Tandem Chemoselective 1,2-/1,4-Migration of the Thio Group in Keto Thioesters: An Efficient Approach to Substituted Butenolides

We report herein an efficient and mechanistically unique tandem chemoselective 1,2-/1,4-migration of the thio group in keto thioesters that provides substituted butenolides in moderate to excellent yields. Thus, alpha-keto thioesters in the presence of stabilized phosphonate carbanions undergo tandem 1,2-sulfur migration; whereas 1,4-migration of the thio group has been achieved with the same thioesters after the treatment with Wittig reagents followed by BF3 center dot OEt2-catalyzed tandem reaction. The crossover experiments and the isolation of intermediates reveal a stepwise mechanism for both of these transformations.