Regioselective [3 + 2] Cycloaddition Reaction of 3-Alkynoates with Seyferth-Gilbert Reagent.

A Et3N-triggered regioselective [3 + 2] cycloaddition reaction of 3-alkynoates with Seyferth-Gilbert reagent has been developed to furnish a series of trisubstituted pyrazole-3-phosphonates. A one-pot cycloaddition/alkylation sequence further offered access to the corresponding fully substituted pyrazoles.