Highly enantioenriched mixed benzoins are obtained selectively through a biocatalytical cross-coupling reaction of aromatic aldehydes using ThDP-dependent enzymes.

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Alteration of the substrate specificity of thiamin diphosphate (ThDP)-dependent benzoylformate decarboxylase (BFD) by error-prone PCR is described. Two mutant enzymes, L476Q and M365L-L461S, were identified that accept ortho-substituted benzaldehyde derivatives as donor substrates, which leads to the formation of 2-hydroxy ketones. Both variants, L476Q and M365L-L461S, selectively catalyze the formation of enantiopure (S)-2-hydroxy-1-(2-methylphenyl)propan-1-one with excellent yields, a reaction which is only poorly catalyzed by the wild-type enzyme. Different ortho-substituted benzaldehyde derivatives, such as 2-chloro-, 2-methoxy-, or 2-bromobenzaldehyde are accepted as donor substrates by both BFD variants as well and conversion with acetaldehyde resulted in the corresponding (S)-2-hydroxy-1-phenylpropan-1-one derivatives. As deduced from modeling studies based on the 3D structure of wild-type BFD, reduction of the side chain size at position L461 probably results in an enlarged substrate binding site and facilitates the initial binding of ortho-substituted benzaldehyde derivatives to the cofactor ThDP.

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