Dithioacetals, alkylation from acetals

Corynebacterium equi IF0 3730 gave high enantioselectivity in the oxidation of aryl alkyl sulfides [106], The results listed in Scheme 6C, 11 arise from experiments with no formation of sulfone, which occurs quite easily in several cases. Thio ketals and thio acetals were oxidized into mono S-oxides by various fungal species with enantioselectivity up to 70% ee [107]. Corynebacterium equi was very successfully used in the oxidation of formaldehyde dithioacetals to mono S-oxide or sulfone-sulfoxide, depending on the substrates. Thus n-Bu-S-CH-S-n-Bu was transformed to n-Bu-S02-CH2-S(0)-n-Bu with more than 95% ee in 70% yield. 

A variation by Potts utilizes a-oxoketene ditioacetals in a condensation-cyclization bipyridine synthesis.49,50 For example, reaction of the Q-oxoketenc dithioacetal 3,3-bis(methylthio)-l-(2-pyridinyl)-2-propen-l-one with acetone and potassium /-butoxide, followed by treatment of the intermediate 1,5-enedione potassium salt, (9), with ammonium acetate produced the unsymmetric 6-methyl-4-(methylthio)-bpy, (10) (Scheme 5). Desulfurization with nickel boride afforded 6-methyl bipyridine in 72% yield. Various substituted bpys with alkyl and alkylsulfonyl groups were similarly produced in yields ranging from 37% to 94%. 

Transacetalization, usually starting from the dimethyl acetal, is an important method for the preparation of amide acetals (465 Scheme 86) with long chain or secondary alkyl groups7 4 83t3235 Recently a method was described which allows the preparation of even the di-r-butyl acetal of DMF by transace-talization. To achieve good yields it is necessary to drive the equilibrium reaction to completion by distilling off the alcohol formed. Cyclic acetals (467) or dithioacetals (466) can be more easily synthesized by transacetalization. 24 28-3i-32,35... 

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