Enantiomeric Excess Of Product Research Articles

Kinetic Rationalization of Nonlinear Effects in Asymmetric Catalytic Cascade Reactions under Curtin-Hammett Conditions.

Observations of nonlinear effects of catalyst enantiopurity on product enantiomeric excess in asymmetric catalysis are often used to infer that more than one catalyst species is involved in one or more reaction steps. We demonstrate here, however, that in the case of asymmetric catalytic cascade reactions, a nonlinear effect may be observed in the absence of any higher order catalyst species or any reaction step involving two catalyst species. We illustrate this concept with an example from a recent report of an organocatalytic enantioselective [10 + 2] stepwise cyclization reaction. The disruption of pre-equilibria (Curtin–Hammett equilibrium) in reversible steps occurring prior to the final irreversible product formation step can result in an alteration of the final product ee from what would be expected based on a linear relationship with the enantiopure catalyst. The treatment accounts for either positive or negative nonlinear effects in systems over a wide range of conditions including “major-minor” kinetics or the more conventional “lock-and-key” kinetics. The mechanistic scenario proposed here may apply generally to other cascade reaction systems exhibiting similar kinetic features and should be considered as a viable alternative model whenever a nonlinear effect is observed in a cascade sequence of reactions.

Minisci‐Type Alkylation of N‐Heteroarenes by N‐(Acyloxy)phthalimide Esters Mediated by a Hantzsch Ester And Blue LED Light

AbstractA synthetic method that enables the Hantzsch ester‐mediated Minisci‐type C2‐alkylation of quinolines, isoquinolines and pyridines by N‐(acyloxy)phthalimide esters (NHPI) under blue LED (light emitting diode) light (456 nm) is described. Achieved under mild reaction conditions at room temperature, the metal‐free synthetic protocol was shown to be applicable to primary, secondary and tertiary NHPIs to give the alkylated N‐heterocyclic products in yields of 21–99%. On introducing a chiral phosphoric acid, an asymmetric version of the reaction was also realised and provided product enantiomeric excess (ee) values of 53–99%. The reaction mechanism was delineated to involve excitation of an electron‐donor acceptor (EDA) complex, formed from weak electrostatic interactions between the Hantzsch ester and NHPI, which generates the posited radical species of the redox active ester that undergoes addition to the N‐heterocycle.magnified image

Kinetics of enzyme-catalysed desymmetrisation of prochiral substrates: product enantiomeric excess is not always constant.

The kinetics of enzymatic desymmetrisation were analysed for the most common kinetic mechanisms: ternary complex ordered (prochiral ketone reduction); ping-pong second (ketone amination, diol esterification, desymmetrisation in the second half reaction); ping-pong first (diol ester hydrolysis) and ping-pong both (prochiral diacids). For plausible values of enzyme kinetic parameters, the product enantiomeric excess (ee) can decline substantially as the reaction proceeds to high conversion. For example, an ee of 0.95 at the start of the reaction can decline to less than 0.5 at 95% of equilibrium conversion, but for different enzyme properties it will remain almost unchanged. For most mechanisms a single function of multiple enzyme rate constants (which can be termed ee decline parameter, eeDP) accounts for the major effect on the tendency for the ee to decline. For some mechanisms, the concentrations or ratios of the starting materials have an important influence on the fall in ee. For the application of enzymatic desymmetrisation it is important to study if and how the product ee declines at high conversion.

Lipase-catalyzed hydrolysis of 2-(4-hydroxyphenyl)propionic acid ethyl ester to (R)-(−)-2-(4-hydroxyphenyl)propanoic acid

Stereoselective hydrolysis of (±)-2-(4-hydroxyphenyl)propionic acid ethyl ester (2-HPPAEE) by lipase catalyzed in aqueous system was investigated. Lipase AK with higher catalytic activity and enantioselectivity was selected as catalyst. Simultaneously, factors affecting the conversion of substrate (c) and the enantiomeric excess of product (eep) were optimized. The optimal conditions were established, involving 45 °C of temperature, 5.5 of pH, 10 mg of lipase AK dosage, 0.04 mmol of substrate dosage and 40 h of reaction time. Under the optimum conditions, c and eep could reach up to 49% and 98%, respectively.

