Enantioselective Resolution Research Articles

Preparation of chiral polyether dendrimer and its non-bonding immobilized CALB for high enantioselective synergy resolution of amines

Monodisperse polyethylene glycol monomethyl ether and chiral glycidyl tosylate were utilized as substrates in the synthesis of the chiral monomer unit epoxypropyl polyethylene glycol monomethyl ether (MPEGn-EPO). The initiator propargyl-terminated polyethylene glycol alkyne-PEG3-OH was employed, while the phosphazene base P4-t-Bu facilitated the controlled ROP of MPEGn-EPO, resulting in clickable dendritic P(MPEGn-EPO) with terminal alkyne. The chiral four-arm dendritic polymer 4-Arm-PEG3-Mn was synthesized through a click reaction between 4-Arm-PEG3-N3 and P(MPEGn-EPO) using Cu(PPh3)3Br/CuBr as a catalyst. Candida antarctica lipase B (CALB) was chosen as a model enzyme for the preparation of a non-covalently immobilized biocatalyst (CALB-M) via a simple adsorption process with magnesium silicate and 4-Arm-PEG3-M4-20. The immobilized lipase CALB-M16 demonstrated high enzyme activity in the resolution of racemic amines, yielding chiral amines with enantioselectivity up to 99 % ee. Circular dichroism (CD) analysis indicated the formation of helical conformation in the dendritic polyethers 4-Arm-PEG3-M16-20 in the presence of magnesium silicate at 5 °C. The polyethylene glycol side groups of 4-Arm-PEG3-M16-20 are suggested to potentially form a crown ether-like structure with magnesium ions, which could enhance steric hindrance and induce a right-handed helix. Furthermore, the helical structure of this dendritic polyether is thought to work in synergy with CALB, improving the enantioselective resolution of amines.

A Homochiral Porous Organic Cage-Polymer Membrane for Enantioselective Resolution.

Membrane-based enantioselective separation is a promising method for chiral resolution due to its low cost and high efficiency. However, scalable fabrication of chiral separation membranes displaying both high enantioselectivity and high flux of enantiomers is still a challenge. Here, the authors report the preparation of homochiral porous organic cage (Covalent cage 3 (CC3)-R)-based enantioselective thin-film-composite membranes using polyamide (PA) as the matrix, where fully organic and solvent-processable cage crystals have good compatibility with the polymer scaffold. The hierarchical CC3-R channels consist of chiral selective windows and inner cavities, leading to favorable chiral resolution and permeation of enantiomers; the CC3-R/PA composite membranes display an enantiomeric excess of 95.2% for R-(+)-limonene over S-(-)-limonene and a high flux of 99.9mg h-1 m-2. This work sheds light on the use of homochiral porous organic cages for preparing enantioselective membranes and demonstrates a new route for the development of next-generation chiral separation membranes.

Enantioselective resolution of two model amino acids using inherently chiral oligomer films with uncorrelated molecular structures.

Preferential crystallization induced by chiral surfaces is an interesting alternative to isolate enantiopure antipodes. Herein, we take advantage of the outstanding enantiorecognition capabilities of inherently chiral oligomers to induce an enantioselective crystallization process. We exemplify this strategy with two amino acid model molecules, asparagine and glutamic acid, having a completely uncorrelated structure with respect to the template. This illustrates the versatility of the approach with potential applications in the resolution of pharmaceutical compounds.

Lipase Immobilized onto Metal‐Organic Frameworks for Enantioselective Resolution of Mandelic Acid

AbstractAn immobilized lipase with metal‐organic frameworks (MOFs) was first used for the kinetic resolution of mandelic acid enantiomers by transesterification reaction using vinyl acetate as acyl donor in organic medium. Notably, the biocompatibility of the carrier and catalytic performance of lipase could be significantly improved via modification of NH2‐Ni‐MOF with polyvinylpyrrolidone. After extensively screening the enzymatic process, the optimal enantioselective resolution conditions and excellent outcomes were obtained for the enzyme‐catalyzed transesterification reaction. In addition, the recovered immobilized lipase displayed outstanding reusability after immobilization.

