Stereocomplementary asymmetric reduction of bulky-bulky ketones by biocatalytic hydrogen transfer

Biocatalysts suitable for the reduction of ketones bearing two sterically demanding substituents (bulky-bulky ketones) at elevated substrate concentration (10 gL^-1) were identified. The biocatalysts can be employed in a substrate-coupled approach; thus, a simple alcohol such as ethanol or 2-propanol serves as a hydrogen donor. Both enantiomers are accessible by using either Rhodococcus ruber DSM 44541 and ethanol or Ralstonia sp. DSM 6428 and Sphingobium yanoikuyae DSM 6900 and ethanol or 2-propanol as the hydrogen source. For Rhodococcus ruber DSM 44541, it was found that bulky-bulky ketones were only transformed when ethanol was used as a hydrogen source, whereas no conversion was observed when 2-propanol was employed. From the substrate spectrum, as well as from the cosubstrate preference, it became clear that a different alcohol dehydrogenase than the previously described ADH-"A" is active in the presence of ethanol in Rhodococcus ruber.