chaptershodhganga.inflibnet.ac.in/bitstream/10603/482/8/08_chapter 2.pdfcould be prepared from...
TRANSCRIPT
Chapter 2 a-OXOKETENEDITHIOACETALS IN ORGANIC SYNTHESIS
2.1 Introduction
The a-oxoketenedithioacetals of general formula 1 have been proved to be
versatile intermediates in organic synthesis. They can be easily synthesized by treating
active rnethylene compounds with two equivalents of base and carbondisulfide
followed by alkylation. a-Oxoketenedithioacetals are considered as synthetic
equivalents of P-ketoesters. Moreover, they provide opportunities for the regioselective
and chemoselective formation of new carbon-carbon and carbon-heteroatom bonds. A
large number ofthem with diverse structural features have been prepared by employing
minor variations of the general strategy
The chemistry of a-oxoketenedithioacetals leading to the formation various
other valuable reactive intermediates, heterocycles, and aromatic compounds has been
studied extensively. Several reviews have appeared on their synthesis and subsequent
applications in organic synthesis.' They also serve as precursors for the synthesis of the
corresponding a-oxoketene-N,S-acetals and aminals by the sequential displacement of
either one or both of the metbythio groups. The displacement of methylthio group is
also facile with carbon nucleophiles yielding the 1,4 adducts which can be further
transformed in to novel heterocyclic and aromatic compounds. a-
Oxoketenedithioacetals are proven three carbon synthons possessing 1,3-electrophilic
centers with differing electrophilic properties. suitable for synthetic manipulations.
2.2 a-Oxoketenedithioacetals: Synthesis
~ e l b e r ~ and co-workers reported the first synthesis a-oxoketenedithioacetals
I . Their procedure involved alkylation of 0-oxodithioicacids with alkyl halides under
basic conditions.
Later Thuillier and ~ i a l l e ' have developed a more convenient method for their
synthesis from ketones. They treated active methylene ketones 2 with carbondisulfide
in presence of sodium I-amylate followed by treatment with two equivalents of the
alkylating agent (Scheme 1).
Scheme 1
Aliphatic, cyclic and aryl alkyl ketones undergo smooth reaction under these
conditions to give the corresponding ketenedithioacetals. a-Oxoketenedithioacetals
could be prepared from several heterocyclic compounds having active methylene
moiety as well. Subsequently other methods also have been introduced, most of them
differing only in the base and the solvent used. Thus, lithium-2,6-di-1-butyl-4-
methylphenoxide4, lithium dialkylamide" sodium hydride6, potassium t-butoxide7 and
~ ~ l ~ l u m i n a ~ have been successfully used for the preparation of a-
oxoketenedithioacetals. Besides these, other methods for their preparation also exist.
For example. aromatic hydroxy compounds such as phenols and naphthols are
converted to the corresponding ketenedithioacetals by their reaction with
trithiocarbonium salts (Scheme 2) '
Scheme 2
They can also be prepared by the Friedel-Crafts acylation of ketenedithioacetals
(Scheme 3 ) "
Scheme 3
Similarly, trithioorthoacetates can be converted to the corresponding a-
oxoketenedithioacetals by Friedel-Crafts type acylation of the intermediate ketene
dithioacetals with acid anhydrides or acid chlorides (Scheme 4)"
9 10
Scheme 4
Though there exists several methods for the preparation of a-
oxoketenedithioacetals, the reaction of enolate anions derived from the active
methylene carbonyl compounds with carbondisulfide followed by alkylation is the
method of choice. A large number of a-oxoketenedithioacetals with diverse structural
features are known and their chemistry has been studied in detail by different groups.