Enantioselective resolution of (R,S)-DMPM to prepare (R)-DMPM by an adsorbed-covalent crosslinked esterase PAE07 from Pseudochrobactrum asaccharolyticum WZZ003.

(R)-N-(2,6-dimethylphenyl) aminopropionic acid methyl ester ((R)-DMPM) is an important chiral intermediate of the fungicide N-(2,6-Dimethylphenyl)-N-(methoxyacetyl)-alanine methyl ester ((R)-Metalaxyl). In this study, (1) D3520 (macroporous acrylic anion resin), selected from the ten resins, was used to immobilize the esterase from Pseudochrobactrum asaccharolyticum WZZ003 (PAE07) for resoluting the (R,S)-DMPM to obtain (R)-DMPM. (2) Up to 20g/L PAE07 could be immobilized onto D3520 with a high enzymatic activity of 32.4 U/g. Moreover, the Km and Vmax values of 19.1mM and 2.8mM/min for D3520-immobilized PAE07 indicated its high activity and stereoselectivity. (3) The optimal temperature and pH for the immobilized PAE07 were 40 ℃ and 8.0, and substrate concentration was up to 0.35M. After 15h reaction, the conversion rate from (R,S)-DMPM to (R)-DMPM was 48.0% and the e.e.p and E values were 99.5% and 1393.0, respectively. In scale-up resolution, 200g/L substrate and 12.5g immobilized esterase PAE07 condition, a conversion rate from substrate to product of 48.1% and a product e.e.p of 98% were obtained within 12h, with the activity of immobilized PAE07 retained 80.2% after 5 cycles of reactions. These results indicated that the D3520-immobilized esterase PAE07 had great potential for enzymatic resolution of (R,S)-DMPM to prepare (R)-Metalaxyl.

Enantioselective Resolution Copolymerization of Racemic cis-Epoxides and Cyclic Anhydrides Mediated by Multichiral Bimetallic Chromium Complexes

Enantioselective Resolution Copolymerization of <i>Racemic cis</i>-Epoxides and Cyclic Anhydrides Mediated by Multichiral Bimetallic Chromium Complexes

Enzymatic Resolution of cis‐Dimethyl‐1‐acetylpiperidine‐2,3‐dicarboxylate for the Preparation of a Moxifloxacin Building Block

AbstractThis work presents the enantioselective resolution of a water‐soluble racemic mixture of cis‐dimethyl 1‐acetylpiperidine‐2,3‐dicarboxylate catalyzed by Candida antarctica lipase B. The separation of the carboxylic acid product in its (2R,3S) configuration from non‐reacted substrate in the (2S,3R) configuration was easily obtained by organic phase extraction. The latter molecule can be used as an enantiomerically pure precursor in the synthesis of (4aS,7aS)‐octahydro‐1H‐pyrrolo[3,4‐b]pyridine, an expensive building block in the production of the antibiotic Moxifloxacin. On the other hand, the hydrolyzed product in the (2R,3S) configuration can be used for the preparation of a neuromediator analogue. The lipase demonstrated enantioselectivity towards the (2R,3S) substrate enantiomer and regioselectivity towards the ester in position 3 of the molecule. Studies of hydrolysis kinetics and the use of the immobilized enzyme were performed to evaluate its industrial application.

A Polymer-based Monolithic Capillary Column with Polymyxin-B Chiral Selector for the Enantioselective Nano-High Performance Liquid Chromatographic Pharmaceutical Analysis

Herein, we report the first use of Polymyxin-B antibiotic as a enantio-selector in polymer monolithic capillary. The capillaries were functionalised, characterized and tested for the enantioselective nano-HPLC separation of 50 racemic pharmaceutical drugs. They have been easily prepared by immobilizing Polymyxin-B over the organic polymer for 48 h (P1) or encapsulating Polymyxin-B within the organic polymer (P2) and tested for the enantioselective resolution of racemic drugs. Acceptable resolution was achieved for 21 drugs using RP-HPLC conditions on both (P1) and (P2) capillary columns, while no separation was observed under NP-HPLC conditions. Polymyxin-B is commercially available, easily solubilized and stable in both acidic and neutral media. The developed Polymyxin-B-based polymer monolithic capillaries provide a promising expansion of platform in enantioselective HPLC separations.

