Liquid Chromatographic Chiral Stationary Phase Research Articles

Polysaccharide and Cyclodextrin-Based Monolithic Chiral Stationary Phases and its Application to Chiral Separation.

The recent development of monolithic chiral stationary phases (CSPs) for liquid chromatography (LC) is mainly focused on reducing backpressure, maximizing flow rates, faster run time, column efficiency, and stability. This review paper emphasizes recent progress in the development of polysaccharide and cyclodextrin-based monolithic CSPs. Further the paper draws attention to competing techniques, like non-porous particle-packed columns, core-shell and monoliths as chromatographic support matrix, available for achieving fast and efficient chromatographic separation. A brief discussion on the three main classes of chiral monolithic stationary phase viz. silica, organic polymer and hybrid-based monolithic stationary phases is also presented. In addition, the paper highlights various studies on the application of monolith chiral CSPs in LC and capillary electrochromrography separation and analysis of chiral compounds.

Synthesis and Application of New Calix[4]Arenes‐Containing (R)‐Phenylglycinol Chiral Stationary Phases for Enantioseparation

N‐3,5‐Dinitrobenzoyl‐(R)‐phenylglycinol silylation product was used as a high‐performance liquid chromatography chiral stationary phase (CSP 1) for the resolution of various racemic samples, and some racemic samples were successfully separated. In this study, instead of the commonly used π‐acidic acyl chloride, calix[4]arenes were introduced to prepare two phenylglycinol CSPs (CSP 2, CSP 3). CSP 3 showed similar separation patterns as CSP 1 but different characteristics in specific samples. The newly prepared CSP 3 separated 10 of 13 π‐acidic, π‐basic, and oxazolidinone chiral samples and was especially useful for separating chiral oxazolidinones. In comparison between CSP 2 and CSP 3, CSP 2 separated fewer chiral samples than CSP 3 because of poor cavity and steric interactions. The newly developed C‐methylcalix[4]resorcinarene derived CSP (CSP 3) will be a good model for the development of new stationary phase of this calixarene series.

Preparation of Poly-(GMA-EDA-β-CD-co-TMPTMA) Monolith as High Performance Liquid Chromatography Chiral Stationary Phase Column

An enantiomer molecule consisted of the chiral atom has different structure conformations, which exhibit different activities as well. Yet, its separation considerably difficult since ordinary separation could not separate both molecules. One of the popular enantioseparations which are often used was using organic polymer monolithic column modified by ethylenediamine-β-cyclodextrin (EDA-β-CD) as the enantioseparations site. The aim of this research was to produce chiral stationary phase column for enantioseparations of (±)-citronellal. It was conducted by preparing monolithic column using monomer glycidyl methacrylate (GMA), trimethylolpropane trimethacrylate (TMPTMA) as crosslinker, 1-propanol, 1,4-butanediol, and water as pore-forming agents (porogens) in the presence of α,α'-azoisobutyronitrile (AIBN) as radical initiator inside polyetheretherketone (PEEK) tubing. It was then modified with EDA-β-CD synthesized from β-CD. Finally, it was installed as a high-performance liquid chromatography column. The result shows the produced chiral stationary phase column could separate (±)-citronellal at a retention time of 44.76 and 45.71 min.

Resolution of methoxyphenamine and its analogues on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid.

A liquid chromatographic chiral stationary phase, which contains two N-CH3 amide connecting groups, based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was first applied to the resolution of methoxyphenamine (2-methoxy-N-methylamphetamine, a β-adrenergic receptor agonist used as a bronchodilator). The resolution of methoxyphenamine on the chiral stationary phase containing two N-CH3 amide connecting groups was quite successful with the separation factor (α) of 1.42 and resolution (RS ) of 4.22 compared with those (α of 1.09 and RS of 1.55) on the chiral stationary phase containing two N-H amide connecting groups. In addition, the chiral stationary phase containing two N-CH3 amide connecting groups was applied to the resolution of methoxyphenamine analogues. From the comparison of the resolution results of methoxyphenamine with those of methoxyphenamine analogues on the chiral stationary phase containing two N-CH3 amide connecting groups, the N-methyl group and the 2-methoxyphenyl group of methoxyphenamine were elucidated to be the structurally essential parts for the resolution on the chiral stationary phase.