2.3 a-Oxoketenedithioacetals: Reactivity a-Oxoketenedithioacetals are considered as synthetic equivalents to (3-
ketoesters, in which the ester functionality is protected as the ketenedithioacetal group
Alternatively, they can be considered as highly functionalized a,P-unsaturated ketones
containing alkylthio substituents that can be displaced with nucleophiles at the P carbon
atom. From either point of view, they possess considerable potential for the
regioselective and stereoselective construction of new bonds via 1,2 or 1.4 nucleophilic
addition reactions. Though the ketenedithioacetal derived from acetone is synthetically
equivalent to ethyl acetoacetate, when we consider the donor-acceptor properties of the
carbons the acyl ketenedithioacetal have a distinct advantage over the (3-ketoester in
reactivity. While the a-carbon can undergo. Lewis acid assisted addition to
electrophiles. a'-carbon can selectively be deprotonated and added to electrophiles in
the presence of base. These reactivity patterns have been exploited in the synthesis of a
number of acyclic, cyclic and heterocyclic systems starting from a-
oxoketenedithioacetals.
2.3.1 Reaction with Binucleophiles The flexibility of functional group manip~~lation along with hard-soft
dissymmetry and its possible inversion makes a-oxoketenedithioacetals a potential 1.3-
electrophilic three-carbon fragment having synthetic importance. Their reactions with a
large number of hetero binucleophiles leading to the formation of a variety of
heterocyclic structures are studied.' Thus reactions of I .3-dinitrogen nucleophiles with
a-oxoketenedithioacetals provide a convenient method for the synthesis of
functionalized pyrimidines. Rudorf and ~ u g u s t i n ' ~ and ~ o t t s ' ~ rt a/. have reported the
synthesis of pyrimidines by treating amidines with ketenedithioacetals derived from
various substituted ketones. For example the ketenedithioacetals 12 derived from a-
cyanoketones on treatment the amidine 13 afford cyano substituted pyrimidines 14
(Scheme 5).
Scheme 5
Ketenedithioacetal 15 prepared from 2,6-diacetyl pyridine affords pyrimidine
derivative 16 on refluxing with acetamidine in DMF in the presence of triethylamine.
These pyrimidine derivatives are valuable ligands in organometallic chemistry (Scheme
6).14
Scheme 6
Other substituted amidines also have been shown to participate in similar
reactions with a-oxoketenedithioacetals. Thus, the reaction of 2-
thiophenecarboxamidine 17 with the ketenedithioacetal 18, prepared from 2-acetyl
thiophene, leads to the formation of the substituted pyrimidine 19 (Scheme 7).13
Scheme 7
Amidines, which form pan of a heterocycle such as, 2-aminopyridine 21 reacts
with a-oxoketenedithioacetals forming annulated pyrimidines 22 (Scheme 8 ) "
20 21 22
Scheme 8
Functionalized pyridones'h could also be prepared from u-
oxoketenedithioacetals The reaction follows a two step process in which the conjugate
addition of the enolates is followed by cyclization. For example. the reaction of
cyanoacetamide with a-oxoketenedithioacetal 23 is shown in Scheme 9. The
ketenedithioacetal was treated with cyanoacetamide in presence of sodium
isopropoxide in refluxing isopropanol.
Scheme 9
Reaction of a-oxoketenedithioacetal with hydrazine hydrate leads to the
formation of substituted pyrazoles (Scheme 10)"
23 26 27
Scheme 10
The reaction of a-ketenedithioacetals with hydroxylamine is an example where
the electrophilicity of the carbonyl carbon and the 13-carbon of the a-
oxoketenedithioacetal could be altered by employing appropriate reaction conditions.
