimine-type

N

O

lycopodine-type

HN

N

lycodine-type

HN

NH

miscellaneous

Lycopodiumalkaloids

H2N

NHO

HN

N

N HN

MeH H

complanadine Astimulates nerve

growth factor (NGF)production in human glial cells

N

O

N

fastigiatine

huperzine Areversible inhibitor of

AChe

N N H

OH

O

H H

H

lycopalhine A

NMe

HN

O

HH

H lyconadin A * cytotoxic to murine lymphoma cells (IC50 = 0.46 µg/mL)

& human epidermoid carcinoma KB cells (IC50 = 1.7 µg/mL)* stimulates NGF production in human glial cells

NMe

HH

H

HN

nankakurine A

phlegmarine

N

HN

O

CO2H

huperzine G

H

Step McCabe @ Wipf Group Page 2 of 15 6/19/2016

•  Isolated in Guizhou Province, China from Palhinhaea cernua along with it’s proposed biosynthetic precursor obscurinine Structure & absolute configuration determined through spectroscopic/computational methods •  Complex hexacyclic ring system containing

•  1x6 + 2x5 membered carbocycles, 1x piperidine ring & 1x hexahydropyrimidine ring on a highly substituted pyrrolidine core •  Sensitive aminal functionality + strained aldol moiety •  9 stereogenic centres, 8 of which are contiguous

•  Weak butyrylcholinesterase (BuChe) inhibitory activity

•  (31.4% at 50 µM) compared to Tacrine (87.8% at 33 µM, + control )

Palhinhaea cernua

Isolation/ Characterization

3

O

H

obscurinine(10 mg)

N

H

N

HN N H

OH

O

H H

H

lycopalhine A(1.5 mg)

Chem. Commun., 2012, 48, 9038

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Biosynthesis

4

•  The biosynthesis of lycopodium alkaloids is not well established due to difficulty cultivating lycopodium species in the lab.

•  Current insight is largely based on feeding experiments with radiolabeled precursors

NH2

H2NO

OH

lysine

NN

H

N

O

lycopodine

HOrearrangement

N

OH O

HH

Fawcettimine

N

O[O]

obscurinineO

H

N

H

N

H

H2NH2Ncadaverine

NH

OCO2H

NH

O

-CO2-CO2

NN

HO

HO

HO2C

lycodane skeleton

N HN

N

OH

41312

13 12 4H N N H

OH

O

H H

H

lycopalhine A

H2O

Step McCabe @ Wipf Group Page 4 of 15 6/19/2016

Proposed Biosynthesis

O

H

obscurinine

N

H

N

H

O

H

N

H

N

H

H

O

O

HN

H

N

H OH

H

O

HN

H

HN

H OH

HO H

O

HN

H

HN

H OH

H

N N H

OH

O

H H

H

[O]aldol

condensation

[O][H]

cyclization

Polonovski-typereaction

N HN H

OH

O

H

H

lycopalhine A

[H]

5 Step McCabe @ Wipf Group Page 5 of 15 6/19/2016

6

•  Trauner group – published 8th January 2016 •  Fukuyama group – 7th March 2016

•  2 total syntheses of (+)-Lycopalhine to date:

