A Surprising Mechanistic Switch in Lewis Acid Activation: A Bifunctional Asymmetric Approach to α-Hydroxy Acid Derivatives

摘要

We report a detailed synthetic and mechanistic study of an unusual bifunctional, sequential hetero-Diels–Alder/ring-opening reaction in which chiral, metal complexed ketene enolates react with o-quinones to afford highly enantioenriched, α-hydroxylated carbonyl derivatives in excellent yield. A number of Lewis acids were screened in tandem with cinchona alkaloid derivatives; surprisingly, trans-(Ph3P)2PdCl2 was found to afford the most dramatic increase in yield and rate of reaction. A series of Lewis acid binding motifs were explored through molecular modeling, as well as IR, UV and NMR spectroscopy. Our observations document a fundamental mechanistic “switch” – namely the formation of a tandem Lewis base/Lewis acid activated metal enolate in preference to a metal-coordinated quinone species (as observed in other reactions of o-quinone derivatives). This new method was applied to the syntheses of several pharmaceutical targets, each of which was obtained in high yield and enantioselectivity.