Bases, reactions with dithioacetals

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. ... 

When thiols are added to substrates susceptible to nucleophilic attack, bases catalyze the reaction and the mechanism is nucleophilic. These substrates may be of the Michael type or may be polyhalo alkenes or alkynes. As with the free-radical mechanism, alkynes can give either vinylic thioethers or dithioacetals ... 

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

The CS2 and COS-based ligands are all dealt with in Section 16.4.3. Dimeric oxidation products of the heteroallyls and other ligands in which a CS2 or COS fragment is found are treated briefly in Section 16.4.4. No further attention is paid to ligands like S—S—Me and methanedithiolate or dithioacetal, which occurs for example in the reaction of two moles of (4) with one mole of CS2 in a two-step hydrogen migration.6... 

With regard to the phosphorus stereochemistry, a new stereospecific synthesis of chiral 0,0-dialkyl thiophosphoric acids (6,7) has been developed which is based on the Horner-Wittig reaction of the optically active phosphonothionate carbanions containing the sulfoxide or dithioacetal moieties. The transformation of (R)-(+) 0-methyl 0-isopropyl methanephosphonothionate (10)(8) into (R)-(-) 0-isopropyl phosphorothioic acid (H) best Illustrates this method. 

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