Liquid chromatographic resolution of proline and pipecolic acid derivatives on chiral stationary phases based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Two liquid chromatographic chiral stationary phases based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid were applied to the resolution of the amide derivatives of cyclic α-amino acids including proline and pipecolic acid. Among the five amide derivatives of proline, aniline amide was resolved best on the first chiral stationary phase, which contains two N-H tethering amide groups, with the separation factor of 1.31 and the resolution of 2.60, and on the second chiral stationary phase, which contains two N-CH3 tethering amide groups, with the separation factor of 1.57 and the resolution of 5.50. Among the five amide derivatives of pipecolic acid, 2-naphthyl amide was resolved best on the first chiral stationary phase with the separation factor of 1.30 and the resolution of 1.75, but 1-naphthylmethyl amide was resolved best on the second chiral stationary phase with the separation factor of 1.30 and the resolution of 2.26. In general, the second chiral stationary phase was found to be better than the first chiral stationary phase in the resolution of the amide derivatives of cyclic α-amino acids. In this study, the second chiral stationary phase was first demonstrated to be useful for the resolution of secondary amino compounds.

Dynamic Behavior of Clobazam on High-Performance Liquid Chromatography Chiral Stationary Phases.

Clobazam, a 1,5-benzodiazepin-2,4-dione, is a chiral molecule because its ground state conformation features a nonplanar seven-membered ring lacking reflection symmetry elements. The two conformational enantiomers of clobazam interconvert at room temperature by a simple ring-flipping process. Variable temperature HPLC on the Pirkle type (R)-N-(3,5-dinitronenzoyl)phenylglycine and (R,R)-Whelk-O1 chiral stationary phases (CSPs) allowed us to separate for the first time the conformational enantiomers of clobazam and to observe peak coalescence-decoalescence phenomena due to concomitant separation and interconversion processes occurring on the same time scale. Clobazam showed temperature dependent dynamic high-performance liquid chromatography (HPLC) profiles with interconversion plateaus on the two CSPs indicative of on-column enantiomer interconversion. (enantiomerization) in the column temperature range between Tcol = 10°C and Tcol = 30°C, whereas on-column interconversion was absent at temperature close to or lower than Tcol = 5°C. Computer simulation of exchange-deformed HPLC profiles using a program based on the stochastic model yielded the apparent rate constants for the on-column enantiomerization and the corresponding free energy activation barriers. At Tcol = 20°C the averaged enantiomerization barriers, ΔG(‡), for clobazam were found in the range 21.08-21.53 kcal mol(-1) on the two CSPs. The experimental dynamic chromatograms and the corresponding interconversion barriers reported in this article are consistent with the literature data measured by DNMR at higher temperatures and in different solvents.

Superficially porous particles vs. fully porous particles for bonded high performance liquid chromatographic chiral stationary phases: Isopropyl cyclofructan 6

This work reports a comparison of HPLC separations of enantiomers with chiral stationary phases (CSPs) prepared by chemically bonding cyclofructan-6, functionalized with isopropyl carbamate groups on fully and superficially porous particles (SPPs). The chromatographic performance of the superficially porous CSP based column was compared with columns packed with 5μm and 3μm fully porous particles (FPPs). At a flow rate of 3.0mL/min the number of plates on column afforded by the SPP column was ∼7× greater than the number of plates on column (same length) obtained when using the 5μm FPP based column. The flow rate providing the highest efficiency separation was ∼1.0mL/min for the SPP column while it was ∼0.5mL/min for both FPP columns. It was found that the selectivity and resolution of the separations were comparable between fully porous and superficially porous based columns (under constant mobile phase conditions), even though the SPP column contained lower absolute amounts of chiral selector. When tested under constant retention conditions, the SPP based CSP greatly improved resolution compared to the FPP based columns. At high flow rates the efficiency gained by using superficially porous CSP was accentuated. The advantages of columns based on SPPs become more obvious from the viewpoint of plate numbers and resolution per analysis time.

