New chiral stationary phases based on xanthone derivatives for liquid chromatography

Six chiral derivatives of xanthones (CDXs) were covalently bonded to silica, yielding the corresponding xanthonic chiral stationary phases (XCSPs). The new XCSPs were packed into stainless‐steel columns with 150 x 4.6 mm i.d. Moreover, the greening of the chromatographic analysis by reducing the internal diameter (150 x 2.1 mm i.d.) of the liquid chromatography (LC) columns was also investigated. The enantioselective capability of these phases was evaluated by LC using different chemical classes of chiral compounds, including several types of drugs. A library of CDXs was evaluated in order to explore the principle of reciprocity as well as the chiral self‐recognition phenomenon. The separation of enantiomeric mixtures of CDXs was investigated under multimodal elution conditions. The XCSPs provided high specificity for the enantiomeric mixtures of CDXs evaluated mainly under normal‐phase elution conditions. Furthermore, two XCSPs were prepared with both enantiomers of the same xanthonic selector in order to confirm the inversion order elution.     

Enantioselective potential of polysaccharide‐based chiral stationary phases in supercritical fluid chromatography

The enantioselective potential of two polysaccharide‐based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5 μm silica particles were tris‐(3,5‐dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose‐based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose‐based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose‐based chiral stationary phase were achieved particularly with propane‐2‐ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO₂, respectively. Methanol and basic additive isopropylamine were preferred on amylose‐based chiral stationary phase. The complementary enantioselectivity of the cellulose‐ and amylose‐based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.     

Immobilized vancomycin as chiral stationary phase in packed capillary liquid chromatography

Fused silica-packed capillary columns containing vancomycin immobilized by reductive amination on an aldehyde-silica were used to separate enantiomers of some non-steroidal anti-inflammatory drugs. Attempts have been made to qualitatively explain the influence of various mobile phase compositions on the enantioselective retention. The effects of mobile phase pH, buffer, and organic modifier concentrations were investigated as well as the influence of salts of hydrophobic ions added to the mobile phase to induce ion pair retention. Chirality 10:273–280, 1998. © 1998 Wiley-Liss, Inc.     

Synthesis of polymer-type chiral stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two new chiral polymers of different molecular weights were synthesized by the copolymerization of (1R,2R)-(+)-1,2-diphenylethylenediamine, phenyl diisocyanate and terephthaloyl chloride. The polymers were immobilized on aminated silica gel to afford two chiral stationary phases. The polymers and the corresponding chiral stationary phases were characterized by Fourier transform-IR, elemental analysis, 1H and 13C NMR. The surface coverages of chiral structural units on the chiral stationary phases were estimated as 0.27 and 0.39 mmol/g, respectively. The enantioseparation ability of these chiral stationary phases was evaluated with a variety of chiral compounds by high-performance liquid chromatography. The effects of the organic additives, the composition of mobile phases, and the injection amount of sample on enantioseparation were investigated. A comparison of enantioseparation ability between these two chiral stationary phases was made. It was believed that the chain length of polymeric chiral selector significantly affected the enantioseparation ability of corresponding chiral stationary phase.     

New chiral stationary phases for liquid chromatography based on small molecules: Development,enantioresolution evaluation and chiral recognition mechanisms

Recently, we reported the development of new chiral stationary phases (CSPs) for liquid chromatography (LC) based on chiral derivatives of xanthones (CDXs). Based on the most promising CDX selectors, 12 new CSPs were successfully prepared starting from suitable functionalized small molecules including xanthone and benzophenone derivatives. The chiral selectors comprising one, two, three, or four chiral moieties were covalently bonded to a chromatographic support and further packed into LC stainless-steel columns (150 × 2.1 mm I.D.). The enantioselective performance of the new CSPs was evaluated by LC using different classes of chiral compounds. Specificity for enantioseparation of some CDXs was observed in the evaluation of the new CSPs. Besides, assessment of chiral recognition mechanisms was performed by computational studies using molecular docking approach, which are in accordance with the chromatographic parameters. X-Ray analysis was used to establish a chiral selector 3D structure.     

