Comparison of the factors that contribute to retention on immobilized polysaccharide-based chiral stationary phases and macrocyclic glycopeptide chiral stationary phases with the Abraham model

The Abraham model of linear solvation energy relationship (LSER) was utilized to characterize three recently commercialized chiral stationary phases (CSPs), the Chiralpak IA, IB and IC. Normal phase system constants were determined by HPLC for these three CSPs and compared to literature values for the Chirobiotic T and V CSPs. The results indicate that the Chirobiotic T and V CSPs participate in more polar interactions, such as hydrogen bonding and dipolar interactions, than the three immobilized derivatized polysaccharide CSPs. Additionally, differences were noted for the e and b terms of the Abraham model (polarizable interactions and hydrogen bond acidity) for the IA and IB CSPs, which are nominally similar CSPs in their chemical makeup.     

A new class of network-polymeric chiral stationary phases

A strategy based on the use of homo bi- and multifunctional building blocks for the synthesis of a new class of network-polymeric chiral stationary phases has been evaluated. The key steps comprise acylation of N,N′-diallyl-L-tartardiamide (DATD) and reaction with a multifunctional hydrosilane, yielding a network polymer incorporating the bifunctional C₂-symmetric chiral selector. Covalent bonding to a functionalized silica takes place during the latter process. Many of these chiral sorbents show interesting enantioselective properties toward a wide variety of racemic solutes under normal-phase (hexane-based) conditions. The retention is mainly caused by the hydrogen-bonding ability of the analyte, which is regulated by mobile phase additives like alcohol or ether cosolvents. The most interesting chiral stationary phases, in terms of broad enantioselectivity, were obtained from O,O′-diaryol-DATD-derivatives, particularly those containing the 3,5-dimethylbenzoyl and the 4-(tert-butyl)benzoyl moieties. Since high column efficiencies can be obtained with these chiral sorbents, an α-value of ca. 1.2 is usually sufficient to produce baseline separation. A large number of neutral as well as acidic or basic drug racemates are resolved without derivatization. © 1995 Wiley-Liss, Inc.     

Chiral discrimination with regioselectively substituted cellulose esters as chiral stationary phases

Four kinds of cellulose derivatives, including two regioselectively substituted cellulose esters (6-O-acetyl-2,3-di-O-benzoyl cellulose and 2,3-di-O-acetyl-6-O-benzoyl cellulose), were synthesized so that the effects of their functional group distribution on their chiral discrimination ability could be examined. The degree of substitution by functional groups appeared to have a critical effect on the separation in most cases, but the type of the functional group at the C-6 position also significantly influenced chiral discrimination when a series of neutral arylalcohol derivatives were used as racemates. Copyright 2000 Wiley-Liss, Inc.     

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.     

Solution-phase synthesis and evaluation of tetraproline chiral stationary phases

A protocol was developed for the solution-phase synthesis of multigram amounts of two 9-fluorenylmethoxycarbonyl (Fmoc)-protected tetraproline peptides. These tetraproline peptides were then attached to amino derivatized silica gel. The replacement of the Fmoc group with the trimethylacetyl group lead to two tetraproline chiral stationary phases (CSPs). A comparison of the chromatographic behavior of these two solution-phase-synthesized tetraproline CSPs with that prepared by stepwise solid-phase synthesis revealed that all three had similar chromatographic performance for resolving 53 model analytes. This suggests that the solution-phase synthesis of oligoprolines, which allows for the specific benefits of good batch reproducibility, selector homogeneity, and possibly low cost, is a feasible alternative to the solid-phase synthesis of oligoproline CSPs.     

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.     

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.     

Recent progresses in protein-based chiral stationary phases for enantioseparations in liquid chromatography

Chiral stationary phases (CSPs) based on proteins orglycoproteins have been developed for the enantioseparations of various compounds. In 2001, a review article [J. Haginaka, J. Chromatogr. A, 906 (2001) 253] dealing with CSPs based on proteins and glycoproteins was published. After that serum albumin from other species, penicillin G-acylase, antibodies, fatty acid binding protein and streptavidin were newly introduced as the chiral selectors in CSPs. This review article deals with recent progresses in CSPs based on protein or glycoproteins in LC after 2001, focusing on their enantioselective properties and chiral recognition mechanisms.     

