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

• Published in: Journal of molecular structure Vol. 892; no. 1; pp. 336 - 342
• Main Authors: Makó, Attila, Menyhárd, Dóra K., Bakó, Péter, Keglevich, György, Tőke, László
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.12.2008
• Subjects: Asymmetric epoxidation; Asymmetry; Carbon; Catalysis; Catalysts; Chiral crown ether; Crown ethers; Enantioselective synthesis; Epoxidation; Mathematical models; Monosaccharides; Theoretical calculations; Chiral crown ether; Theoretical calculations; Enantioselective synthesis; Asymmetric epoxidation
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2008.05.057

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 (2 R,3 S and 2 S,3 R, 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.