Evaluation of dalbavancin as chiral selector for HPLC and comparison with teicoplanin‐based chiral stationary phases

Dalbavancin is a new compound of the macrocyclic glycopeptide family. It was covalently linked to 5 μm silica particles using two different binding chemistries. Approximately 250 racemates including (a) heterocyclic compounds, (b) chiral acids, (c) chiral amines, (d) chiral alcohols, (e) chiral sulfoxides and sulfilimines, (f) amino acids and amino acid derivatives, and (g) other chiral compounds were tested on the two new chiral stationary phases (CSPs) using three different mobile phases. As dalbavancin is structurally related to teicoplanin, the same set of chiral compounds was screened on two commercially available teicoplanin CSPs for comparison. The dalbavancin CSPs were able to separate some enantiomers that were not separated by the teicoplanin CSPs and also showed improved separations for many racemates. However, there were other compounds only separated or better separated on teicoplanin CSPs. Therefore, the dalbavancin CSPs are complementary to the teicoplanin CSPs. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Evaluation of the chiral recognition properties and the column performances of three chiral stationary phases based on cellulose for the enantioseparation of six dihydropyridines by high‐performance liquid chromatography

Separations of six dihydropyridine enantiomers on three commercially available cellulose‐based chiral stationary phases (Chiralcel OD‐RH, Chiralpak IB, and Chiralpak IC) were evaluated with high‐performance liquid chromatography (HPLC). The best enantioseparation of the six chiral drugs was obtained with a Chiralpak IC (250 × 4.6 mm i.d., 5 μm) column. Then the influence of the mobile phase including an alcohol‐modifying agent and alkaline additive on the enantioseparation were investigated and optimized. The optimal mobile phase conditions and maximum resolution for every analyte were as follows respectively: n‐hexane/isopropanol (85:15, v /v) for nimodipine (R  = 5.80) and cinildilpine (R  = 5.65); n‐hexane/isopropanol (92:8, v /v) for nicardipine (R  = 1.76) and nisoldipine (R  = 1.92); and n‐hexane/isopropanol/ethanol (97:2:1, v /v/v) for felodipine (R  = 1.84) and lercanidipine (R  = 1.47). Relative separation mechanisms are discussed based on the separation results, and indicate that the achiral parts in the analytes' structure showed an important influence on the separation of the chiral column.     

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.     

Chiral separation of aliphatic primary amino alcohols as o‐phthaldialdehyde/mercaptoethanol derivatives on polysaccharide‐based chiral stationary phases

A sensitive chiral high performance liquid chromatography (HPLC) method for the determination of aliphatic primary amino alcohol isomers with o‐phthaldialdehyde/mercaptoethanol precolumn derivatization has been developed and validated. Seven chiral columns were tested in a reversed phase mode. Excellent enantioseparation with the resolution more than 2.0 was achieved on Chiralcel OJ‐3R. The effect of various chromatographic conditions including column temperature, acetonitrile content in the mobile phase, buffer pH, buffer concentration, and buffer type in the mobile phase on the retention and the selectivity was investigated. The final mobile phase consisted of binary mixture of 20mM ammonium formate solution with acetonitrile (75:25; v/v). The analyses were performed at mobile phase flow rate of 1.0 mL/min and the column temperature of 40°C. The fluorescence detection was performed at excitation wavelength of 345 nm and emission wavelength of 450 nm. The developed method was fully validated in terms of linearity, sensitivity, accuracy, precision, intermediate precision, and selectivity according to International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use guidelines using internal normalization procedure. The proposed chiral method was proved to be highly sensitive, simple, and rapid and was successfully applied to the determination of D‐Valinol content in commercially available samples of L‐Valinol.     

Novel chiral ionic liquids stationary phases for the enantiomer separation of chiral acid by high‐performance liquid chromatography

Novel chiral ionic liquid stationary phases based on chiral imidazolium were prepared. The ionic liquid chiral selector was synthesized by ring opening of cyclohexene oxide with imidazole or 5,6‐dimethylbenzimidazole, and then chemically modified by different substitute groups. Chiral stationary phases were prepared by bonding to the surface of silica sphere through thioene “click” reaction. Their enantioselective separations of chiral acids were evaluated by high‐performance liquid chromatography. The retention of acid sample was related to the counterion concentration and showed a typical ion exchange process. The chiral separation abilities of chiral stationary phases were greatly influenced by the substituent group on the chiral selector as well as the mobile phase, which indicated that, besides ion exchange, other interactions such as steric hindrance, π‐π interaction, and hydrogen bonding are important for the enantioselectivity. In this report, the influence of bulk solvent components, the effects of varying concentration, and the type of the counterion as well as the proportion of acid and basic additives were investigated in detail.     

Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high‐performance liquid chromatography on immobilized polysaccharide‐type chiral stationary phases and off‐column racemization experiments

Direct enantiomer separation of hypericin, pseudohypericin, and protohypericin was accomplished by high‐performance liquid chromatography (HPLC) using immobilized polysaccharide‐type chiral stationary phases (CSPs). Enantioselectivities up to 1.30 were obtained in the polar‐organic elution mode whereby for hypericin and pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5‐dichlorophenylcarbamate)] and for protohypericin Chiralpak IA (chiral selector being the 3,5‐dimethylphenylcarbamate of amylose) gave favorable results. Enantiomers were distinguished by on‐line electronic circular dichroism detection. Optimized enantioselective chromatographic conditions were the basis for determining stereodynamic parameters of the enantiomer interconversion process of hypericin and pseudohypericin. Rate constants delivered by computational simulation of dynamic HPLC elution profiles (stochastic model, consideration of peak tailing) were used to calculate averaged enantiomerization barriers (ΔG) of 97.6–99.6 kJ/mol for both compounds (investigated temperature range 25–45°C). Complementary variable temperature off‐column (i.e., in solution) racemization experiments delivered ΔG = 97.1–98.0 kJ/mol (27–45°C) for hypericin and ΔG = 98.9–101.4 kJ/mol (25–55°C) for pseudohypericin. An activation enthalpy of ΔH^# = 86.0 kJ/mol and an activation entropy of ΔS^# = ?37.7 J/(K mol) were calculated from hypericin racemization kinetics in solution, whereas for pseudohypericin these figures amounted to 74.1 kJ/mol and ?82.6 J/(K mol), respectively. Although the natural phenanthroperylene quinone pigments hypericin and pseudohypericin as well as their biological precursor protohypericin are chiral and can be separated by enantioselective HPLC low enantiomerization barriers seem to prevent the occurrence of an excess of one enantiomer under typical physiological conditions—at least as long as stereoselective intermolecular interactions with other chiral entities are absent. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Enantiomeric separation of prothioconazole and prothioconazole‐desthio on chiral stationary phases

Prothioconazole is a type of broad‐spectrum triazole thione fungicide developed by the Bayer Company. Prothioconazole‐desthio is the main metabolite of prothioconazole in the environment. In our study, enantiomeric separation of prothioconazole and prothioconazole‐desthio was performed on various chiral stationary phases (CSPs) by high‐performance liquid chromatography (HPLC). It was found that polysaccharide CSPs showed better ability than brushing CSPs in enantiomeric separation. The successful chiral separation of prothioconazole could be achieved on self‐made Chiralcel OD, commercialized Chiralcel OJ‐H and Lux Cellulose‐1. Chiralpak IA, Chiralpak IB, Chiralpak IC, Chiralcel OD, Chiralpak AY‐H, Chiralpak AZ‐H, and Lux Cellulose‐1 realized the baseline separation of prothioconazole‐desthio enantiomers. Simultaneous enantiomeric separation of prothioconazole and prothioconazole‐desthio was performed on Lux Cellulose‐1 using acetonitrile (ACN) and water as mobile phase. In most cases, low temperature favored the separation of two compounds. The influence of the mobile phase ratio or type was deeply discussed. We obtained larger Rs and longer analysis time with a smaller proportion of isopropanol (IPA) or ethanol and more water content at the same temperature. The ratio of ACN and water had influences on the outflow orders of prothioconazole‐desthio enantiomers. This work provides a new approach for chiral separation of prothioconazole and prothioconazole‐desthio with a discussion of chiral separation mechanism on different CSPs.     

Enantiomeric separation of type I and type II pyrethroid insecticides with different chiral stationary phases by reversed‐phase high‐performance liquid chromatography

The enantiomeric separation of type I (bifenthrin, BF) and type II (lambda‐cyhalothrin, LCT) pyrethroid insecticides on Lux Cellulose‐1, Lux Cellulose‐3, and Chiralpak IC chiral columns was investigated by reversed‐phase high‐performance liquid chromatography. Methanol/water or acetonitrile/water was used as mobile phase at a flow rate of 0.8 mL/min. The effects of chiral stationary phase, mobile phase composition, column temperature, and thermodynamic parameters on enantiomer separation were carefully studied. Bifenthrin got a partial separation on Lux Cellulose‐1 column and baseline separation on Lux Cellulose‐3 column, while LCT enantiomers could be completely separated on both Lux Cellulose‐1 and Lux Cellulose‐3 columns. Chiralpak IC provided no separation ability for both BF and LCT. Retention factor (k) and selectivity factor (α) decreased with the column temperature increasing from 10°C to 40°C for both BF and LCT enantiomers. Thermodynamic parameters including ?H and ?S were also calculated, and the maximum R_s were not always obtained at lowest temperature. Furthermore, the quantitative analysis methods for BF and LCT enantiomers in soil and water were also established. Such results provide a new approach for pyrethroid separation under reversed‐phase condition and contribute to environmental risk assessment of pyrethroids at enantiomer level.     