N N H

OH

O

H

H

aminal condensation

biomimeticaldolH

N N H

OH

O

H

aminal condensation

biomimeticaldol

H

Michael addition

cyclopropanationorganocatalyzedintramolecular

Mannich reaction

Pauson-Khand

Step McCabe @ Wipf Group Page 6 of 15 6/19/2016

7

Fukuyama’s Retrosynthesis

Org. Lett., 2016, 18 (6),1494–1496

O O

OEtN2

OPMP

H

O

CO2Et

OPMP

H

O

O

NH

H

H

NHMe

H

O

N

H

H

HO

NMe

H

(+)-lycopalhine A

OHC O

O

OPMP

O OH

aminalformation

aldol

cyclopropanationcyclopropanecleavage/Michael addition

Johnson-Claisen

rearrangement

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Fukuyama’s Approach

8

OHO

OH

N2

OO

N

ClO2C

NHSO2Tol

N

N

OCuN

OtBu

tBu

Tetrahedron Lett., 1984, 25, 3559E. J. Corey and Andrew G. Myers

bis-(N-t-butylsalicylaldiminato) copper (II) catalyst

- soluble catalyst --> higher yields on scale compared to

heterogenous Cu powder

PhMe, reflux

NHNSO2Tol

ClO2C

Et3NN

HC•

O

N STol

O O

O

O S Tol

O

N2

ROHO

ORN2

RO S Tol

Oor

weak base preventedbasic side reaction observed with NEt3

OPMPTBSO

OH

OAcTBSO

11 steps

O O

OEtNN

N3 PF6

Et3N, MeCN, THF, rt

TBSO

O O

OEtN2

OPMP

PhMe, reflux, 55%H

TBSO

O

CO2Et H

HO

O

OPMP OPMP

O

NH

SO2Ar

Ar = C6H4-p-CF3

R

O O

OEtH N

NN

MeN NMe

Synlett, 2009, 2009, 2943Synthesis, 2011, 7, 1037

N

O CuN

OtBu

tBu

3 steps

synthesized 26.1 g scale

mild diazo transfer for 1,3-dicarbonylswater soluble guanidine byproduct

2 steps(lit. procedure)

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Fukuyama’s Approach

9

H

O

OPMP

NBocOHC

H

H

NNsMe

15 stepsKOH, MeOH

0 °C to rt, 98% H

O

OPMP

NBoc

H

H

NNsMeHO

6 steps H

O

N

H

H

HO

NMe

H H

O

N

H

H

OH

NMe

H+

lycopalhine A epi-lycopalhine A

DMP, pyridineCH2Cl2, rt to 40 °C

H

O

O

OPMP

O

NHSO2Ar H

O

O

OPMP

NSO2Ar

O

Ar = C6H4-p-CF3

H

HO

O

OPMP

O

NH

SO2Ar

synthesized on 490 mg scale

H

80%

Step McCabe @ Wipf Group Page 9 of 15 6/19/2016

Retrosynthesis

10

Angew. Chem. Int. Ed., 2016, 128, 2231-2234

N N H

OH

O

H

H

aminal condensation

biomimeticaldol

BocN HN

O

H

H

OH

O

Mannich cyclization O

H

H2N

OHNBoc

CHO

conjugate addition

OH

H2N

OTBSPauson-Khand

reaction NHOTBS

Boc

NH2

O

OH

O

HO

diastereoselectiveallylation

L-glutamic acid

H

Step McCabe @ Wipf Group Page 10 of 15 6/19/2016

Synthesis of Core Bicycle

11

MeO2C

NHBoc

CO2Me

LiHMDS, THF, - 78 °C

90%Br

OHO

BocHN

DIBAL (5 equiv.)THF, PhMe, -78 °C

83%NH

CO2MeMeO2C

Boc

1. K2CO3, MeOH

2. TBSCl, imidazoleCH2Cl2

61% (2 steps)

NH OTBSBoc

OPMeO

MeON2

O

Co2(CO)8PhMe, 70 °C

72% NHBocOTBS

H

Co2(CO)8

O

H

BocHN OTBS

dr C(3) 10:1

H

nOe

O

H

BocHN

OTBS

H

MgBr

CuBr•SMe2TMSCl, HMPA,

78%

AcClMeOH92%

O

H

H2N

OH

H

gram quantities(up to 1.3 grams 1 batch)

3

7

Ohira-Bestmann reagent

Pauson-Khand reaction

anti diastereoselectivity

dr C(3) 10:1

3

chlorosilane accelerated

conjugate addition

Step McCabe @ Wipf Group Page 11 of 15 6/19/2016

propargylic carbamate 11. This intermediate would ultimatelybe derived from l-glutamic acid.

