Enantioselective intramolecular [2,3]‐sigmatropic rearrangement of aldehydes via a sulfonium enamine intermediate

We report the rearrangement of sulfur-containing aldehydes using a sulfonium enamine intermediate as a formylcarbene mimetic. This is an enantioselective, organocatalytic [2,3]-sigmatropic rearrangement enabling chiral cyclic sulfides bearing an α-quaternary chiral center to be prepared in high optical purity. The enantioselectivity is controlled with a cooperative organocatalyst pair consisting of a chiral amine and a chiral phosphoric acid (CPA). The synthetic versatility of this method is demonstrated by its rapid access to structurally diverse chiral spiro S-heterocycles.