Catalytic enantioselective 1,3-dipolar cycloadditions of nitrones with propioloylpyrazoles and acryloylpyrazoles induced by chiral π-cation catalysts.

A chiral copper(II) complex of 3-(2-naphthyl)-l-alanine amide successfully catalyzes the enantioselective 1,3-dipolar cycloaddition reaction of nitrones with propioloylpyrazole and acryloylpyrazole derivatives. The asymmetric environment created by intramolecular π-cation interaction gives the corresponding adducts in high yields with excellent enantioselectivity. This is the first successful method for the catalytic enantioselective 1,3-dipolar cycloaddition of nitrones with acetylene derivatives. The 1,3-dipolar cycloadducts can be stereoselectively converted to β-lactams via reductive cleavage of the N-O bond using SmI(2).