Cooperative Photoinduced/Brønsted Acid Catalyzed Cycloaddition of Transient Thioaldehydes and -Quinone Methides toward a Synthesis of Benzo[][1,3]oxathiines.

A cooperative, one-pot approach for the in situ generation and ensuing cycloaddition of thioaldehydes and -quinone methides transiently formed under irradiation with UV-A light and Brønsted acid catalysis, respectively, has been developed giving direct access to benzo[][1,3]oxathiines in good to excellent yields and diastereoselectivity. Both electron-rich and electron-poor thioaldehydes easily react with a broad range of -quinone methides at ambient temperature in a short reaction time to furnish a wide variety of ,-heterocycles.