A simple approach to pyrazol-3-ones via diazenes

An efficient entry into pyrazol-3-ones is described starting from propenoic acids that were first transformed into the corresponding hydrazides. Oxidation of the hydrazides gave the diazenes and the latter cyclized to pyrazol-3-ones on treatment with ZrCl4. The methoxycarbonyl protection of the N-1 of the pyrazolone derivatives was easily removed under mild reaction conditions.