Diastereoselective formation of dithioacetal oxides from aliphatic sulfines under thermodynamic control

Protonation of alpha-sulfinyl carbanions 3, resulting from the selective th iophilic addition of organolithium compounds to aliphatic sulfines 2, provi ded dithioacetal oxides 4 (63-94% yield). The diastereomeric ratio ranged f rom 52:48 up to 100:0 with sterically hindered substituents. Thus, (1S*,SS* )-2,2-dimethyl-1-methylsulfany1-1-methylsulfinylpropane (4ca) was obtained as a single isomer. The origin of the stereoselectivity is rationalized in terms of a thermodynamic equilibrium between the two diastereomers of alpha -sulfinyl carbanions 3, at which protonation occurs under kinetic control a nd with retention of configuration with the assistance of the Lithium catio n. Moreover, the stereochemistry could be totally reversed by converting th e Lithiated anion intermediate into an aluminium "ate" complex. Thus, (1R*, SS*)-2,2-dimethyl-1-methylsulfanyl-1-methylsulfinylpropane (4ca) was select ively obtained by this method (dr = 0:100).