Stereoinversions using microbial redox-reactions

This paper aims to provide a summary of the recent literature on the use of redox enzymes to carry out stereoinversion reactions on chiral secondary alcohols. Emphasis has been placed on biotransformations which result in the deracemization of a racemic substrate to give high value synthetic intermediates in a theoretical 100% yield. Most of the biocatalysts which are competent to carry out such transformations are whole cell systems, which contain the necessary cofactor recycling machinery to facilitate this otherwise entropically disfavoured process. The first section deals with deracemization of compounds such as mandelic acid and pantoyl lactone using two microorganisms which display enantiocomplementary stereospecificity. The deracemization of chiral alcohols such as beta-hydroxyesters, aryl ethanols and terminal 1,2-diols with single microorganisms will then be discussed and the influence of growth and reaction conditions on the selectivity observed will be emphasised. Then the ability of several microorganisms to deracemize by double stereoinversion substrates with two stereocentres such as cyclohexan-1,2-diol, cis and trans indan-1,2-diol and pentan-2,4-diol will be presented and some mechanistic rationale proposed. Lastly enzymes known as epimerases which are important in sugar and deoxysugar biosynthesis will be discussed with reference to some recent work on the mechanism of UDP-glucose epimerase.