Efficient and regioselective syntheses of 1-phenyl-5-pyrazolones substituted 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 of functionalized hydrazones by [2+ 2] cycloaddition reaction of phosphazenes derived from hydrazines with acetylenic esters. Subsequent cyclization of these compounds with butyl lithium affords substituted 5- ...
