Asymmetric Retro-Henry Reaction Catalyzed by Hydroxynitrile Lyase from Hevea brasiliensis

Hydroxynitrile lyase from Hevea brasiliensis (HbHNL) is a promiscuous biocatalyst that, besides the native cyanohydrin reaction, also catalyzes the asymmetric Henry reaction yielding (S)-beta-nitroalcohols with high enantiomeric excess. Since the Henry reaction is reversible, the enzyme can be also utilized for the production (R)-enantiomers by means of resolution of racemic beta-nitroalcohols. Herein the biocatalytic retro-Henry reaction is studied using the cleavage of 2-nitro-1-phenylethanol as a model system. The main problem that prevents high levels of conversion or high ee values during the cleavage of the beta-nitroalcohol is the formation of benzaldehyde, which is known to be a strong enzyme inhibitor. The product inhibition is overcome by performing the biocatalytic retro-Henry reaction in the presence of HCN, which reacts in situ with benzaldehyde and converts it to the less-inhibitive mandelonitrile. By using such a reaction cascade, it was possible to conduct the resolution practically to completion (95% ee, 49% conversion). Furthermore, the catalyst productivity achieved during the resolution was ten times higher than that in the HbHNL-catalyzed synthesis of (S)-2-nitro-1-phenylethanol by condensation of benzaldehyde and nitromethane.