Comparison of enantioseparation of selected benzodiazepine and phenothiazine derivatives on chiral stationary phases based on β‐cyclodextrin and macrocyclic antibiotics

Enantioselective separation of some phenothiazine and benzodiazepine derivatives was studied on six different chiral stationary phases (CSPs) in HPLC. Selected CSPs, with respect to the structure of the separated compounds, were either based on β‐cyclodextrin chiral selectors – underivatized β‐cyclodextrin and hydroxypropyl ether β‐cyclodextrin, or on macrocyclic antibiotics – vancomycin, teicoplanin, teicoplanin aglycone, and ristocetin A. Measurements were carried out in a reversed‐phase separation mode. The influence of mobile phase composition on retention and enantioseparation was studied. Benzodiazepines could be enantioresolved with almost all the chiral stationary phases used, except for the vancomycin‐bonded CSP. Peak coalescence of oxazepam and lorazepam was observed if separation was carried out at laboratory temperature. Reduced temperature was required in some instances in order to avoid the on‐column racemization. Separation systems composed of teicoplanin‐bonded CSP and buffer‐methanolic or pure methanolic mobile phases were shown to be suitable even for preparative purposes due to high resolution values of the enantiomers. Enantioseparation of phenothiazine derivatives was more difficult to achieve but it was successful, at least partly, also with both types of the CSPs used (except for levomepromazine).     

Separation of chiral sulfoxides by liquid chromatography using macrocyclic glycopeptide chiral stationary phases

A set of 42 chiral compounds containing stereogenic sulfur was prepared. There were 31 chiral sulfoxide compounds, three tosylated sulfilimines and eight sulfinate esters. The separations were done using five different macrocyclic glycopeptide chiral stationary phases (CSPs), namely ristocetin A, teicoplanin, teicoplanin aglycone (TAG), vancomycin and vancomycin aglycone (VAG) and seven eluents, three normal-phase mobile phases, two reversed phases and two polar organic mobile phases. Altogether the macrocyclic glycopeptide CSPs were able to separate the whole set of the 34 sulfoxide enantiomers and tosylated derivatives. Five of the eight sulfinate esters were also separated. The teicoplanin and TAG CSPs were the most effective CSPs able to resolve 35 and 33 of the 42 compounds. The three other CSPs each were able to resolve more than 27 compounds. The normal-phase mode was the most effective followed by the reversed-phase mode with methanol-water mobile phases. Few of these compounds could be separated in the polar organic mode with 100% methanol mobile phases. Acetonitrile was also not a good solvent for the resolution of enantiomers of sulfur-containing compounds, neither in the reversed-phase nor in the polar organic mode. The structure of the chiral molecules was compared to the enantioselectivity factors obtained with the teicoplanin and TAG CSP. It is shown that the polarity, volume and shape of the sulfoxide substituents influence the solute enantioselectivity factor. Changing the oxidation state of the sulfur atom from sulfoxides to sulfinate esters is detrimental to the compound's enantioselectivity. The enantiomeric retention order on the teicoplanin and TAG CSPs was very consistent: the (S)-(+)-sulfoxide enantiomer was always the less retained enantiomer. In contrast, the (R)-(-)-enantiomer was less retained by the ristocetin A, vancomycin and vancomycin aglycone columns, showing the complementarity of these CSPs. The macrocyclic glycopeptide CSPs provided broad selectivity and effective separations of chiral sulfoxides.     

Enantioseparation of vesamicol and novel vesamicol analogs by high-performance liquid chromatography on different chiral stationary phases

High-performance liquid chromatography enantioseparation of vesamicol and six novel azaspirovesamicols (amino alcohols) was accomplished on different chiral stationary phases (CSPs) by using an optical rotation based chiral detector for identification of the resolved enantiomers. The Pirkle-type column Reprosil Chiral-NR was found to be most suitable for chiral resolution in normal phase (NP) mode; all compounds could be enantioseparated successfully. Also the cellulose-based column Reprosil Chiral-OM showed appropriate separation properties by using NP conditions. The amylose-type column Reprosil Chiral-AM-RP was most suitable for enantioseparation in reversed phase (RP) mode; five out of seven compounds were resolved. This CSP showed a considerably higher capability for chiral recognition of vesamicol derivatives in RP mode than the corresponding cellulose-based column Reprosil Chiral-OM-RP. Enantioseparation with the teicoplanin aglycone-based column Reprosil Chiral-AA was successful under polar ionic mobile phase conditions.     