Our actual synthesis of lycopalhine A, guided by theseconsiderations, is presented in Scheme 2. To begin, dimethylester 12, prepared from l-glutamic acid on a decagram scalein a one-pot procedure (see the Supporting Information),[7]

was allylated with high anti diastereoselectivity in a proceduredescribed by Hanessian and co-workers.[8] Chemoselectivereduction of 13 by controlled addition of diisobutylaluminumhydride (DIBAL) afforded lactol 14 as a 1:1 mixture ofdiastereomers (C1). Alkynylation of 14 was complicated bythe epimerization of the neighboring amine stereocenterunder the basic conditions required to open the six-memberedlactol. Optimized conditions using Ohira–Bestmann reagent15[9] and silyl protection of the resulting alcohol affordedalkyne 11 and its C3 epimer as an inseparable 10:1 mixture.[10]

The ensuing Pauson–Khand reaction proceeded under ther-mal conditions and afforded enone 10 with the desired C7stereoselectivity, confirmed by nuclear Overhauser effect(NOE) analysis, as a 10:1 mixture with the cyclized C3epimer. The diastereoselectivity of related intramolecularPauson–Khand reactions in the context of Lycopodiumalkaloid synthesis has previously been explored by thegroups of Zard,[11] Takayama,[2d] and Mukai.[2i] The authorspropose that chair-like conformations of intermediate 16translate to the desired configuration of the bicyclo-[4.3.0]nonenone.[12] TMSCl-promoted conjugate addition ofbutenylmagnesium bromide to the enone[13] and cleavage ofthe resulting silyl enol ether with acetic acid afforded 17,featuring the sole quaternary stereocenter of the molecule, asa single diastereomer upon purification. The first six steps of

the synthesis were easily scalable and provided gram quanti-ties of advanced intermediate 17. Global deprotection withacetyl chloride in methanol then produced free aminoketone8.

The intramolecular 5-endo-trig Mannich cyclizationproved to be a challenging transformation. Acidic conditionsresulted mainly in self-condensation of the aldehyde. How-ever, mixing amine 8 and aldehyde 9 with triethylaminefollowed by concentration under reduced pressure granted

Scheme 2. Total synthesis of lycopalhine A. a) LiHMDS, THF, ˇ78 88C, then allyl bromide; b) DIBAL (5.0 equiv), THF/PhMe, ˇ78 88C; c) dimethyl(1-diazo-2-oxopropyl)phosphonate 15, K2CO3, MeOH, 0 88C!RT; d) TBSCl, imidazole, CH2Cl2, 0 88C!RT; e) Co2(CO)8, PhMe, 70 88C; f) 3-butenylmagnesium bromide (4.5 equiv), CuBr·SMe2 (20 mol %), TMSCl, HMPA, THF, ˇ78 88C!ˇ30 88C, then AcOH, RT; g) AcCl (10 equiv),MeOH, 45 88C; h) 9 (1.1 equiv), Et3N, CH2Cl2, then l-proline, DMF, RT; i) Boc2O, CH2Cl2, RT, 72 h; j) IBX, EtOAc, 80 88C; k) K2CO3, MeOH, RT,l) OsO4, NaIO4, 2,6-lutidine, dioxane/H2O (3:1 v/v), RT, then TFA, CH2Cl2, 0 88C!RT. Boc = tert-butyloxycarbonyl, DIBAL= diisobutylaluminumhydride, DMF=dimethylformamide, HMPA= hexamethylphosphoramide, IBX =2-iodoxybenzoic acid, LiHMDS = lithium bis(trimethylsilyl)amide,TBS = tert-butyldimethylsilyl, TFA = trifluoroacetic acid, THF = tetrahydrofuran, TMS= trimethylsilyl.

Table 1: Conditions for the intramolecular Mannich cyclization.

