A New Chiral BLA Promoter for Asymmetric Aza Diels-Alder and Aldol-Type Reactions of Imines

Abstract

A new type of chiral promoter for double asymmetric inductions of aza Diels-Alder and aldol-type reactions of imines is prepared from trialkyl borates (B(OMe)3 or B(OPh> 3) and optically pure binaphthol; X-ray analysis of the boron complex demonstrates that it exists as a Brønsted acid-assisted chiral Lewis acid (BLA). The aldol-type reactions of a number of N-benzhydrylimines derived from aromatic aldehydes with the ketene silyl acetalderived from terf-butyl acetate mediated by the chiral BLA afford β-amino acid esters with high enantioselectivity. The solution conformations of the BLA·imine complexes have been studied using 1H NMR analysis and difference NOE measurements. The absolute configurations of the adducts can be understood in terms of a rational model involving an intramolecular hydrogen binding interaction via a Brønsted acid. © 1994, American Chemical Society. All rights reserved.