New routes to aroylthiadiazolines and arylazothiazoles from phenylglyoxalyl bromide arylhydrazones and phenacyl thiocyanate

Reaction of phenylglyoxalyl bromide arylhydrazones (III) with thiourea in ethanol produces 2‐amino‐4‐phenyl‐5‐arylazothiazoles (XI) instead of the expected 2‐benzoyl‐4‐aryl‐5‐imino‐Δ 2− 1,3,4‐thiadiazolines (V) obtained from III and potassium thiocyanate. Phenacyl thiocyanate (IV) couples with diazotized anilines to give V. The mechanisms of formation of V and XI from VI and III, respectively, are postulated. Nitrosation of V gives the corresponding N ‐nitroso derivatives (VII), which decompose upon refluxing in xylene to give 2,4‐disubstituted Δ 2 −1,3,4‐thiadiazolin‐5‐ones (VIII). The thiadiazolines V give the respective N ‐aeyl derivatives IX and X with acetic anhydride and benzoyl chloride in pyridine.