A Ring-Enlargement Reaction Yielding 1,2,5-Benzothiadiazonin-6-one 1,1-Dioxides

At room temperature or under reflux in MeCN, 3‐amino‐2H‐azirines 2 and 3,4‐dihydro‐2H‐1,2‐benzothiazin‐3‐one 1,1‐dioxide (4) give 1,2,5‐benzothiadiazonin‐6‐one 1,1‐dioxides5 in fair‐to‐good yield (Scheme 2). The structure of this novel type of heterocyclic compounds has been established by X‐ray crystallography of 5a (Fig.). A ring expansion via a zwitterionic intermediate of type A' is proposed as the reaction mechanism of the formation of 5.