Phenylcarbamoylated β‐CD: π‐Acidic and π‐basic chiral selectors for HPLC

Two types of chiral stationary phases for HPLC based on π‐acidic or π‐basic perphenylcarbamoylated β‐CDs were synthesized. The relative structural features of the two effective chiral selectors are discussed and compared in both normal‐phase and RP modes. In addition, the nature and concentration of alcoholic modifiers were varied for optimal separation in normal phase and the structural variation of the analytes was also examined. The results showed that hydrogen bonding, steric effect and π‐acidic–π‐basic interaction contributed greatly to enantioseparation. Upon comparison, some of the differences in the separation behavior of the two types of chiral stationary phases might be due to the π‐acidic or π‐basic phenylcarbamate groups.     

Recent development trends for chiral stationary phases based on chitosan derivatives,cyclofructan derivatives and chiral porous materials in high performance liquid chromatography

The separation of enantiomers by chromatographic methods, such as gas chromatography, high‐performance liquid chromatography and capillary electrochromatography, has become an increasingly significant challenge over the past few decades due to the demand of pharmaceutical, agrochemical, and food analysis. Among these chromatographic resolution methods, high‐performance liquid chromatography based on chiral stationary phases has become the most popular and effective method used for the analytical and preparative separation of optically active compounds. This review mainly focuses on the recent development trends for novel chiral stationary phases based on chitosan derivatives, cyclofructan derivatives, and chiral porous materials that include metal‐organic frameworks and covalent organic frameworks in high‐performance liquid chromatography. The enantioseparation performance and chiral recognition mechanisms of these newly developed chiral selectors toward enantiomers are discussed in detail.     

Last ten years (2008–2018) of chiral ligand‐exchange chromatography in HPLC: An updated review

Chiral ligand‐exchange chromatography is one of the elective strategies for the direct enantioresolution of small chelating compounds: amino acids, diamines, amino alcohols, diols, small peptides, etc. Unlike other methods, the interaction between chiral selector and analyte enantiomers is mediated by a cation, thus producing diastereomeric ternary complexes. Two main approaches are conventionally applied in chiral ligand‐exchange chromatography. The first relies upon chiral stationary phases where the chiral selector is either covalently immobilized or physically adsorbed onto suitable packing materials (coated phases). In the second approach, chiral molecules are added to the eluent, thus generating chiral eluent systems. Among the advantages of chiral ligand‐exchange chromatography, the generation of UV/vis‐active metal complexes, and the use of commercially available or easy‐to‐synthesize chiral selectors, in combination to rather inexpensive achiral columns for coated phases and chiral eluents, are noteworthy. Besides amino acids and amino alcohols, other species have proven suitable for chiral ligand‐exchange chromatography applications. Recently, the use of either chiral ionic liquids or micellar liquid chromatography systems as well as the successful off‐column formation of diastereomeric complexes have expanded the selectivity profiles and application fields. All of these issues are touched in the review, shedding light to the contributions appeared in the last decade.     

Adjusting the chromatographic properties of poly(ionic liquid)‐modified stationary phases by substitution on the imidazolium cation

Poly(ionic liquid)‐modified stationary phases can have multiple interactions with solutes. However, in most stationary phases, separation selectivity is adjusted by changing the poly(ionic liquid) anions. In this work, two poly(ionic liquid)‐modified silica stationary phases were prepared by introducing the cyano or tetrazolyl group on the pendant imidazolium cation on the polymer chains. Various analytes were selected to investigate their mechanism of retention in the stationary phases using different mobile phases. Two poly(ionic liquid)‐modified stationary phases can provide various interactions toward solutes. Compared to the cyano‐functionalized poly(ionic liquid) stationary phase, the tetrazolyl‐functionalized poly(ionic liquid) stationary phase provides additional cation‐exchange and π‐π interactions, resulting in different separation selectivity toward analytes. Finally, applicability of the developed stationary phases was demonstrated by the efficient separation of nonsteroidal anti‐inflammatory drugs.     