Junjappa and co-workers have successfully demonstrated that a-oxoketenedithioacetals
could be transformed to either 3-alkylthio or 5-alkylthio isoxazoles by simply choosing
reagent combinations of appropriate pH. Thus, the 5-alkylthioisoxazole 28 can be
obtained by the reaction of hydroxylamine hydrochloride with a-
oxoketenedithioacetals in the presence of barium hydroxide or sodium methoxide at the
pH range 5 to 9. On the other hand in the presence of sodium acetate-acetic acid, the B- carbon is more electrophilic and the reaction yields 3-alkylthio isoxazole 29 (Scheme
I I) . '%
29
Scheme 11
The enamine moiety of amino uracils and amino pyrazoles also act as 1.3
binucleophiles in their reactions with a-oxoketenedithioacetals. Thus the reactions of
N,N-dimethyl-6-aminouracil 30 with a-oxoketenedithioacetals 31 afford pyrido[2,3-
dlpyrimidine derivatives 32 (Scheme 12) l9
Scheme 12
I,4-Binucleophiles such as ethylene diamine, ethanolamine and 2-
arninoethanethiol sequentially displace both the alkylthio groups of a-
oxoketenedithioacetals leading to the formation of imidazolidines, oxazolidines, and
thiazolidines respectively (Scheme 13) .~" Addition of amines to ketenedithioacetals
afford the corresponding N, S acetals which could be converted in to the ketene arninals
by reaction with a second equivalent of the a~nine.".~'
Scheme 13
2.3.2 1,2-Addition Reactions of Carbon Nucleophiles
An interesting aspect of the a-oxoketenedithioacetal chemistry is their
application in the synthesis of substituted aromatic compounds. The Grignard reagents
prepared from ally1 magnesium bromide undergo selective 1.2-addition to a-
oxoketenedithioacetals. The intermediate carbinol acetal 37 on cycloaromatization in
the presence of boron trifluoride etherate in refluxing benzene leads to the formation of
benzenoids 38 (Scheme 14) .~ '
38
Scheme 14
Dieter and co-workers have also reported similar results on the addition of
methallyl magnesium bromide to a-oxoketenedithioacetals followed by
cycloaromatisation employing HBF4 in aqueous ~ ~ ~ ~ " u n j a ~ ~ a ' s method for the
preparation of highly substituted aromatic compounds has been hrther extended to
cinnammoyl ketenedithioacetals as well for the synthesis of substituted stilbenes.
Substituted cinnamoyl ketenedithioacetals 39 also undergo selective addition of ally1
magnesium bromide on to the carbonyl group. Cycloaromatization of the intermediate
carbinol acetal 40 upon treatment with a Lewis acid such as boron trifluoride etherate
gave the corresponding alkylthio substituted stilbene 41 in good yields (Scheme IS).^^
M@r 4 Me etherfI'HFI0 oC)
SMe Ar SMe R R'
41
R'=H, Me; @ = H , Me
Scheme 15
By using benzyl magnesium chloride, the cycloaromatization route was
extended for the preparation of substituted naphthalenes as well." Propargyl
magnesium bromide also reacts in 1,2 fashion giving the intermediate carbinol, which
undergo benzoannulation on treatment with BF3 etherakz6
Dieter's group have also developed a method for the synthesis of substituted
phenols starting from a-oxoketenedithioacetals. The addition of trimethyl silyl
substituted allyl lithium 43 to the ketenedithioacetal 42 derived from cyclohexanone
gave the a,P-unsaturated thiolester 44, which could be cyclized to the phenol 45 in
56% yield on treatement with dimethyl methylthio sulfonium fluoroborate in CH~CIZ
(Scheme 16).17
Scheme 16
Other than organomagnesium reagents, nucleophilic addition reactions of other
hnctionalized carbanions to a-oxoketenedithioacetals also have studied in detail. They
include reactions with Reformatsky reagents2x, with enolates derived from esters and
ketones29 and with imine anions3' prepared from hydrazones etc. An interesting
transformation of a-oxoketenedithioacetals to substituted salisylates 46 involves a
sequential addition of two molecules of he Reformatsky reagent prepared from ethyl
bromoacetate followed by i,i.silr, cyclisation and aromatization." Similar reactions have
been further extended to cinnamoyl ketenedithioacetals also (Scheme 17)."
OZnBr SCHl ~ 2 c < C02Et &vscH3 R OEt , R SMe
The addition of lithioacetonitrile to a-oxoketenedithioacetals follows a 1.2-
addition pathway. The intermediate carbinol acetals 47 formed on treatment with
phosphoric acid undergo ring closure to afford substituted pyridine derivatives 48
(Scheme 18).
