The Use of Cyclodextrin-Based LC Stationary Phases for the Separation of Chiral Dihydrobenzofuran Derivatives

The enantiomeric separation of 12 chiral dihydrobenzofurans is reported on derivatized -cyclodextrin stationary phases using high performance liquid chromatography. The hydroxypropyl -cyclodextrin chiral stationary phase (CSP) with acetonitrile/water mobile phases was the most effective combination as it baseline separated 9 of the 12 compounds. The acetyl -cyclodextrin and 2,3-dimethyl -cyclodextrin CSPs were also effective in the reverse phase mode. The native -cyclodextrin was far less effective than the non-aromatic derivatized CSPs. The aromatic functionalized CSPs showed no selectivity in the normal phase mode. Structural characteristics, such as substituent polarity and ring location, were important in the observed enantioselectivity.     

Chiral separation on achiral stationary phases with different functionalities using β-cyclodextrin in the mobile phase and application to bioanalysis and coupled columns

Summary -cyclodextrin was used in the mobile phase as chiral selector for separating the enantiomers of terbutaline, chlorthalidone and oxazepam. The effect on chiral resolution using e.g. hydrophobic, polar or cation exchanging stationary phases was investigated. Both the chiral separation factor and retention level were affected by the concentration of methanol and -cyclodextrin. The stationary phase had no effect on the chiral separation only on the level of retention. By tuning the concentration of -cyclodextrin and methanol in the mobile phase chiral separation could be obtained on most stationary phases. By changing the stationary phase while adjusting the mobile phase composition to maintain the chiral selectivity, improvements of the selectivity towards e.g. endogenous compounds can be obtained when separating enantiomers in complex matrixes as biological fluids. Further improvement on selectivity can be obtained if coupled columns are used. This is examplified for separation of chlorthalidone and terbutaline enantiomers in biological fluids by coupling an achiral column to another achiral column and using a mobile phase containing -cyclodextrin on the last column.     

Separation of Enantiomers of Isochromene Derivatives by HPLC Using Cyclodextrin-Based Stationary Phases

Twenty chiral isochromene derivatives have been chromatographed on native and derivatized cyclodextrin stationary phases using HPLC. The most effective CSPs for the enantioresolution of these analytes in the reverse phase mode are the hydroxypropyl--cyclodextrin (Cyclobond RSP), the 2,3-dimethyl--cyclodextrin (Cyclobond DM), and the -cyclodextrin (Cyclobond II) stationary phases. The -cyclodextrin (Cyclobond III), -cyclodextrin (Cyclobond I), acetyl--cyclodextrin (Cyclobond AC), S-1-naphthylethyl carbamate--cyclodextrin (Cyclobond SN), and 3,5-dimethylphenyl carbamate--cyclodextrin (Cyclobond DMP) stationary phases also show enantioselectivities for some analytes. No enantioseparations were observed in the polar organic mode and only a few separations were found in the normal phase mode. The Cyclobond RSP CSP provided the best overall separations of these analytes in the reverse phase mode. The pH of the mobile phase and the nature of organic modifiers have little effect on the enantioresolution. The substituents on the isochromene ring greatly affect the chiral recognition.     

Chiral separations by host-guest complexation with cyclodextrin and crown ether in capillary zone electrophoresis

Summary Capillary zone electrophoresis using cyclodextrins and a chiral crown ether as buffer constituents was studied for the enantiomeric separation of drugs and amino acids. Based on results obtained from separation of racemic -amino acids both chiral selectors are compared with respect to resolution, efficiency and retention time. For (±)-Quinagolide effects of buffer composition and temperature are examined using -cyclodextrin as chiral agent. Optimum conditions were pH 2.5 at 30 mmol L^–1 -cyclodextrin. A linear dependence of retention on -cyclodextrin concentration allowed calculation of formation constants of the host-guest complexes. Buffer concentration and temperature also influence resolution. The application of a chiral crown ether to the separation of optical isomers in capillary zone electrophoresis is described for the first time. Chiral recognition of solutes depends on the formation of protonated alkyl amines and separation is attributed to the formation of diastereomeric host-guest complexes with different interactions for each enantiomer. The effects of crown ether concentration on resolution are presented.     

