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.