Comparison of performance of Chirobiotic T, T2 and TAG columns in the separation of beta2- and beta3-homoamino acids

The enantiomers of eight unusual beta(2)- and beta(3)-homoamino acids were separated on chiral stationary phases containing the macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T or T2) or teicoplanin aglycone (Chirobiotic TAG) as chiral selectors. The effects of the organic modifier, the mobile phase composition, and temperature on the separations were investigated. Linear van't Hoff plots were observed in the studied temperature range, 280-318 K, and the changes in enthalpy, Delta(DeltaH(o)), entropy, Delta(DeltaS(o)), and free energy, Delta(DeltaG(o)) were calculated. The values of the thermodynamic parameters depended on the nature of the selectors, the structures of the analytes, and especially the positions of the substituents on the analytes. A comparison of the separation performances of the macrocyclic glycopeptide stationary phases revealed that the Chirobiotic TAG column exhibited much better selectivity for beta(2)-homoamino acids, while the separation of beta(3)-homoamino acid enantiomers was better on Chirobiotic T or T2. The elution sequence was determined in some cases and was observed to be R < S.     

Chiral high‐performance liquid and supercritical fluid chromatographic enantioseparations of limonene‐based bicyclic aminoalcohols and aminodiols on polysaccharide‐based chiral stationary phases

Enantioseparation of limonene‐based bicyclic 1,3‐aminoalcohols and 1,3,5‐ and 1,3,6‐aminodiols was performed by normal‐phase high‐performance liquid chromatographic and supercritical fluid chromatographic (SFC) methods on polysaccharide‐based chiral stationary phases. The effects of the composition of the mobile phase, the column temperature and the structures of the analytes and chiral selectors on retention and selectivity were investigated by normal‐phase LC and SFC technique. Thermodynamic parameters derived from selectivity–temperature‐dependence studies were found to be dependent on the chromatographic method applied, the nature of the chiral selector and the structural details of the analytes. Enantiorecognition in most cases was enthalpically driven but an unusual temperature behavior was also observed: decreased retention times were accompanied by improved separation factors with increasing temperature, i.e. some entropically driven separations were also observed. The elution sequence was determined in all cases. The separation of the stereoisomers was optimized in both chromatographic modalities.     

A novel C2 symmetric chiral stationary phase with N‐[(4‐Methylphenyl)sulfonyl]‐l‐leucine as chiral side chains

In this study, a series of chiral stationary phases based on N‐[(4‐methylphenyl)sulfonyl]‐l ‐leucine amide, whose enantiorecognition property has never been studied, were synthesized. Their enantioseparation abilities were chromatographically evaluated by 67 enantiomers. The chiral stationary phase derived from N‐[(4‐methylphenyl)sulfonyl]‐l ‐leucine showed much better enantioselectivities than that based on N‐(4‐methylbenzoyl)‐l ‐leucine amide. The construction of C₂ symmetric chiral structure greatly improved the enantiorecognition performance of the stationary phase. The C₂ symmetric chiral stationary phase exhibited superior enantioresolutions to other chiral stationary phases for most of the chiral analytes, especially for the chiral analytes with C₂ symmetric structures. By comparing the enantioseparations of the enantiomers with similar structures, the importance of hydrogen bond interaction, π–π interaction, and steric hindrance on enantiorecognition was elucidated. The enantiorecognition mechanism of trans‐N,N′‐(1,2‐diphenyl‐1,2‐ethanediyl)bis‐acetamide, which had an excellent separation factor on the C₂ symmetric chiral stationary phase, was investigated by ¹H‐NMR spectroscopy and 2D ¹H‐¹H nuclear overhauser enhancement spectroscopy.     

