Of dithioacetals

TABLE 21. Addition of dithioacetal monoxide lithium salts 376 to cyclic g, /1-unsaturated ketones... 

Acylation of the anions of dithioacetal monoxides proceeds in a similar way leading to the desired products 470 in 83-92% yield505,542 (equation 281). 

Partial hydrolysis of dithioacetals has been found useful for the preparation of anomers not obtained by the foregoing methods and also for preparation of fu-ranosidic 1-thioglycosides. 

The Knoevenagel condensation of a-lithiosulphoxides with hemiacetal 437 has been used to synthesize PGI2 analogues 438 (equation 261). The Knoevenagel-type condensation of dithioacetal monoxides with substituted benzaldehydes has been performed using Triton B as a base and gave the corresponding ketene dithioacetal monoxides 4395 2,513 (equation 262). 

This procedure has been recently applied to the synthesis of L-lyxitol and the polyhydroxylated chain of amphotericin Interesting results have also been obtained in the reduction of ) -oxo derivatives of dithioacetal monoxides. In the reaction sequence of equation 322 two successive asymmetric inductions are involved. After the first reaction, involving acylation of the carbanion, a diastereoisomeric mixture in a 65 35 ratio is produced. When this mixture is reduced with NaBH4 in MeOH-conc. aqueous solution of ammonia, among four possible diastereoisomeric alcohols, the stereoisomer 523 is obtained with a stereoselectivity of 98% . Guanti and coworkers have found that the LiAlH4 reduction of the same substrates at — 78° in THF/ether leads to 523 with a stereoselectivity 99 i6i3.6i4... 

The differing nucleophilicity of acetate and trifluoroacetate anion determined the manner in which naphtho[l,8-/yt]-l,5-dithiocinc sulfoxide 127 rearranged on treatment with acetic and trifluoroacetic anhydrides. In both cases, the reaction proceeded through formation of a disulfonium dication 128, but the final products were different. When acetic anhydride was used, the reaction afforded the corresponding a-acetylsulfide 130, a normal product of the Pummerer rearrangement, while trifluoroacetic anhydride caused isomerization with formation of dithioacetal 132 (see Scheme 16) <1995HAC559>. 

The conventional conversion of carbonyl systems by sulphur tetrafluoride into difluoroalkyl compounds has been modified in the reaction of dithioacetals with tetra-n-butylammonium dihydrogentrifluoride and A -bromo- or AModosuccinimide. Yields of the difluoroalkanes are generally in excess of 70% [35]. 

Cyanogen Iodide (ICN) has been used extensively for the cyanation of alkenes and aromatic compounds [12], iodination of aromatic compounds [13], formation of disulfide bonds in peptides [14], conversion of dithioacetals to cyanothioacetals [15], formation of trans-olefins from dialkylvinylboranes [16], lactonization of alkene esters [17], formation of guanidines [18], lactamization [19], formation of a-thioethter nitriles [20], iodocyanation of alkenes [21], conversion of alkynes to alkyl-iodo alkenes [22], cyanation/iodination of P-diketones [23], and formation of alkynyl iodides [24]. The products obtained from the reaction of ICN with MFA in refluxing chloroform were rrans-16-iodo-17-cyanomarcfortine A (14)... 

This article is limited to the reactions involving cyclic acetals of aldoses and aldosides. However, for comprehensiveness, or for their potential usefulness in carbohydrate chemistry, some properties of cyclic acetals not directly relevant to aldoses and aldosides are included. It may be recalled that the chemistry of dithioacetals had already been reviewed in this Series.7... 

L-Arabinoae diethyl dithioacetal appears to be more thermodynamically stable than the tfaioglycogidet, for it is the only prodnct isolated from mercaptalation of L-arabinose, even after extended exposure to the reagents at room temperature [34]. In general, however, the best yields of dithioacetal ptfcdocts are obtained after short reaction Himg at room... 

Laszlo introduced an oxidative cleavage of dithioacetals by their clayfen (4) and claycop reagents under mild conditions with excellent yields [46, 47]. These reagents are convenient sources of the nitrosonium ion NO+, a soft reactive Lewis acid species, well adapted for attack of the soft sulfur atom. 

Photo lytic transformation of dithioacetals into carbonyl compounds... 

Dithioacids themselves (R CSSH) could be prepared in about a 40-50% yield by addition of Grignard reagents to caibon disulfide in THF as the solvent, followed by acidification in the presence of ether or pentane [143], Hartke has reported that the yields of dithioacids could be improved by reverse addition of the Grignard reagent to carbon disulfide [144], a protocol previously used by Julia [145] in the preparation of dithioesters from allylic organometallics. A 70-80% yield of dithioacetic acid was thus achieved on a molar scale. 

Preparation of dithioacetic acid by addition of methylmagnesium iodide to carbon disulfide... 

If the reaction is carried out on dithioesters, carbanions stabilized by two a-sulfur atoms are formed. Such carbanions are analogous to those obtained by deprotonation of dithioacetals, such as Corey-Seebach... 

Under the same conditions a low yield of dithioacetal was isolated for R1 = t-Bu (23% with R3 = Et). Only dimerization products of carbene intermediates were obtained for R1 = phenyl however, the corresponding unstable magnesiodithioacetal could be trapped by addition of a good alkylating species before that of the Grignard reagent. An 80% yield of dithioacetal (R1 = Ph, R3 = Et or i-Pr, E = Me) was thus isolated. 

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