A comparison of LC and SFC for cellulose- and amylose-derived chiral stationary phases

This paper presents a systematic comparison of liquid chromatography (LC) and supercritical fluid chromatography (SFC) for Chiralcel OD and Chiralpak AD chiral stationary phases (CSPs), performed using various chiral compounds having a known or potential pharmaceutical activity. The chiral recognition mechanisms involved in LC and SFC for the enantiomeric separation of β-blockers have been studied more particularly. As a general rule, it appears that the presence of polar functions, like primary or secondary hydroxyl or amine functions, may result in marked discrepancies in selectivity between LC and SFC. This result is peculiar to cellulose- and amylose-derived CSPs, for which the interactions involved in chiral recognition mechanism are not always well balanced, contrary to what happens for independent CSPs. In the case of chiral resolution of polar solutes or polymer-type CSPs, the analyst should try both the LC and SFC techniques to be able to choose the more stereoselective one. © 1995 Wiley-Liss, Inc.     

Microscale HPLC enables a new paradigm for commercialization of complex chiral stationary phases

The small column size (0.3 mm i.d. x 15 cm) used in microscale HPLC contains only a small fraction (<1%) of the chromatographic packing material of a typical analytical HPLC column. Consequently, chromatographic stationary phases that are prohibitively expensive in conventional HPLC, owing either to synthetic complexity or costly starting materials, may become commercially viable in the microscale format. To illustrate this point, a previously described, synthetically complex, crown ether chiral stationary phase was prepared and evaluated in the microscale format, showing excellent separation of the enantiomers of underivatized amine analytes.     

Use of cellulose-based stationary phases for chiral separation in open tubular column chromatography

Application of cellulose-based chiral stationary phases was extended to open tubular columns. These chiral materials were mixed with achiral matrix stationary phases. Compromises were found among the polarity and the ratio of achiral matrix polymers against the content of the chiral cellulose derivative in order to optimize the resolution of the investigated racemates. In GC, the high efficiency feature of open tubular columns allows fast analysis, however, compounds which express strong H-bond interaction with cellulose derivatives elute with a bad peak shape. The application of these stationary phases for open tubular SFC was more successful, because the solvation power of the mobile phase can compensate the strong interaction between the solute and the cellulose derivative. Immobilization of the stationary phases were achieved for SFC purposes. © 1992 Wiley-Liss, Inc.     

Chromatographic properties of composite chiral stationary phases based on cellulose derivatives

Chiral stationary phases (CSPs) prepared by mixing together two different cellulose derivatives, before or after being coated on macroporous silica gel, were developed in order to determine the mutual influence of two different polymers on global chiral recognition capacity. The chromatographic properties of these CSPs were evaluated using a wide range of racemic test solutes. The mixing method does not significantly affect the enantioselectivities. The composite CSPs obtained by cocoating of two different cellulose derivatives on silica generally exhibit chiral recognition capacities intermediate between those of the two individual phases, and thus broadening the application range of a single column. These results indicate that the simultaneous coating of two different cellulose derivatives does not significantly alter the optical resolution power of each chiral material and are discussed in relationship with the supramolecular structure of the polymeric stationary phases. © 1995 Wiley-Liss, Inc.     

Enantiomeric LC separation of calcium antagonists on protein-based chiral stationary phases

Three chiral calcium antagonist drugs, bepridil and two dihydropyridine derivatives (nicardipine and REC 15/2375), have been successfully separated within short retention times using either the α₁-acid glycoprotein chiral stationary phase (Chiral AGP) or the ovomucoid column (Ultron ES-OVM). Aqueous buffer at defined pH is modified by the addition of an organic component (propan-2-ol, acetonitrile, ethanol) in order to modulate the retention properties of each system. The influence of pH and percentage of organic modifier on retention, selectivity, resolution, and column performance are discussed for bepridil analyzed on Chiral AGP and for the two dihydropyridines (nicardipine and REC 15/2375) analyzed on Ultron ES-OVM stationary phases. © 1993 Wiley-Liss, Inc.     

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.     

Comparative HPLC enantioseparation of ferrocenylalcohols on two cellulose-based chiral stationary phases

The direct HPLC enantiomeric separation of several ferrocenylalcohols on the commercially available Chiralcel OD and Chiralcel OJ columns has been evaluated in normal-phase mode. Almost all the compounds were resolved on one or both chiral stationary phases (CSPs) with separation factor (alpha) ranging from 1.06 to 2.88 while the resolution (R(s)) varied from 0.63 to 12.70 In the separation of the alpha-ferrocenylalcohols 1a-e and the phenyl analogues 2a-e, which were all resolved except 1c, a similar trend in the retention behavior for the two series of alcohols was evidenced and the selectivity was roughly complementary on the two investigated CSP. For three ferrocenylacohols, chosen as model compounds, the influence of the mobile phase composition and temperature on the enantioseparation were investigated and additional information on the chiral recognition mechanism were deduced from the chromatographic behavior of their acetylderivatives.     

