Synthesis of pyrazolines by the condensation of monoalkyl hydrazones with aldehydes

Abstract

Acetaldehyde readily condenses with its monalkyl hydrazones with the formation of 1-alkyl-5-methyl-2-pyrazolines. A number of pure 2pyrazolines have been synthesized in this way with yields of 40–60%. In the condensation of propionaldehyde with its isopropyl hydrazone, because of the occurrence of rearrangements and the nonstereospecificity of the process, a mixture of structural and stereoisomeric 2-pyrazolines is formed. The promising condensation of aldehydes with monalkyl hydrazones of other aldehydes and ketones is accompanied by “transhydrazonation,” which limits the preparative possibilities of this reaction.