Highly Regioselective Synthesis of 3,5-Substituted Pyrazoles from Bromovinyl Acetals and N-Tosylhydrazones

A regioselective synthesis of 3,5-disubstituted pyrazoles was achieved by 1,3-dipolar cycloaddition of diazo compounds, generated in situ from N-tosylhydrazones, with unactivated bromovinyl acetals, which served as alkyne surrogates. The reaction tolerated N-tosylhydrazones bearing various substituted benzylidene groups, and a range of 3,5-disubstituted pyrazoles were obtained in yields of up to 92%.