Performance of brush-type HPLC chiral stationary phases with tertiary amide in the connecting tether

The replacement of the N-H hydrogen of the secondary amide-tethered Pirkle-concept N-(3,5-dinitrobenzoyl)-L-leucine derived chiral stationary phase with various pi-basic or aliphatic groups improved the chiral discrimination ability of new chiral stationary phases, based on the leucine- or alanine-derived chiral selector, for the enantiomers of various racemic neutral analytes with amide functional groups. Retention times decreased while separation and resolution factors increased, thus proving the role of pi-donor aromatic unit as an electron-rich shield in the front of a silica surface. In general, chiral stationary phase (CSP) 5 with the 3,5-dimethylphenyl unit showed best performance, while CSP 3, with phenyl unit, and CSP 7, with 1-naphthyl unit in the tertiary amide connecting tether, were less efficient.     

Direct HPLC enantioseparation of chiral aptazepine derivatives on coated and immobilized polysaccharide-based chiral stationary phases

The direct HPLC enantioseparation of Mianserin and a series of aptazepine derivatives is accomplished on polysaccharide-based chiral stationary phases (CSPs). The resolutions are performed on the coated-type Chiralcel OD and Chiralpak AD CSPs and on the first commercially available immobilized-type Chiralpak IA CSP, in normal-phase and polar-organic modes. The complete separation of enantiomers of all racemates investigated was successfully achieved under at least one of CSP/eluent combinations employed. Pure alcohols such ethanol or 2-propanol, with a fixed percentage of DEA added, serve as valuable alternatives to the more common n-hexane-based normal-phase eluents in resolution of Mianserin on the AD CSP. In order to study the chiroptical properties of aptazepine derivatives, chromatographic resolutions are carried out at semipreparative scale using Chiralpak AD and Chiralpak IA as CSPs. Nonconventional dichloromethane-based eluents have permitted to expand the chiral resolving ability of the immobilized Chiralpak IA CSP and to perform mg-scale enantioseparations with an analytical-size column. Assignment of the absolute configuration of the separated enantiomers is empirically established by comparing their chiroptical data with those of structurally related Mianserin.     

Normal phase chiral HPLC of methylphenidate: comparison of different polysaccharide-based chiral stationary phases.

A comparison of the enantiomeric resolution of (+/-)-threo-methylphenidate (MPH) (Ritalin) was achieved on different polysaccharide based chiral stationary phases. The mobile phase used was hexane-ethanol-methanol-trifluoroacetic acid (480:9.75:9.75:0.5, v/v/v/v). Benzoic acid and phenol were used as the mobile phase additives for the enantiomeric resolution of MPH on Chiralcel OB column only. The alpha values for the resolved enantiomers were 1.34, 1.29, 1.30, and 1.24 on Chiralpak AD, Chiralcel OD, Chiralcel OB (containing 0.2 mM benzoic acid in mobile phase), and Chiralcel OB (containing 0.2 mM phenol in mobile phase) columns, respectively. The R(s) values were 1.82, 1.53, 1.19, and 1.10 on Chiralpak AD, Chiralcel OD, Chiralcel OB (containing 0.2 mM benzoic acid in mobile phase), and Chiralcel OB (containing 0.2 mM phenol in mobile phase), respectively. The role of benzoic acid and phenol as mobile phase additives is discussed.     

Vancomycin‐based chiral stationary phases for micro‐column liquid chromatography

Vancomycin selectively immobilized to silica via either one of its two amino groups has been investigated and compared with columns made from native vancomycin. The chemical modification of vancomycin prior to immobilization involved protection of one amino group as a 9‐fluorenylmethyl carbamate. The immobilization and the subsequent cleavage of the protecting group was performed on‐column. The types of compounds that can be separated with the vancomycin chiral stationary phases resemble those separated previously by capillary electrophoresis and thin‐layer chromatography. The protected chiral stationary phases were also investigated and in some cases very high enantioselectivity were obtained. One example of this is a separation of thalidomide with an α‐value as high as 5.4. The soft immobilization procedure preserves the structure of native vancomycin, in contrast to other approaches. Good repeatability and stability of the columns have also been obtained. Chirality 11:121–128, 1999. © 1999 Wiley‐Liss, Inc.     

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.