Asymmetric Reduction of (R)‐Carvone through a Thermostable and Organic‐Solvent‐Tolerant Ene‐Reductase

Ene‐reductases allow regio‐ and stereoselective reduction of activated C=C double bonds at the expense of nicotinamide adenine dinucleotide cofactors [NAD(P)H]. Biological NAD(P)H can be replaced by synthetic mimics to facilitate enzyme screening and process optimization. The ene‐reductase FOYE‐1, originating from an acidophilic iron oxidizer, has been described as a promising candidate and is now being explored for applied biocatalysis. Biological and synthetic nicotinamide cofactors were evaluated to fuel FOYE‐1 to produce valuable compounds. A maximum activity of (319.7±3.2) U mg−1 with NADPH or of (206.7±3.4) U mg−1 with 1‐benzyl‐1,4‐dihydronicotinamide (BNAH) for the reduction of N‐methylmaleimide was observed at 30 °C. Notably, BNAH was found to be a promising reductant but exhibits poor solubility in water. Different organic solvents were therefore assayed: FOYE‐1 showed excellent performance in most systems with up to 20 vol% solvent and at temperatures up to 40 °C. Purification and application strategies were evaluated on a small scale to optimize the process. Finally, a 200 mL biotransformation of 750 mg (R)‐carvone afforded 495 mg of (2R,5R)‐dihydrocarvone (>95 % ee), demonstrating the simplicity of handling and application of FOYE‐1. Enantioselective hydrogenation: Ene‐reductases (ERs, OYEs) are versatile biocatalysts for selective hydrogenation of activated C=C double bonds. FOYE‐1 was characterized with respect to solvent stability for the conversion of (R)‐carvone into (2R,5R)‐dihydrocarvone at the expense of the synthetic nicotinamide cofactor 1‐benzyl‐1,4‐dihydronicotinamide, the activity of which is compared with that of the natural electron donor NAD(P)H.