Efficient Biocatalytic Preparation of Optically Pure (R)-1-[4-(Trifluoromethyl)phenyl]ethanol by Recombinant Whole-Cell-Mediated Reduction

(R)-1-[4-(Trifluoromethyl)phenyl]ethanol is an important pharmaceutical intermediate of a chemokine CCR5 antagonist. In the present study, a bioprocess for the asymmetric reduction of 4-(trifluoromethyl)acetophenone to (R)-1-[4-(trifluoromethyl)phenyl]ethanol was developed by recombinant Escherichia coli cells with excellent enantioselectivity. In order to overcome the conversion limitation performed in the conventional buffer medium resulting from poor solubility of non-natural substrate, we subsequently established a polar organic solvent-aqueous medium to improve the efficacy. Isopropanol was selected as the most suitable cosolvent candidate, based on the investigation on a substrate solubility test and cell membrane permeability assay in different organic solvent-buffer media. Under the optimum conditions, the preparative-scale asymmetric reduction generated a 99.1% yield with >99.9% product enantiomeric excess (ee) in a 15% (v/v) isopropanol proportion, at 100 mM of 4-(trifluoromethyl)acetophenone within 3 h. Compared to bioconversion in the buffer medium, the developed isopropanol-aqueous system enhanced the substrate concentration by 2-fold with a remarkably improved yield (from 62.5% to 99.1%), and shortened the reaction time by 21 h. Our study gave the first example for a highly enantioselective production of (R)-1-[4-(trifluoromethyl)phenyl]ethanol by a biological method, and the bioreduction of 4-(trifluoromethyl)acetophenone in a polar organic solvent-aqueous system was more efficient than that in the buffer solution only. This process is also scalable and has potential in application.

Lipase-Catalyzed Production of (S)-Carprofen Enhanced by Hydroxyethyl-β-cyclodextrins: Experiment and Optimization

Stereoselective resolution of (R,S)-carprofen methyl ester (CPOMe) by lipase-catalyzed hydrolysis to (S)-carprofen (CP) was investigated in an aqueous medium. With the highest catalytic activity, Candida antarctica Lipase A (CALA) was selected as catalyst compared with eight other lipases. Hydroxyethyl-β-cyclodextrin (HE-β-CD) was added to enhance the solubility of (R,S)-CPOMe, which significantly raised the conversion of substrate from 11.12% to 30.84%. Response surface methodology (RSM) was adopted to evaluate the influence of factors on the substrate conversion (c) and enantiomeric excess of product (eep), such as pH, concentrations of enzyme and HE-β-CD, temperature, substrate loading, and reaction time. The optimal conditions were obtained, including pH 6.0, 40 mg/mL CALA, 0.05 mmol substrate, 35 mmol/L HE-β-CD, agitation speed of 600 rpm, temperature of 76 °C, and reaction time of 30 h. Under the above conditions, (S)-CP as the desired product was obtained with an enantiomeric excess of 96.24% and o...

CALB Immobilized onto Magnetic Nanoparticles for Efficient Kinetic Resolution of Racemic Secondary Alcohols: Long-Term Stability and Reusability.

In this study, an immobilization strategy for magnetic cross-linking enzyme aggregates of lipase B from Candida antarctica (CALB) was developed and investigated. Magnetic particles were prepared by conventional co-precipitation. The magnetic nanoparticles were modified with 3-aminopropyltriethoxysilane (APTES) to obtain surface amino-functionalized magnetic nanoparticles (APTES–Fe3O4) as immobilization materials. Glutaraldehyde was used as a crosslinker to covalently bind CALB to APTES–Fe3O4. The optimal conditions of immobilization of lipase and resolution of racemic 1-phenylethanol were investigated. Under optimal conditions, esters could be obtained with conversion of 50%, enantiomeric excess of product (eep) > 99%, enantiomeric excess of substrate (ees) > 99%, and enantiomeric ratio (E) > 1000. The magnetic CALB CLEAs were successfully used for enzymatic kinetic resolution of fifteen secondary alcohols. Compared with Novozym 435, the magnetic CALB CLEAs exhibited a better enantioselectivity for most substrates. The conversion was still greater than 49% after the magnetic CALB CLEAs had been reused 10 times in a 48 h reaction cycle; both ees and eep were close to 99%. Furthermore, there was little decrease in catalytic activity and enantioselectivity after being stored at −20 °C for 90 days.