Liquid chromatographic resolution of racemic rasagiline and its analogues on a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

A liquid chromatographic chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied to the resolution of 15 analytes, including racemic rasagiline, a chiral drug for the treatment of Parkinson's disease, and its analogues. The composition of mobile phase was optimized to be ethanol/acetonitrile/acetic acid/triethylamine (80:20:0.2:0.3, v/v/v/v) by evaluating the chromatographic results for the resolution of five selected analytes under various mobile phase conditions. Under the optimized mobile phase conditions, racemic rasagiline was resolved quite well with a separation factor of 1.48 and resolution of 2.71 and its 14 analogues were also resolved reasonably well with separation factors of 1.06-1.54 and resolutions of 0.54-2.11. Among 15 analytes, racemic rasagiline was resolved best except for just one analyte. The analyte structure-enantioselectivity relationship indicated that racemic rasagiline has the most appropriate structural characteristics for resolution on the chiral stationary phase.

Bidirectional Chiral Inversion of Trantinterol Enantiomers After Separate Doses to Rats

The chiral inversion and pharmacokinetics of two enantiomers of trantinterol, a new β2 agonist, were studied in rats dosed (+)- or (-)-trantinterol separately. Plasma concentrations of (+)- and (-)-trantinterol were measured by chiral stationary phase liquid chromatography tandem mass spectroscopy (LC-MS/MS). The apparent inversion ratio was calculated as the ratio of AUC0-t of (-)-trantinterol or (+)-trantinterol inverted from their antipodes to the sum of the AUC0-t of (-)- and (+)-trantinterol. Following single intravenous administration, both given enantiomers declined in similar plasma concentrations, suggesting that the two enantiomers have approximately the same disposition kinetics by the route of intravenous administration. However, after single oral administration, plasma concentrations of uninverted (-)-trantinterol at many timepoints were significantly higher than those of uninverted (+)-trantinterol, suggesting that the two enantiomers undergo apparently different absorption or metabolism after oral administration. Significant bidirectional chiral inversion occurred after intravenous and oral administration of (+)- or (-)-trantinterol. After dosing with optically pure enantiomer, the concentration of the administered enantiomer predominated in vivo. The AUC0-36 of (+)-trantinterol after intravenous and oral dosing of (-)-trantinterol were 16.6 ± 5.2 and 33.3 ± 16%, respectively of those of total [(+) + (-)] trantinterol. The AUC0-36 of (-)-trantinterol after intravenous and oral dosing of (+)-trantinterol were 19.6 ± 8.8 and 37.9 ± 4.5%, respectively, of those of total [(-) + (+)] trantinterol. After intravenous administration of (+)- and (-)-trantinterol the chiral inversion ratios of the two enantiomers were not significantly different and similar results were found for oral administration. The extent of chiral inversion after intravenous administration was apparently lower, indicating that the bidirectional chiral inversion was not only systemic but also presystemic.

Comparison of polarimetry and crown ether-based HPLC chiral stationary phase method to determine (l)-amino acid optical purity

Although various pharmacopoeias provide titration methods to assay (l)-amino acid content, none of these methods distinguish between (l)- and (d)-amino acids and do not consider the presence of enantiomeric impurities. Consequently, these methods are limited in scope to describe the relationship between content and specific rotation, [α]. In this study, the US Pharmacopoeia method was compared with the crown ether-based high performance liquid chromatographic (HPLC) chiral stationary phase (CSP) method to determine (l)-amino acid content and specific rotation. The (l)-amino acid content specified by the US Pharmacopoeia method was not consistent with the specific rotation in the presence of enantiomeric impurities, whereas the HPLC-CSP method was very effective for determining the (l)-amino acid content and the optical purity. The other advantage is that the HPLC-CSP method requires amino acid samples of quite low concentration (as low as 1μg/mL), whereas the pharmacopoeia method requires higher concentrations (20–110mg/mL).

Resolution of Aryl α-Aminoalkyl Ketones on a Doubly Tethered Liquid Chromatographic Chiral Stationary Phase Based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic Acid

A doubly tethered chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied to the resolution of various aryl <TEX>${\alpha}$</TEX>-aminoalkyl ketones with the use of 80% ethanol in water containing 10 mM sulfuric acid as a mobile phase. The chiral resolution was quite successful, the separation factors (<TEX>${\alpha}$</TEX>) and the resolutions (<TEX>$R_S$</TEX>) being in the range of 1.39-2.05 and 3.18-5.22, respectively. The separation factors (<TEX>${\alpha}$</TEX>) on the doubly tethered CSP were slightly worse than those on the corresponding singly tethered CSP. However, the resolutions (<TEX>$R_S$</TEX>) on the doubly tethered CSP were generally greater than those on the corresponding singly tethered CSP. The chromatographic behaviors for the resolution of aryl <TEX>${\alpha}$</TEX>-aminoalkyl ketones on the doubly tethered CSP were demonstrated to be dependent on the type and the content of the organic and acidic modifiers in aqueous mobile phase and the column temperature.

