Direct enantiomeric resolution of (+/-)-atenolol, (+/-)-metoprolol,and (+/-)-propranolol by impregnated TLC using L-aspartic acid as chiral selector

Resolution of three commonly used beta-blockers, (+/-)-atenolol, (+/-)-metoprolol and (+/-)-propranolol, into their enantiomers has been achieved using normal-phase TLC on silica gel plates impregnated with L-aspartic acid as the chiral selector. Different combinations of acetonitrile-methanol-water as mobile phase were found to be successful in resolving the enantiomers. The spots were detected with iodine and the detection limits were found to be 0.26 microg for atenolol and 0.23 microg for each of metoprolol and propranolol as racemate.     

Chiral separation with ligand-exchange micellar electrokinetic chromatography using a D-penicillamine-copper(II) ternary complex as chiral selector

D-Penicillamine is demonstrated for the first time as a chiral ligand for the enantioseparation of dansyl amino acids based on ligand-exchange micellar electrokinetic chromatography (LE-MEKC). Copper(II) was used as the central ion in the ternary complex. The effect of surfactant on the resolution was significant. A concentration of 20 mM sodium dodecyl sulfate (SDS) was shown to be necessary for the separation. Other important parameters, such as the concentration ratio of D-penicillamine (D-PEN) to Cu2+, the kind of metal central ion, the type and pH value of buffer, were also investigated. N-Acetyl-D-penicillamine and L-valine (Val), with similar structure to D-penicillamine, were applied as their copper(II) complexes as chiral selector and the chiral recognition mechanism is briefly discussed. Under optimum experimental conditions, i.e., 20 mM NH4OAc, pH 6.5, a 2:1 concentration ratio of D-penicillamine to Cu(II), 4 mM CuSO4 and 8 mM D-penicillamine, the chiral separation of eight pairs of different dansyl amino acid enantiomers was accomplished with resolution ranging from 1.1 to 5.9. When L-PEN was used instead of D-PEN, reversal of the migration order was observed.     

Direct enantiomeric TLC resolution of dl-penicillamine using (R)-mandelic acid and l-tartaric acid as chiral impregnating reagents and as chiral mobile phase additive

DL-Penicillamine has been resolved into its enantiomers by normal-phase TLC using L-tartaric acid as chiral impregnating reagent as well as chiral mobile phase additive, while (R)-mandelic acid has been found to be successful as a chiral impregnating reagent. The solvent system acetonitrile-methanol-water (5:1:1, v/v) was found to be successful when L-tartaric acid was used as impregnating agent while the solvent combination acetonitrile-methanol-(0.5% l-tartaric acid in water, pH 5)-glacial acetic acid (7:1:1.1:0.7, v/v) was successful as mobile phase as it contained L-tartaric acid as the chiral additive. (R)-mandelic acid was successful as chiral impregnating reagent with ethyl acetate-methanol-water (3:1:1, v/v), as the mobile phase. The effects of concentration of chiral selectors, temperature and pH were examined on enantiomeric resolution. The spots were detected with iodine vapors and the detection limits were found to be 0.12 microg for each enantiomer of penicillamine with L-tartaric acid, under both the conditions, and 0.11 microg with (R)-mandelic acid.     

Direct TLC resolution of atenolol and propranolol into their enantiomers using three different chiral selectors as impregnating reagents

Direct resolution of racemic atenolol and propranolol into their enantiomers was achieved by normal phase TLC on silica gel plates impregnated with optically pure L-tartaric acid, (R)-mandelic acid and (-)-erythromycin as chiral selectors. Different solvent systems were worked out to resolve the enantiomers. Spots were detected using iodine vapour. The TLC method was validated for linearity, limit of detection and limit of quantification. The influence of pH, temperature and concentration of chiral selector was studied.     

Multiple dual-mode countercurrent chromatography applied to chiral separations using a (S)-naproxen derivative as chiral selector

Countercurrent chromatography (CCC) is a liquid–liquid chromatographic technique without a solid support. Several alternative elution modes can be applied to take advantage of the special nature of the liquid stationary phase. Among these dual-mode (DM) and multiple dual-mode (MDM) consist of switching alternatively between Reversed and Normal Phase operation during the experiment (once for DM and several times for MDM). In this paper, MDM has been applied to the chiral CCC separations of two racemic mixtures, (±)-N-(3,4-cis-3-decyl-1,2,3,4-tetrahydrophenanthren-4-yl)-3,5-dinitrobenzamide and N-(3,5-dinitrobenzoyl)-(±)-leucine, using (S)-naproxen N,N-diethylamide as chiral selector (CS). Although the behaviour of the two analytes differed, improved resolution factors were successfully obtained. Results are rationalized on the basis of the distinct partition behaviour of the CS/enantiomer complexes in the biphasic system.     

