Separation Of Racemates Research Articles

Combining enzymatic hydrolysis with magnetic nanoparticles for resolution of chiral substances

At present, the separation of racemates is still a great challenge, since they are enantiomers, having similar chemical structures and physical properties. Here, we proposed a new strategy for efficiently separating chiral compounds, i.e., stereoselectively hydrolyzing the individual enantiomer of racemates via enzyme, followed by a simple magnetic separation using Fe3O4 nanoparticles, may implement a complete resolution of racemates. In this work, d-aminoacylase was selected as enzyme, and phenylalanine and methionine as chiral models. The use of d-aminoacylase can hydrolyze d-enantiomer, while l-enantiomer cannot. Magnetic nanoparticles of Fe3O4@SiO2-NH2-COOH composites were designed and synthesized for the amidation reaction with the hydrolyzed d-amino acids from the mixture. With the help of a magnetic field, d-form amino acids can be readily separated at a large scale and the resultant resolving performances indicated that this method exhibited a higher selectivity, compared with common chiral separation techniques. The enantiomeric excess (e.e.) values are 75% and 98% for racemic N-acyl-phenylalanine and methionine, respectively, which solely rely on the hydrolysis capability of used enzymes. This result suggests that our method is feasible, simple, easy to scale-up and high-efficient for the chiral resolution. These findings demonstrate that this strategy could provide new opportunities for improving enantioseparation performances for a host of enantio- and bio-substance with both high-yield and high-selectivity.

Homochiral metal-organic framework for HPLC separation of enantiomers

In recent years, homochiral metal-organic frameworks (MOFs) have been widely used as novel media for separation of chiral compounds. Herein, we report a homochiral MOF [Cu(S-mal)(bpe)]n (S-mal=S-malic acid, bpe=trans-1,2-bis(4-pyridyl)ethylene) packed column for HPLC separation of racemates. A variety of different types of racemic compounds such as alcohols, acids, ketones and phenols can be separated on the homochiral MOF column with high enantioselectivity. Compared with the chiral recognition ability of another homochiral MOF [Cu(S-mal)(bpy)]n column previously reported by our group, the results show that the chiral microenvironment and pore sizes of homochiral MOFs play a vital role in chiral recognition process. The repeatability of homochiral MOF [Cu(S-mal)(bpe)]n column was investigated. The relative standard deviation (RSD) for the retention time of DNB-leucine was less than 1.1% (n=5), indicating the good stability and repeatability of the homochiral MOF [Cu(S-mal)(bpe)]n column for HPLC enantioseparation. These results demonstrated that the homochiral MOF [Cu(S-mal)(bpe)]n is promising for enantioseparation, which can be complementary to the currently reported homochiral MOFs columns in HPLC.

Post-synthetic modification of metal–organic frameworks for chiral gas chromatography

A post-synthetic modification strategy was established to synthesize a series of chiral metal–organic frameworks for gas chromatographic separation of diverse racemates.

Enantioselective separation of racemates using CHIRALPAK IG amylose-based chiral stationary phase under normal standard, non-standard and reversed phase high performance liquid chromatography

We have previously reported on the solvent versatility of immobilized amylose and cellulose-based chiral stationary phases in enantioselective liquid chromatographic separation of racemates. The studies were mainly focusing on the tris substituted 3,5-dimethylphenylcarbamate polysaccharide-based chiral stationary phases namely CHIRALPAK IA® [Amylose tris (3,5-dimethylphenylcarbamate)] or ADMPC and CHIRALPAK IB® [Cellulose tris (3,5-dimethylphenylcarbamate)] or CDMPC. Here we focus on the application of the recently introduced amylose tris (3-chloro-5-methylphenylcarbamate) or ACMPC and brand name CHIRALPAK IG® with a chlorine substituent replacing the methyl group in CHIRALPAK IA®. This was investigated for the enantioslective separation of different classes of pharmaceuticals namely β- and α-blockers, anti-inflammatory and antifungal drugs, norepinephrine-dopamine reuptake inhibitor, catecholamines, sedative hypnotics, anti-histaminics, anticancer drugs, antiarrhythmic drugs, flavonoids, amino acids, alpha-2 adrenergic agonist, adrenaline and miscellaneous.A brief comparison between CHIRALPAK IG® and CHIRALPAK IA® under normal standard, non-standard and reversed mobile phase is demonstrated. The results revealed the versatility of the CHIRALPAK IG® column, its compatibility with a wide ranges of solvent and operation modes and its ability to separate chiral compounds not separated with other amylose based chiral stationary phases.

