Enantiomeric separation of imidazolinone herbicides using chiral high-performance liquid chromatography

Chiral high-performance liquid chromatography (HPLC) is one of the most powerful tools to prepare enantiopure standards of chiral compounds. In this study, the enantiomeric separation of imidazolinone herbicides, i.e., imazethapyr, imazapyr, and imazaquin, was investigated using chiral HPLC. The enantioselectivity of Chiralpak AS, Chiralpak AD, Chiralcel OD, and Chiralcel OJ columns for the three analytes was compared under similar chromatographic conditions. Chiralcel OJ column showed the best chiral resolving capacity among the test columns. The resolved enantiomers were distinguished by their signs of circular dichroism detected at 275 nm and their structures confirmed with LC-mass spectrometric analysis. Factors affecting the chiral separation of imidazolinones on Chiralcel OJ column were characterized. Ethanol acted as a better polar modifier than the other alcohols including 2-propanol, 1-butanol, and 1-pentanol. Although the acidic modifier in the mobile phase did not influence chiral recognition, it was necessary for reducing the retention time of enantiomers and suppressing their peak tailing. Thermodynamic evaluation suggests that enantiomeric separation of imidazolinones on Chiralcel OJ column is an enthalpy-driven process from 10 to 40 degrees C. This study also shows that small amounts of pure enantiomers of imidazolinones may be obtained by using the analytical chiral HPLC approach.     

Enantiomeric separation of 2-(phenoxy)propionate derivatives by chiral high-performance liquid chromatography

2-(Phenoxy)propionate derivatives were separated on three chiral columns, OD, OK, and chiral-2 columns. The chlorine substitution in the phenyl ring and the alcohol moiety of the ester groups of the derivatives had great influence for separation on the OD and OK columns, but little effect on the chiral-2 column.     

Enantiomeric separation of vilanterol trifenatate by chiral liquid chromatography

Vilanterol trifenatate is a novel chiral long‐acting β2‐agonist developed. Vilanterol combined with inhaled corticosteroids can treat COPD and asthma. A simple liquid chromatographic method is developed for the quantitative determination of R‐vilanterol and S‐vilanterol (impurity S). HPLC separation was achieved on Chiralpak ID (250 × 4.6 mm; particle size 5 μm) column using hexane‐ethanol‐ethanolamine (75:25:0.1, v/v/v) as mobile phase at a flow rate of 1.0 mL/min. The resolution is greater than 3.3. Ethanolamine in the mobile phase is vital to enhance chromatographic efficiency and resolution between the isomers. The method was validated with respect to accuracy, specificity, precision, LOD, LOQ, linearity, and robustness as ICH guidelines.     

Enantioselective and diastereoselective separation of synthetic pyrethroid insecticides on a novel chiral stationary phase by high-performance liquid chromatography

A novel chiral packing material for high-performance liquid chromatography (HPLC) was prepared by connecting (R)-1-phenyl-2-(4-methylphenyl) ethylamine (PTE) amide derivative of (S)-isoleucine to aminopropyl silica gel through 2-amino-3,5-dinitro-1-carboxamido-benzene unit. This chiral stationary phase was applied to the enantioselective and diastereoselective separation of five pyrethroid insecticides by HPLC under normal phase condition. To achieve satisfactory baseline separation an optimization of the variables of mobile phase composition was required. The two enantiomers of fenpropathrin and four stereoisomers of fenvalerate were baseline separated using hexane-1,2-dichloroethane-2-propanol as mobile phase. The results show that the enantioselectivity of CSP is better than Pirkle type 1-A column for these compounds. Only partial separations for the cypermethrin and cyfluthrin stereoisomers were observed. Seven peaks and eight peaks were observed for cypermethrin and cyfluthrin, respectively. The elution orders were assigned by using different stereoisomer-enriched products.     

