Regio- and stereoselective ring contraction of 1,2-dithianes to 1,2-dithiolanes

[Display omitted] •A novel ring contraction reaction of 4,5-disubstituted 1,2-dithianes is discovered.•The 3,4-disubstituted 1,2 dithiolanes products are formed stereoselectively.•Mitsunobu reaction conditions are used with overall retention of configuration.•Representative examples of O, N, and S nucleophiles are demonstrated.•S-episulfonium intermediates are postulated. Monobenzoylated 1,2-dithiane-4,5 diols undergo a novel ring contraction reaction under Mitsunobu conditions to produce 3,4-disubstituted 1,2-dithiolanes. This transformation is stereoselective and proceeds with retention of configuration. The yields for the trans isomers are higher than those of the cis series for a variety of O, N and S nucleophiles. A plausible pathway for forming the 1,2-dithiolanes involves the formation of a S-episulfonium intermediates.