Photoinduced Reversible Semicrystalline‐to‐Amorphous State Transitions of Stereoregular Azopolyesters

AbstractWe report the synthesis of isotactic azobenzene‐based polyesters (azopolyesters) with main‐chain chirality via highly enantioselective resolution copolymerization of racemic azobenzene‐containing epoxides with cyclic anhydrides. All polyesters with trans‐azobenzene moieties were found to be semicrystalline materials with melting temperatures of 153–231 °C, while the corresponding isotactic cis‐azopolyesters were amorphous. The azobenzene groups in the copolymers exhibited reversible trans‐to‐cis and cis‐to‐trans photoisomerization upon irradiation with light. This demonstrates that the crystallinity of isotactic azopolyesters can be manipulated via photoinduced reversible isomerization. In addition, mixing isotactic trans‐polyesters with different enantiomeric configurations in a 1:1 mass ratio afforded crystalline stereocomplexes for which the crystalline behavior differed significantly from those of the component enantiomer. Also, photoinduced reversible transitions between semicrystalline and amorphous states were observed in various stereocomplexes of isotactic trans‐azopolyesters, similar to the isotactic azopolyesters themselves.

Enantioselective Resolution Copolymerization of Racemic 2,3‐Disubstituted cis‐Epoxides with CO2 Mediated by Binuclear Cobalt(III) Catalyst†

Main observation and conclusionEnantioselective resolution copolymerization of racemic internal epoxides with carbon dioxide (CO2) is a challenging issue because of their poor reactivity and complicated regio/stereoselectivity. Herein, we describe the first enantioselective resolution copolymerization of racemic aromatic 2,3‐disubstituted cis‐epoxides and CO2 using enantiopure dinuclear cobalt(III) complexes as catalyst under mild conditions, affording the corresponding polycarbonates with completely alternating structure and good enantioselectivity in the range of 70% to 97% ee. The isotactic polycarbonate from β‐methylstyrene oxide is a typical semicrystalline material, possessing a melting point of 241 oC; while other isotactic‐enriched copolymers from 2,3‐disubstituted cis‐epoxides bearing a substitute group on the aromatic ring are amorphous, having glass transition temperatures between 86 oC and 124 oC. Interestingly, the copolymer selectivity is correlated well with the Hammett substituent constant, and the highest polymer selectivity of 98% was found in the system using the epoxides bearing an electron‐donating group on the benzene ring.

Characterization of an Immobilized Amino Acid Racemase for Potential Application in Enantioselective Chromatographic Resolution Processes

Enantioselective resolution processes can be improved by integration of racemization. Applying environmentally friendly enzymatic racemization under mild conditions is in particular attractive. Owing to the variety of enzymes and the progress in enzyme engineering, suitable racemases can be found for many chiral systems. An amino acid racemase (AAR) from P. putida KT2440 is capable of processing a broad spectrum of amino acids at fast conversion rates. The focus of this study is the evaluation of the potential of integrating AAR immobilized on Purolite ECR 8309 to racemize L- or D-methionine (Met) within an enantioselective chromatographic resolution process. Racemization rates were studied for different temperatures, pH values, and fractions of organic co-solvents. The long-term stability of the immobilized enzyme at operating and storage conditions was found to be excellent and recyclability using water with up to 5 vol% ethanol at 20 °C could be demonstrated. Packed as an enzymatic fixed bed reactor, the immobilized AAR can be coupled with different resolution processes; for instance, with chromatography or with preferential crystallization. The performance of coupling it with enantioselective chromatography is estimated quantitatively, exploiting parametrized sub-models. To indicate the large potential of the AAR, racemization rates are finally given for lysine, arginine, serine, glutamine, and asparagine.

