Steric Effects on the Stereochemistry of Old Yellow Enzyme-Mediated Reductions of Unsaturated Diesters: Flipping of the Substrate within the Enzyme Active Site Induced by Structural Modifications

The ene-reductase-mediated reduction of the carbon-carbon double bond of some alkyl 2-substituted butenedioates was investigated. The stereochemical outcome of the reaction was found to be influenced by steric effects. Ethyl and butyl citraconates were converted into the corresponding alkyl (R)-2-methylsuccinates with excellent enantioselectivity, whereas ethyl and butyl mesaconates were completely unreactive. Methyl 2-substituted fumarates were reduced to enantiomerically enriched methyl (S)-2-substituted succinates, whereas the (Z)-stereoisomers were left unreacted by ene-reductases. Labelling experiments were performed to investigate the mechanism of these bioreductions and explain their stereochemical outcome.