Comparison of vancomycin-based stationary phases with different chiral selector coverage for enantioselective separation of selected drugs in high-performance liquid chromatography

Two vancomycin-based chiral stationary phases (CSPs) with different coverage of the chiral selector vancomycin (Chirobiotic V and Chirobiotic V2) were compared. beta-Blockers and profens, as structurally diverse groups of drugs, were chosen as analytes. Retention and enantioseparation of beta-blockers were studied in reversed-phase (RP) and polar-organic (PO) separation modes. Higher retention and better enantioresolution were obtained on the CSP with higher coverage of vancomycin in the both separation modes. Baseline separation of four beta-blockers (eight enantiomers) in the PO mode was achieved on the Chirobiotic V2 column within 15 min. The enantioseparation of profens did not bring so excellent and easy to interpret results. Higher retention of profens on the Chirobiotic V2 column was not always accompanied by an improvement of their chiral separation in the RP mode. The polar-organic mode was not suitable for these derivatives at all. The most interesting result was obtained with flobufen; its chiral center is further away from the rigid part of the molecule, which mostly causes difficulties in enantioselective recognition. Nevertheless, the enantiomers of flobufen were shown to be much better (baseline) resolved on the CSP with lower coverage of the chiral selector (Chirobiotic V).     

淀粉及纤维素三(3-三氟甲基苯基氨基甲酸酯)手性固定相的制备及其性能评价

为了扩展多糖类手性固定相的种类,制备了基于淀粉及纤维素三(3-三氟甲基苯基氨基甲酸酯)的涂敷型手性固定相,以正己烷-异丙醇混合液为流动相,对8种手性化合物进行了高效液相色谱拆分。研究表明: 虽然与应用最广泛的分别以淀粉及纤维素三(3,5-二甲基苯基氨基甲酸酯)为手性选择因子的商品化手性柱Chiralpak AD和Chiralcel OD相比,所制备的手性固定相的手性分离能力较低,但纤维素三(3-三氟甲基苯基氨基甲酸酯)手性固定相显示出特异的手性识别能力,一些手性化合物在此固定相上得到了比在Chiracel OD上更好的分离;所制备的手性固定相的手性识别能力随流动相中异丙醇含量的降低而变好,当流动相中正己烷与异丙醇的体积比为95:5时所制备的手性固定相显示出相对较高的手性识别能力;总体来说,淀粉三(3-三氟甲基苯基氨基甲酸酯)手性固定相的手性识别能力稍强于纤维素三(3-三氟甲基苯基氨基甲酸酯)手性固定相,同时两种手性固定相的手性识别能力具有一定的互补性。     

Direct high-performance liquid chromatographic separation of unusual secondary amino acids and a comparison of the performances of Chirobiotic T and TAG columns

Two macrocyclic glycopeptide antibiotic-type chiral stationary phases (CSPs) based on native teicoplanin and teicoplanin aglycone, Chirobiotic T and TAG, respectively, were evaluated with regard to the high-performance liquid chromatographic separation of the enantiomers of 10 secondary alpha-amino acids (imino acids). The chromatographic results are given as the retention, separation and resolution factors, together with the enantioselective free energy difference corresponding to the separation of the enantiomers. By application of these two CSPs, excellent resolutions were achieved for the investigated compounds by using reversed-phase mobile mode systems. The separation conditions were optimized by variation of the mobile phase composition. The difference in enantioselective free energy between the aglycone CSP and the teicoplanin CSP for these particular amino acids ranged between 0.70 and -1.83 kJ mol(-1). It was established that better enantioseparations of the secondary alpha-amino acids were attained in most cases on the aglycone CSP.     

Expanding the enantioselectivity of the gas-chromatographic chiral stationary phase chirasil-val-C11 by doping it with octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin

Combination of the enantioselective properties of the two versatile gas-chromatographic chiral stationary phases (CSPs) octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-CD (Lipodex E) 1 and L-valine-diamide-based CSP Chirasil-Val-C11 2 has been realized by doping the chiral polymer 2 with the nonpolymeric selector 1. The resulting mixed-mode CSP Chirasil-Val(gamma-Dex) 3 was found to have a greatly improved enantioselectivity toward proline and aspartic acid (as N-trifluoroacetyl ethyl or methyl esters) in comparison to the single-mode CSP 2. The presence of the CD selector in 3 extended the scope of gas-chromatographic enantioseparations achievable on 2 to underivatized alcohols, terpenes, and other chiral compounds that are exclusively enantioseparated on 1.     