SMe LCH,N/-~U O C
SMe
SMe
H,PO, - SMe
48
Scheme 18
Junjappa and co-workers have extended their cycloaromatization methodology
for the synthesis of a variety of benzoheterocycles Here, a-oxoketenedithioacetals are
allowed to react with allylic anions which form part of a heterocyclic system. The
allylic anion undergo either a I,?-addition or a 1.4-addition followed by
cycloaromatization For example, when 3-methyl-5-lithiomethylisoxazole 49 was
treated with a-oxoketene-dithioacetals and the intermediate carbinols were allowed to
undergo cycloaromatisation on treatment with boron trifluoride etherate, the
corresponding substituted benzisoxazoles 50 were formed in good yields (Scheme
19).'2.'' This method has been further extended of a large variety of substituted
Scheme 19
Dieter and co-workers have developed several methods for the synthesis of
substituted 2-pyrons from a-oxoketenedithioacetals. Addition of enolates derived from
esters and ketones to a-oxoketenedithioacetal followed by acid catalyzed cyclization is
highly useful in the synthesis of substituted pyrones.29 A typical example for this
strategy is given in Scheme 20. The method involves 1,2 nucleophilic addition of ester
or ketone enolate anions to ketenedithioacetals followed by acid assisted 1.3- carbonyl
group transposition to give the 6-keto-a,D-unsaturated acids or esters 51. They were
subjected to enol lactonization to the respective pyrones 52.29.34
Scheme 20
This reaction gives better results when cyclic ketenedithioacetals are allowed to
react with ester or ketone enolates. Reaction with acyclic ketenedithioacetals suffered
from lower yields of pyrons An alternative method involves the 1.2-addition of
enolates derived from hydrazone and the resulting carbinol was hydrolyzed and
rearranged to pyrons by a sequential treatment with CU(OAC)~ and HBFJHgO or
treatment with trifluoroacetic acidz9
2.3.3 1,4-Addition Reactions of Carbon Nucleophies
l,4-Addition reactions of carbon nucleophiles to a-oxoketenedithioacetals
follow an addition-elimination pathway resulting in the substitution of the alkylthio
group or cyclization of the intermediate depending on the nature of the
ketenedithioacetal and the nucleophile. Active methylene compounds undergo addition
to oxoketenedithioacetals through enolate anions. Potassium enolate derived from
active methylene ketones prefer conjugate addition to polarised ketenedithioacetals. If
the starting ketenedithioacetal has an ester hnctionality at the a-psition, the adduct on
subsequent cyclization afford substituted 2-pyrons (Scheme 2 1 ) . ~ ~ Enolate anions
derived from heterocyclic compounds that posses active methylene moiety have also
been shown to participate in similar reactions leading to the formation of hetero
annulated pyron derivatives. ' 6
53 2
X= CN, COzEt; R', 2 = aiyl, alkyl, cyclic
Scheme 21
Potts and co-workers have studied the conjugated addition of active methylene
ketones to a-oxoketenedithioacetals in the presence of potassium t-butoxide in THF.
The 1,5-enediones formed in these reactions have been subsequently exploited for the
synthesis of substituted pyridines. Scheme 22 shows an application of this methodology
for the synthesis of oligopyridines 57.
fi \ SMe + % \
MeS / C H N THF
SMe SMe 0
Scheme 22
Organocopper reagents add to a-oxoketenedithioacetals stereoselectively,
giving the 1,4-addition product^.^ Thus, the ketenedithioacetal 42 reacts wi th two
equivalents of methylcopperlithium to afford the a-alkylidine ketone 58 (Scheme 23).
42 58
Scheme 23
2.3.4 Miscellaneous Reactions Leading to Heterocycles Reaction with phosphorous pentasulfide leads to the formation of 3-thione-1.2-
dithiols 60 (Scheme 24).'".'7
Scheme 24
If an active methylene moiety is attached to sulhr atom of the a-
oxoketenedithioacetals, they can be converted in to thiophene derivatives in the
presence of a suitable base.38 The reaction proceeds through the initial deprotonation of
the alkylthio group followed by intramolecular condensation involving carbonyl group
(Scheme 25).
