Ene-Reductase-Catalyzed Enantioselective Desymmetrization of Cyclohexadienones: Straightforward Access to All-Carbon Quaternary Stereocenters

An unprecedented enzyme-catalyzed enantioselective desymmetrization of achiral 2,5-cyclohexadienones has been reported. Using ene-reductases as the biocatalysts, a variety of gamma,gamma-disubstituted cyclohexadienones were reduced to the respective chiral cyclohexenones bearing an all-carbon quaternary stereocenter in high yields (up to 96%) along with low levels of over-reduction (less than 4% cyclohexanones in most cases) and good enantioselectivities (mostly 99% ee), which are superior to the enantioselectivities obtained with the chemocatalysis. A mutagenesis study indicated residue Q232 was likely important for NCR in providing cyclohexenones selectively over cyclohexanones, and molecular dynamics (MD) simulations were performed to rationalize the good enantioselectivity and low level of over-reduction observed for this enzyme. The deuterium-labeling experiment suggested the hydrogen at the N5 atom of the reduced flavin cofactor added to the face of the substrate 1a that has the bulkier phenyl group facing toward it. The current work expands the substrate scope of ene-reductases, providing an efficient, stereoselective access to valuable chiral gamma,gamma-disubstituted cyclohexenones, and will stimulate the development of other classes of enzyme-catalyzed enantioselective desymmetrization of cyclohexadienones.