Comparison of Separation Efficiency of Macrocyclic Glycopeptide-Based Chiral Stationary Phases for the LC Enantioseparation of β-Amino Acids

Direct reversed-phase high-performance liquid chromatographic methods were developed for the separation of enantiomers of eighteen unnatural β-amino acids, including several β-3-homo-amino acids. The direct separations of the underivatized analytes were performed on chiral stationary phases containing macrocyclic glycopeptide antibiotics such as teicoplanin (Chirobiotic T and T2), teicoplanin aglycone (Chirobiotic TAG), vancomycin (Chirobiotic V and V2), and ristocetin A (Chirobiotic R) as chiral selectors. The effects of the organic modifier, mobile phase composition and pH on the separations were investigated. A comparison of the separation performances of the macrocyclic glycopeptide stationary phases revealed that the Chirobiotic T2 column exhibited better selectivity than the Chirobiotic T column for the separation of β-3-homo-amino acid enantiomers; vancomycin or ristocetin A exhibited lower selectivity. The elution sequence was determined in some cases: the S enantiomers eluted before the R enantiomers, with the exception of the Chirobiotic R column, where the elution sequence R < S was observed.     

Structure–property relationship study of the HPLC enantioselective retention of neuroprotective 7‐[(1‐alkylpiperidin‐3‐yl)methoxy]coumarin derivatives on an amylose‐based chiral stationary phase

The enantiomer separation of a number of racemic 7‐[(1‐alkylpiperidin‐3‐yl)methoxy]coumarin derivatives, some of which show outstanding in vitro multitarget neuroprotective activities, was successfully achieved on a polysaccharide‐based chiral stationary phase, bearing amylose tris(3,5‐dimethylphenylcarbamate) as a chiral selector, in normal polar mode (methanol and acetonitrile as the mobile phases). The majority of the screened selectands, especially those bearing 1‐(3‐X‐benzyl)piperidin‐3‐yl moieties, showed baseline enantiomer separations, and compound 8 (X = NO₂) was the best resolved (α = 2.01; R_S = 4.27). Linear free energy relationships, usefully complemented by molecular docking calculations, have the key role in enantioselective retention of aromatic interactions between π‐donor moieties in the chiral selector and π‐acceptor moieties in selectand, strengthened by hydrogen bond interaction between a hydrogen bond donor in the chiral selector and the hydrogen bond acceptor group(s) in the selectand. Statistically, reliable equations highlighted the importance of the substituent's size and substitution pattern (meta better than para) to affect the enantiorecognition of the title compounds. The chromatographic data support the scalability of the optimized experimental conditions for preparative purposes.     

Separation of ofloxacin enantiomers by capillary electrophoresis with fluorescence detection using deoxyribonucleic acid oligonucleotides as chiral selectors

This study used capillary electrophoresis with fluorescence detection- and a partial-filling mode-based method for chiral separation of ofloxacin. The deoxyribonucleic acid oligonucleotides with different base sequences were studied as potential chiral selectors including deoxyribonucleic acid tetrahedron, G-quadruplex, and G-riched double-strand deoxyribonucleic acid. Under the optimized conditions, all the deoxyribonucleic acid chiral selectors exhibited excellent chiral separation capabilities with a resolution higher than 1.5. The electrophoretic behavior of the ofloxacin enantiomer might result from the intermediate conjugate with different stabilities between chiral selectors and analytes by a combination of the hydrogen bond and spatial recognition structure. Moreover, satisfactory repeatability regarding run-to-run and interday repeatability was obtained, and all the relative standard deviation values of migration times and resolutions were below 4% (n = 6). Conclusively, both spatial structure and arrangement of the G bases potentiated the chiral separation capability of deoxyribonucleic acid for ofloxacin enantiomer. This work offered a stepping stone for enantioseparation using deoxyribonucleic acid as chiral selectors.     