R1 R'
Base ___C
R S ds
Scheme 25
~ h i o ~ h e n e s ' ~ are also formed from the reaction of a-oxoketenedithioacetals
with Simmon-Smith reagent. The reaction proceeds through the intermediate formation
of an ylide by the addition of the carbenoid methylene to the alkylthio group (Scheme
26).
Scheme 26
Aziridines undergo substitution reaction with the alkylthio group giving the 6-
azirido aceta~s,~' which in the presence of K1 undergo ring expansion to afford the
corresponding 2-thiornethyl 3.3-disubstituted pyrrolines 66. The reaction of the cyclic
ketenedithioacetal 64, derived from pyrazoline, with aziridine is shown in Schcrne 27.
Scheme 27
2.3.5 Solvolysis and Hydrolysis of a -0x0 and a-Hydroxyketenedithioacetals:
a-Oxoketenedithioacetals are considered as protected p-ketoesters and hence
their complete hydrolysis should give the corresponding P-ketoesters. Partial hydrolysis
of ketenedithioacetals to P-ketothiolesters in low yields is known to proceed in
presence of mineral acid and water (Scheme 28).
Scheme 28
Shahak and co-workers have reported the complete solvolysis of
ketenedithioacetal 68 to the respective P-ketoester 69 in the presence of PTSA and
ethanol (Scheme ~ 9 ) . ~
CH, S FTSAIEiOH
CH3S
CH3 CH3
68 69
Scheme 29
The reductive and alkylative 1,3-carbonyl group transpositions involving a-
oxoketenedithioacetals have been extensively studied. The general procedure for these
reactions include 1.2-nucleophilic addition of borohydride, Grignard or organo lithium
reagents to ketenedithioacetals followed by acid assisted solvolysis of the resulting
carbinol acetals.
a-Oxoketenedithioacetals undergo 1.2-reduction with sodium borohydride to
give the allylic alcohols in good yields." These carbinol acetals on treatment with p-
toluenesulfonic acid gave low yields of a,P-unsaturated thiolesters 70 along with P- methylthio thiolesters 71, formed by the conjugate addition of methanethiol to the
initially formed a,P-unsaturated thiolester (Schesme 30). 2
Scheme 30
Further developments in this area came from Junjappa's group, who reported
that treatment of the allylic alcohol, obtained by the 1,2-reduction of a-
oxoketenedithioacetals, with boron trifluoride etherate in methanol afforded a-
unsaturated methylester 72 in good yields (Scheme 3
Scheme 31
Later ~ieter~'. '" found that HBF4 in THF could be effectively used to get high
yields of the unsaturated thiolester together with the methyl sulfide.
The effectiveness and stereoselectivity of HBF4 or BF3 assisted solvolysis is
attributed to a six membered transition state 73 formed by the complexion of boron to
oxygen and sulfur. The proposed involvement of a cyclic transition state also mles out
the possibility of an intramolecular 1.3-migration of the alkylthio group leading to the
formation of the P-alkylthio substituted product.
73
The reductive 1.3-carbonyl group transposition methodology was further
extended to cinnamoyl ketenedithioacetals as well. Thus the ketenedithioacetal 74 on
1.2-reduction with sodium borohydride and subsequent methanolysis gave the 5-aryl
pentadienoate 75 (Scheme 321.~' Similarly pentadienoyl ketenedithioacetal 76 also
yielded the corresponding heptatrienoates 77 (Scheme 33) .45
1- NaB&/ELOH Ar SCH3 2BF3.E20/MeOH A,. OMe
Scheme 32
SCH-, + I - NaBHJEiOH Ar SCH3 2. BF3.E20/MeOHAr *+ OMe
R. R
Scheme 33
However, the carbinol acetal obtained by the 1,2-reduction of the alkenyl
ketenedithioacetal 78 did not give the expected dienester, instead the cyclopentenone
derivatives 79 were isolated after boron trifluoride assisted methanolylsis (Scheme
34).4"
78
Scheme 34
Under similar conditions, 2,4-dimethyl-7-aryI-2,4,6-heptatrienoy ketene-
dithioacetals afforded the styryl cyclopentenones.