Ionic liquids as cosolvents for the lipase-catalyzed kinetic resolution of ketoprofen

This study examined the use of ionic liquids (ILs) as cosolvents for the Candida rugosa lipase-catalyzed enantioselective hydrolysis of racemic ketoprofen ethyl ester. To determine the effect of the IL anion structures on the lipase activity and enantioselectivity, six ILs containing the 1-butyl-3-methylimidazolium ([Bmim]) cation were used as cosolvents and their solvent properties were correlated with various reaction characteristics. The highest lipase activities were obtained using [Bmim][BF4] and [Bmim][MeSO4] as cosolvents. The enantioselectivity (E) of lipase was enhanced by a factor of 50 with 5% [Bmim][MeSO4]. Although the conversion, enantiomeric excess of product (eep), and E value in 5% [Bmim][MeSO4] were 47.3%, >99%, and ∼300, respectively, the corresponding lipase stability is inadequate for commercial application. Excluding [Bmim][MeSO4], the lipase enantioselectivity increased with decreasing hydrogen bond basicity (β) of the IL when the log E values of lipase in 5% ILs were correlated with IL solvent parameters. With [Bmim][PF6] as the cosolvent, which has the lowest β value among all ILs examined, the E value was enhanced by a factor of 32. The conversion and eep in 20% [Bmim][PF6] were 48.0% and 96.9%, respectively. Additionally, the lipase stability in aqueous [Bmim][PF6] solution was much higher than that in aqueous [Bmim][MeSO4] solution.

Efficient production of (R)-(-)-2-hydroxy-4-phenylbutyric acid by recombinant Pichia pastoris expressing engineered D-lactate dehydrogenase from Lactobacillus plantarum with a single-site mutation.

(R)-2-hydroxy-4-phenylbutyric acid (R-HPBA) is a valuable intermediate for the synthesis of angiotensin-converting enzyme inhibitors. The asymmetric reduction of 2-oxo-4-phenylbutyric acid (OPBA) by oxidoreductases is an efficient approach for its synthesis. Here, we report a novel biocatalytic approach for asymmetric synthesis of R-HPBA using recombinant Pichia pastoris expressing the Tyr52Leu variant of D-lactate dehydrogenase (D-LDH) from Lactobacillus plantarum. The recombinant yeast cells showed impressive catalytic activity at a high concentration of NaOPBA (380mM, 76g/L) and achieved full conversion starting with 40g/L NaOPBA or even at higher concentration. Under optimized reaction conditions (pH 7.5, 37°C, and 2% glucose), a full conversion with > 95% reaction yield and ~ 100% product enantiomeric excess (ee) was achieved for the preparation of R-HPBA on a 2-g scale. The findings of this study promote both the biotransformation of R-HPBA and an extension of the application of recombinant yeast as biocatalysts.

Computational redesign of enzymes for regio- and enantioselective hydroamination.

Introduction of innovative biocatalytic processes offers great promise for applications in green chemistry. However, owing to limited catalytic performance, the enzymes harvested from nature's biodiversity often need to be improved for their desired functions by time-consuming iterative rounds of laboratory evolution. Here we describe the use of structure-based computational enzyme design to convert Bacillus sp. YM55-1 aspartase, an enzyme with a very narrow substrate scope, to a set of complementary hydroamination biocatalysts. The redesigned enzymes catalyze asymmetric addition of ammonia to substituted acrylates, affording enantiopure aliphatic, polar and aromatic β-amino acids that are valuable building blocks for the synthesis of pharmaceuticals and bioactive compounds. Without a requirement for further optimization by laboratory evolution, the redesigned enzymes exhibit substrate tolerance up to a concentration of 300 g/L, conversion up to 99%, β-regioselectivity >99% and product enantiomeric excess >99%. The results highlight the use of computational design to rapidly adapt an enzyme to industrially viable reactions.

