Synthesis of heterocycles. Part II. New routes to acetylthiadiazolines and alkylazothiazoles

Methylglyoxalyl chloride arylhydrazones (III) react with an ethanolic solution of thiourea to give 2‐amino‐4‐methyl‐5‐arylazothiazoles (XII) instead of the expected 2‐acetyl‐4‐aryl‐5‐imino‐Δ 2 ‐1,3,4‐thiadiazolines (V) which were obtained from III and potassium thiocyanate. 3‐Thiocyanato‐2,4‐pentanedione (IV) coupled with diazotized anilines to give V. The postulated routes to formation of V and XII from III are given. Nitrosation of V gave the corresponding N ‐nitroso derivatives (VI) which decomposed upon refluxing in dry xylene to give 2,4‐disubstituted‐Δ 2 ‐1,3,4‐thiadiazolin‐5‐ones (VII). Boiling of either V or VI with hydrochloric acid gave the hydrochloride salt (VIII). The thiadiazolines V gave the respective N ‐acyl derivatives (IX) and (X) with acetic anhydride and benzoyl chloride in pyridine.