Doubly tethered tertiary amide linked and ionically bonded diproline chiral stationary phases

Oligoproline chiral stationary phase (CSP) is a new class peptide chiral stationary phase. Many proline chiral stationary phases with different proline chain lengths and linkers have been prepared and evaluated. However, the doubly tethered and ionic type linkers have not been adequately investigated. In this study, covalently and ionically bonded chiral stationary phases with doubly tethered linker were prepared and characterized. The new covalently bonded doubly tethered diproline CSP was applied successfully to resolve various enantiomers of acidic, basic, and neutral compounds with phenyl, naphthyl, anthryl, or similarly sized groups. The enantiorecognition performances of singly and doubly tethered diproline CSPs were comparable. Variation of the type and content of organic modifiers in hexane or heptane mobile phase showed that the branch alcohols such as 2‐propanol and 2‐butanol, 1,2‐dichloroethane, methyl tert‐butyl ether, and ethyl acetate in the mobile phase enhanced chiral separation. End‐capping on doubly tethered diproline CSP did not always improve the separation factor and resolution. Due to the rigid structure of the double tether, the enantioseparation ability of ionically bonded diproline CSP was well expressed to some analytes.     

Recent advances in peptide chiral selectors for electrophoresis and liquid chromatography

The application of peptides in chiral separations using techniques such as capillary electrophoresis (CE), electrokinetic capillary chromatography (EKC) and liquid chromatography is the focus of this review. Methods for finding peptide selectors using combinatorial library approaches are discussed, as well as recent advances in the use of peptides as general chiral selectors for electrophoresis and liquid chromatography. One example shows the effectiveness of polymeric dipeptide surfactants as general chiral selectors for electrophoresis. Another example shows the versatility of oligoproline chiral stationary phases, exhibiting resolution for a number of racemic analytes comparable to other well-established chiral stationary phases.     

Comparative Enantioseparation of Seven Amino Alcohols on Teicoplanin‐Based Chiral Stationary Phases

Abstract

The macrocyclic antibiotics represent a relatively new class of chiral selectors in separation science and teicoplanin‐based chiral stationary phases (CSP) have been used successfully in a number of applications in high‐performance liquid chromatography. In the present studies, we self‐prepared two bonded CSPs–teicoplanin (TE) and teicoplanin phenyl isocyanate (TE‐Phe). Seven amino alcohols, propranolol, bisoprolol fumarate, atenolol, salbutamol, isoproterenol, metoprolol, and labetalol were enantioseparated on both self‐made CSPs using methanol as mobile phase and acetic acid (HOAc) and triethylamine (TEA) as mobile phase additives. On both CSPs, the different enantioseparation behavior of analytes with different structure was compared. The influence of the concentration of mobile phase additives (HOAc and TEA) on the enantioseparation was investigated. In all conditions, the retention factors (k′) of seven analytes on TE‐Phe CSP were larger than that on TE CSP. However, the separation factors (α) and resolutions (Rs) on TE‐Phe CSP were smaller than that on TE CSP. The results indicated that the derivatized TE‐Phe CSP is not efficient as original teicoplanin CSP. Our observations also suggested that, for teicoplanin‐based CSPs, π‐π interactions and dipole‐dipole between solutes and CSPs mainly contribute to the retention of solutes on CSPs while hydrogen bonding and steric interactions play important roles in the chiral recognition for teicoplanin‐based CSPs.     

Chiral separation of phenylalanine and tryptophan by capillary electrophoresis using a mixture of β‐CD and chiral ionic liquid ([TBA] [l‐ASP]) as selectors

In this paper, a simple, effective and green capillary electrophoresis separation and detection method was developed for the quantification of underivatized amino acids (dl ‐phenylalanine; dl ‐tryptophan) using β‐Cyclodextrin and chiral ionic liquid ([TBA] [l ‐ASP]) as selectors. Separation parameters such as buffer concentrations, pH, β‐CD and chiral ionic liquid concentrations and separation voltage were investigated for the enantioseparation in order to achieve the maximum possible resolution. A good separation was achieved in a background electrolyte composed of 15 mm sodium tetraborate, 5 mm β‐CD and 4 mm chiral ionic liquid at pH 9.5, and an applied voltage of 10 kV. Under optimum conditions, linearity was achieved within concentration ranges from 0.08 to 10 µg/mL for the analytes with correlation coefficients from 0.9956 to 0.9998, and the analytes were separated in less than 6 min with efficiencies up to 970,000 plates/m. The proposed method was successfully applied to the determination of amino acid enantiomers in compound amino acids injections, such as 18AA‐I, 18AA‐II and 3AA. Copyright © 2013 John Wiley & Sons, Ltd.     

