Unexpected Ring Enlargement of 2‐Hydrazono‐2,3‐dihydro‐1,3‐thiazoles to 1,3,4‐Thiadiazines

Abstract The cyclization of thiosemicarbazide with α ‐bromoacetophenone can result in the formation of isomeric 1,3,4‐thiadiazines and two different thiazoles. We studied the use of 4‐methyl‐ and 4‐ethylthiosemicarbazide as dinucleophilic building blocks. In this context, we observed an unprecedented rearrangement of a 2‐hydrazono‐2,3‐dihydrothiazole to a 1,3,4‐thiadiazine. While ring contractions of 1,3,4‐thiadiazines to thiazoles are quite common, ring enlargements are new. The course of the reaction depends on the substitution pattern of the substrate.