Copper(I)-Catalyzed Cycloaddition of 4-Bromosydnones and Alkynes for the Regioselective Synthesis of 1,4,5-Trisubstituted Pyrazoles

Copper-catalyzed cycloaddition of alkynes with 4-bromosydnones provides a convenient, mild, and regioselective method for the synthesis of a wide range of bromopyrazoles. The broad functional group tolerance of the cycloaddition reaction and further palladium-catalyzed cross-coupling reactions allowed the preparation of polyfunctionalized 1,4,5-pyrazoles that are otherwise difficult to obtain by conventional methods.