Chromatographic Separation of Racemates of Alcohols Using Porous Coordination Compounds of Zinc and Vanadium(IV) with Lactate and β-Cyclodextrin

It was shown that the efficiency of chromatographic separation of the racemates of 2-butanol, 2-hexanol, 3-hexanol, and 1-phenylethanol on columns with coordination compounds [Zn2(bdc)(S-Lact)(DMF)]n (bdc2– = 1,4-benzenedicarboxylate, S-Lact– = (S)-lactate) and Na7{(VO)7Na7(H2O)7(β-CD)2} (β-CD = β-cyclodextrin) does not depend on the size of the alcohol molecule. It was found that in the case of the 1-phenylethanol racemate separation with [Zn2(bdc)(S-Lact)(DMF)]n the enantiomeric excess of the alcohol varies inversely with the viscosity of the solvent.

Stereoselective Crystallization as a Basis for Single‐Enantiomer Drug Production

AbstractIn the 21st century, single enantiomeric chiral active pharmaceutical ingredients (APIs) prevail among new drugs. Their preparation is made easier by using direct methods for the separation of racemates. The history of this issue is summarized and information about new and updated methods of conglomerate identification and practical implementation options for direct resolutions is provided. Examples are given for the application of direct methods for obtaining enantiopure APIs by the resolution of their racemic synthetic precursors, using a slight modification of the target product and finally by direct resolution of the racemic chiral drugs as such.

Proof of concept for continuous enantioselective liquid–liquid extraction in capillary microreactors using 1-octanol as a sustainable solvent

Separation of racemates in microreactors using liquid–liquid extraction with 1-octanol as the solvent is successfully demonstrated.

Thermodynamic Study of Racemic Ibuprofen Separation by Liquid Chromatography Using Cellulose-Based Stationary Phase

Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID), also known for its significant antipyretic and analgesic properties. This chiral drug is commercialized in racemic form; however, only S-(+)-ibuprofen has clinical activities. In this paper the effect of temperature change (from 288.15 to 308.15 K) on the ibuprofen resolution was studied. A column (250×4.6 mm) packed with tris(3,5-dimethylphenylcarbamate) was used to obtain the thermodynamic parameters, such as enthalpy change (ΔH), entropy change (ΔS), variation enthalpy change (ΔΔH), variation entropy change (ΔΔS), and isoenantioselective temperature (Tiso). The mobile phase was a combination of hexane (99%), isopropyl alcohol (1%), and TFA (0.1%), as an additive. The conditions led to a selectivity of 1.20 and resolution of 4.55. The first peak, R-(−)-ibuprofen, presented an enthalpy change of 7.21 kJ/mol and entropy change of 42.88 kJ/K·mol; the last peak, S-(+)-ibuprofen, has an enthalpy change of 8.76 kJ/mol and 49.40 kJ/K·mol of entropy change.

Stereoselective quantification of triticonazole in vegetables by supercritical fluid chromatography

A highly fast analytical method though supercritical fluid chromatography (SFC) has been developed to quantify triticonazole enantiomers in cucumbers and tomatoes. Effects of organic modifier type and concentration on chiral separation and quantification of standard solution as well as matrix-matched standard solutions have been studied in detail. Among three organic modifiers, better separation of triticonazole racemate was achieved with 20% ethanol (v/v). The run time in SFC (ca 3min) with CO2-ethanol (80:20, v/v) as the mobile phase was six-fold shorter than HPLC analysis (about 18min). Then, QuEChERS (quick, easy, cheap, effective, rugged and safe) extraction procedure was used for triticonazole in vegetables. The residue analysis method was validated. Good linearity (R2≥0.9988) and recoveries (81.62–106.21%, RSD≤7.30%) for the two enantiomers were achieved. This developed method described herein is convenient and reliable for enantioselective detection of triticonazole in vegetables, which might provide additional information for reliable risk assessment of chiral pesticides.

Chiral separation of ketoprofen on an achiral NH2 column by HPLC using vancomycin as chiral mobile phase additive

A high-performance liquid chromatographic method for chiral separation of ketoprofen racemate was developed. (R)- and (S)-ketoprofen enantiomers were separated on a LiChrosorb NH2 column (250 mm × 4.6 mm, i.d 5 µm) at 20 °C, using 2-propanol/potassium dihydrogen phosphate buffer (pH 6.0, 0.05 M) (50:50 v/v). Containing vancomycin as the mobile phase, at a flow rate of 0.8 ml min−1 and detection wavelength of UV, the detector was set at 310 nm. Under these conditions, ketoprofen enantiomers could be separated with a selectivity factor (α) of 2.172 and a resolution (Rs) of 4.78 using extremely low concentrations of the vancomycin chiral additive.

Metal-Organic Framework [Cd(LTP)2]n for Improved Enantioseparations on a Chiral Cyclodextrin Stationary Phase in GC.

