Solvent-controlled, regio-switchable formation of 3-/5-arylaminopyrazole isomer in cyclocondensation of β-aminoenones with hydrazides: intermolecular hydrogen bonding plays a role

An unprecedented solvent-controlled, regio-switchable Knorr-type reaction for the synthesis of pyrazoles has been realized in the cyclocondensation of β-aminoenones with hydrazides. Single regioisomer of 3- or 5-arylaminopyrazole can be obtained simply by changing the reaction solvent. Using hydrazine hydrate, only 3-arylaminopyrazoles were produced.