Phase-transfer catalyzed asymmetric epoxidation of chalcones using chiral crown ethers derived from d-glucose, d-galactose, and d-mannitol

Abstract

Chiral monoaza-15-crown-5 lariat ethers synthesized from d-glucose, d-galactose, and d-mannitol have been applied as phase-transfer catalysts in the enantioselective epoxidation of chalcones with tert-butylhydroperoxide. The type of monosaccharide on the crown ether and the substituents at the nitrogen atom of the crown-ring has a major influence on both the chemical yield and enantioselectivity. Among the catalysts, the crown ether annellated to methyl-4,6- O-benzylidene-α- d-glucopyranoside, with 3-hydroxypropyl side arm at the nitrogen atom 1f proved to be the most effective (92% ee). The enantioselectivity was also affected by the substituents on the aromatic rings of the chalcone. The absolute configurations of epoxyketones 6a, 6b, 6d, 6i, 6k, and 6m were determined by CD spectroscopy; the complete stereostucture of 6b was determined by single crystal X-ray analysis.