High-performance liquid chromatographic and capillary electrophoretic enantioseparation of plant growth regulators and related indole compounds using macrocyclic antibiotics as chiral selectors

Enantioseparation of plant growth regulators, such as 3-(3-indolyl)-butyric acid, abscisic acid and structurally related molecules including a variety of substituted tryptophan compounds, has been achieved by HPLC and/or CE. The covalently bonded macrocyclic antibiotics, teicoplanin, ristocetin A and vancomycin, were used as chiral stationary phases (CSPs) in HPLC. Most of the racemates were baseline resolved in the reversed-phase mode (EtOH-H2O) using the teicoplanin CSP. The chiral recognition mechanism is discussed in regard to the structure of the analytes. In CE, the three aforementioned macrocyclic antibiotics were used as chiral additives in a phosphate run buffer. The effect of pH and the concentration of the organic modifiers were considered. The results obtained by HPLC and CE were compared.     

Evaluation of the macrocyclic glycopeptide A-40,926 as a high-performance liquid chromatographic chiral selector and comparison with teicoplanin chiral stationary phase

A new macrocyclic antibiotic of the vancomycin family, referred to by its industrial designation as A-40,926, was bonded to 5 microm silica particles and utilised as a chiral stationary phase (CSP). Since A-40,926 is structurally related to teicoplanin, the A-40,926 CSP was compared to a commercially available teicoplanin CSP. A set of 28 chiral compounds, including amino-acids and related compounds, compounds with a ring containing the stereogenic centre, compounds bearing aromatic structures near their stereogenic centres and alcohols, was tested for enantioseparation on the two CSPs. The results are compared and discussed in terms of enantioselective Gibbs energy difference. The A-40,926 CSP was able to resolve one compound that was not resolved by the teicoplanin CSP. However, it could not separate four compounds that the teicoplanin CSP did separate. It is shown that the A-40,926 CSP is complementary to the teicoplanin CSP, thereby enlarging the number of enantiomers that can be separated by the macrocyclic glycopeptide based CSPs.     

Triazolo-linked cinchona alkaloid carbamate anion exchange-type chiral stationary phases: Synthesis by click chemistry and evaluation

Immobilization strategy based on Huisgen 1,3-dipolar cycloaddition (click chemistry) of 10,11-didehydrocinchona tert-butylcarbamates to azido-grafted silica gels has been evaluated for preparation of novel chiral stationary phases (CSP 1-3). The resultant 1,2,3-triazole-linked CSPs were tested under various mobile phase conditions (polar organic and reversed phase mode) with a representative set of structurally diverse racemic acids including N-protected aminoacids, aromatic and aryloxycarboxylic acids as well as binaphthol phosphate. The chiral recognition performance of the C3-triazole-linked CSPs was found to mirror largely that of the known C3-thioether-linked CSP in terms of elution order, enantioselectivity and retention behavior. In an effort to assess the non-specific binding expressed as retention increment of these triazole-linked CSPs, the parent azidopropyl- and triazole-modified silica materials (thus not containing the chiral head ligand) were studied independently. Compared with the corresponding CSPs, the analyte retention on the azidopropyl control column was very low, and practically negligible on the corresponding triazole-modified reference column. Only minor losses in analyte retention behavior (<5%) were observed with triazole-linked CSPs after two month of continuous use with polar-organic and reversed-phase-type mobile phases, highlighting the excellent stability of the 1,2,3-triazole linker.     

Comparison of the separation efficiencies of chirobiotic T and TAG columns in the separation of unusual amino acids

Two macrocyclic antibiotic type chiral stationary phases (CSPs), based on native teicoplanin and teicoplanin aglycone, Chirobiotic T and Chirobiotic TAG, respectively, were evaluated for the high-performance liquid chromatographic separation of enantiomers of 15 unnatural conformationally constrained alpha-amino acids, Phe and Tyr analogs, and 12 beta-amino acids having cycloalkane or cycloalkene skeletons. The chromatographic results are given as the retention, separation and resolution factors along with the enantioselective free energy difference corresponding to the separation of the enantiomers. It is clearly established that in most cases the aglycone is responsible for the enantioseparation of amino acids. The difference in enantioselective free energy between the aglycone CSP and the teicoplanin CSP was between 0.02 and 0.30 kcal mol(-1) for these particular amino acids. The resolution factors are higher with the aglycone CSP. Although the sugar units generally decrease the resolution of amino acid enantiomers, they can contribute significantly to the resolution of some unusual amino acid analogs. By application of these two CSPs excellent resolutions were achieved for most of the investigated compounds by using reversed phase or polar organic mobile mode systems. The separation conditions were optimized by variation of the mobile phase composition.     