Entry[a] Additives (equiv) Solvent T [88C] Yield[b] [%]

1 Yb(OTf)3 (1.0) MeCN 0–RT –2 TiCl4 (2.0)/ Et3N (4.0) CH2Cl2 ˇ30–RT –3 K2CO3 (3.0) MeOH RT –4[c] Et3N (3.0) PhMe RT–80 –5 pyrrolidine (1.0) DMF RT 116 pyrrolidine (1.0)/

AcOH (1.0)DMF RT 10

7 d-proline (1.0) DMF RT 208 l-proline (1.0) DMF RT 609[c] l-proline (0.5) DMF RT 3910 l-phenylalanine (1.0) MeCN RT 30

[a] Reactions conducted under nitrogen atmosphere for 18–24 h.[b] Yield of isolated product. [c] Reaction performed without preforma-tion of imine by treatment with Et3N.

AngewandteChemieZuschriften

2232 www.angewandte.de ⌫ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2016, 128, 2231 –2234

5-endo-trig Mannich reaction

12

O

H

H2N

OH

H

BocN O

H BocN N

O

H

H

OHconditionsEt3N, CH2Cl2

BocN HN

O

H

H

OH7

HN

O

H

H

OH7

HN

Boc

(not observed)

Step McCabe @ Wipf Group Page 12 of 15 6/19/2016

Completion of the Synthesis via a biomimetic aldol reaction

13

N HN

O

H

H

OH BocN N

O

H

H

OH

BocN N H

OH

O

H

H

Boc

BocN N H

OH

O

H

HN N H

OH

O

H

H

1. OsO4 (0.1 equiv.) NaIO4 (10 equiv.)2,6-lutidine, dioxane/ H2O

Lemieux-Johnson oxidation

2. TFA, CH2Cl2, 0 °C to rt56% (2 steps)

+

Boc2O, CH2Cl272 h, 93%

Boc1. IBX, EtOAc, 80 °C

2. K2CO3, MeOH98% (2 steps)

biomimetic aldol

1616

dr C(16) 5.5: 1

H H H

HN HN H

OH

O

H

H

O H

Step McCabe @ Wipf Group Page 13 of 15 6/19/2016

14

Expedient Synthesis of (+)-Lycopalhine A – Supporting Information

S41

6. Deuterium exchange studies of lycopalhine A

Figure S7. Deuterium exchange studies of lycopalhine A and epi-lycopalhine A

Note: No additional deuterium exchange was observed after stirring for >24 h in MeOD with K2CO3 or after repeating the experiment over 3Å molecular sieves.

b. 1 and epi-1 (pyridine-d5, 600 MHz) after stirring in MeOD with K2CO3 at rt for 2h

a. 1 and epi-1 (pyridine-d5, 600 MHz) before treatment with MeOD/K2CO3

H-1a H-6

H-6‘

H-15 H-4

H-4 H-8a

H-1a

H-6

H-6‘

H-15 H-14a

H-8a H-14a

H-7

H-7

Deuterium Study

•  Deuterium exchange between C6/C15 under basic conditions + the existence of both epimers in the experimental and isolated samples suggests equilibration with a thermodynamic preference for closed aldol product.

N N H

OH

O

H

HN N H

OH

O

H

H+ 1616

H H

MeOD, K2CO3

15

6

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Conclusions

•  First total synthesis of lycopalhine A •  First asymmetric synthesis utilizing a chiral pool approach

•  Key steps:

•  Comparison of synthetic routes

•  Application of P-K/ organocatalytic Mannich approach to other lycopodium alkaloids

Synthesis Steps Overall Yield (mixture of C(16)

epimers)

Quantity

Fukuyama 41 ~2% 13.0 mg

Trauner 13 ~5.7% 4.0 mg

15

NH2

O

OMe

O

MeON N H

OH

O

H

H

N N H

OH

O

H

H

biomimeticaldol

Pauson-Khand

reaction

5-endo-trigorganocatalytic

Mannich cyclization

16

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