Enantioselective resolution of (±)-1-phenylethyl acetate using the immobilized extracellular proteases from deep-sea Bacillus sp. DL-1

Bacillus sp. DL-1 was isolated from the deep sea of the Western Pacific Ocean and behaved very good resistance to NaCl. The extracellular proteases of Bacillus sp. DL-1 were found to exhibit excellent enantioselectivity for the kinetic resolution of (±)-1-phenylethyl acetate. To improve the stability of the enzyme, the immobilized extracellular proteases were preparated by using 60 g kieselguhr and 1-L crude fermentation broth containing extracellular proteases of Bacillus sp. DL-1, shaking at 25 °C, 200 r/min for 10 h. Every gram of kieselguhr adsorbed 25.7 mg extracellular protease and the enzymatic activity recovery was 79.86%. The immobilized proteases preserved about 35.8% of its activity after 6 repeated uses, which were also used as biocatalyst to asymmetrically hydrolyse (±)-1-phenylethyl acetate for the preparation of (R)-1-phenylethanol and (S)-1-phenylethyl acetate with high optical purities. The effects of pH, temperature, enzyme concentration, substrate concentration, reaction time and additives (metal ions/surfactants) on the resolution were investigated by single factor experiments. Under the optimal reaction conditions (10 mM (±)-1-phenylethyl acetate, 40 mg/mL immobilized extracellular proteases, pH 7.5 (Tris-HCl), 5% (v/v) methanol and 45 °C for 2 h), (R)-1-phenylethanol was generated with the e.e.p being > 97%, and the yield being 53%, respectively. Analogously, under the optimal reaction conditions (10-mM (±)-1-phenylethyl acetate, 360 mg/mL immobilized extracellular proteases, pH 6.0 (PB), 5% (v/v) DMSO and 35 °C for 1.5 h), (S)-1-phenylethyl acetate was generated with the e.e.s being over 99% and the yield being 79%, respectively. Compared with the extracellular proteases from Bacillus sp. DL-2, the immobilized extracellular proteases from Bacillus sp. DL-1 exhibited higher hydrolytic activity and could asymmetrically hydrolyse (±)-1-phenylethyl acetate by using higher substrate concentrations, shorter reaction times to obtain higher yields. Notably, the extracellular proteases of Bacillus sp. DL-1 were demonstrated to behave the same enantio-preference as those of most other reported esterases/lipases. Proteases from deep-sea Bacillus sp. DL-1 are promising biocatalysts for the synthesis of valuable chiral chemicals.

Enantioselective, Stereoconvergent Resolution Copolymerization of Racemic cis-Internal Epoxides and Anhydrides.

Unprecedented enantioselective resolution copolymerization of racemic cis-internal epoxides and anhydrides was mediated by dinuclear aluminum complexes with multiple chirality, affording optically active polyesters with two contiguous stereogenic centers, and the unreacted substrates in good enantioselectivity. Unexpected stereoconvergence is observed in this resolution copolymerization, where the selectivity factor for the enantioselective formation of copolymer significantly exceeds the kinetic resolution coefficient based on the unreacted epoxide at various conversions. Catalytic activity and copolymer enantioselectivity are strongly influenced by the phenolate ortho-substituents of the ligand set, as well as the axial linker and its chirality. An enantiopure binaphthol-linked bimetallic AlIII complex allows stereoconvergent access to the stereoregular semi-crystalline polyesters and a concomitant kinetic resolution of the epoxide substrates.

Enantioselective, Stereoconvergent Resolution Copolymerization of Racemic cis‐Internal Epoxides and Anhydrides

AbstractUnprecedented enantioselective resolution copolymerization of racemic cis‐internal epoxides and anhydrides was mediated by dinuclear aluminum complexes with multiple chirality, affording optically active polyesters with two contiguous stereogenic centers, and the unreacted substrates in good enantioselectivity. Unexpected stereoconvergence is observed in this resolution copolymerization, where the selectivity factor for the enantioselective formation of copolymer significantly exceeds the kinetic resolution coefficient based on the unreacted epoxide at various conversions. Catalytic activity and copolymer enantioselectivity are strongly influenced by the phenolate ortho‐substituents of the ligand set, as well as the axial linker and its chirality. An enantiopure binaphthol‐linked bimetallic AlIII complex allows stereoconvergent access to the stereoregular semi‐crystalline polyesters and a concomitant kinetic resolution of the epoxide substrates.

