Biocatalytic Deracemization Techniques: Dynamic Resolutions and Stereoinversions

Biocatalytic deracemization techniques, i.e. reactions allowing the transformation of a racemic mixture into one single enantiomeric product in quantitative yield, are reviewed. This goal may be achieved via two different principles: (i) Dynamic resolution: If a (biocatalytic) kinetic resolution of a substrate is performed under conditions where the (chirally labile) substrate enantiomers are equilibrating (racemizing), all of the substrate is transformed into a single product enantiomer in 100% theoretical yield. Substrate racemization may be achieved either by chemical or biocatalysis. The major advantages of these so-called 'Dynamic Resolutions' are two-fold. They exhibit higher enantio-selectivities as compared to classic resolutions and due to the absence of an unwanted remaining (substrate) enantiomer the separation of the substrate-product mixture can be omitted. This concept has been applied to a variety of compounds such as α-amino acids, hemithioacetal esters, α-(hetero)arylcarboxylic acids, α-substituted nitriles, cyanohydrin esters, α-substituted thioesters, as well as 4-substituted oxazolin-5-ones and thiazolin-5-ones. (ii) Stereoinversion: If one enantiomer from a racemic substrate is transformed into its mirror image by leaving the other enantiomer untouched, the reaction comprises a formal stereoinversion leading to a single product enantiomer in 100% theoretical yield. This concept has been verified with the transformation of racemic secondary alcohols via biocatalytic inversion of one enantiomer by means of a two-step oxidation-reduction sequence catalyzed by whole microbial cells.

biocatalysis - dynamic kinetic resolution - stereoinversion