Bio-preparation of (R)-DMPM using whole cells of Pseudochrobactrum asaccharolyticum WZZ003 and its application on kilogram-scale synthesis of fungicide (R)-metalaxyl.

Methyl (R)-N-(2,6-dimethylphenyl)alaninate ((R)-DMPM) is a key chiral intermediate for the production of (R)-metalaxyl, which is one of the best-selling fungicides. A new strain, Pseudochrobactrum asaccharolyticum WZZ003, was identified as a biocatalyst for the enantioselective hydrolysis of (R,S)-DMPM. The key parameters including pH, temperature, rotation speed and substrate concentrations were optimized in the enantioselective hydrolysis of (R,S)-DMPM. After the 48h hydrolysis of 256mM (R,S)-DMPM under the optimized reaction conditions, the enantiomeric excess of product (e.e.p ) was up to 99% and the conversion was nearly 50%. Subsequently, the unhydrolyzed (S)-DMPM was converted to (R,S)-DMPM through the n-butanal-catalyzed racemization. Furthermore, stereoselective hydrolysis of (R,S)-DMPM catalyzed by whole cells of P. asaccharolyticum WZZ003 was scaled up to kilogram-scale, offering (R)-MAP-acid with 98.6% e.e.p and 48.0% yield. Moreover, (R)-metalaxyl was prepared at kilogram scale after subsequent esterification and coupling reactions. Therefore, a practical production process of (R)-DMPM and (R)-metalaxyl with the prospect of industrialization was developed in this study. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:921-928, 2018.

The effect of deep eutectic solvents on the asymmetric hydrolysis of styrene oxide by mung bean epoxide hydrolases

BackgroundDeep eutectic solvents have attracted considerable attention in numerous fields. There is little information on mung bean epoxide hydrolase-catalyzed epoxides in deep eutectic solvent-containing system.ResultsAdding deep eutectic solvents with hydrogen bond donor of acids to phosphate buffer resulted in an obvious decrease in the optical purity of product; nevertheless, the relative slight change was observed by the addition of the deep eutectic solvents with hydrogen bond donor of alcohols and urea. Of the tested deep eutectic solvents, 10% additional amount of choline chloride/trithylene glycol can cause a significant improvement in the enantiopurity of product, from 83.2 ± 1.3% to 87.9 ± 0.3%. Moreover, with the increase in the addition amount of choline chloride/triethylene glycol from 10 to 30%, the enantiomeric excess of product enhanced from 87.9 to 94%, but a decline of product yield was observed. Finally, the evaluation of enzyme stability showed that the additional amount from 10% to 30% was not beneficial to the activity recovery.ConclusionIn short, adding choline chloride/triethylene glycol contributes to the improvement in the optical purity of (R)-1-phenyl-1, 2-ethanediol in the catalysis of styrene oxide by mung bean epoxide hydrolases; meanwhile, the enzyme immobilization could be essential.

Covalently Immobilized Lipase on a Thermoresponsive Polymer with an Upper Critical Solution Temperature as an Efficient and Recyclable Asymmetric Catalyst in Aqueous Media

AbstractA thermoresponsive lipase catalyst with an upper critical solution temperature (UCST) of about 26 °C was exploited by covalent immobilization of an enzyme, Pseudomonas cepacia lipase (PSL), onto poly(acrylamide‐co‐acrylonitrile) by glutaraldehyde coupling. The experimental conditions for the PSL immobilization were optimized. The immobilized PSL was much more stable for wide ranges of temperature and pH than free PSL. The material was also evaluated as an asymmetric catalyst in the kinetic resolution of racemic α‐methylbenzyl butyrate at 55 °C in an aqueous medium and exhibited high catalytic performance and stability. Up to 50 % conversion and 99.5 % product enantiomeric excess were achieved, thus providing highly pure enantiomers. This biocatalyst could be easily recovered by simple decantation for reuse based on temperature‐induced precipitation. It showed good reusability and retained 80.5 % of its original activity with a well reserved enantioselectivity in the 6th cycle. This work would shed light on the future development of new UCST‐type enzyme catalysts.