Interaction of cyclodextrins with drugs. Chromatographic investigation using a cyclodextrin bonded-phase and its application to chiral separations

Summary The interaction between cyclodextrin and the drug (1R,2S,3S,4S)-(5Z)-7-(3-((phenylsulfonyl)amino)bicyclo[2.2.1]hept-2-yl)hept-5-enoic acid ((+)-S-145), was studied using -, -, and -cyclodextrin bonded-phase columns. Retention behavior of (+)-S-145 on these columns revealed that the strength of inclusion was -cyclodextrin. Interaction between -cyclodextrin and (+)-S-145 was found to increase as the proportion of carboxylic ion in the (+)-S-145 molecule increased. Comparison of binding capacities of these bondedsilica gels obtained by frontal analysis and surface coverage indicated that availability of the immobilized - and -cyclodextrin was 20–25%. The synthesized -cyclodextrin bonded-phase column was superior to that of commercial columns in terms of chiral separation of (±)-S-145. A typical usage of the -cyclodextrin column is discussed for separation of (±)-S-145 in plasma samples.     

Monolithic silica columns with chemically bonded β-cyclodextrin as a stationary phase for enantiomer separations of chiral pharmaceuticals

An enantioselective silica rod type chiral stationary phase (CSP) is presented; a novel combination of the well known enantiomer separation properties of -cyclodextrin and the unique properties concerning the flow behavior of silica monoliths. Two different synthesis routes are described, and it was found that the in situ modification of a plain silica rod column turned out to be the best. The chromatographic behaviour of the -cyclodextrin silica rod was studied and compared with a very similar commercially available -cyclodextrin bonded particulate material (ChiraDex^®). Even if the amount of -cyclodextrin bound to the silica rod was only about half of the amount of -cyclodextrin bound to ChiraDex^® particles, good resolutions were achieved for a set of chiral test components like Chromakalin, Prominal, Oxazepam, Methadone and some other drugs. By taking advantage of the unique features of the silica rods relating to their flat H/u (Van Deemter) curves, fast enantiomer separations could be demonstrated.     

Synthesis of Some Allyl-β-Cyclodextrin Derivatives and their Properties as Capillary GC Stationary Phases

Summary Three -Cyclodextrin derivatives (-CDs) were synthesized by substituting the 2,6-OH groups of -CD with allyl groups and the 3-OH groups with three different acyl groups (valeryl, heptanonyl, octanoyl). The chromatographic properties of these -CDs as stationary phases for capillary gas chromatography (CGC) were studied. The test results showed that the three -CDs possessed good coating properties and that the capillary columns coated with them exhibited high column efficiency. These -CDs can separate not only the disubstituted benzene isomers but also some racemic compounds.     

Separation of diastereomers of a 1,3-disubstituted tetrahydro-β-carboline by capillary electrophoresis with β-cyclodextrin

Summary Capillary zone electrophoresis with -cyclodextrin as electrolyte additive has been investigated for the separation of the diastereomers of a 1,3-disubstituted tetrahydro--carboline. The effects of pH, buffer concentration, -cyclodextrin concentration and applied voltage on resolution were investigated. This study shows that optimizing the assay conditions leads to maximum chiral separation or resolution. The method enables further research on the quantitative analysis of these compounds.     

Enantiomer separations with chromatographic supports based on β-cyclodextrin polymers immobilized on porous silica. Role of the polymer structure in separating ability

Summary Two -cyclodextrin (-CD)-containing polymers have been prepared either by condensation of -CD molecules with a bifunctional reagent or by grafting a -CD derivative on to a linear polymer (polyvinylimidazole). HPLC stationary phases were obtained by adsorption of the -CD polymers on to silica. The ability of these chromatographic supports to resolve racemic mixtures of organic compounds such as amino acid derivatives, phenylhydantoins, barbiturates, and hydroxycoumarin derivatives has been investigated. Results were found to depend on the chemical structure of the -CD polymers     

Enantiomeric separation of amphetamine,methamphetamine and ring substituted amphetamines by means of a β-cyclodextrin-chiral stationary phase

Summary The enantioseparation of amphetamine, methamphetamine and various ring-substituted amphetamines by use of a chiral stationary phase carrying immobilized native -cyclodextrin (-CyD) selectors is reported. The system is evaluated for resolving the specified compounds directly without any derivatization and after derivatization with phenyl isothiocyanate (PITC), naphthyl isothiocyanate (NITC) and 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). This direct enantioseparation is compared with the features of indirect separation of diasteromeric derivatives after reaction with the optically pure Marfey's reagent employing a simple non-chiral alkyl-silica (RP-8) column. A selection of those methods best suited for each single amphetamine is given.Seventeen different samples of amphetamine, confiscated by the Swedisch police, were analyzed with respect to their enantiomeric composition. Within this set of samples synthesized by the same method no significant deviation from a racemic ratio could be observed.     