Liquid chromatographic resolution of 1-aryl-1,2,3,4-tetrahydroisoquinolines on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

A liquid chromatographic chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied for the first time to the resolution of biologically important 1-aryl-1,2,3,4-tetrahydroisoquinolines. The unusual resolution of cyclic secondary amino compounds on a chiral crown ether-based CSP was quite successful with the use of a mixture of methanol–acetonitrile–triethylamine at a ratio of 30/70/0.5 (v/v/v) as a mobile phase. From the chromatographic behaviours for the resolution of seven 1-aryl-1,2,3,4-tetrahydroisoquinolines, the steric bulkiness of the 1-phenyl ring at the chiral center of analytes was concluded to play an important role in the chiral recognition.

Development of Vancomycin-Capped β-CD-Bonded Silica Particles as Chiral Stationary Phase for LC

Vancomycin-capped (3-(2-O-β-cyclodextrin)-2-hydroxypropoxy)-propylsilyl-appended silica particles (VCD-HPS), a new type of substituted β-cyclodextrin-bonded chiral stationary phase (CSP) for liquid chromatography (LC), have been synthesized by treatment of bromoacetate-substituted-(3-(2-O-β-cyclodextrin)-2-hydroxypropoxy)-propylsilyl-appended silica particles (BACD-HPS) with vancomycin in anhydrous methanol. The stationary phase is characterized by elemental analysis. This new CSP has a chiral selector with two recognition sites: vancomycin and β-cyclodextrin (β-CD), which can provide multiple interactions with the solutes. The chromatographic performance of VCD-HPS was studied with several disubstituted benzenes and some chiral drug compounds as solutes in reversed-phase LC. The results show that VCD-HPS has excellent selectivity for the separation of aromatic positional isomers and enantiomers of chiral compounds.

Separation mechanism of chiral compounds in chiral stationary phase liquid chromatography

In this paper, the concept of reversed- or normal-phase chiral stationary phase liquid chromatography has been put forward according to the polar strength of mobile and stationary phases. The statistical model developed in HPLC has been used to investigate the separation mechanism of D- and L-enantiomer in chiral stationary phase liquid chromatography. It has been observed that the variation of capacity factor of enantiomers with mobile phase composition in both reversed-phase and normal-phase chiral stationary phase liquid chromatography can be described by the fundamental elution equation lnk' = a + blnCb + cCb. The effect of mobile phase composition on the selectivity of enantiomers D and L in normal-phase chiral stationary phase liquid chromatography can be described by the equation lnα = Δa + ΔblnCb, but in reversed-phase chiral stationary phase liquid chromatography the selectivity is almost independant of the mobile phase composition.

Liquid chromatographic direct resolution of flecainide and its analogs on a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Flecainide, an antiarrythmic agent, and its analogs were resolved on a high performance liquid chromatographic chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid with the use of a mobile phase consisting of methanol-acetonitrile-trifluoroacetic acid-triethylamine (80/20/0.1/0.3, v/v/v/v). The chiral resolution was quite successful, the separation factors (alpha) and the resolutions (R(S)) for 20 analytes including flecainide being in the range of 1.19-1.82 and 1.73-6.80, respectively. The ortho-substituent of the benzoyl group of analytes was found to cause decrease in the retention times of analytes probably because of the conformational deformation of analytes originated from the steric hindrance exerted by the ortho-substituent.

