Phase-Transfer Catalysis in the Asymmetric Synthesis of Pyrazolines

Abstract

The enantioselective synthesis of 3,5-diarylpyrazolines is accomplished by means of asymmetric phase-transfer catalysis. The authors found the cinchona-derived quaternary ammonium salt 1 bearing an ortho-methoxybenzyl group at the quinuclidine nitrogen to efficiently promote the enantioselective conjugate addition of Boc-protected hydrazine to various chalcones. The combination of catalyst 1, hydrazine, and an inorganic base leads to an efficient ammonium-amide ion pairing accounting for the high enantio­selectivities obtained. Subsequent imine formation ensures the irreversible formation of the desired products in good yields and enantio-selectivities.