Diastereomeric difference of inclusion modes between (-)-epicatechin gallate, (-)-epigallocatechin gallate and (+)-gallocatechin gallate, with beta-cyclodextrin in aqueous solvent

Inclusion complexes of (-)-epicatechin gallate (ECg) as well as (+)-gallocatechin gallate (GCg) and beta-cyclodextrin (beta-CD) in an aqueous solution were investigated using several NMR techniques and a computational method. ECg and EGCg formed a 1:1 complex with beta-CD, in which the A ring and a portion of the C ring were included from the wide secondary hydroxyl group side of the beta-CD cavity, and the B and B' rings were left outside the cavity. GCg formed a 1:2 complex with beta-CD, in which the A and B rings of GCg were included by two molecules of beta-CD. The difference between the two modes of inclusion of the 1:1 complex of ECg, EGCg.beta-CD and the 1:2 complex of GCg.beta-CD might have resulted from the size of the space between the B and B' rings in aqueous solution. As a result of nuclear Overhauser effect (NOE) experiments, GCg was considered to have a large enough space between the B and B' rings to include the B ring in the beta-CD cavity; on the other hand, ECg and EGCg have no such large space.     

磺丁基醚-β-环糊精手性流动相添加剂法分离兰索拉唑对映体

采用磺丁基醚-β-环糊精(SBE-β-CD)为手性流动相添加剂,建立了兰索拉唑对映体的高效液相色谱分离分析方法.对影响兰索拉唑对映体分离的主要因素:环糊精种类和浓度、缓冲溶液pH以及有机改性剂种类和含量进行考察.确定最优色谱条件:色谱柱为Spherigel C18 (150 mm×4.6 mm,5 μm),流动相为V(乙腈):V(水相)=20:80(水相含10 mmol/L SBE-β-CD、 10 mmol/L NaH2PO4缓冲液、 pH 2.5),流速为0.9 mL/min,检测波长为288 nm.在此条件下,兰索拉唑对映体的保留时间分别为14.4和15.8 min,分离度为2.0.两对映体质量浓度在0.2～50 μg/mL范围内线性关系良好(r≥0.9996),保留时间的RSD分别为0.27%和0.26%,峰面积的RSD分别为0.65%和0.68%.     

High performance liquid chromatographic separation of dipeptide and tripeptide enantiomers using a chiral crown ether stationary phase

A chiral stationary phase (CSP) based on (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was evaluated for the direct resolution of the enantiomers of dipeptides and tripeptides. The type and concentration of the acid and the methanol content were optimized with regard to retention time and resolution using Ala-Phe as model peptide. A mobile phase consisting of 10 mM sulfuric acid in 70% aqueous methanol was applied to the separation of a set of 16 structurally diverse dipeptides and tripeptides. Generally, the configuration of the amino acid at the N-terminus determined the enantiomer elution order. With a few exceptions the LL- and LD-enantiomers interacted stronger with the CSP compared to the corresponding DD- or DL-enantiomers. The experimental conditions also allowed the simultaneous separation of all four stereoisomers of Ala-Phe. Addition of ammonium sulfate generally reduced retention times and enantiomer resolution. Addition of triethylamine as modifier led to an overall increase of the retention times while the resolution did not show a general trend, increasing in the case of Ala-Ala but decreasing in the case of Ala-Phe.     

S-trityl-(R)-cysteine, a powerful chiral selector for the analytical and preparative ligand-exchange chromatography of amino acids

S-trityl-(R)-cysteine [(R)-STC] is the new selector of a dynamically coated, chiral ligand-exchange stationary phase which proved to be highly effective in both analytical and preparative-scale separation of enantiomers of some natural and unnatural underivatized amino acids, with good separation and resolution factors. With the aim of identifying the best chromatographic conditions suitable for the preparative-scale separations, some parameters controlling retention, separation and resolution factors (such as the type and amount of cupric salt and the eluent pH) were investigated. The relatively easy removal of the Cu(II) ions renders this technique suitable for obtaining small amounts of enantiomerically pure samples for preliminary biological evaluations.     

Enantioseparation in capillary electrophoresis using 2-O-(2-hydroxybutyl)-beta-CD as a chiral selector

The resolving ability of 2-O-(2-hydroxybutyl)-beta-CD (HB-beta-CD) with different degrees of substitution (DS = 2.9 and 4.0) as a chiral selector in CZE is reported in this work. Fourteen chiral drugs belonging to different classes of compounds of pharmaceutical interest such as beta-agonists, antifungal agents, ageneric agents, etc., were resolved. The effects of the DS of HB-beta-CD on separations were also investigated. The chiral resolution (R(s)) was strongly influenced by the concentrations of the CD derivative, the BGE, and the pH of the BGE. Under the conditions of 50 mmol/L Tris-phosphate buffer at pH 2.5 containing 5 mmol/L HB-beta-CD, all 14 analytes were separated. The very low concentration necessary to obtain separation was particularly impressive. The DS had a significant effect on the resolution of the chiral drugs and the ionic strength of the separation media; hence, the use of a well-characterized CD derivative is crucial.     

