Dynamic kinetic resolution of phenylglycine esters via lipase-catalysed ammonolysis

An Aminocaprolactam Racemase from Ochrobactrum anthropi with Promiscuous Amino Acid Ester Racemase Activity

The kinetic resolution of amino acid esters (AAEs) is a useful synthetic strategy for the preparation of single-enantiomer amino acids. The development of an enzymatic dynamic kinetic resolution (DKR) process for AAEs, which would give a theoretical yield of 100 % of the enantiopure product, would require an amino acid ester racemase (AAER); however, no such enzyme has been described. We have identified low AAER activity of 15 U mg-1 in a homologue of a PLP-dependent α-amino ϵ-caprolactam racemase (ACLR) from Ochrobactrum anthropi. We have determined the structure of this enzyme, OaACLR, to a resolution of 1.87 Å and, by using structure-guided saturation mutagenesis, in combination with a colorimetric screen for AAER activity, we have identified a mutant, L293C, in which the promiscuous AAER activity of this enzyme towards l-phenylalanine methyl ester is improved 3.7-fold.

Photoredox activation of carbon dioxide for amino acid synthesis in continuous flow

Although carbon dioxide (CO₂) is highly abundant, its low reactivity has limited its use in chemical synthesis. In particular, methods for carbon–carbon bond formation generally rely on two-electron mechanisms for CO₂ activation and require highly activated reaction partners. Alternatively, radical pathways accessed via photoredox catalysis could provide new reactivity under milder conditions. Here we demonstrate the direct coupling of CO₂ and amines via the single-electron reduction of CO₂ for the photoredox-catalyzed, continuous flow synthesis of α-amino acids. By leveraging advantages for utilizing gases and photochemistry in flow, a commercially available organic photoredox catalyst effects the selective α-carboxylation of amines bearing various functional groups and heterocycles. Preliminary mechanistic studies support CO₂ activation and carbon–carbon bond formation via single-electron pathways, and we expect that this strategy will inspire new perspectives on using this feedstock chemical in organic synthesis.

Dynamic kinetic asymmetric transformations of β-stereogenic α-ketoesters by direct aldolization

Dynamic kinetic asymmetric transformations (DyKAT) of racemic β-bromo-α-keto esters by direct aldolization of nitromethane and acetone provide access to fully substituted α-glycolic acid derivatives bearing a β-stereocenter. The aldol adducts are obtained in excellent yield with high relative and absolute stereocontrol under mild reaction conditions. Mechanistic studies determined that the reactions proceed through a facile catalyst-mediated racemization of the β-bromo-α-keto esters under a DyKAT Type I manifold.

Silica-immobilized His6-tagged enzyme: Alanine racemase in hydrophobic solvent

A new immobilization method for enzymes is presented to facilitate synthetic applications in aqueous as well as organic media. The enzyme Alanine racemase (AlaR) from Geobacillus stearothermophilus was cloned, overexpressed and then immobilized on a silica-coated thin-layer chromatography plate to create an enzyme surface. The enzyme, fused to a His(6)-tag at its N-terminal, was tethered to the chemically modified silica-coated TLC plate through cobalt ions. The immobilized enzyme showed unaltered kinetic parameters in small-scale stirred reactions and retained its activity after rinsing, drying, freezing or immersion in n-hexane. This practical method is a first step towards a general immobilization method for synthesis applications with any enzyme suitable for His6-tagging.

Efficient enantioselective hydrolysis of d,l-phenylglycine methyl ester catalyzed by immobilized Candida antarctica lipase B in ionic liquid containing systems

Immobilized Candida antarctica lipase B (Novozym 435)-catalyzed enantioselective hydrolysis of D,L-phenylglycine methyl ester to enatiopure D-phenylglycine was successfully conducted in the systems with ionic liquids (ILs). Novozym 435 exhibited excellent activity and enantioselectivity in the system containing the IL BMIMxBF(4) compared to several typical organic solvents tested. It has been found that the cations and, particularly, the anions of ILs have a significant effect on the reaction, and the IL BMIMxBF(4), which shows to be the most suitable for the reaction, gave the highest initial rate and enantioselectivity among various ILs examined. The reaction became much less active and enantioselective in the systems with BMIMxHSO(4). Also, it was noticed that the enzymatic hydrolysis was strongly dependent on BMIMxBF(4) content in the co-solvent systems and the favorable content of the IL was 20% (v/v). Of the assayed four co-solvents and phosphate buffer, the lowest apparent K(m) and activation energy, and the highest V(max) of the reaction were achieved using 20% (v/v) BMIMxBF(4) co-solvent with phosphate buffer. Additionally, various influential variables were investigated. The optimum pH, substrate concentration, reaction temperature and shaking rate were 8.0, 80mM, 25-30 degrees Celsius and 150rpm, respectively, under which the initial rate, the residual substrate e.e. and the enantioselectivity were 2.46mM/min, 93.8% (at substrate conversion of 53.0%) and 38, respectively. When the hydrolysis was performed under reduced pressure, the initial rate (2.64mM/min) and the enantioselectivity (E=43) were boosted.

