Asymmetric dithioacetals II: A novel and versatile method for the preparation of chiral dithioacetals

Reaction of aldehydes with one equivalent each of a thiol and a chiral thiolacid such as (R)-(-)-α-methoxyphenylthiolacetic acid in the presence of an acidic catalyst such as ZnI₂ or p-toluenesulfonic acid yields diastereomeric mixed thioacetals in good yields which are generally readily separable. Subsequent deacylation at low temperature with sodium methoxide and alkylation of the resulting thiolate anion with a variety of electrophiles provides chiral dithioacetals with no loss of enantiomeric purity.