Separation of chiral furan derivatives by liquid chromatography using cyclodextrin-based chiral stationary phases

The enantiomeric separation of a set of 30 new chiral furan derivatives has been achieved on native and derivatized beta-cyclodextrin stationary phases using high performance liquid chromatography (HPLC). The hydroxypropyl-beta-cyclodextrin (Cyclobond RSP), the 2,3-dimethyl-beta-cyclodextrin (Cyclobond DM), and the acetyl-beta-cyclodextrin (Cyclobond AC) stationary phases are the most effective chiral stationary phases (CSPs) for the separation of these racemates in the reverse phase mode. No enantioseparations have been observed on the native beta-cyclodextrin chiral stationary phase (Cyclobond I 2000) and only a few separations have been attained on the S-naphthylethyl carbamate beta-cyclodextrin (Cyclobond SN) and 3,5-dimethylphenyl carbamate beta-cyclodextrin (Cyclobond DMP) chiral stationary phases in the reverse phase mode. The polar organic and the normal phase mode on these CSPs are not effective for separation of these compounds. The characteristics of the analytes, including steric bulk, hydrogen bonding ability, and geometry, play an important role in the chiral recognition process. The pH affects the enantioseparation of compounds with ionizable groups and the addition of 0.5% methyl tert-butyl ether to the mobile phase significantly enhances the separation efficiency for some highly retained compounds.     

Synthesis and evaluation of a new biselector s-triazine based chiral stationary phase for enantioselective HPLC: potentiality of the approach and perspectives

A s-triazine scaffold bearing a free and a protected amino group was used for connecting two different chiral auxiliaries, whose enantiodiscriminating capabilities in enantioselective chromatography are well documented. A biselector system was synthesized and, after linkage to silica gel, used for the chromatographic resolution of different racemic analytes, chosen among the compounds resolved by the two different isolated chiral auxiliaries. The obtained chromatographic results were compared with literature data related to the use of the two chiral stationary phases (CSPs) whose chiral moieties constitute the biselector CSP, allowing us to gain useful information about potentialities and limitations of this approach for obtaining independent CSPs having broader applicability with respect to the classical independent monoselector CSPs.     

Comparison of enantiomer separation on two chiral stationary phases derived from (+)-18-crown-6-2,3,11,12-tetracarboxylic acid of the same chiral selector

Liquid chromatographic comparisons for enantiomer resolution of α-amino acids and chiral primary amino compounds were made using chiral stationary phases (CSPs) prepared by covalently bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA) of the same chiral selector. The resolution of all α-amino acids on CSP 1 developed in our group was found to be better than that on CSP 2 reported by Machida et al. All α-amino acids examined in this study were well enantioseparated on CSP 1 (α=1.22–2.47), while four analytes were not resolved or all the other analytes were poorly resolved on CSP 2 than on CSP 1. However, in resolving the primary amino compounds without a carbonyl group, CSP 1 was comparable with CSP 2. Although (+)-18-C-6-TA of the same chiral selector was used to prepare CSP 1 and CSP 2, this study showed that different connecting methods for the CSPs might influence their ability to resolve the analytes depending on their structures related to the chiral recognition mechanism.     

The Resolution of Enantiomeric Ibuprofen on Chiral Stationary Phases Containing N-Arylcarbamoyl Derivatives of Phenylglycine

A series of fourteen anilide derivatives of ibuprofen were resolved on six chiral stationary phases (CSPs) derived from N-arylcarbamoyl derivatives of (S)-phenylglycine. Excellent chiral resolutions were achieved on these CSPs. The ionic-type CSPs showed better chiral recognition abilities than the corresponding covalent-type CSPs, and the CSP bearing two chiral centers has better performance than the CSPs bearing only one chiral center. The highest separation factor was achieved using the ionic-type CSP bearing two chiral centers for the resolution of the 3,5-dinitroanilide derivative of ibuprofen. This result is better than those reported in literature for the resolution of ibuprofen on the CSPs derived from amino acids, According to the chromatographic behaviors, the hydrogen bonding interaction, the π-π interactions provided by the phenyl groups in CSPs bearing one chiral center, and the phenylethylcarbamoyl moiety in CSPs bearing two chiral centers dominate the chiral recognition.     

