Cascade Coupling of Ene Reductases with Alcohol Dehydrogenases: Enantioselective Reduction of Prochiral Unsaturated Aldehydes

The baker’s yeast (BY)‐mediated reduction of a set of α‐substituted derivatives of cinnamaldehyde to give the corresponding saturated alcohols was investigated. Our study demonstrates that the ene reductases of BY (OYE 2 or OYE 3) preferentially reduce the CC double bond of stereoisomers with the phenyl group trans to the carbonyl group. Moreover, the alcohol dehydrogenases (ADHs) chemoselectively reduce (oxidise) the carbonyl group (hydroxyl group) of saturated aldehydes (alcohols) if the side chain does not contain a heteroatom such as oxygen or sulphur and is longer than an ethyl group. Using isolated OYEs instead of BY, the saturated aldehydes partially lost their optical purity during the biotransformation, even with the in situ substrate feeding product removal (SFPR) technology. This limitation can be overcome by the combination of an isolated ene reductase (OYE 2 or OYE 3) together with an ADH (horse liver alcohol dehydrogenase) in a cascade system which allowed both yields and enantioselectivities to improve.