Dimroth rearrangement of 5‐hydrazino‐1,2,3‐thiadiazoles

The reversible isomerization of 5‐hydrazino‐1,2,3‐thiadiazoles and 1‐amino‐5‐mercapto‐1,2,3‐triazoles has been established for an ester function at the 4‐position ( 4a ⇌ 5a ). Thus, 4a was isolated by reacting 3a with two equivalents of hydrazine, whereas 5a resulted when an excess of hydrazine was used. Acid treatment of 5a yielded 4a again, and its hydrazone 8 was obtained from 5a and benzaldehyde. Compound 5a was also converted into the derivatives 6a, 7, 9 and 10a‐c. The less reactive 5‐chloro‐1,2,3‐thiadiazoles 3b,c afforded the bis‐heterocycles 6b,c on heating with hydrazine.