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

Abstract

AbstractHydroxynitrile lyase from Hevea brasiliensis (HbHNL) is a promiscuous biocatalyst that, besides the native cyanohydrin reaction, also catalyzes the asymmetric Henry reaction yielding (S)‐β‐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 β‐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 β‐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.