Biocatalytic ammonolysis of (5S)-4,5-dihydro-1H-pyrrole-1,5-dicarboxylic acid, 1-(1,1-dimethylethyl)-5-ethyl ester: Preparation of an intermediate to the dipeptidyl peptidase IV inhibitor Saxagliptin

An efficient biocatalytic method has been developed for the conversion of (5S)-4,5-dihydro-1H-pyrrole-1,5-dicarboxylic acid, 1-(1,1-dimethylethyl)-5-ethyl ester (1) into the corresponding amide (5S)-5-aminocarbonyl-4,5-dihydro-1H-pyrrole-1-carboxylic acid, 1-(1,1-dimethylethyl)ester (2), which is a critical intermediate in the synthesis of the dipeptidyl peptidase IV (DPP4) inhibitor Saxagliptin (3). Candida antartica lipase B mediates ammonolysis of the ester with ammonium carbamate as ammonia donor to yield up to 71% of the amide. The inclusion of Ascarite and calcium chloride as adsorbents for carbon dioxide and ethanol byproducts, respectively, increases the yield to 98%, thereby offering an efficient and practical alternative to chemical routes which yield 57-64%.

Chemoenzymatic Dynamic Kinetic Resolution of Primary Amines

An efficient process for dynamic kinetic resolution of amines was developed by combining a ruthenium-catalyzed racemization with a lipase-catalyzed resolution. A variety of unfunctionalized primary amines were transformed into one enantiomer of the amide in high yield and high enantioselectivity.

Chiral ion-exchange capillary electrochromatography of arylglycine amides with dextran sulfate as a pseudostationary phase

A low-cost tunable chiral ion-exchange capillary electrochromatographic method has been developed for the separation of arylglycine amide racemic mixtures with dextran sulfate (DS) as an anionic and chiral pseudostationary phase and Tris-tartrate as a buffer system. The concentrations of DS and Tris had opposite influences on retention and resolution and could serve as ideal factors to finely tune the running speed and chiral resolution. Tartrate and pH largely impact the separation but pH should be confined within 3.0-5.5, only suitable for coarse tuning, while tartrate was preserved as the key buffering reagent, normally maintained at 40 mmol/L. With a working system composed of 0.1-1.0% DS, 20-60 mmol/L Tris, and 40 mmol/L tartrate at pH 3.50-4.50, the enantioresolution of arylglycine amides was shown to be dependent on their chemical structure: The chiral resolution increased when the hydrogen at the alpha-amino group or at the p-position of phenyl ring was replaced by other larger group(s) but the resolution decreased when the group at the o- or m-site on the phenyl ring was enlarged. Further, the electronegative substitute of -Cl had larger resolution increment than methyl or methoxy at the position p- of phenyl ring but much lower increment at position m-. It is possible to well explain the resolution variation phenomenon by considering the group resistance and the variation of hydrogen-bonds formed inside the amino amides and between the solutes and DS. The amido group was shown irreplaceable to have chiral resolution with DS alone as an ionic and chiral pseudostationary phase.

Chemoenzymatic dynamic kinetic resolution

During the past decade a new concept has appeared in asymmetric catalysis involving the combination of a biocatalyst and a chemocatalyst in one 'pot' leading to efficient deracemization via dynamic kinetic resolution (DKR). Here, we outline the different strategies that have been developed for efficient chemoenzymatic DKR, in particular the powerful combination of an enzyme and a metal catalyst.

Fast enantioseparation of arylglycine amides by capillary electrophoresis with highly sulfated-beta-cyclodextrin as a chiral selector

Nine racemic arylglycine amides were synthesized and successfully enantioseparated by capillary electrophoresis (CE) using highly sulfated beta-cyclodextrin (HS-beta-CD) as a chiral selector. Baseline enantioseparation of the analytes was obtained around neutral pH but not in the acidic conditions that are commonly used. HS-beta-CD content, buffer pH, type and concentration, and organic modifier concentration were studied and optimized for fast and efficient separation. A chiral CE separation system composed of 1.5% (w/v) HS-beta-CD, 0 to 10% (v/v) methanol and 20 mM 3-(N-morpholino)propanesulfonic acid at pH 6.5 was shown suitable for baseline enantioseparation of the mentioned amides within 6 min, including simultaneous enantioseparation of three positional isomer series (methyl-, methoxyl or chloro-substituted). By using this system, D-enantiomers migrated ahead of the L-enantiomers and the enantiomeric resolution order of arylglycine amides was more or less parallel to the pK(a), order of the analytes.

The production of fine chemicals by biotransformations

Today, biocatalysis is a standard technology for the production of chemicals. An analysis of 134 industrial biotransformations reveals that hydrolases (44%) and redox biocatalysts (30%) are the most prominent categories. Most products are chiral (89%) and are used as fine chemicals. In the chemical industry, successful product developments involve on average a yield of 78%, a volumetric productivity of 15.5 g/(L.h) and a final product concentration of 108 g/L. By contrast, the pharmaceutical industry focuses on time-to-market. The implications of this for future research and development on biocatalysis are discussed.

Production of fine chemicals using biocatalysis

Presently, a large number of biotransformations are carried out on an industrial scale and are discussed in a fast increasing number of reviews. Besides this, a significant number of biotransformations have been investigated over the past year, from degrading to transforming and synthetic reactions. The development of more specific and stable biocatalysts, either isolated enzymes or whole cells, generated by the new methods of genetic engineering and improved by reaction engineering have led to new industrial biotransformations.