Chemoselective synthesis of pyrazole derivatives via beta-enamino keto esters

beta -Enamino keto ester (2a) reacts with hydroxylamine to give an isoxazole derivative (3). Compound (2a) reacts with hydrazine, alkyl- and arylhydrazines to give pyrazole derivatives (4a-e) as only reaction products. Methylation of compound (4a) afforded the two isomeric pyrazole derivatives (4b) and (5).