Chemistry of ethanediyl S,S-acetals 4. A promising way to cis substituted olefins, stereoselectively from carbonyl compounds.

Aldehydes and methyl ketones readily afford 5,6-dihydro-1,4-dithiins that can be converted by n-butyllithium into their corresponding sulfur stabilized carbanions. Coupling of the latter with alkyl halides leads to species having a cis-configurated, disubstituted double bond tied up by the sulfur-containing ring which is known to be susceptible of selective removal.