Ring Expansion of Ketones to 1,2-Keto Thioketals. Control of Bond Migration

Article abstract of DOI:10.1021/jo00178a006

A practical two-step procedure for the title transformation has been developed.Treatment of cyclic ketones with (CH3S)3CLi gave adducts 6 and 13-17, which underwent ring expansion at 75 deg C in the presence of CuBF4*4CH3CN to the keto thioketals 7 and 18-21.In the cases examined the reaction was highly regioselective, giving the product resulting from migration of the more substituted carbon.The procedure has been used to prepare a key bicyclic intermediate (33) for a total synthesis of (+/-)-coriolin (1).An alternative synthesis of 33 suggests that were there is steric crowding in the vicinity of the tris(methylthio)methyl group of the adduct, the bond migration is controlled by the location of the obtruding group.