Study on Interactions of Several Psychopharmaceutical Drugs with β-Cyclodextrin by Affinity Capillary Electrophoresis and their Application in Separation

The binding of psychopharmaceutical drugs by -cyclodextrin (-CD) changed their effective electrophoretic mobilities. Based on this phenomenon the interactions between some psychopharmaceutical drugs and -cyclodextrin were studied and the binding constants of -CD with tiapride, imipramine, clomipramine, amitriptyline, trifluoperazine, perphenazine and carbamazepine were determined by affinity capillary electrophoresis. The presence of -CD improved the separation of these psychopharmaceutical drugs. Organic modifier was used as another kind of additive to be added in the running buffer to improve further the separation. 70 mM pH 7.2 phosphate buffer solution containing 12 mM -cyclodextrin and 15% (v/v) acetone was selected as optimal running buffer for separation by capillary electrophoresis.     

Use of methylamino-β-cyclodextrin in capillary electrophoresis. Resolution of acidic and basic enantiomers

Summary -cyclodextrin derivatives, namely 6^A methylamino--CD and hepta-methylamino--CD have been used as chiral selectors for the enantiomeric separation of a number of acidic and basic compounds by capillary electrophoresis employing either coated or uncoated capillaries. The effects of the CD type and concentration and the pH of the background electrolyte on the mobility and chiral resolution of the analytes have been studied. The use of monomethylamino--CD in a coated capillary allowed the enantiomeric resolution of phenyl lactic acid, warfarin and acenocoumarol but was not successful for tiaprofen and its 3-isomer. The heptamethylamino derivative, under the same experimental conditions, was a better chiral selector than the monosubstuted CD toward the arylpropionic acids and phenyl lactic acid while the anticoagulant drugs showed poor or no chiral resolution. Inversion of migration order was obtained for phenyl lactic acid, warfarin and acenocoumarol enantiomers. The enantiomers of basic compounds of pharmaceutical interest, namely propranolol, terbutaline, ketamine, chlorpheriramine and isoproterenol were only resolved using monomethylamino--CD dissolved in a phosphate buffer containing tetramethylammonium ions.Part of this work was presented at Eighth International Symposium on High Performance Capillary Electrophoresis, HPCE '96, January 21–25, 1996, Orlando, FL, USA.     

The GC separation of C2-naphthalene isomers using cyclodextrins and crown ethers

Summary The twelve structural C₂ isomers of naphthalene are resolved on a single capillary column using , , or cyclodextrin as the stationary phase. A change in elution order is seen upon going from to and then cyclodextrin which can be related to the size of the cyclodextrin cavity and the stereochemical fit of the isomer. A crown-ether column and a liquid-crystal column were found to be unable to separate all of the isomers.     

Chiral separation of β-amino alcohols by capillary electrophoresis using cyclodextrins as buffer additives. I. Effect of varying operating parameters

Summary Capillary electrophoresis has been used for the chiral analysis of two -amino alcohol pharmaceutical compounds. Capillary zone electrophoresis conditions were used with -cyclodextrin as a chiral mobile phase additive. The effects of variation of -cyclodextrin concentration, temperature, pH, background electrolyte composition and concentration have been investigated. Optimum separations were achieved for clenbuterol using -cyclodextrin at its solubility limit (16mM), the lowest practicable temperature (19°C), pH 4.0 and an electrolyte solution with a high ionic strength prepared from 0.1 M citric acid and 0.3 M Na₂HPO₄. For the development compound picumeterol and its (S)-enationmer, the optimum pH 4.0 buffer was prepared from 0.1 M citric acid and 0.2 M sodium acetate. Baseline separation with resolution greater than 2 was achieved for both compounds.This work was presented in part at the 2nd International Symposium on Chiral Discrimination, Rome, May 27–31, 1991.     

Separation and estimation of lindane from a mixture of hexachlorocyclohexane isomers via formation of an inclusion compound withβ-cyclodextrin

The treatment of a mixture of hexachlorocyclohexane isomers containing 16.03% lindane (-isomer) with-cyclodextrin results in an effective separation of lindane as indicated by a 50.4% lindane content in the total included guest. An assembly for a semimicro quantitative extraction for the separation of guest components in the inclusion compounds of cyclodextrins has been designed. The separation by inclusion has been confirmed by the preparation of an inclusion compound of-cyclodextrin with pure lindane whose guest content has been found to be 10.5% using three independent methods.     

