Asymmetric Reduction of alpha-Keto Esters with Thermus thermophilus NADHDependent Carbonyl Reductase using Glucose Dehydrogenase and Alcohol Dehydrogenase for Cofactor Regeneration

The enantioselective synthesis of methyl (R)-mandelate and methyl (R)-o-chloromandelate was investigated using an NADH-dependent carbonyl reductase from Thermus thermophilus (TtADH) and, separately, archaeal glucose dehydrogenase and Bacillus stearothermophilus alcohol dehydrogenase (BsADH) for NADH regeneration. Optimal reaction times and substrate concentrations in the absence and presence of organic solvents were determined. The enantiofacial selectivity of TtADH was shown to be inversely proportional to the hydrophobicity of the short-chain linear alcohols employed as co-substrates of the bacillar ADH. The bioreduction of methyl benzoylformate yielded the (R)-alcohol with a 77 % yield (ee = 96 %) using glucose dehydrogenase and glucose, and 81 % yield (ee = 94 %) applying BsADH and ethanol. The bioreduction of methyl o-chlorobenzoylformate yielded the halogenated (R)-alcohol with 95 % and 92 % ee, and 62 % and 78 % yield using glucose dehydrogenase and BsADH, respectively.