Metal-organic frameworks (MOFs) are a class of novel porous materials consisting of clusters or chains of metal ions and organic linkers. The chiral MOF [Cd(LTP)2]n (LTP = l(-)-thiazolidine-4-carboxylic acid) possesses infinite extension three-dimensional supramolecular reticular structure with right-handed helix. In this work, three capillary columns (A, B and C) containing MOF [Cd(LTP)2]n, permethylated β-cyclodextrin (CD) and sodium chloride, and MOF [Cd(LTP)2]n and permethylated β-CD, respectively, have been prepared and their respective enantioseparation abilities have been investigated. The polarities of all of the MOFs and permethylated β-CDs are moderate. The numbers of theoretical plates (plate m-1) of the three columns have been measured using n-dodecane at 120°C, which followed an increasing order of A (3100) < B (3800) < C (4300). Some racemates were separated with different resolutions on Columns A, B and C. The results indicated that the incorporated MOF [Cd(LTP)2]n enhanced the separations of racemates on a permethylated β-CD stationary phase with high column efficiency and good reproducibility in gas chromatography.

Analytical and semipreparative chiral separation of cis-itraconazole on cellulose stationary phases by high-performance liquid chromatography.

cis-Itraconazole is a chiral antifungal drug administered as a racemate. The knowledge of properties of individual cis-itraconazole stereoisomers is vital information for medicine and biosciences as different stereoisomers of cis-itraconazole may possess different affinity to certain biological pathways in the human body. For this purpose, either chiral synthesis of enantiomers or chiral separation of racemate can be used. This paper presents a two-step high-performance liquid chromatography approach for the semipreparative isolation of four stereoisomers (two enantiomeric pairs) of itraconazole using polysaccharide stationary phases and volatile organic mobile phases without additives in isocratic mode. The approach used involves the separation of the racemate into three fractions (i.e. two pure stereoisomers and one mixed fraction containing the remaining two stereoisomers) in the first run and consequent separation of the collected mixed fraction in the second one. For this purpose, combination of cellulose tris-(4-methylbenzoate) and cellulose tris-(3,5-dimehylphenylcarbamate) columns with complementary selectivity for cis-itraconazole provided full separation of all four stereoisomers (with purity of each isomer > 97%). The stereoisomers were collected, their optical rotation determined and their identity confirmed based on the results of a previously published study. Pure separated stereoisomers are subjected to further biological studies.

Synthesis of Metal-Organic Zeolites with Homochirality and High Porosity for Enantioselective Separation.

Using lactic acid derivatives as chiral ligands, a pair of unprecedented homochiral metal-organic zeolites have been synthesized that feature zeotype CAN topology and have high porosity for enantioselective separation of racemates.

Three chiral metal organic frameworks used as stationary phases for the separation of racemates by open tubular capillary electrochromatography

Three chiral metal organic frameworks used as stationary phases for the separation of racemates by open tubular capillary electrochromatography

Homochiral Porous Organic Cage with High Selectivity for the Separation of Racemates in Gas Chromatography.

Porous organic molecular cages as a new type of porous materials have attracted a tremendous attention for their potential applications in recent years. Here we report the use of a homochiral porous organic cage (POC) (CC3-R) diluted with a polysiloxane (OV-1701) as a stationary phase for high-resolution gas chromatography (GC) with excellent enantioselectivity. A large number of optical isomers have been resolved without derivatization, including chiral alcohols, diols, amines, alcohol amines, esters, ketones, ethers, halohydrocarbons, organic acids, amino acid methyl esters, and sulfoxides. Compared with commercial β-DEX 120 and Chirasil-L-Val columns, the CC3-R coated capillary column offered more preeminent enantioselectivity. In addition, CC3-R also exhibits good selectivity for the separation of isomers, linear alkanes, alcohols, and aromatic hydrocarbons. The excellent resolution ability, repeatability, and thermal stability make CC3-R a promising candidate as a novel stationary phase for GC. The study described herein first proves useful commercially. This work also indicates that porous organic molecular materials will become more attractive in separation science.

Fabrication of chiral amino acid ionic liquid modified magnetic multifunctional nanospheres for centrifugal chiral chromatography separation of racemates

As the rapid development of nanotechnology, the magnetic nanospheres modified with special chiral selective ligands show a great potentiality in enantiomeric separation. In this study, magnetic nanospheres modified with task-specific chiral ionic liquid were designed for the separation of chiral amino acids. These modified magnetic nanospheres were effective in a direct chiral separation of five racemic amino acids (d- and l-cysteine, d- and l-arginine, d- and l-leucine, d- and l-glutamine and d- and l-tryptophan). Furthermore, a new online method for complete separation of the enantiomers via the magnetic nanospheres was established with centrifugal chiral chromatography using a spiral tube assembly mounted on a type-J coil planet centrifuge. One kind of chiral compounds, d- and l-tryptophan was resolved well using this method. These results demonstrated that the modified nanospheres display a good chiral recognition ability, and can be used as a potential material for chiral separation of various racemates.