Evaluation of the enantioselectivity of carbon nanoparticles‐modified chiral separation systems using dextrin as chiral selector by capillary electrokinetic chromatography

Nanoparticles (NPs) can be used as pseudostationary phases (PSPs) in EKC, which is similar to the use of micelle additives as applied in MEKC. To date, the use of NPs to enhance enantiomeric separation by EKC with β‐CD or its derivative as chiral selector has been reported only in two papers. However, to the best of our knowledge, there has been no prior effort to use NPs for achieving enantioseparation with polysaccharides as chiral selector. This paper describes for the first time the use of carbon nanoparticles (CNPs) as PSPs to modify chiral separation system employing dextrin as chiral selector for the enantioseparations of several basic drugs in capillary EKC. Three different types of CNPs, including carbogenic nanoparticles (NPs), carboxylated single‐walled carbon nanotubes, and carboxylated multiwalled carbon nanotubes, were used as running buffer additives, respectively. The potential of the PSPs and the effects of dextrin concentration, buffer pH, and buffer concentration on the enantioseparations were evaluated. Four pairs of tested enantiomers were successfully resolved in less than 15 min with the resolution values in the range of 1.41–4.52 under optimized conditions. Compared to the buffer without NPs, the introduction of NPs into the buffer enhanced the separation of the enantiomers.     

Enatiomeric analysis of simendan by CE with beta-CD as chiral selector compared with CMPA-HPLC

The chiral separation of simendan enantiomers using capillary electrophoresis was studied with beta-cyclodextrin (beta-CD) as chiral selector. The influences of the concentration and pH of borate buffer solution, beta-CD concentration and methanol content in the background electrolyte were investigated. These factors were compared with those in an HPLC with beta-CD as chiral mobile phase additive (CMPA-HPLC). The quantification properties of the developed CE method were examined. A baseline separation of simendan enantiomers was achieved in the background electrolyte of 20 mmol/L borate buffer (pH 11.0) containing 12 mmol/L beta-CD-methanol (50:50 in volume ratio). The CE method is comparable with CMPA-HPLC in chiral resolution, although the optimal pH in CE (11.0) is much higher than that (6.0) in CMPA-HPLC. This chiral CE method is applicable to the quantitative ananlysis and enantiomeric excess value determination of L-simendan.     

Total synthesis of (-)-dendrobine

Cascading to alkaloids: An 18-step total synthesis of (-)-dendrobine is based on a reaction cascade with a key amine group (see scheme, Bn=benzyl). The amine is the initiator of the cascade and provides an efficient method for installing the stereocenters at C11 and C3. The overall transformation occurs stereoselectively only when the conversion is carried out without the isolation of intermediates.     

(S)-(-)-alpha,alpha-Di(2-naphthyl)-2-pyrrolidinemethanol, a useful tool to study the recognition mechanism in chiral ligand-exchange chromatography

A dynamic coating of the RP-18 carbon chain layers with the new chiral selector (S)-(-)-alpha,alpha-di(2-naphthyl)-2-pyrrolidinemethanol allowed the formation of a mixed chiral stationary phase that has been used in the separation of a selected set of amino acid racemates. Both a representative model and classification structure-property relationship studies have been performed in order to study the contribution of hydrophobic, bulky and electron-donating groups in the side chain of the chiral selector to the mechanism of chiral recognition.     

Enantiomeric separation of D,L-tryptophan and D,L-kynurenine by HPLC using pre-column fluorescence derivatization with R(-)-DBD-PyNCS

The enantiomeric separation of d ,l ‐tryptophan (Trp) and d ,l ‐kynurenine (KYN) was investigated by high‐performance liquid chromatography using pre‐column fluorescence derivatization with a chiral fluorescent labeling reagent, R(−)‐4‐(3‐isothiocyanatopyrrolidin‐1‐yl)‐7‐ (N,N‐dimethylaminosulfonyl)‐2,1,3‐benzoxadiazole [R(−)‐DBD‐PyNCS]. Using an octadecylsilica column, namely, an Inertsil ODS‐3 column (250 × 2.0 mm; i.d., 3 µm), four fluorescence peaks of D‐ and l ‐Trp as well as d ‐ and l ‐KYN derivatized with R(−)‐DBD‐PyNCS were clearly observed, and their chemical structures were confirmed by HPLC–time‐of‐flight–mass spectrometry. Simultaneous separation was achieved under the mobile phase condition of 1.5% acetic acid in H₂O–CH₃CN (60:40), and the separation factors of d ,l ‐Trp and d ,l ‐KYN derivatized with R(−)‐DBD‐PyNCS were 1.22 and 1.19, respectively. Fluorescence detection was carried out by setting the emission wavelength at 565 nm, and the excitation wavelength at 440 nm, and the detection limits were approximately 0.3–0.5 pmol (signal‐to‐noise ratio of 3). Copyright © 2010 John Wiley & Sons, Ltd.     