The formation of cyclopentenones results from an electrocyclic ring closure of
the intermediate pentadienoyl cation. The presence of methyl substituents at the 2 and 4
positions force the cation to form a 'U' conformation. which is the stereochemical
requirement for the cyclization.
a-0x0, a-alkenyl ketenedithioacetals 80 were also subjected to 1.2-reduction
followed by methanolysis under boron trifluoride assisted conditions. These reactions
resulted in the formation of a-ylidene-y-6-unsaturated esters 81, which are
subsequently converted to the y-butyrolactones 82 (Scheme 35).j7
I NaBkEkOH 2.BF,.El20IMeOH
A OMe
Scheme 35
a-oxoketenedithioacetals are also subjected to 1.2-addition of organometallic
reagents and further solvolytic studies resulted in the formation a number of interesting
intermediates. Ofien the initial adducts formed were subjected to additional synthetic
manipulations leading to the formation of a variety of useful products. Junjappa and Ila
have studied the reaction of methylmagnesium iodide with a-oxoketenedithioacetal to
afford the carbinol acetal 83 which on BF, assisted methanolysis gave the
corresponding P-methyl a,P-unsaturated esters 84, exclusively as E isomers (Scheme
36).48
Scheme 36
However, the carbinol acetal derived from the addition of methyl Grignard to
aroylketenedithioacetals 85, having an a-alkyl sybstituent gave the corresponding 2-
alkyl-3-methyl indenones 86 under the solvolysis conditions(Scheme 371.~"
Scheme 37
The proposed mechanism for the formation of the indenone involves a boat like
transition state 87. Under solvolytic condition, this will lead to the Z-cinnamate 88
wherein the phenyl and the ester groups are cis to each other which would subsequently
cyclize to the indenone 86 (Scheme 38). This transition state is preferentially formed
when the a-position of the ketenedithioacetal is substituted with a bulky group.
Scheme 38
Similarly, a-allyl substituted aroyl ketenedithioacetal 89 gave the
corresponding 2-allyl indenone 90 under similar conditions (Scheme 39).47
iMcMel 2. BFz.E1201McOH
MeS SMe
0
89 90
Scheme 39
The a-allyl substituted acyl ketenedithioacetals 91 where the aromatic
participation is absent, gave the enesters 92 on the addition of methyl Grignard
followed by Lewis acid assisted methanolysis (Scheme 40). The enesters 92 could be
subsequently cyclized to the butyrolactones under acid catalyzed conditions.
I-MeMel 2 BF3.EtZO/MeOH
MeS SMe
91
Scheme 40
The addition of Grignard reagents derived from higher alkyl halides to a-
oxoketenedithioacetals was also studied. Here. the reaction proceeds by a successive
1.4 and 1,2 addition leading to the formation of the cerbinol acetals 94. This under BF1
catalyzed methanolysis gave the corresponding a,P-unsaturated ketones 95 (Scheme
4 I ) ~ ' .
SCH3
d J Q SCH3 1.' RMgBr 3 addn. * SCH3
R
Scheme 41
The brief profile given above exemplifies the synthetic potential of a-
oxoketenedithioacetals Several methods are available for their preparation and the
possibility of easy hnctional group manipulation makes them an important building
block in organic chemistry.
2.4 References
a) Dieter, R. K. Tetrahedron 1986,42. 3029; b) Junjappa, H.; Ila, H.; Asokan, C. V.
Tetrahedron 1990.46, 5423; c ) Kolb, M. Synthesis 1990, 171
Kelber, C. Chem. Ber. 1910, 43, 1252; b) Kelber, C.; Schwarz. A. Chern. Ber.
