Pyrazoline ring formation. Part I. The action of phenylhydrazine on some saturated and unsaturated β-amino-ketones

Phenylhydrazine, acting on ß-sec.-aminoethyl ketones in the form of their hydrochlorides, gives first the phenylhydrazone, which by extrusion of a sec.-base residue yields a pyrazoline derivative. Examination of the possible reaction mechanisms suggests the intermediate formation of a vinyl derivative which undergoes prototropic rearrangement to the pyrazoline. The phenylhydrazones of a-unsaturated ß-sec.-aminoethyl ketone hydrochlorides on treatment with acid yield in the main 3-ß-sec.- aminoethylpyrazolines. In addition, however, by extrusion of a sec.-base residue certain styryl and substituted styryl ketone phenylhydrazones of this type have been shown to yield minute quantities of 1 : 5-diphenyl-3-vinylpyrazoline derivatives. The formation of the phenylhydrazones of styryl vinyl ketones as intermediates is postulated.