Chiral counter-current chromatography of gemifloxacin guided by capillary electrophoresis using (+)-(18-crown-6)-tetracarboxylic acid as a chiral selector

(+)-(18-crown-6)-tetracarboxylic acid (18C6H4) has been known as a highly efficient chiral selector for resolving primary amine enantiomers in capillary electrophoresis (CE). We investigated the chiral separation of gemifloxacin using 18C6H4 in analytical counter-current chromatography (CCC). The separation conditions for CE, including the binding constant, pH, and run buffer constituents, provided a helpful guideline for chiral CCC. A successful separation of gemifloxacin enantiomers could be achieved using a two-phase solvent system composed of 1-butanol-ethyl-acetate-bis(2-hydroxyethyl)aminotris(hydroxymethyl)methane acetate buffer with a small amount of 18C6H4. The hydrophobicity of the solvent system and the 18C6H4 concentration were varied to optimize the chiral separation.     

Chiral separation of dansyl amino acids in capillary electrophoresis using mono-(3-methyl-imidazolium)-beta-cyclodextrin chloride as selector

Enantioseparations of fourteen dansyl amino acids were achieved by using a positively-charged single-isomer beta-cyclodextrin, mono-(3-methyl-imidazolium)-beta-cyclodextrin chloride, as a chiral selector. Separation parameters such as buffer pH, selector concentration, separation temperature, and organic modifier were investigated for the enantioseparation in order to achieve the maximum possible resolution. Chiral separation of dansyl amino acids was found to be highly dependent on pH since the degree of protonation of these amino acids can alter the strength of electrostatic interaction and/or inclusion complexation between each enantiomer and chiral selector. In general, the chiral resolution of dansyl amino acids was enhanced at higher pH, which indicates that the carboxylate group on the analytes may interact with the imidazolium group of cationic cyclodextrin. For most analytes, a distinct maximum in enantioresolution was obtained at pH 8.0. Moreover, the chiral separation can be further improved by careful tuning of the separation parameters such as higher selector concentration (e.g. 10 mM), lower temperature, and addition of methanol. Enantioseparation of a standard mixture of these dansyl amino acids was further achieved in a single run within 30 min.     

HPLC enantioseparation of phenylalanine analogs by application of (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester as a new chiral derivatizing agent

Summary The indirect stereochemical resolution of the enantiomers of phenylalanine analogs was studied by high-performance liquid chromatographic methods. The resolution was performed by applying pre-column derivatization with (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester, (S)-NIFE, as a new chiral derivatizing agent. Its applicability is demonstrated by the derivatization and separation of a series of ring-and α-methyl-substituted phenylalanines and phenylalanine amides, and especially by the derivatization of sterically hindered, less reactive α-methyl-substituted phenylalanine analogs. The diastereomeric, derivatives produced were separated by reversed-phase high-performance liquid, chromatography. The conditions of both derivatization and separation were optimized. The method described offers good enantioselectivity for various chiral amino compounds derived from chemo-enzymatic processes.     

Resolution of beta-amino acids on a high performance liquid chromatographic doubly tethered chiral stationary phase containing N-CH3 amide linkage based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

A doubly tethered chiral stationary phase (CSP) containing N-CH(3 )amide linkage based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was applied to the resolution of various beta-amino acids. The chiral recognition behaviors for the resolution of beta-amino acids on the doubly tethered CSP were consistent with those on the corresponding singly tethered CSP while the chiral recognition ability of the doubly tethered CSP was generally greater in terms of both the separation (alpha) and the resolution factors (R(S)) than that of the corresponding singly tethered CSP. From these results, it was concluded that attaching the second tethering group to silica gel through a carbon atom of the first tethering group of the CSP improves the chiral recognition ability for the resolution of beta-amino acids without any change in the chiral recognition mode. The retention factors (k(1)) on the doubly tethered CSP were larger than those on the corresponding singly tethered CSP and these retention factors were found to be controllable with the variation of the type and the content of the organic and/or acidic modifier in the aqueous mobile phase without significant change in the separation and the resolution factors.