Dynamic Ligand Exchange as a Mechanistic Probe in Pd-Catalyzed Enantioselective C-H Functionalization Reactions Using Monoprotected Amino Acid Ligands.

A new tool for probing enantioselective reaction mechanisms is introduced. Monitoring the temporal change in product enantiomeric excess after addition of the opposite enantiomer of the ligand during the reaction provides a means of probing dynamic ligand exchange in enantioselective C-H iodination catalyzed by Pd with monoprotected amino acid ligands (MPAAs). This work has general potential to provide insights about the dynamics of catalyst and ligand molecularity and exchange.

Enantioselective hydrolysis of ibuprofen ethyl ester by a thermophilic immobilized lipase, ELT, from Rhodothermus marinus

Production of the optically pure enantiomers has attracted much attention for both agrochemicals and drugs due to their more target-specific feature and fewer side effects than racemic mixtures. In the case of ibuprofen, an anti-inflammatory drug, the (S)-(+)-form is remarkably active than (R)-(−)-enantiomer. Lipolytic enzymes were considered as excellent biocatalysts for enantiomeric resolution of ibuprofen. In this study, the gene encoding a thermo-halophile putative esterase/lipase/thioesterase, briefly named ELT, from Rhodothermus marinus DSM 4252 was cloned and overexpressed in Escherichia coli. Reusability of the enzyme was provided by the immobilization onto biocompatible supports of SiO2 and chitosan coated Fe3O4-nanoparticles (CS-MNPs). Immobilized enzymes were recovered from the reaction mixture and employed to catalyze at least ten reactions with a residual activity above 80%. The storage stability increased from 45% for the free enzyme to 75 and 92% for ELT-SiO2 and ELT-MNPs, respectively, after 14days incubation at 25°C. Kinetic studies showed that the ELT-SiO2 had higher substrate affinity and turnover number in comparison to the free enzyme. ELT-SiO2 and ELT-MNPs showed enantioselective potential towards hydrolysis of ibuprofen ethyl ester with the conversion percent of 44 and 42% and the product enantiomeric excess of 40 and 43, respectively. The present reusable biocatalyst can be considered to obtain enantiopure ibuprofen with invaluable impact in pharmaceutical industry.

Extended Catalytic Scope of a Well-Known Enzyme: Asymmetric Reduction of Iminium Substrates by Glucose Dehydrogenase.

NADP(H)-dependent imine reductases (IREDs) are of interest in biocatalytic research due to their ability to generate chiral amines from imine/iminium substrates. In reaction protocols involving IREDs, glucose dehydrogenase (GDH) is generally used to regenerate the expensive cofactor NADPH by oxidation of d-glucose to gluconolactone. We have characterized different IREDs with regard to reduction of a set of bicyclic iminium compounds and have utilized 1 H NMR and GC analyses to determine degree of substrate conversion and product enantiomeric excess (ee). All IREDs reduced the tested iminium compounds to the corresponding chiral amines. Blank experiments without IREDs also showed substrate conversion, however, thus suggesting an iminium reductase activity of GDH. This unexpected observation was confirmed by additional experiments with GDHs of different origin. The reduction of C=N bonds with good levels of conversion (>50 %) and excellent enantioselectivity (up to >99 % ee) by GDH represents a promiscuous catalytic activity of this enzyme.

An NADPH-dependent Lactobacillus composti short-chain dehydrogenase/reductase: characterization and application to (R)-1-phenylethanol synthesis.

A new anti-Prelog short-chain dehydrogenase/reductase (SDR) encoding gene lcsdr was cloned from Lactobacillus composti DSM 18527, and heterologously expressed in Escherichia coli. LcSDR is nicotinamide adenine dinucleotide phosphate (NADPH)-dependent and has a molecular weight of approximately 30kDa. The optimal pH and temperature were 6.5 and 30 °C, respectively. The maximal reaction rate Vmax was 133.9 Umg-1; the Michaelis-Menten constant K m of LcSDR were 0.345mM for acetophenone (1a), and 0.085mM for NADPH. Through introducing an EsGDH-catalyzed NADPH regeneration system, a biocatalytic process for (R)-1-phenylethanol ((R)-1b) was developed with outstanding time-space yield. Under the optimized conditions, 50 gl-1 1a was converted to (R)-1b in 2h with a yield of 93.8%, enantiomeric excess of product (e.e.p) above 99% and space-time yield of 562.8 gl-1d-1.