Enantiomeric separation of five amino acids by high-performance liquid chromatography on a chiral crown ether column

A chiral liquid chromatographic method was validated to analyze the D-and L-enantiomers of five amino acids contained in a commercial solution: aspartic acid, leucine, lysine, phenylalanine, and valine. These 10 compounds were separated on a chiral crown ether column with a mobile phase composed of water adjusted to pH 1.5 with perchloric acid, with ultraviolet detection at 220 nm. The method was applied to the commercial amino acid solution before and after sterilization by 5 kGy irradiation; no stereoconversion was observed following sterilization. Chirality 9:150–152, 1997. © 1997 Wiley-Liss, Inc.     

Enantiomeric separation of dansyl amino acids using macrocyclic antibiotics as chiral mobile phase additives by narrow-bore high-performance liquid chromatography

Seven macrocyclic antibiotics were evaluated as chiral selectors for the enantiomeric separation of 11 dansyl amino acids using narrow-bore high-performance liquid chromatography (HPLC). The macrocyclic antibiotics were incorporated as mobile phase additives to determine the enantioselective effects on the chiral analytes. The resolution and capacity factor (k') of each analyte were assessed while varying the structure of macrocyclic antibiotic and the mobile phase buffer pH. The selectivity of the chiral selectors was measured as a function of changes in these parameters. All 11 dansyl amino acids were separated by at least one of the chiral selectors. Three-dimensional computer modeling of the more effective chiral selectors illustrated the importance of macrocyclic antibiotic structure concerning stereospecific analyte interaction.     

Enantiomer separation of triazole fungicides by high-performance liquid chromatography

Enantiomer separation is one of the most important prerequisites for the investigation of environmental enantioselective behaviors for chiral pesticides. In the present study, the enantiomer separation of 7 triazole fungicides, i.e., hexaconazole (1), triadimefon (2), tebuconazole (3), diniconazole (4), flutriafol (5), propiconazole (6), and difenoconazole (7), were evaluated using normal phase high-performance liquid chromatography. Chrialcel OD column and Chrialcel OJ column were used. The influence of column temperature was studied for the optimization of the resolution as well as the type and percentage of organic modifier in the mobile phase. The retention factors for the enantiomers of all investigated compounds decreased as the temperature increased. The natural logarithms of the selectivity factors (lnalpha) of hexaconazole (1), tebuconazole (3), flutriafol (5), propiconazole (6) and difenoconazole (7) depended linearly on the inverse of temperature (1/T) while the corresponding values for triadimefon (2) and diniconazole (4) kept unchanged in the studied temperature range 10-35 degrees C. Van't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy DeltaH degrees , the apparent change in entropy DeltaS degrees and the apparent change in DeltaDeltaH degrees and DeltaDeltaS degrees . The thermodynamic parameters (DeltaH degrees , DeltaS degrees , DeltaDeltaH degrees and DeltaDeltaS degrees ) were calculated in order to provide an understanding of the thermosynamic driving forces for enantioseparation. The established method shows perspective to be used for preparing micro-scale amount of pure enantiomers of the chiral triazoles studied.     

Enantioseparation on 4-halogen-substituted phenylcarbamates of amylose as chiral stationary phases for high-performance liquid chromatography

Four 4-halogen-substituted phenylcarbamate derivatives of amylose were prepared and their chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were evaluated and compared with those of the corresponding cellulose derivatives. The amylose derivatives with fluoro, chloro, bromo, or iodo group at the four-position on the phenyl group were found to show higher chiral resolving ability than the corresponding cellulose derivatives. Among four amylose derivatives 4-fluoro- and 4-chlorophenylcarbamates showed an excellent chiral recognition ability. Especially, amylose tris(4-chlorophenylcarbamate) resolved (±)-1,2,2,2-tetraphenylethanol with a very high α value (α = 8.29). In order to obtain useful information concerning the chiral recognition mechanism of this resolution, we also performed enantioseparation of a variety of analogous racemic alcohols, and found that both the hydroxy and bulky triphenylmethyl groups of the racemate are essential for the effective chiral recognition. Chirality 9:63–68, 1997. © 1997 Wiley-Liss, Inc.     

