Theoretical study of the asymmetric phase-transfer mediated epoxidation of chalcone catalyzed by chiral crown ethers derived from monosaccharides

The synthesis of a novel, optically active crown ether derived from α-d-altropyranoside is described. A significantly different asymmetric induction was generated by the α-d-glucopyranoside-, α-d-mannopyranoside- and α-d-altropyranoside-based chiral crown catalysts in the epoxidation of trans-chalcone with tert-butyl hydroperoxide under phase-transfer catalytic conditions. It was shown that absolute configuration of the crown-ring fused carbon atoms of the monosaccharides has a great impact on the enantioselectivity. The asymmetric induction could be well explained by considering the possible mechanistic pathway. Molecular modeling (MCMM) and subsequent DFT calculations - in accordance with the experimental results - indicate that the use of glucopyranoside-based catalyst 1 and that of mannopyranoside-based crown ether 2 results in the preferred formation of the opposite antipodes (2R,3S and 2S,3R, respectively) of the corresponding epoxyketone. At the same time, practically no asymmetric induction was proved if altropyranoside-based crown 3 is applied as the catalyst. The computational results are in qualitative agreement with the experimental data.