Synthesis of 1,3,5-trisubstituted pyrazoles via 1,3-dipolar cycloaddition of nitrile imines with ninhydrin-derived Morita-Baylis-Hillman carbonates.

An effective synthetic method for 1,3,5-trisubstituted pyrazoles via 1,3-dipolar cycloaddition reaction has been developed. This reaction could smoothly proceed between ninhydrin-derived Morita-Baylis-Hillman carbonates and nitrilimines to provide a wide scope of differently substituted pyrazoles in high yields (up to 95%). In addition, the reaction mechanism was also proposed to explain its regioselectivity.