A regioselective synthesis of 5-pyrazolones and pyrazoles from phosphazenes derived from hydrazines and acetylenic esters

Efficient and regioselective syntheses of 1-phenyl-5-pyrazolones substitute d with a phosphoranylidene group in the 3-position and of 3-alkoxycarbonyl- 5-methoxy-1-phenyl pyrazoles are described. The key step is the formation o f functionalized hydrazones by [2+2] cycloaddition reaction of phosphazenes derived from hydrazines with acetylenic esters. Subsequent cyclization of these compounds with butyllithium affords substituted 5-pyrazolones, while their heating with acetonitrile leads to the formation of 3-alkoxycarbonyl- 5-methoxy-1-phenyl pyrazoles in a regioselective fashion. (C) 1999 Elsevier Science Ltd. All rights reserved.