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.

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