Solid-phase synthesis of amino acid derived N-unsubstituted pyrazoles via sydnones.

A new method to synthesize N-unsubstituted pyrazoles on a solid support is described. The solid support acts as a protecting group for the amino acid. N-Protected amino acid is N-nitrosated, and the subsequent treatment with acetic anhydride in a microwave reactor yields mesoionic sydnones that react in situ in 1,3-dipolar cycloaddition reactions with alkynes. Traceless cleavage of the products gives N-unsubstituted pyrazoles in high overall yields.