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.
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