Elucidation of the active conformation of cinchona alkaloid catalyst and chemical mechanism of alcoholysis of meso anhydrides.

Abstract

Complementary to enantioselective transformations of planar functionalities, catalytic desymmetrization of meso compounds is another fundamentally important strategy for asymmetric synthesis. However, experimentally established stereochemical models on how a chiral catalyst discriminates between two enantiotopic functional groups in the desymmetrization of a meso substrate are particularly lacking. This article describes our endeavor to elucidate the chemical mechanism and characterization of the active conformation of the cinchona alkaloid-derived catalyst for a desymmetrization of meso cyclic anhydrides via asymmetric alcoholysis. First, our kinetic studies indicate that the cinchona alkaloid-catalyzed alcoholysis proceeds by a general base catalysis mechanism. Furthermore, the active conformer of the cinchona alkaloid-derived catalyst DHQD-PHN was clarified by catalyst conformation studies with a designed, rigid cinchona alkaloid derivative as a probe. These key mechanistic insights enabled us to construct a stereochemical model to rationalize how DHQD-PHN differentiates the two enantiotopic carbonyl groups in the transition state of the asymmetric alcoholysis of meso cyclic anhydrides. This model not only is consistent with the sense of asymmetric induction of the asymmetric alcoholysis but also provides a rationale on how the catalyst tolerates a broad range of cyclic anhydrides. These mechanistic insights further guided us to develop a novel practical catalyst for the enantioselective alcoholysis of meso cyclic anhydrides.