1911. 44, 1693; c) Kelber. C.; Schwarz, ('hem. Her. 1912. 45,137
a) Thuillier. A,; Vialle. J. Hlrll. Soc. ('him. I.?. 1962, 2187; b) Thuillier. A ;
Vialle, J. Bull. Soc. ('him. I.?. 1962. 2194
Corey, E. J . ; Chen, R. H. K. 7i.trahedrorr Leti. 1973. 38 17
a) Konnen, D. A ; Pfeffer, P. E.; Silbert, L. S. 7i.trhedrotr 1976, 32, 2507; b)
Arjona, 0.; Cereceda, J. A ; Quiroga, M. I . Telrahedrotr 1980, 36, 2137; c)
Dieter, R. K. J. Org. (-hem. 1981, 46, 503 1
Shahak. I . ; Sasson, Y. Teirahedrorr I.eti. 1973, 4207
a) Kurnar, A.; Ila, H.; Junjappa, H. Syrrthe.sis 1976, 324; b) Aapparao, S.; Ila, H.;
Junjappa, H. Sytlihesis 1981, 65; c) Thuillier, A.; Vialle, J. Hull. Soc. ('him. I-?.
1959 1398
Villernin. D.; Alloum, A. B. Syrtihesis 1991, 301
a) Gompper, R.; Schmidt, R. R.; Kutter, E. Liebig.?. Ajar. C'hem. 1965, 37, 684;
b) Gornpper, R; Kutter, E. ('hem. Her. 1965. 98, 1365; c) Gompper, R.; Kutter,
E.; Schmidt, R. R. ('hem. Ber. 1965, 98. 1374
a) Stachel, H. -D ('hem. Her. 1962, 95, 2166; b) Hojo, M.; Masuda, R.;
Kamitori. Y. Tetrahedron I,e//. 1976, 1009
Hojo, M.; Masuda. R. .I. Org. ('hmi. 1975, 40, 963
Rudorf, W. - D ; Augustin, M. .I. Praki. ('hml. 1978, 320, 576
Potts, K. T.; Cipullo. M. J.; Ralli. P.; Theodoridis, G. J. Org. ('hem. 1983, 48.
4841
a) Tominaga. Y.; Kohra, S. , Okuda, H ; Ushirogochi, A,; Matsuda, Y. ;
Kobayashi, G. ('heni. l ' h ~ ~ r n ~ . H t r N 1984, 32, 122; b) Potts, K.T.; Cipullo. M. J . ;
Ralli, P ; Theodoridis, G. .I. Org. ('hem. 1983, 48, 4841
Gompper. R.; Topfl, W. ('hem. Her. 1962. 95, 2871
Rastogi. R. R.; Kumar. A , Ila, H.: Junjappa, H. .I. ('hem. Soc. I'erkitr. 7i.atr.s. I
1978.519
Chauhan. S. M. S.. Junjappa, H. .S)~tr/hesi.s 1975, 798
Purkayastha, M.; Ila. H.. Junjappa. H. Sytr/he.sis 1989. 20
Tomiga, Y.; Kohra. S.; Okuda, H.: Ushirogochi, A ; Matsuda, Y.; Kobayashi,
G. ('hem. Pharm. Hull. 1984, 32. 122
a) Augustin, M.; Groth, Ch. 1. I r k . ' h e m 1979, 321, 2 15, b) Rudorf, D.;
Augustin. M. .I. J'r.uk/. ('hem. 1977, 319. 545; c) Augustin, M.; Jahreis, G . .I.
Prakr. Chem. 1979. 321. 699 (d)
a) Augustin, M.; Groth, Ch.; Kristen. H.; Peseke, K.; Wiechman, Ch. .I. Praki.
('hem. 1979, 321. 205; b) Kumar, A ; Agganval, V.; Ila, H.; Junjappa, H.
Sytrihesis 1980. 748
Singh, G.; Ila, H.; Junjappa, H. Syt7thesi.s 1986, 744
Dieter, R. K.; Jenkitkasemwong. L. Y. Etrahedrorr i .e / / . 1985. 26, 39
Asokan, C. V.; Ila, H.; Junjappa, H. .Sytr/he.sis 1987. 284
Balu, M. P.; Singh, G.; Ila, H.; Junjappa, H. Teirahedrotr Lett. 1986, 27, 1 17.
