One‐Pot Enzyme Cascade Catalyzed Asymmetrization of Primary Alcohols: Synthesis of Enantiocomplementary Chiral β‐Nitroalcohols

Abstract

Biocatalytic asymmetrization of inexpensive and stable primary alcohols to prepare enantioenriched β-nitroalcohols is an important development in green chemistry for the production of chiral pharmaceutical intermediates. Herein, we report a one-pot, two-step cascade reaction sequence in which first benzylic alcohols were oxidized to produce corresponding benzaldehydes using horse liver alcohol dehydrogenase (HLADH). The in situ generated aldehydes were then reacted in a biphasic medium with nitromethane by Arabidopsis thaliana hydroxynitrile lyase (AtHNL) or Baliospermum montanum HNL (BmHNL) catalyzed Henry reaction to produce stereoselective β-nitroalcohols with (R) or (S) configuration, respectively. Using HLADH-AtHNL, (R)-β-nitroalcohols were obtained in up to 64% conversion, and HLADH-BmHNL, (S)-β-nitroalcohols in up to 70% conversion, while in both cases excellent stereoselectivity (up to > 99% ee) was achieved. The concept was proven by functionalization of sp 3 CÀ H bond of ten simple achiral benzylic alcohols to enantiocomplementary chiral β-nitroalcohols.

Hydroxynitrile Lyase Discovery, Engineering, and Promiscuity towards Asymmetric Synthesis: Recent Progress

“…A stereocomplementary synthesis could be achieved by using the HLADH-AtHNL cascade that synthesized different (R)-β-nitroalcohols in up to 64 % and > 99 % ee, from their corresponding primary alcohols. [64] Unlike the conventional stereoselective hydrocyanation and Henry reaction, a few HNLs have been used in asymmetric synthesis using different approaches. They are enantioselective cleavage and enzyme cascade.…”

“…and >99 % ee (Scheme 18). A stereocomplementary synthesis could be achieved by using the HLADH‐ At HNL cascade that synthesized different ( R )‐β‐nitroalcohols in up to 64 % and >99 % ee, from their corresponding primary alcohols [64] …”

Hydroxynitrile lyase engineering for promiscuous asymmetric Henry reaction with enhanced conversion, enantioselectivity and catalytic efficiency

“…They are Hevea brasiliensis HNL (HbHNL), AtHNL, Granulicella tundricola (GtHNL), Acidobacterium capsulatum (AcHNL) and Baliospermum montanum HNL (BmHNL). [13][14][15][16][17][18][19] Among them, only AtHNL, and two metal dependent cupin fold HNLs, GtHNL, and AcHNL exhibit (R)-selectivity. Enantioselective Henry reaction of GtHNL, AcHNL, and their variants is restricted to only four substrates.…”

“…The WT conversion was limited to 18% with 86% ee, which improved to 53% with 91% ee by the Y14F and 56% with 95% ee by 125 U of Y14M-F179W in 3 h. Both high conversion and % ee were achieved in less time and amount of enzyme than the previous reports using AtHNL variants. 15,18,20 Similarly, 125 U of Y14F could produce 2i in 61% conversion with 99% ee against only 38% conversion by the WT. The p-fluoro benzaldehyde (1c), unlike 1i, could be converted only up to 4% with 77% ee by the WT.…”

“…In a recent study, we used At HNL in a cascade reaction and found poor enantioselectivity in the synthesis of several ( R )-β-nitroalcohols. 18 An acyl-peptide releasing enzyme from Sulfolobus tokodaii (ST0779) was studied in the enantioselective synthesis of β-nitroalcohols, which used only 0.1 mM scale reaction with 20 mg of the purified enzyme at 40 °C. 20 The catalyst, however, did not show uniform enantiopreference, and its enantioselectivity varied with the electronic effects of the substituents on the benzaldehyde ring (Fig.…”