1,4-Unsubstituted 2-phosphorylated vinylacetylenes as valuable phosphorus-containing dipolarophiles

2-(Diethoxyphosphoryl)- and 2-(diphenylphosphoryl)but-2-en-3-ynes, accesible from 4-mesyloxy-3-phosphorylbuta-1,2-dienes, undergo [2+3] cycloaddition with benzonitrile N-oxide or azides to afford isoxazole or 1,2,3-triazole derivatives, respectively.