Use of Unmodified and Aminated β-Cyclodextrins for the Separation of Enantiomers of Amino Acid Derivatives by High-Performance Liquid Chromatography

The separation of enantiomers of amino acid derivatives with modifiers of different structures on -cyclodextrin and aminated -cyclodextrin by reversed-phase high-performance liquid chromatography was studied. The dependence of the capacity factors of adsorbates on the concentration of the organic component in the mobile phase was examined. It was found that the retention of amino acid derivatives on unmodified -cyclodextrin and aminated -cyclodextrin increases with the hydrophobicity of the modifying agent of amino acid. In addition, for aminated -cyclodextrin, electrostatic interactions of ionized adsorbates and protonated surface amino groups substantially contribute to the retention. It was demonstrated that the recognition ability of aminated -cyclodextrin is affected by the structure of amino acid and its degree of dissociation.     

Separations of a-hexachlorocyclohexane (a-HCH) and pentachlorocyclohexene (PCCH) enantiomers on a cyclodextrin-phase (Cyclodex-B) by HRGC/ECD

The separation of a-hexachlorocyclohexane and of all eight possible pentachlorocyclohexene isomers into their enantiomers by high-resolution gas chromatography and electron capture detection was achieved by using permethylated -cyclodextrin in DB 1701 (Cyclodex-B, J & W) as chiral stationary phase. The first step in the metabolism of 1,2,3,4,5,6-hexachlorocyclohexane (HCH) is the formation of 1,3,4,5,6-pentachlorocyclohexene-1 (PCCH). Various degradation experiments were carried out with -, -, -, and -HCH. Whereas -HCH gave no degradation products, the isomers -, -, and -HCH degraded into various PCCHs and trichlorobenzenes, when using pyridine as dehydrochlorination reagent in xylene at 60°C. Isomerization into other HCH isomers was observed only for -HCH. Enantioselective dehydrochlorination was observed for a- and -HCH using the alkaloid (–)-brucine, R-(+)- or S-(–)-a-phenylethylamine as chiral base.     

Interactions of Nabumetone with Cyclodextrins in Solution and in the Solid State

The interactions of nabumetone (NAB) with -cyclodextrin (-CD) and-cyclodextrin (-CD) were studied in aqueous solution by meansof phase-solubility analysis. Solid dispersions of NAB with -cyclodextrin(-CD), -cyclodextrin (-CD), methyl- (M-CD),hydroxypropyl-cyclodextrin (HP-CD) were prepared by coevaporationand kneading and also by coprecipitation in the case of -CD. X-ray diffractometry, thermal analysis and infrared spectroscopy (FTIR) were used to study the possibility of complexation of the drug with the different cyclodextrins. Solid dispersions of nabumetone with -CD showed a remarkable improvement in the dissolution rate of nabumetone.     

Solvent-generated HPLC systems with β-cyclodextrin derivatives as chiral additives

It is shown that enantioselective solvent-generated liquid-liquid and liquid-solid Chromatographic systems with -cyclodextrin derivatives can be realized. The liquid-liquid systems are based on ternary solvent systems of limited miscibility, containing chiral additives; the chiral stationary phase is spontaneously generated on the porous microparticulate silica used as a solid support. In the solvent-generated adsorption systems a very small proportion of -cyclodextrin derivatives is added to the mobile phase. These components act as column activators, since they are strongly adsorbed on the RP solid support where they form new stereoselective adsorption sites. It is demonstrated that dynamically generated systems with -cyclodextrin derivatives as chiral agents are powerful and versatile tools for enantioseparation of different types of compounds. This technique creates not only a new type of selectivity but also leads to very stable and reproducible Chromatographic columns.Dedicated to Professor J. F. K. Huber on the occasion of his 65th birthday     

Asymmetric halogenation and hydrohalogenation of styrene in crystalline cyclodextrin complexes

Asymmetric halogenation and hydrohalogenation of styrene in microcrystalline cyclodextrin complexes were studied in the gas-solid state, and compared with the homogenous reactions in aqueous or dimethyl sulfoxide solutions. The gas-solid brominations in the - and -cyclodextrin complexes produced predominantly (–)-1,2-dibromo-l-phenylethane. The chiral induction for the reaction of the -cyclodextrin complex rose to 9 times that of the -cyclodextrin complex. Brominations in the homogenous solutions containing the - or -cyclodextrin complexes gave no dibromide but racemic bromohydrin. In the gas-solid chlorination, the -cyclodextrin complex gave (–)-dichloride,S-(+)-2-chloro-l-phenylethanol (14% ee) and (+)-1,2,2-trichloro-l-phenylethane, and the -cyclodextrin complex produced (+)-dichloride,S-(+)-chlorohydrin (8% ee) and (+)-trichloride. The chiral induction of the gas-solid halogenation using the solid cyclodextrin complexes is attributed to the ability to hold rigidly a chiral conformation of the crystalline state. However, the gas-solid hydrohalogenation all gave racemic products.Presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on Cyclodextrins, Lancaster, U.K., 20–25 July 1986.