Reaction of Isocyanates and Isothiocyanates with Butyraldazine; Formation of 1‐Substituted 2‐Pyrazolines

Phenyl isothiocyanate reacts with butyraldazine in presence of acidic catalysts to form a 2‐pyrazoline (IV) whereas phenyl or benzyl isocyanates under similar conditions yield a mixture of the two isomeric 2‐pyrazolines XIa and XII. On reaction with Ac 2 O, IV gave the 1‐acetyl derivative V. Reduction with LiAlH 4 led to the corresponding pyrazolidines. Reexamination of Kost 's three step preparation of a 1‐phenylthiocarbamoyl 2‐pyrazoline [19] such as X via formic acid cyclization [18] of butyraldazine revealed that his first step product was apparently a mixture of two stereoisomeric 1‐formyl‐2‐pyrazolines VIIIa and VIIIb. A mechanism for the formation of these pyrazolines is proposed.