Guptha, A. K.; Ila, H.; Junjappa, H. Teirahedrorr Lett. 1987, 28, 1459
Dieter, R. K.; Jenkitkasemwong, L. Y. J. Org. ('hem. 1984. 49, 3 183
Apparao. S.; Datta, A ; Ila. H.; Junjappa, H. Sytrthe.sis 1985, 169; b)
Dieter, R. K.; Fishpaugh, J . R. J. Org. ('hem 1988. 53, 203 1
Dieter. R. K.; Fishpaugh, J . R. .I. Org. ('hem. 1983, 48,4439
Datta, A.; Ila, H.; Junjappa, H. Tktrahedrotr Let/. 1988, 29, 497
Balu, M. P.; Pooranchand. D.; Ila, H.; Junjappa, H. Tetrahedron I,et/. 1988,
29,501
Pooranchand. D.; Satyanarayana, J . ; Ila, H.; Junjappa, H. .Syrr/he.si.s 1993, 241
Dieter, R. K.; Fishpaugh, J . R.; 7i.trahedrorr Leii. 1986, 27, 3823
a) Tominaga, Y.; Ushirogochi, A ; Matsuda, Y. Heterocycl. Chem 1987, 24,
1557; b) Tominaga. Y.; Ushirogochi, A,; Matsuda, Y.; Kobayashi, G. ('hem.
Phurm. BzrN 1984, 32, 3384; c) Tominaga, Y . ; Matsuda, Y.; Kobayashi, G.
(:hem. Pharm. BtrN. 1984, 32, 1665; d) ) Tominaga, Y.; Ushirogochi, A ;
Matsuda, Y.; Kobayashi, G. Heterocycles 1977, 8. 193; e ) Tominaga, Y.;
Matsuda, Y.; Kobayashi, G. Heterocyc1r.s. 1976, 4, 1493
Tominaga, Y.; Natsuki, R.; Matsuda, Y.; Kobayashi, G. Chem. Pharm. RIIN.
a) Thuillier, A,; Vialle. J. Btrll. Soc. Chim. Fr. 1959, 1398; b ) Rioult, P.; Vialle.
J. BUN. Soc. Chim. Fr 1965, 33 15; c) Saquet, M.; Thuillier, A. J. BIIII. Chem.
Soc. Chim. Fr. 1966, 1582; d ) Rioult. P.; Vialle. J. R~rll. Soc. Chim. Fr.1968,
4483; e) Maijnan. J.; Vialle, 1. Bull. Soc. Chim. Fr.1973. 2388
a) Augustin, M.; Rudorf, W. - D ; Schmidt, U. Tetrahedron 1976, 32, 3055; b)
Gompper, R.; Schafer, H. Chem. Ber. 1967, 591; d) Mansour, N. B.; Rudorf, W.
- D ; Augustin, M. Z. Chem. 1981. 21, 69
Thomas, A : Ila, H.; Junjappa, H. Tetrahedron Lett. 1989, 30, 3093
Kumar, A ; Mahatre, S.; Ila, H.; Junjappa, H. .I ('hem. Soc. Chem.Commutt.
1976, 592
Saquet, M.; Thuillier, A. Bull. Soc. Chim. Fr. 1966, 3969
Myrboh, B.; Ila, H.; Junjappa, H. J. Org. ('hem. 1983, 48, 5327
Dieter, R. K.; Jenkitkasemwong, L Y. Tetrahedrot~ Lett. 1982, 23, 3747
Myrboh, B.; Asokan, C. V.; Ila, H.; Junjappa, H. Synthesis 1984, 50
Asokan, C . V.; Ila, H.; Junjappa, H. Sy~~thesis 1985, 163
Asokan, C . V.; Ila, H.; Junjappa, H. Terahedror~ Lett. 1985, 26, 1087
Dana, A,; Ila, H.; Junjappa, H. Tetrahedron. 1987, 43. 5367
Singh, G.; Purkayastha, M. L.; Ila, H.; Junjappa, H. J. (%em. Soc. Perkin Trans. 11985, 1289