Determination of enantiomeric impurity in besifloxacin hydrochloride by chiral high-performance liquid chromatography with precolumn derivatization

Besifloxacin hydrochloride is a novel chiral broad-spectrum fluoroquinolone developed for the treatment of bacterial conjunctivitis. R-besifloxacin hydrochloride is used in clinics as a consequence of its higher antibacterial activity. To establish an enantiomeric impurity determination method, some chiral stationary phases (CSPs) were screened. Besifloxacin enantiomers can be separated to a certain extent on Chiral CD-Ph (Shiseido Co., Ltd., Japan), Chiral AGP, and Crownpak CR (+) (Daicel Chemical IND., Ltd., Japan). However, the selectivity and sensitivity were both unsatisfactory on these three CSPs. Therefore, Chiral AGP, Chiral CD-Ph, and Crownpak CR (+) were not used in the enantiomeric impurity determination of besifloxacin hydrochloride. The separation of enantiomers of besifloxacin was further performed using a precolumn derivatization chiral high-performance liquid chromatography method. 2,3,4,6-Tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate was used as the derivatization reagent. Besifloxacin enantiomer derivates were well separated on a C(18) column (250 × 4.6 mm, 5 μm) with a mobile phase that consisted of methanol-KH(2)PO(4) buffer solution (20 mM; pH 3.0) (50:50, v/v). Selectivity, sensitivity, linearity, accuracy, precision, stability, and robustness of this method were all satisfied with the method validation requirement. The method was suitable for the quality control of enantiomeric impurity in besifloxacin hydrochloride.     

High‐performance liquid chromatography separation of enantiomers of mandelic acid and its analogs on a chiral stationary phase

The enantiomers of mandelic acid and its analogs have been chromatographically separated on a chiral stationary phase (CSP) derived from 4‐(3,5‐dinitrobenzamido) tetrahydrophenanthrene. The rationale of separations of these compounds is discussed with respect to the method development for determining enantiomeric purity and possibility of obtaining enantiomerically pure materials by high‐pressure liquid chromatography. The relationship of analyte structure to the extent of enantiomeric separation has been examined and separation factors (α) are presented for various groups of structurally related compounds. Chiral recognition models have been suggested to account for the observed separations. These models provide mechanistic insights into the chiral recognition process. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Optimization of the chiral separation of some 2-arylpropionic acids on an avidin column by modeling a combined response

The enantiomeric separation of some nonsteroidal antiinflammatory drugs was investigated on an avidin column. An experimental design approach (central composite design) was used to evaluate the effects of three method parameters (pH, concentration of organic modifier, and buffer concentration) on the analysis time and the resolution, as well as to model these responses. This revealed that the organic modifier concentration and sometimes the pH are significant parameters to control because of their influence on both analysis time and resolution. Furthermore, the central composite design results were combined in a multicriteria decision-making approach in order to obtain a set of optimal experimental conditions leading to the most desirable compromise between resolution and analysis time.     

Direct high-performance liquid chromatography (HPLC) separation of etodolac enantiomers using chiral stationary phases

The enantiomers of the antiinflammatory drug Etodolac were separated without derivatization on Chiralcel OD and Pirkle (R)-DNBPG columns. Enantiomeric purity can be determined in less than 10 min. Optimization of separation was evaluated using various concentrations of 2-propanol (doped with TFA) in hexane as the mobile phase. © 1993 Wiley-Liss, Inc.     

Direct chromatographic separation of 2-arylpropionic acid enantiomers using tris(3,5-dimethylphenylcarbamate)s of cellulose and amylose as chiral stationary phases

Direct optical resolution of antiinflammatory drugs such as ibuprofen, ketoprofen, flurbiprofen, and tiaprofenic acid were attempted by high-performance liquid chromatography using tris(3,5-dimethylphenylcarbamate)s of cellulose and amylose as chiral stationary phases. Although ibuprofen was not sufficiently resolved, the other three 2-arylpropionic acids were completely resolved by amylose tris(3,5-dimethylphenylcarbamate). Ibuprofen was resolved as anilide derivative.     