Metal–Organic Framework InH(d-C10H14O4)2 for Improved Enantioseparations on a Chiral Cyclodextrin Stationary Phase in GC

The chiral MOF InH(d-C10H14O4)2 has a left-handed helical structure and adopts a diamond network. In this work, three capillary columns (A, B, and C) containing InH(d-C10H14O4)2, peramylated β-cyclodextrin and sodium chloride, and InH(d-C10H14O4)2 and peramylated β-cyclodextrin have been prepared and their respective enantioseparation abilities have been investigated. The polarity of the MOF is moderate, whereas peramylated β-cyclodextrin provides a stationary phase of low polarity. The numbers of theoretical plates (plate m−1) of the three columns have been measured with n-dodecane at 120 °C, which followed an increasing order of A (2,973) < B (3,563) < C (3,665). 1-Phenylethanol, dihydrocarvyl acetate, citronellal, leucine, methyl l-β-hydroxyisobutyrate, limonene, and menthol were separated with different resolutions on columns A, B, and C. The relative standard deviations (RSDs) in the run-to-run and column-to-column retention times were 0.50 and 2.50 %, respectively. The test results indicated that the incorporated InH(d-C10H14O4)2 enhanced the separations of racemates on a peramylated β-cyclodextrin stationary phase, resulting in high column efficiency and good reproducibility in gas chromatography.

Preparation of molecularly imprinted polymer for chiral recognition of racemic 1,1′-binaphthalene-2,2′-diamine by HPLC

Molecularly imprinted polymers (MIPs) were synthesized by imprinting a new template—S(-)-1,1′-binaphthalene-2,2′-diamine (S-DABN) and applied as chiral stationary phases for chiral separation of DABN racemates by high-performance liquid chromatography (HPLC). The influence of some key factors on the chiral recognition ability of MIPs, such as the type of functional monomers and porogen and the molar ratio of template to monomer, was systematically investigated. The chromatographic conditions, such as mobile phase composition, sample loading, and flow rate, were also measured. The chiral separation for DABN racemates under the optimum chromatographic conditions by using MIP chiral stationary phase (CSP) of P3, prepared with the S-DABN/MAA ratio = 1/4 and used acetonitrile (2 mL) and chloroform (4 mL) as porogen, showed the highest separation factor (2.14). Frontal analysis was used to evaluate affinity to the target molecule of MIPs. The binding sites (Bt) of MIPs and dissociation constant (Kd) were estimated as 4.56 μmol g−1 and 1.40 mmol L−1, respectively. In comparison with the previous studies, this approach had the advantages, such as the higher separation factor, easy preparation, and cost-effectiveness, it not only has the value for research but also has a potential in industrial application.

Separation and Determination of Cold Medicine Ingredients by Capillary Zone Electrophoresis Using Sulfated β‐Cyclodextrin as an Electrolyte Modifier and Chiral Selector

AbstractCapillary zone electrophoretic separations of cold medicine ingredients, including acetaminophen (AC), dextromethorphan (DMF) and racemates of phenylpropanolamine (PPA) and chlorpheniramine maleate (CPM) using randomly sulfated‐β‐CD (S‐β‐CD) as an electrolyte modifier and a chiral selector were investigated. The results indicate that S‐β‐CD is an excellent chiral selector and a suitable electrolyte modifier as well for the separation of those cold medicine ingredients. Influences of S‐β‐CD concentration and buffer concentration on the separation were examined. Baseline separation of these cold medicine ingredients with 1.0 % (w/v) S‐β‐CD could be simultaneously and successfully achieved within 11.8 minutes. In addition, S‐β‐CD could also act as a chiral selector for enantioseparation of PPA and CPM. A high enantioselectivity was obtained for these two analytes. Linear relationships between the peak area and its concentration for the calibration curves of AC and DMF were obtained (correlation coefficients: 0.9987 for AC, 0.9965 for DMF, respectively). The relative standard deviations of the migration time and peak area of AC and DMF were 0.19, 2.44 % and 0.34, 2.99 %, respectively. Detection limits were 0.93 and 2.57 μg/mL for AC and DMF, respectively. Recoveries of AC and DMF ranging between 102.42 and 97.28 % were observed. The proposed method was successfully applied to the determination of AC and DMF in cold medicines.

Chiral porous organic frameworks for asymmetric heterogeneous catalysis and gas chromatographic separation.

Three chiral robust diene-based porous organic frameworks (POFs) are prepared. POF- is shown to be an efficient heterogeneous catalyst after metallation for asymmetric conjugation addition with up to 93% ee, and it can also function as a new chiral stationary phase for gas chromatographic separation of racemates.