Access to Functionalized 1,4-Oxathiines through [3 + 3] Annulation of Dicyanoepoxides with Pyridinium 1,4-Zwitterionic Thiolates

A metal- and base-free [3 + 3] annulation of readily accessible dicyanoepoxides and pyridinium 1,4-zwitterionic thiolates has been developed. The protocol enables the efficient synthesis of highly substituted 1,4-oxathiine derivatives under mild conditions and in green media with a broad substrate scope. Furthermore, the synthetic utility of the method was demonstrated through a post-transformation involving a ring-opening/recyclization sequence with primary amine derivatives, furnishing a range of 1,4-thiazine scaffolds.