Enantioselective resolutions by high‐performance liquid choromatography using chiral inorganic mesoporous silica

AbstractFour chiral hexagonal ordered mesoporous silica were synthesized by self‐assembly and templated by surface active agents prepared from d‐phenylalanine, l‐phenylglycine, l‐proline, and l‐isoleucine, with tetraethoxylsilane as the silica source and N‐trimethoxysilylpropyl‐N,N,N‐trimethylammonium chloride as the structure promoter under the induction of templates. The four generated mesoporous silica materials were used to prepare chromatographic columns to separate chiral compounds and positional isomers of benzene series with a good separation effect and a complementary effect on species separation. The experiment suggests that the four chromatographic columns have a wide range of applications and potential practical value, providing a possible direction to explore new chiral stationary phases.

(S)-Thienyl and (R)-Pirydyl phosphonate Derivatives Synthesized by Stereoselective Resolution of Their Racemic Mixtures With Rhodotorula mucilaginosa (DSM 70403) - Scaling Approaches.

Rhodotorula mucilaginosa was successfully applied as a biocatalyst for the enantioselective resolution of the racemic mixtures of heteroatom phosphonates derivatives, resulting in receiving the following enantiomers: (S)-1-amino-1(2-thienyl)methylphosphonic acid (Product 1) and (R)-1-amino-1-(3′pirydyl) methylphosphonic acid (Product 2). Biological synthesis of both products is reported for the first time. Pure (S)-1-amino-1-(2-thienyl)methylphosphonic acid (Product 1) was isolated with a conversion degree of 50% after 24 h of biotransformation was conducted on a laboratory scale under moderate conditions (1.55 mM of substrate 1, 100 mL of distilled water, 135 rpm, 25°C; Method A). The scale was enlarged to semi-preparative one, using a simplified flow-reactor (Method C; 3.10 mM of substrate 1) and immobilized biocatalyst. The product was isolated with a conversion degree of 50% just after 4 h of biotransformation. Amino-1-(3′pirydyl)methylphosphonic acid (Substrate 2) was converted according to novel procedure, by the immobilized biocatalyst - Rhodotorula mucilaginosa. The process was carried out under moderate conditions (3.19 mM – substrate 2 solution; Method C1) with the application of a simplified flow reactor system, packed with the yeasts biomass entrapped in 4% agar-agar solution. Pure (R)-amino-1-(3′pirydyl)methylphosphonic (50% of conversion degree) was received within only 48 h.

PEG-modified lipase immobilized onto NH2-MIL-53 MOF for efficient resolution of 4-fluoromandelic acid enantiomers

A new heterogeneous bio-catalyst was prepared by the immobilization of lipase from Pseudomonas fluorescents (PFL) onto metal-organic frameworks (MOF), NH2-MIL-53(Fe), using covalent cross-linking. The immobilized lipase [PEG-PFL@NH2-MIL-53(Fe)] was firstly applied in enantioselective resolution of 4-fluoromandelic acid (4-FMA) enantiomers. After optimization of the immobilization PFL onto NH2-MIL-53, its loading capacity is 224.5 mg PFL/g MOF. The optimal enzymatic conditions are temperature of 50 °C, VA/4-FMA substrate ratio of 6:1, immobilized lipase loading of 60 mg and reaction time of 12 h. Experimental results show that the catalytic activity and thermal stability of PFL are significantly improved by polyethylene glycol (PEG) modification and immobilization. At 65 °C, the catalytic activity of immobilized lipase retains 86.0% of initial activity. Under the optimal conditions, the excellent results were obtained with conversion of 49.6% and enantiomer excess of 98.0% for the immobilized PFL catalyzed transesterification reaction. Furthermore, the immobilized lipase exhibits excellent cycle stability with 83% of its initial activity after four cycle.

