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 Ac2O, IV gave the 1-acetyl derivative V. Reduction with LiAlH4 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.