Enantiomeric bioanalysis of simendan and levosimendan by chiral high-performance liquid chromatography

rac-Simendan, (±)-(R, S)-[[4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)-phenyl]hydrazono]propanedinitrile, and the levorotatory enantiomer levosimendan, are drug candidates intended for the treatment of congestive heart failure. An enantiospecific high-performance liquid chromatographic (HPLC) method suitable for determination of the ratio of the enantiomer concentrations in blood plasma samples was developed. Direct resolution of the enantiomers was achieved by using a chiral β-cyclodextrin stationary phase in reversed phase mode. With an eluent containing 24–33% of methanol in a 0.5% (v/v) triethylammonium acetate buffer, pH 6.0, and a flow rate of 1 ml/min, a resolution (1.2–1.6) adequate for the determinations was achieved. By using UV detection, the relative concentration of the enantiomers in plasma was assessed down to 10 ng/ml. For the racemate, the results indicated a slightly enantioselective disposition and plasma protein binding in rat, dog, and man. The pure enantiomer, levosimendan, was found not to isomerize in vivo. © 1996 Wiley-Liss, Inc.     

Dichloro-, dimethyl-, and chloromethylphenylcarbamate derivatives of cyclodextrins as chiral stationary phases for high-performance liquid chromatography

New dichloro-, dimethyl-, and chloromethylphenylcarbamate derivatives of cyclodextrins (CDs) were prepared and their enantiomeric recognition abilities were evaluated as chiral stationary phases (CSPs) in normal phase high-performance liquid chromatography (HPLC). The effects of the type of cyclodextrins, the nature and position of the substituents on the phenyl ring, binding mode and spacer on the chiral recognition were studied in detail. No marked change of chiral recognition abilities was established by reversing the binding side of CDs (i.e., by the narrower [primary] opening of the cone-shaped CD to silica gel with the wider [secondary] opening sides). This result indirectly proves the previously drawn conclusion about the minor role of inclusion phenomena in chiral recognition in this case. Nevertheless, chiral recognition of these CSPs toward some compounds critically depends on the type of CDs used. All CD derivatives described in this study show rather low enantiomeric resolving abilities compared with corresponding polysaccharide (cellulose and amylose) derivatives, although very high enantioselectivity of separation was observed for a few compounds, such as racemic flavanone and cyclopropanedicarboxilic acid dianilide. © 1996 Wiley-Liss, Inc.     

Resolution of terfenadine enantiomers by beta-cyclodextrin chiral stationary phase high-performance liquid chromatography.

Enantiomers of terfenadine were resolved by high-performance liquid chromatography (HPLC) using a chiral stationary phase (CSP) column packed with beta-cyclodextrin (beta-CD) covalently bound to silica. Separation was achieved in both the reverse phase and normal phase modes. Resolution of enantiomers was confirmed by ultraviolet-visible absorption, circular dichroism, and mass spectral analysis.     

Enantioseparation of chiral pharmaceuticals by vancomycin‐bonded stationary phase and analysis of chiral recognition mechanism

The drug chirality is attracting increasing attention because of different biological activities, metabolic pathways, and toxicities of chiral enantiomers. The chiral separation has been a great challenge. Optimized high‐performance liquid chromatography (HPLC) methods based on vancomycin chiral stationary phase (CSP) were developed for the enantioseparation of propranolol, atenolol, metoprolol, venlafaxine, fluoxetine, and amlodipine. The retention and enantioseparation properties of these analytes were investigated in the variety of mobile phase additives, flow rate, and column temperature. As a result, the optimal chromatographic condition was achieved using methanol as a main mobile phase with triethylamine (TEA) and glacial acetic acid (HOAc) added as modifiers in a volume ratio of 0.01% at a flow rate of 0.3 mL/minute and at a column temperature of 5°C. The thermodynamic parameters (eg, ΔH, ΔΔH, and ΔΔS) from linear van 't Hoff plots revealed that the retention of investigated pharmaceuticals on vancomycin CSP was an exothermic process. The nonlinear behavior of lnk′ against 1/T for propranolol, atenolol, and metoprolol suggested the presence of multiple binding mechanisms for these analytes on CSP with variation of temperature. The simulated interaction processes between vancomycin and pharmaceutical enantiomers using molecular docking technique and binding energy calculations indicated that the calculated magnitudes of steady combination energy (ΔG) coincided with experimental elution order for most of these enantiomers.     

