Enantioselective enzymatic desymmetrization catalyzed by oxidoreductases. Reduction reactions. Part 2

Biotransformation reactions of many organic compounds under the influence of enzymes take place with the high selectivity, rarely achieved by other methods. Ketoesters represent an extensive group of selectively bioreduced compounds. Chiral hydroxyesters and, subsequently, hydroxyacids are valuable intermediates in the syntheses of various biologically active compounds. Acyclic α- and β-ketoesters are transformed to the corresponding (R)- and (S)-hydroxyesters by using a specific dehydrogenases. The whole-cells enzymes, e.g. baker’s yeast, may exhibit a different catalytic activity depending on the substrate structure. Baker’s yeast enzymes selectively reduce the cyclic β-ketoesters providing mainly anti diastereomers due to the lack of rotation around the single α,β carbon-carbon bond. The enzymatic reduction of the esters, cyclopentanone, and cyclohexanone derivatives gave the optically active anti-alcohol enantiomers. The reductive EED of prochiral α-ketoesters, as well as β-ketoesters is an interesting transformation in organic chemistry due to the importance of the resulting chiral α-hydroxy acids and their derivatives used as building blocks. Baker’s yeast-catalyzed reduction of alkyl esters derived from pyruvate and benzoylformate allows the preparation of the (R)-alcohols. Polyketones can also be subjected to the reductive EED to give different compounds bearing the quaternary stereogenic centers which are broadly applied in asymmetric synthesis. In asymmetric synthesis, similarly to carbon-oxygen double bonds, carbon-carbon double bonds of prochiral alkanes can be reduced to obtain the optically active saturated compounds. The reduction of alkenes is catalyzed by both, the whole cells (microorganisms, plant cells) as well as isolated enzymes belonging to the oxydoreductases, so-called ene-reductases. The whole-cell catalysts are suitable, most frequently, for the preparative scale syntheses, but they are less chemoselective in comparison to the isolated reductases. In the case of polyfunctionalized alkenes, microorganisms can cause the additional side reaction reducing the desired product yield