Dithioacetals, alkylation reaction with base

In dithioacetals the proton geminal to the sulfur atoms can be abstracted at low temperature with bases such as Bu"Li. Lithium ion complexing bases such as DABCO, HMPA and TMEDA enhance the process. The resulting anion is a masked acyl carbanion, which enables an assortment of synthetic sequences to be realized via reaction with electrophiles. Thus, a dithioacetal derived from an aldehyde can be further functionalized at the aldehyde carbon with an alkyl halide, followed by thioacetal cleavage to produce a ketone. Dithiane carbanions allow the assemblage of polyfunctional systems in ways complementary to traditional synthetic routes. For instance, the p-hydtoxy ketone systems, conventionally obtained by an aldol process, can now be constructed from different sets of carbon groups. ... 

The preparation of linear monothioacetals or dithioacetals may be accomplished by either the reaction of cysteine hydrochloride with the appropriate a-halo ether or sulphide in methanol or better by the base catalyzed alkylation of cysteine in liquid ammonia. Young and associates [11, 115] have utilized alkylation reactions of this type to prepare several... 

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