Chiral Template-Directed Regio-, Diastereo-, and Enantioselective Photodimerization of an Anthracene Derivative Assisted by Complementary Amidinium–Carboxylate Salt Bridge Formation

A series of optically active amidine dimers composed of m-terphenyl backbones joined by a variety of linkers, such as achiral and chiral p-phenylene and chiral amide linkers, were synthesized and used as templates for the regio- (head-to-tail (HT) or head-to-head (HH)), diastereo- (anti or syn), and enantioselective [4 + 4] photocyclodimerization of an achiral m-terphenyl-based carboxylic acid monomer bearing a prochiral 2-substituted anthracene at one end (1) through complementary amidinium-carboxylate salt bridges. The amidine dimers linked by p-phenylene linkages almost exclusively afforded the chiral syn-HT and anti-HH dimers at 25 °C, while those joined by amide linkers produced all four dimers. The p-phenylene-linked templates tended to enhance the syn-HT-photodimer formation at high temperatures with no significant changes in the product enantiomeric excess (ee), while the anti-HH-photodimer formation remarkably increased with the decreasing temperature accompanied by a significant enhancement of the product ee up to -86% at -50 °C. Temperature-dependent inversion of the chirality of the anti-HH dimer was observed when the chiral phenylene-linked amidine dimer was used and the product ee was changed from 22% at 50 °C to -86% at -50 °C. A similar enhancement of the enantioselectivity of the anti-HH dimer was also observed for the chiral amide-linked template, producing the anti-HH dimer with up to -88% ee at -50 °C. The observed difference in the regio-, diastereo-, and enantioselectivities due to the difference in the linker structures of the amidine dimers during the template-directed photodimerization of 1 was discussed on the basis of a reversible conformational change in the amidine dimers complexed with 1.

Enantioselective acetylation of (R,S)-atenolol: The use of Candida rugosa lipases immobilized onto magnetic chitosan nanoparticles in enzyme-catalyzed biotransformation

This paper describes the enzyme immobilization protocol as well as the enzymatic method for the direct resolution of (R,S)-atenolol. The used magnetic enzyme carriers possess on their surface new-synthetized chitosan derivatives with free amine groups distanced by ethyl or butyl chain. Additionally the catalytic activity of two types of commercially available lipases from Candida rugosa immobilized onto two different magnetic nanoparticles were compared. The highest values of enantioselectivity (E=66.9), enantiomeric excess of product (eep=94.1%) and conversion (c=41.84%) were obtained by using lipase from Candida rugosa OF immobilized onto Fe3O4-CS-EtNH2. The study confirmed that even after 5 reaction cycles the immobilized lipase maintain its high catalytic activity.

Nonlinear Effects in Asymmetric Synthesis: A Practical Tool for the Discrimination between Monomer and Dimer Catalysis

AbstractNonlinear effects in asymmetric synthesis are identified by a nonlinear relationship between the enantiomeric excess of the product versus that of the chiral catalyst. Such information is not always sufficient to reveal if the active catalyst acts in its monomeric or dimeric form. Numerical simulations of two kinetic models including monomer and dimer catalysis with a Michaelis–Menten‐type reaction mechanism suggest that the nonlinear effects are mainly sensitive to an analogue of the Michaelis–Menten constant as well as to the diastereomeric energy difference between homochiral and heterochiral catalyst dimers. A practical monomer–dimer discrimination table is presented showing that the influence of the initial catalyst enantiomeric excess and the catalytic charge on the product enantiomeric excess and on the reaction half‐time together with the assessment of the kinetic order of the prochiral substrate are sufficient to assess whether the active catalyst acts in its monomeric or dimeric form.