Deracemisation of Mandelic Acid to Optically Pure Non‐Natural L‐Phenylglycine via a Redox‐Neutral Biocatalytic Cascade

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Abstract
A biocatalytic redox‐neutral reaction cascade was designed for the deracemisation of racemic mandelic acid to yield optically pure L‐phenylglycine employing three enzymes. The cascade consisted of three steps: a racemisation, an enantioselective oxidation and a stereoselective reductive amination. The enantioselective oxidation of D‐mandelic acid to the corresponding oxo acid was coupled with the stereoselective reductive amination of the latter; thus the oxidation as well as the reduction reactions were performed simultaneously. The formal hydrogen abstracted in the first step – the oxidation – was consumed in the reductive amination allowing a redox‐neutral cascade due to a cascade‐internal cofactor recycling. The enantiomers of the starting material were interconverted by a racemase (mandelate racemase) ensuring that in theory 100% of the starting material can be transformed. Using this set‐up racemic mandelic acid was transformed to optically pure L‐phenylglycine (ee >97%) at 94% conversion without the requirement of any additional redox reagents in stoichiometric amounts.