Photografted methacrylate‐based monolithic columns coated with cellulose tris(3,5‐dimethylphenylcarbamate) for chiral separation in CEC

A chiral capillary monolithic column for enantiomer separation in capillary electrochromatography was prepared by coating cellulose tris(3,5‐dimethylphenylcarbamate) on porous glycidyl methacrylate‐co‐ethylene dimethacrylate monolith in capillary format grafted with chains of [2(methacryloyloxy)ethyl] trimethylammonium chloride. The surface modification of the monolith by the photografting of [2(methacryloyloxy)ethyl] trimethylammonium chloride monomer as well as the coating conditions of cellulose tris(3,5‐dimethylphenylcarbamate) onto the grafted monolithic scaffold were optimized to obtain a stable and reproducible chiral stationary phase for capillary electrochromatography. The effect of organic modifier (acetonitrile) in aqueous mobile phase for the enantiomer separation by capillary electrochromatography was also investigated. Several pairs of enantiomers including acidic, neutral, and basic analytes were tested and most of them were partially or completely resolved under aqueous mobile phases. The prepared monolithic chiral stationary phases exhibited a good stability, repeatability, and column‐to‐column reproducibility, with relative standard deviations below 11% in the studied electrochromatographic parameters.     

双手性选择单元手性固定相的研究进展

手性固定相（chiral stationary phase,CSP）作为手性色谱分离的核心技术,在手性化合物的识别和分离中得到广泛应用。以双手性选择单元结合作为CSP是近些年的研究热点,研究表明,两种手性选择单元相结合的CSP可增加手性识别位点,显著提高分离效果。本文介绍了近几年双手性选择单元手性固定相在手性分离中的研究进展,并对其发展前景进行了展望。     

Comparison of HPLC enantioseparation of substituted binaphthyls on CD‐, polysaccharide‐ and synthetic polymer‐based chiral stationary phases

Retention and enantioseparation behavior of ten 2,2′‐disubstituted or 2,3,2′‐trisubstituted 1,1′‐binaphthyls and 8,3′‐disubstituted 1,2′‐binaphthyls, which are used as catalysts in asymmetric synthesis, was investigated on eight chiral stationary phases (CSPs) based on β‐CD, polysaccharides (tris(3,5‐dimethylphenylcarbamate) cellulose or amylose CSPs) and new synthetic polymers (trans‐1,2‐diamino‐cyclohexane, trans‐1,2‐diphenylethylenediamine and trans‐9,10‐dihydro‐9,10‐ethanoanthracene‐(11S,12S)‐11,12‐dicarboxylic acid CSPs). Normal‐, reversed‐phase and polar‐organic separation modes were employed. The effect of the mobile phase composition was examined. The enantiomeric separation of binaphthyl derivatives, which possess quite similar structures, was possible in different enantioselective environments. The substituents and their positions on the binaphthyl skeleton affect their properties and, as a consequence, the separation system suitable for their enantioseparation. In general, the presence of ionizable groups on the binaphthyl skeleton, substitution with non‐identical groups and a chiral axis in the 1,2′ position had the greatest impact on the enantiomeric discrimination. The 8,3′‐disubstituted 1,2′‐binaphthyl derivatives were the most easily separated compounds in several separation systems. From all the chiral stationary phases tested, cellulose‐based columns were shown to be the most convenient for enantioseparation of the studied analytes. However, the polymeric CSPs with their complementary behavior provided good enantioselective environments for some derivatives that could be hardly separated in any other chromatographic system.     

