Difluorocarbene-Triggered [1+5] Annulation: Access to Functionalized 1,1-Difluoro-1,9a-dihydropyrido[2,1‑c][1,4]thiazine Derivatives

Difluorocarbene-triggered [1+5] annulation is developed to access 1,1-difluoro-1,9a-dihydropyrido[2,1-c][1,4]thiazine-3,4-dicarboxylate derivatives in satisfactory to good yields via the direct reaction of potassium bromodifluoroacetate and pyridinium 1,4-zwitterionic thiolates under heating. Pyridinium 1,4-zwitterionic thiolates first nucleophilically attack difluorocarbene generated from potassium bromodifluoroacetate followed by an intramolecular nucleophilic addition to pyridiniums. This method provides an expeditious route to introduce the difluoromethyl group into the 1,9a-dihydropyrido[2,1-c][1,4]thiazine ring, even to modify drug molecules.