Preparation and enantioseparation characteristics of three chiral stationary phases based on modified β-cyclodextrin for liquid chromatography

In order to study the effect of the nature and the length of the spacer, three mixed 10-undecenoate/phenylcarbamate derivatives of beta-cyclodextrin have been prepared and linked to allylsilica gel by means of a radical reaction. The chiral recognition ability of the resulting materials, when used as liquid chromatography chiral stationary phases (CSPs), was evaluated using heptane and either 2-propanol or chloroform as organic mobile-phase modifiers. A large variety of racemic compounds have been separated successfully on these CSPs (mainly pharmaceuticals and herbicides). Optimization of these separations was discussed in terms of mobile-phase composition and structural patterns of the injected analytes. The efficiencies of the three prepared materials were compared to those of previously described perphenylated-beta-cyclodextrin column and to analogous cellulose derivative-based CSPs.

Preparation of a New π-Basic Chiral Stationary Phase Based on Cefaclor and the Liquid Chromatographic Resolution of π-Acidic Analytes

A new liquid chromatographic chiral stationary phase (CSP) was prepared by bonding N-(3,5-dimethoxybenzoyl)cefaclor to spherical silica gel. The new CSP was successfully utilized for the resolution of racemic N-(3,5-dinitrobenzoyl)-α-amino acid derivatives and racemic 3-substituted 1,4-benzodiazepin-2-ones. For the resolution of racemic N-(3,5-dinitrobenzoyl)-α-amino acid derivatives on the CSP, a chiral recognition mechanism utilizing the enantioselective π−π donor–acceptor interaction between the π-basic 3,5-dimethoxybenzoyl group of the CSP and the π-acidic 3,5-dinitrobenzoyl group of analytes was proposed.

Preparation and Application of a New Liquid Chromatographic Chiral Stationary Phase Based on Cefaclor

A new HPLC chiral stationary phase (CSP) was prepared by bonding N-(3,5-dinitrobenzoyl)cefaclor to silica gel. The effective π-π donor-acceptor interaction between the CSP and analytes was demonstrated to be important for the chiral recognition by applying the new CSP to the resolution of N-(substituted benzoyl)leucine ethyl esters and N-(substituted benzoyl)leucine propyl amides. The new CSP was also successfully applied to the resolution of anilide derivatives of N-acetyl-α-amino acids, anilide derivatives of Oethoxycarbonyl-2-hydroxycarboxylic acids and 3-substituted 1,4-benzodiazepin-2-ones.

Preparation and Application of a New Ion-Pairing Chiral Stationary Phase for the Liquid Chromatographic Resolution of N-(3,5-Dinitrobenzoyl)-α-amino Acids

A liquid chromatographic chiral stationary phase (CSP) was prepared starting from (1S, 2S)-1,2-diaminocyclohexane. The CSP containing two simple functional groups, such as a primary amino group and an ureide tethering linkage on a cyclohexane ring, was quite successful with the use of 60% 2-propanol in hexane containing 0.5% acetic acid as a mobile phase for the resolution of various N-(3,5-dinitrobenzoyl)-α-amino acids, the separation (α) and the resolution factors (R S ) being in the range of 1.12–1.23 and 1.33–2.99, respectively. In contrast, N-(3,5-dinitrobenzoyl)-α-amino acid ethyl esters and N-(3,5-dinitrobenzoyl)-α-amino acid amides were not resolved at all, or resolved very poorly on the CSP. From these results, it was concluded that the ionic or ion-paring interaction between the CSP and analytes plays a very important role for the chiral recognition. The chromatographic behaviors for the resolution of N-(3,5-dinitrobenzoyl)-α-amino acids on the CSP were found to be controlled with the variation of the type and the content of alcohol and/or acid in hexane.

Liquid chromatographic resolution of vigabatrin and its analogue γ-amino acids on chiral stationary phases based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

Racemic vigabatrin, an anticonvulsant drug used for the treatment of epilepsy, and its analogue γ-amino acids were resolved without derivatization on liquid chromatographic chiral stationary phases (CSPs) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. Between the two CSPs which contain 3 methylene-unit or 11 methylene-unit spacer group, the latter was found to be greater than the former in the resolution of vigabatrin and its analogue γ-amino acids, the separation, α, and the resolution factor, R S, for the resolution of vigabatrin on the latter being 1.91 and 4.57, respectively. The chromatographic behaviors for the resolution of vigabatrin and its analogue γ-amino acids on the two CSPs were found to be dependent on the type and the content of organic and acidic modifiers in aqueous mobile phase.