Stereoselective synthesis of bulky 1,2-diols with alcohol dehydrogenases

Although biotransformations implementing alcohol dehydrogenases (ADHs) are widespread, enzymes which catalyse the reduction and oxidation of sterically demanding substrates, especially 2-hydroxy ketones, are still rare. To fill this gap eight ADHs were investigated concerning their potential to reduce bulky 2-hydroxy ketones. All of these enzymes showed good activities along with excellent enantio- (ee > 99%) and diastereoselectivities (de > 99%). Due to their differences in substrate preferences and stereoselectivity a broad range of diastereomerically pure 1,2-diols is now accessible via biotransformation. Best results were obtained using the alcohol dehydrogenase from Ralstonia sp. (Cupriavidus sp.) (RADH), which showed a broad substrate range, especially for sterically demanding compounds. Araliphatic 2-hydroxy ketones, like (R)-2-hydroxy-1-phenylpropan-1-one ((R)-2-HPP), were reduced much faster than aliphatic or aromatic aldehydes (e.g. benzaldehyde) under the applied conditions. Additionally (R)- as well as (S)-2-hydroxy ketones were converted with high diastereoselectivities (de > 99%). RADH, which was up to now only studied as a whole cell biocatalyst overexpressed in E. coli, was purified and thoroughly characterised concerning its catalytic properties.