Chiral stationary phases based on chitosan bis(4‐methylphenylcarbamate)‐(alkoxyformamide)

Highly N‐deacetylated chitosan was chosen as a natural chiral origin for the synthesis of the selectors of chiral stationary phases. Therefore, chitosan was firstly acylated by various alkyl chloroformates yielding chitosan alkoxyformamides, and then these resulting products were further derivatized with 4‐methylphenyl isocyanate to afford chitosan bis(4‐methylphenylcarbamate)‐(alkoxyformamide). A series of chiral stationary phases was prepared by coating these derivatives on 3‐aminopropyl silica gel. The content of the derivatives on the chiral stationary phases was nearly 20% by weight. The chiral stationary phases prepared from chitosan bis(4‐methylphenylcarbamate)‐(ethoxyformamide) and chitosan bis(4‐methylphenylcarbamate)‐(isopropoxyformamide) comparatively showed better enantioseparation capability than those prepared from chitosan bis(4‐methylphenylcarbamate)‐(n‐pentoxyformamide) and chitosan bis(4‐methylphenylcarbamate)‐(benzoxyformamide). The tolerance against organic solvents of the chiral stationary phase of chitosan bis(4‐methylphenylcarbamate)‐(ethoxyformamide) was investigated, and the results revealed that this phase can work in 100% ethyl acetate and 100% chloroform mobile phases. Because as‐synthesized chiral selectors did not dissolve in many common organic solvents, the corresponding chiral stationary phases can be utilized in a wider range of mobile phases in comparison with conventional coating type chiral stationary phases of cellulose and amylose derivatives.     

Novel chiral stationary phases based on peptoid combining a quinine/quinidine moiety through a C9‐position carbamate group

By connecting a quinine or quinidine moiety to the peptoid chain through the C9‐position carbamate group, we synthesized two new chiral selectors. After immobilizing them onto 3‐mercaptopropyl‐modified silica gel, two novel chiral stationary phases were prepared. With neutral, acid, and basic chiral compounds as analytes, we evaluated these two stationary phases and compared their chromatographic performance with chiral columns based on quinine tert‐butyl carbamate and the previous peptoid. From the resolution of neutral and basic analytes under normal‐phase mode, it was found that the new stationary phases exhibited much better enantioselectivity than the quinine tert‐butyl carbamate column; the peptoid moiety played an important role in enantiorecognition, which controlled the elution orders of enantiomers; the assisting role of the cinchona alkaloid moieties was observed in some separations. Under acid polar organic phase mode, it was proved that cinchona alkaloid moieties introduced excellent enantiorecognitions for chiral acid compounds; in some separations, the peptoid moiety affected enantioseparations as well. Overall, chiral moieties with specific enantioselectivity were demonstrated to improve the performance of peptoid chiral stationary phase efficiently.     

Evaluation of silica from different vendors as the solid support of anion‐exchange chiral stationary phases by means of preferential sorption and liquid chromatography

Chromatographic performance of a chiral stationary phase is significantly influenced by the employed solid support. Properties of the most commonly used support, silica particles, such as size and size distribution, and pore size are of utmost importance for both superficially porous particles and fully porous particles. In this work, we have focused on evaluation of fully porous particles from three different vendors as solid supports for a brush‐type chiral stationary phase based on 9‐O‐tert‐butylcarbamoyl quinidine. We have prepared corresponding stationary phases under identical experimental conditions and determined the parameters of the modified silica by physisorption measurements and scanning electron microscopy. Enantiorecognition properties of the chiral stationary phases have been studied using preferential sorption experiments. The same material was slurry‐packed into chromatographic columns and the chromatographic properties have been evaluated in liquid chromatography. We show that preferential sorption can provide valuable information about the influence of the pore size and total pore volume on the interaction of analytes of different size with the chirally‐modified silica surface. The data can be used to understand differences observed in chromatographic evaluation of the chiral stationary phases. The combination of preferential sorption and liquid chromatography separation can provide detailed information on new chiral stationary phases.