Immobilized Arabidopsis thaliana Hydroxynitrile Lyase-Catalyzed Retro-Henry Reaction in the Synthesis of (S)-β-Nitroalcohols

Enantiopure β-nitroalcohols are versatile intermediates used in the synthesis of important pharmaceuticals and chiral synthons. In this article, immobilized Arabidopsis thaliana HNL (AtHNL)-catalyzed preparation of (S)-β-nitroalcohols from their racemic mixtures via retro-Henry reaction was studied. AtHNL used in biocatalysis was immobilized by physical adsorption in inexpensive celite®545. Under optimized biocatalytic conditions, the total turnover number of the catalyst has improved 2.3-fold for (S)-2-nitro-1-phenylethanol (NPE) synthesis, than free enzyme catalysis. This study reported for the first time celite-AtHNL-catalyzed retro-Henry reaction at low pH. At pH 4.5 and 5.0, 62% ee and 41% conversion, and 97% ee and 42% conversion of (S)-NPE were obtained respectively, while the free enzyme inactivates at pH < 5.0. The increased catalytic efficiency and pH stability of the catalyst could be possibly due to increased stability of AtHNL by immobilization. A dozen of racemic β-nitroalcohols were converted into their corresponding (S)-β-nitroalcohols using this reaction; among them, eight were not tested earlier. The immobilized enzyme has showed broad substrate selectivity in the retro-Henry reaction, and products were obtained up to 98.5% ee.

Keywords: Biocatalysis; Hydroxynitrile lyase; Immobilization; Retro-Henry reaction; β-Nitroalcohols.