Enantioselective reduction of fluorenones in surfactant-aqueous solution by fruits and vegetables

Various fruits and vegetables have been evaluated as biocatalysts in the enantioselective reduction of substituted fluorenones under mild and eco-friendly conditions. Grape exhibited the best results with 99% ee and 97% conversion. The conversion rate varies in the presence of different surfactants, but the enantioselectivity remains unchanged. That is, enantiomeric selectivity is from biocatalysts and the conversion is related to the aqueous solubility of substrates. The surfactant, Triton X-100, is the best for improving the biotransformation and behaved in a concentration-dependent manner. The high enantioselectivity of halogen substituents is attributed to halogen electron-withdrawing effects rather than electron-donating effects like –CH3, but the yield is not dependent on the size of the halogens. It is interesting that the absolute configuration of the product, 2-chloro-fluorenol, is R not S, indicating its biocatalytic reduction following anti-Prelog's rule. This study provided a useful method for the reduction of rigid macro-cyclic aromatic ketones.