Enantiomeric separation of tetrahydroisoquinoline alkaloids by high-performance liquid chromatography with β-cyclodextrin as chiral selector

Tetrahydroisoquinoline alkaloids, which are known to be present not only in plants but also in animals, including mammals, can be considered as condensation products of 2-phenylethylamines (e.g., catecholamines) with aldehydes (e.g., acetaldehyde) or 2-oxo acids (e.g., pyruvic acid). In this study the possibility of separating the optical isomers of several tetrahydroisoquinolines by high-performance liquid chromatography was investigated. For isosalsoline, tetrahydropapaveroline and laudanosoline a good enantiomeric separation could be achieved by applying β-cyclodextrin-bonded silica as stationary phase in connection with various mobile phases. With respect to laudanosoline, the addition of β-cyclodextrin as chiral selector to the mobile phase using a C₁₈ reversed-phase column as stationary phase revealed an even higher resolution when compared with the chiral columns. All tested tetrahydroisoquinolines which could be well separated into enantiomers bear a hydroxyl group at carbon atom 7 as a common structural feature. Those alkaloids substituted with a methoxy group on position 7 instead of a hydroxyl group (e.g., salsolidine) failed to be resolved into their optical isomers. Therefore, the presence of a hydroxyl group on C7 of the aromatic ring seems to be conducive to steric discrimination. However, the separation results for 1-carboxysalsolinol were unsatisfactory although this molecule possesses a 7-hydroxyl group. In this case the existence of a carboxyl group on C1 reduced the chiral recognition and thus the enantiomeric resolution. © 1995 Wiley-Liss, Inc.     

Enantioseparation of 2-aryl-1,3-dicarbonyl analogues by high performance liquid chromatography using polysaccharide type chiral stationary phase

The HPLC chiral separation of 21 kinds of 2-aryl-1,3-dicarbonyl analogues was investigated in normal phase mode with amylose tris(3,5-dimethylphenylcarbamate), amylose tris((S)-1-phenylethylcarbamate), cellulose tris(3,5-dimethylphenylcarbamate), and cellulose tris(4-methylbenzoate) chiral stationary phases, respectively. The whole set of 2-aryl-1,3-dicarbonyl analogues shows better enantioselectivity and enantioseparation on amylose tris(3,5-dimethylphenyl carbamate) (Chiralpak AD-H). The temperature dependence of enantioselectivity was studied to improve the enantioseparation. In addition, efforts are made to relate analyte structure with the quality of the achieved chiral separation.     

Fluorenone 1,4-dihydropyridine derivatives with cardiodepressant activity: Enantiomeric separation by chiral HPLC and conformational aspects

The direct HPLC enantiomeric separation of five fluorenone-1,4-dihydropyridine-3,5-dicarboxylic diesters has been achieved using a Chiralpak AD stationary phase obtaining simultaneously good enantioselectivities, resolution factors, and elution times. CD spectra of the individual enantiomers for two compounds were measured. Thermodynamic parameters associated with the adsorption equilibria of the enantiomers with the chiral stationary phase were obtained from HPLC runs at various temperatures. The conformational preferences of the synperiplanar fluorenone group and of the cis/cis ester groups were obtained by ¹H NMR spectra, including NOE experiments. © 1996 Wiley-Liss, Inc.