Chiral Phosphoric Acid Catalyzed Highly Enantioselective Desymmetrization of 2-Substituted and 2,2-Disubstituted 1,3-Diols via Oxidative Cleavage of Benzylidene Acetals

A highly enantioselective catalytic protocol for the desymmetrization of a wide variety of 2-substituted and 2,2-disubstituted 1,3-diols is reported. This reaction proceeds through the formation of an "ortho ester" intermediate via oxidation of 1,3-diol benzylidene acetal by dimethyldioxirane (DMDO) and the subsequent proton transfer catalyzed by chiral phosphoric acid (CPA). The mechanism and origins of enantioselectivity of this reaction are identified using DFT calculations. The oxidation by DMDO is rate-determining, and the phosphoric acid significantly accelerates the proton transfer; the attractive interactions between the benzylidene part of the substrate and the 2,4,6-triisopropyl group of CPA are the key to high enantioselectivity.