One-step eantioselective bioresolution for (S)-2-chlorophenylglycine methyl ester catalyzed by the immobilized Protease 6SD on multi-walled carbon nanotubes in a triphasic system

(S)-2-chlorophenylglycine methyl ester ((S)-1) is a key chiral building block of clopidogrel, which is a widely administered antiaggregatory and antithrombotic drug. Herein, Protease 6SD was covalently immobilized on multi-walled carbon nanotubes (MWCNT), and the as-prepared immobilizate P-6SD@NH2-MWCNT was applied in the enantioselective resolution of (R,S)-1 to yield (S)-1. In order to overcome the poor solubility of (R,S)-1 in aqueous solution, a novel triphasic reaction system constituting P-6SD@NH2-MWCNT, aqueous phase and methyl tert-butyl ether (MTBE) as the organic phase was constructed, which simultaneously improved the substrate solubility and the immobilizate recyclability. Under the optimized reaction conditions, P-6SD@NH2-MWCNT catalyzed 10 mM (R,S)-1 for 2 h, yielding optically pure (S)-1 (>99.0 % ees) with 70.74 % conversion of the (R,S)-1. Moreover, P-6SD@NH2-MWCNT can be reused for 15 batches, displaying an exquisite recycling performance. It is for the first time that enantiomerically pure (S)-1 was successfully synthesized by protease-catalyzed one-step resolution.

Chiral Mesoporous Silica Materials: A Review on Synthetic Strategies and Applications.

Because of its tunable textural properties and chirality feature, chiral mesoporous silica (CMS) gained significant consideration in many fields and has been developed rapidly in recent years. In this review, we provide an overview of synthesis strategies for fabricating CMS together with its main applications. The properties of CMS, including morphology and mesostructures and enantiomer excess (ee), can be altered according to the synthetic conditions during the synthesis process. Despite its primary stage, CMS has attracted extensive attention in many fields. In particular, CMS nanoparticles are widely used for enantioselective resolution and adsorption of chiral compounds with desirable separation capability. Also, CMS acts as a promising candidate for the effective delivery of chiral or achiral drugs to produce a chiral-responsive manner. Moreover, CMS also plays an important role in chromatographic separations and asymmetric catalysis. There has been an in-depth review of the synthetic methods and mechanisms of CMS. And this review aims to give a deep insight into the synthesis and application of CMS, especially in recent years, and highlights the significance that it may have in the future.

Enantioseparation and quantitative determination of ibuprofen using vancomycin-mediated capillary electrophoresis combined with molybdenum disulfide-assisted dispersive solid-phase extraction: optimization using experimental design

A dispersive solid-phase extraction procedure using molybdenum disulfide as an adsorbent was developed for pretreatment and preconcentration of low quantity of ibuprofen in natural samples prior to enantioseparation. The chiral separation was carried out by vancomycin-mediated capillary electrophoresis coupled with UV detector. The effect of the main factors on the extraction recovery and enantioselective resolution was investigated using a central composite design. The best enantioseparation of ibuprofen was achieved on an uncoated capillary at 20 °C using phosphate buffer (pH 6.5) containing 1 mM vancomycin and 20 kV applied voltage. The optimum condition of the dispersive solid-phase extraction was obtained at the sample solution with pH 4.0 containing 1.4 M NaCl, 0.12 g adsorbent, 10 min adsorption time, 25 °C adsorption temperature, 1 mL the elution solvent (acetone-0.25 M NaOH (aq) (2:1 (v v−1)), 5 min desorption time, 50 °C desorption temperature, and the stirring speed of 900 rpm for adsorption and desorption steps. Good values of linearity range (0.125–5 µg mL−1, R2 = 0.999), preconcentration factor (33.4), precision (less than 2.3%), and recovery (93.2%) were achieved. The limit of detection and quantification of, respectively, 0.025 and 0.070 μg mL−1 was provided, which enabled a satisfactory trace determination of ibuprofen in urban water and human urine samples.