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  • Title: Oxyanion hole stabilization by C-H···O interaction in a transition state--a three-point interaction model for Cinchona alkaloid-catalyzed asymmetric methanolysis of meso-cyclic anhydrides.
    Author: Yang H, Wong MW.
    Journal: J Am Chem Soc; 2013 Apr 17; 135(15):5808-18. PubMed ID: 23517148.
    Abstract:
    Oxyanion holes are commonly found in many enzyme structures. They are crucial for the stabilization of high-energy oxyanion intermediates or transition states through hydrogen bonding. Typical functionalities found in enzyme oxyanion holes or chemically designed oxyanion-hole mimics are N-H and O-H groups. Through DFT calculations, we show that asymmetric methanolysis of meso-cyclic anhydrides (AMMA) catalyzed by a class of cinchona alkaloid catalysts involves an oxyanion hole consisting of purely C-H functionality. This C-H oxyanion hole is found to play a pivotal role for stabilizing the developing oxyanion, via C-H···O hydrogen bonds, in our newly proposed three-point interaction transition-state model for AMMA reactions, and is the key reason for the catalyst to adopt the gauche-open conformation in the transition state. Predicted enantioselectivities of three cinchona alkaloid catalysts, namely DHQD-PHN, DHQD-MEQ, and DHQD-CLB, based on calculations of our transition-state model, agree well with experimental findings.
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