Biocatalytic Route to Chiral 2-Substituted-1,2,3,4-Tetrahydroquinolines Using Cyclohexylamine Oxidase Muteins

Abstract

Chiral 2-substituted-1,2,3,4-tetrahydroquinolines (2-substituted-THQs) can serve as invaluable building blocks for certain pharmaceutical agents. This study was conducted to expand the biocatalytic repertoire of a bacterial cyclohexylamine oxidase (CHAO) to include rare access to some key enantiomers. Principal results of this work are the generation of six key 2-substituted-THQs derivatives with high ee values (up to 99%) and isolated yield in the range of 58%–92%. Synthesis of these compounds was made possible by derived muteins of CHAO by directed evolution and in combination with the Turner-deracemization strategy. Interestingly, the L225A mutein for 2-isopropyl-THQ and 2-cyclopropyl-THQ reversed the enantiopreference relative to other muteins. Four structural models were built for variant L225A and Q233A rendering (R)-cyclopropyl-THQ and (S)-cyclopropyl-THQ, and computational results provided support for the experimentally observed stereoselectivity.