Lipase catalysed resolution of nitro aldol adducts

Enzyme engineering improves catalytic efficiency and enantioselectivity of hydroxynitrile lyase for promiscuous retro-nitroaldolase activity

Protein engineering to improve promiscuous catalytic activity is important for biocatalytic application of enzymes in green synthesis. We uncovered the significance of binding site residues in Arabidopsis thaliana hydroxynitrile lyase (AtHNL) for promiscuous retro-nitroaldolase activity. Engineering of AtHNL has improved enantioselective retro-nitroaldolase activity, a synthetically important biotransformation, for the production of enantiopure β-nitroalcohols having absolute configuration opposite to that of the stereopreference of the HNL. The variant F179A has shown ∼ 12 fold increased selectivity towards the retro-nitroaldol reaction over cyanogenesis, the natural activity of the parent enzyme. Screening of the two saturation libraries of Phe179 and Tyr14 revealed several variants with higher k_cat, while F179N showed ∼ 2.4-fold k_cat/K_m than the native enzyme towards retro-nitroaldol reaction. Variants F179N, F179M, F179W, F179V, F179I, Y14L, and Y14M have shown > 99% ee in the preparation of (S)-2-nitro-1-phenylethanol (NPE) from the racemic substrate, while F179N has shown the E value of 138 vs. 81 by the wild type. Our molecular docking and dynamics simulations (MDS) studies results provided insights into the molecular basis of higher enantioselectivity by the F179N toward the retro-nitroaldolase activity than the other mutants. Binding energy calculations also showed the higher negative binding free energy in the case of F179N-(R)-NPE compared to other complexes that support our experimental low K_m by the F179N for NPE. A plausible retro-nitroaldol reaction mechanism was proposed based on the MDS study of enzyme-substrate interaction.

Biocatalytic approaches for enantio and diastereoselective synthesis of chiral β-nitroalcohols

Chiral β-nitroalcohols find significant application in organic synthesis due to the versatile reactivity of hydroxyl and nitro functionalities attached to one or two vicinal asymmetric centers. They are key building blocks of several important pharmaceuticals, bioactive molecules, and fine chemicals. With the growing demand to develop clean and green methods for their synthesis, biocatalytic methods have gained tremendous importance among the existing asymmetric synthesis routes. Over the years, different biocatalytic strategies for the asymmetric synthesis of β-nitroalcohol stereoisomers have been developed. They can be majorly classified as (a) kinetic resolution, (b) dynamic kinetic resolution, (c) Henry reaction, (d) retro-Henry reaction, (e) asymmetric reduction, and (f) enantioselective epoxide ring-opening. This review aims to provide an overview of the above biocatalytic strategies, and their comparison along with future prospects. Essentially, it presents an enzyme-toolbox for the asymmetric synthesis of β-nitroalcohol enantiomers and diastereomers.

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.

Production of (S)-β-Nitro Alcohols by Enantioselective C-C Bond Cleavage with an R-Selective Hydroxynitrile Lyase

Hydroxynitrile lyase (HNL)-catalysed stereoselective synthesis of β-nitro alcohols from aldehydes and nitroalkanes is considered an efficient biocatalytic approach. However, only one S-selective HNL-Hevea brasiliensis (HbHNL)-exists that is appropriate for the synthesis of (S)-β-nitro alcohols from the corresponding aldehydes. Further, synthesis catalysed by HbHNL is limited by low specific activity and moderate yields. We have prepared a number of (S)-β-nitro alcohols, by kinetic resolution with the aid of an R-selective HNL from Arabidopsis thaliana (AtHNL). Optimization of the reaction conditions for AtHNL-catalysed stereoselective C-C bond cleavage of racemic 2-nitro-1-phenylethanol (NPE) produced (S)-NPE (together with benzaldehyde and nitromethane, largely from the R enantiomer) in up to 99 % ee and with 47 % conversion. This is the fastest HNL-catalysed route known so far for the synthesis of a series of (S)-β-nitro alcohols. This approach widens the application of AtHNL for the synthesis not only of (R)- but also of (S)-β-nitro alcohols from the appropriate substrates. Without the need for the discovery of a new enzyme, but rather by use of a retro-Henry approach, it was used to generate a number of (S)-β-nitro alcohols by taking advantage of the substrate selectivity of AtHNL.

One-pot synthesis and application of novel amino-functionalized silica nanoparticles using guanidine as amino group

The preparation of modified silica nanoparticles with guanidine was developed and used to catalyze the Henry reaction and fix quantum dots.

Enantio- & chemo-selective preparation of enantiomerically enriched aliphatic nitro alcohols using Candida parapsilosis ATCC 7330

Enantiomerically pure β- and γ-nitro alcohols were prepared from their respective nitro ketones by asymmetric reduction using Candida parapsilosis ATCC 7330 under optimized reaction conditions (ee up to >99%; yields up to 76%).

Synthesis of (R)-β-nitro alcohols catalyzed by R-selective hydroxynitrile lyase from Arabidopsis thaliana in the aqueous-organic biphasic system

Both enantiomers of β-nitro alcohols are versatile chiral building blocks. However, their synthesis using enzymes as catalysts has received little attention, with the exception of (S)-β-nitro alcohols produced in a reaction catalyzed by an S-selective hydroxynitrile lyase (HNL) from Hevea brasiliensis (HbHNL). An R-selective HNL containing an α/β-hydrolase fold from the noncyanogenic plant Arabidopsis thaliana (AtHNL) accepts nitromethane (MeNO₂) as a donor in a reaction with aromatic aldehydes to yield (R)-β-nitro alcohols (Henry reaction; nitro aldol reaction). This reaction proceeded in an aqueous-organic biphasic system. The organic solvent giving the highest enantioselectivity was n-butyl acetate (AcOBu) with an optimum aqueous phase content of 50% (v/v). This is the first example of the R-HNL-catalyzed synthesis of (R)-β-nitro alcohols.

A chemoenzymatic approach to the synthesis of enantiomerically pure (S)-3-hydroxy-gamma-butyrolactone

Optically pure (S)-3-hydroxy-gamma-butyrolactone, an important chiral building block in the pharmaceutical industry, was synthesized from L: -malic acid by combining a selective hydrogenation and a lipase-catalyzed hydrolysis. Lipase from Candida rugosa was found to be the most efficient enzyme for the hydrolysis of (S)-beta-benzoyloxy-gamma-butyrolactone. The use of organic solvent-aqueous two-phase system was employed to extract benzoic acid generated from enzymatic hydrolysis of the substrate. Tert-butyl methyl ether as an organic solvent was effective to extract the reaction product, benzoic acid, and stably maintained the enzyme activity of Lipase OF immobilized on polymeric supports Amberlite XAD-7. The immobilization made the recovery of the product simpler and prevented the formation of the emulsion. The pH adjustment was unnecessary with the immobilized Lipase OF. The scale-up of the enzymatic hydrolysis of S-BBL at 1,850-kg scale was carried out without problems to give 728.5 kg of S-HGB at 80% isolated yield. The scale-up results are similar to those of bench scale reactions. Racemic (R,S)-beta-benzoyloxy-gamma-butyrolactone was prepared from D-, L: -malic acid and was found to be hydrolyzed nonselectively by the enzyme.