High‐performance liquid chromatographic enantioseparation of N‐aryl‐thioureidoalkylphosphonates and thiourylenedi(alkylphosphonates) on polysaccharide‐based chiral stationary phases

The first successful enantioseparation of representative O,O‐diphenyl‐N‐arylthioureidoalkylphosphonates, (±)‐Ptc‐Val^P(OPh)₂ & (±)‐Ptc‐Leu^P(OPh)₂ and thiourylenedi(isobutyl phosphonate), Tcm[Val^P(OPh)₂]₂ on analytical and semipreparative scale was achieved by high‐performance liquid chromatography using polysaccharide‐based chiral stationary phases (CPs). Atc‐AA^P(OPh)₂ was obtained using modified tricomponent condensations of the corresponding aldehydes, N‐arylthiourea and triphenyl phosphite whereas Tcm[Val^P(OPh)₂]₂ by the condensations of aldehydes, thiourea, and triphenyl phosphite. The prepared, racemic (±)‐Atc‐AA^P(OPh)₂ [(±)‐Ptc‐Val^P(OPh)₂, (±)‐Ptc‐Leu^P(OPh)₂, (±)‐Ptc‐Pgly^P(OPh)₂ and (±)‐Ntc‐Pgly^P(OPh)₂] and racemic (±)‐Tcm[AA^P(OPh)₂]₂ [(±)‐Tcm[Nva^P(OPh)₂]₂ & (±)‐Tcm[Val^P(OPh)₂]₂] were adequately characterized and used for chromatographic separations on high‐performance liquid chromatography–chiral stationary phases. The best results were obtained for (±)‐Ptc‐Val^P(OPh)₂, (±)‐Ptc‐Leu^P(OPh)₂ and (±)‐Tcm[Val^P(OPh)₂]₂.     

High-performance liquid chromatography of imine stereoisomers on cellulose-based chiral stationary phases

A high-performance liquid chromatographic method has been developed for the analysis of the intermediate imines and end products in an asymmetric isomerization route toward optically active amines. Separation of the imine enantiomers was performed on commercially available Chiralcel OD-H, Chiralcel OJ, and Chiralpak AD chiral stationary phases. All substituted imine enantiomers could be readily resolved with selectivities (α) higher than 1.10 using the Chiralpak AD column. By derivatization with ring-substituted benzaldehydes, aromatic amines were converted into Schiff base derivatives and the enantiopurity of these amines was determined. Chirality 9:727–731, 1997. © 1997 Wiley-Liss, Inc.     

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.     

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.     

Evaluation of "click" binaphthyl chiral stationary phases by liquid chromatography

Two "click" binaphthyl chiral stationary phases were synthesized and evaluated by liquid chromatography. Their structures incorporate S-(-)-1,1'-binaphthyl moiety as the chiral selector and 1,2,3-triazole ring as the spacer. These chiral stationary phases (CSPs) allowed the efficient resolution for a wide range of racemic BINOL derivatives, particularly for nonpolar diether derivatives and 3-phenyl indolin-2-one analogs. The chromatographic data showed that the π-π interaction was crucial for enantiorecognition of these CSPs. Loss of enantioselectivity observed on CSP3, which are lacking the triazole ring linkage, indicated that the triazole ring linkage took part in the enantioseparation process, although it was remote from the chiral selector of the CSP. The substitution of the phenyl group at 6 and 6' positions can significantly improve the separation ability of the CSP. The chiral recognition mechanism was also investigated by tracking the elution orders and studying the thermodynamic parameters.     

Benzoylcellulose derivatives as chiral stationary phases for open tubular column chromatography

The application of cellulose-based stationary phases for chiral separations has been extended to open tubular column chromatography. Efficient columns were obtained by coating the capillaries with mixtures of chiral cellulose materials and conventional achiral stationary phases for gas chromatography. In this study, various siloxane and polyethylene glycol polymers were used as achiral components and mixed with different substituted benzoylcellulose derivatives as chiral components. Systematic investigations were carried out to determine the optimal ratio for the components of the stationary phase. Depending on the chromatographic mode—gas chromatography (GC) or supercritical fluid chromatography (SFC)—the stationary phases were found to behave differently. The applicability of the technique was demonstrated by the resolution of various racemic compounds. © 1993 Wiley-Liss, Inc.     

HPLC‐method for determination of permethrin enantiomers using chiral β‐cyclodextrin‐based stationary phase

The liquid chromatographic separation of permethrin enantiomers on chiral β‐cyclodextrin‐based stationary phase has been investigated. All four enantiomers are obtained by using simple methanol and water mobile phase, under gradient mode. The method was optimized and validated. The relationship between temperature and chromatographic parameters: k′ (capacity factor), α (separation factor) and Rs (resolution factor) was studied. Van't Hoff's curves for each enantiomer were plotted for temperature range 288–318 K. It was noticed that the response factor ratio of permethrin isomers differ and calculated value is found to be 1.66 (cis/trans, for n = 5). This method has been used for determining permethrin enantiomer ratio for a few samples of working standards and one formulation. Chirality 2010. © 2009 Wiley‐Liss, Inc.