Synthesis of chemically bonded cellulose trisphenylcarbamate chiral stationary phases for enantiomeric separation

Cellulose trisphenylcarbamate is regioselectively bonded to 3-aminopropyl silica gel and underivatized silica gel, respectively, at the 6-position of the primary hydroxyl group on the glucose unit of cellulose with 4,4'-diphenylmethane diisocyanate (DPDI) as a spacer. Enantioseparations are evaluated on these prepared chiral stationary phases (CSPs) with several organic acids as the modifiers in the mobile phase by high-performance liquid chromatography. The influence of the amount of DPDI used on chiral resolution is investigated. Also, the corresponding coated-type phase is also prepared for the aim of comparison. It is observed that the bonded-type phase shows a lower chiral recognition power but a better column efficiency than the coated-type phase under the liquid chromatographic mobile phase with hexane-alcohol. However, the bonded-type CSPs are compatible with a wider number of solvents such as tetrahydrofuran (THF) or chloroform, which generally result in the solubility or swelling of the cellulose derivatives on the coated-type CSPs. The results obtained from this study indicate that the bonded-type CSP may provide complementary enantioselectivity over the coated-type phase by adopting THF as a component in the mobile phase.     

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.     

Separation of the enantiomers of 2-phenylcyclopropanecarboxylate esters by capillary gas chromatography on derivatized cyclodextrin stationary phases

Summary Separation of the enantiomers of 2-phenylcyclopropanecarboxylate esters has been investigated on derivatized cyclodextrin chiral stationary phases (CD CSPs) to enable direct determination of the enantiomeric purity of the products of enantioselective cyclopropanation. Four stereoisomers of these chiral compounds could be resolved to baseline on permethylated β-cyclodextrin CSP. Some unusual phenomena, iso-enthalpy retention behavior and entropically driven chiral separation, were observed for the enantioseparation of 2-phenylcyclo-propanecarboxylates on the CD CSPs. Thermodynamic parameters were evaluated and an enthalpy-entropy compensation effect was observed forn-alkyl esters of 2-phenylcyclopropanecarboxylate separated on CD CSPs.     

Chiral separation of beta-adrenergic antagonists, profen non-steroidal anti-inflammatory drugs and chlorophenoxypropionic acid herbicides using teicoplanin as the chiral selector in capillary liquid chromatography

Three groups of structurally diverse chiral compounds were used to study the interaction mechanism responsible for stereoselective recognition with teicoplanin as chiral selector in capillary liquid chromatography. Teicoplanin-based chiral stationary phase (CSP) was used. The effect of the variation of mobile phase composition on retention and enantioselective separation was studied. The mobile phase composition suitable for enantioresolution of the various chiral compounds differed according to the interaction forces needed for chiral recognition. Mobile phases with high buffer portion (70-90 vol.%) were preferred for separation of enantiomers of profen non-steroidal anti-inflammatory drugs and chlorophenoxypropionic acid herbicides that require hydrophobic interactions, inclusion and pi-pi interactions for stereoselective recognition with teicoplanin. Higher concentration triethylamine in the buffer (0.5-1.0%) increased resolution of these acids. On the other hand, H-bonding and electrostatic interactions are important in stereoselective interaction mechanism of beta-adrenergic antagonists with teicoplanin. These interaction types predominate in the reversed phase separation mode with high organic modifier content (95% methanol) and in polar organic mobile phases. For this reason beta-adrenergic antagonists were best enantioresolved in the polar organic mode. The mobile phase composed of methanol/acetic acid/triethylamine, 100/0.01/0.01 (v/v/v), provided enantioresolution values of all the studied beta-adrenergic antagonists in the range 1.1-1.9. Addition of teicoplanin to the mobile phase, which was suitable for enantioseparation of certain compounds on the CSP, was also investigated. This system was used to dispose of nonstereoselective interactions of analytes with silica gel support that often participate in the interaction with CSPs. Very low concentration of teicoplanin in the mobile phase (0.1 mM) resulted in enantioselective separation of 2,2- and 2,4-chlorophenoxypropionic acids.     

The First Chromatographic Resolution of a Chiral Phosphane. An Efficient Synthesis of (R,R)- and (S,S)-DIPAMP

Abstract

Resolution of racemic compounds by chromatography on chiral stationary phases (CSPs) offers several advantages compared with “classical” resolution: no waste of either antipode, no dependence on the e.e. of the chiral auxiliary (CSP), no necessity for chemical transformation. The mean loadability of most CSPs can be compensated if a racemic catalyst is resolved: the chromatographically produced “chiral information” will subsequently be multiplied.