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 Δ² ?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.