Separation of chiral phosphorus compounds on the substituted β‐cyclodextrin stationary phase in normal‐phase liquid chromatography

The separation of enantiomers of a series of eighteen novel nitrogen mustard linked phosphoryl diamide derivatives was investigated on the prepared phenyl carbamate derivative β‐cyclodextrin bonded phase in normal‐phase HPLC. Some of the enantiomers could be separated in baseline. The chiral recognition mechanism was also suggested for the separation of chiral phosphorus organic compounds.     

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.     

Preparation and evaluation of monodispersed,submicron, non‐porous silica particles functionalized with β‐CD derivatives for chiral‐pressurized capillary electrochromatography

Submicron, non‐porous, chiral silica stationary phase has been prepared by the immobilization of functionalized β‐CD derivatives to isocyanate‐modified silica via chemical reaction and applied to the pressurized capillary electrochromatography (pCEC) enantio‐separation of various chiral compounds. The submicron, non‐porous, cyclodextrin‐based chiral stationary phases (sub_μm‐CSP2) exhibited excellent chiral recognition of a wide range of analytes including clenbuterol hydrochloride, mexiletine hydrochloride, chlorpheniramine maleate, esmolol hydrochloride, and metoprolol tartrate. The synthesized submicron particles were regularly spherical and uniformly non‐porous with an average diameter of around 800 nm and a mean pore size of less than 2 nm. The synthesized chiral stationary phase was packed into 10 cm × 100 μm id capillary columns. The sub_μm‐CSP2 column used in the pCEC system showed better separation of the racemates and at a higher rate compared to those used in the capillary liquid chromatography mode (cLC) system. The sub_μm‐CSP2 possessed high mechanical strength, high stereoselectivity, and long lifespan, demonstrating rapid enantio‐separation and good resolution of samples. The column provided an efficiency of up to 170 000 plates/m for n‐propylbenzene.     

Effect of analyte lipophilicity on the resolution of α‐ and β‐amino acids on liquid chromatographic ligand exchange chiral stationary phases

Separation of the two enantiomers of racemic α‐ and β‐amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N‐((S)‐1‐hydroxymethy‐3‐methylbutyl)‐N‐undecylaminoacetate or sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.     

HPLC enantioseparation of β2‐homoamino acids using crown ether‐based chiral stationary phase

RP high‐performance liquid chromatographic methods were developed for the enantioseparation of eleven unusual β²‐homoamino acids. The underivatized analytes were separated on a chiral stationary phase containing (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid as chiral selector. The effects of organic (alcoholic) and acidic modifiers, the mobile phase composition and temperature on the separation were investigated. The structures of the substituents in the α‐position of the analytes substantially influenced the retention and resolution. The elution sequence was determined in some cases: the S enantiomers eluted before the R enantiomers.     

Enantiomeric separation of 2‐arylpropionic acid nonsteroidal anti‐inflammatory drugs by HPLC with hydroxypropyl‐β‐cyclodextrin as chiral mobile phase additive

The enantio‐separations of eight 2‐arylpropionic acid nonsteroidal anti‐inflammatory drugs (2‐APA NSAIDs) were established using reversed‐phase high‐performance liquid chromatography with hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) as chiral mobile phase additive for studying the stereoselective skin permeation of suprofen, ketoprofen, naproxen, indoprofen, fenoprofen, furbiprofen, ibuprofen and carprofen. The effects of the mobile phase composition, concentration of HP‐β‐CD and column temperature on retention and enantioselective separation were investigated. With 2‐APA NSAIDs as acidic analytes, the retention times and resolutions of the enantiomers were strongly related to the pH of the mobile phase. In addition, both the concentration of HP‐β‐CD and temperature had a great effect on retention time, but only a slight or almost no effect on resolutions of the analytes. Enantioseparations were achieved on a Shimpack CLC‐ODS (150 × 4.6 mm i.d., 5 μm) column. The mobile phase was a mixture of methanol and phosphate buffer (pH 4.0–5.5, 20 mM) containing 25 mM HP‐β‐CD. This method was flexible, simple and economically advantageous over the use of chiral stationary phase, and was successfully applied to the enantioselective determination of the racemic 2‐APA NSAIDs in an enantioselective skin permeation study. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of analyte lipophilicity on the resolution of α-amino acids on a HPLC chiral stationary phase based on crown ether

The effect of analyte lipophilicity on the resolution of α-amino acids on a chiral stationary phase based on chiral crown ether has been examined by the chromatographic resolution trends for the resolution of a homologous series of five α-amino acids with an alkyl group of different length at the chiral center. The retention factors (k₁ and k₂) for the two enantiomers and the separation factors (α) were found to depend on the lipophilicity of the α-amino acid. In general, the retention factors increased as the organic modifier content in the mobile phase increased, the degree of the enhancement of retention factors being dependent on the analyte lipophilicity. The separation factors also increased as the analyte lipophilicity and the organic modifier content in the mobile phase increased. Possible rationales for these behaviors have been proposed.     

Preparation of a permethylated β‐cyclodextrin chiral stationary phase by one‐pot hydrosilylation and immobilization at the C2 position for chiral high‐performance liquid chromatography

A novel cyclodextrin intermediate, mono‐2^A‐allylcarbamido‐2^A‐deoxy‐permethylated β‐cyclodextrin, was synthesized by reacting allylamine and newly prepared mono‐2^A‐azido‐2^A‐deoxy‐permethylated β‐cyclodextrin by the Staudinger reaction and anchored onto porous silica beads by a one‐pot hydrosilylation and immobilization procedure to afford a novel chiral stationary phase. This stationary phase acts as a new member of the previous chiral stationary phase series immobilized on the cyclodextrin C2 position. This stationary phase depicted enantiomeric separation abilities toward a series of bicyclic and tricyclic racemates under reversed‐phase conditions. The resolutions for hesperetin and naringenin achieved on the current phase reached 3.91 and 1.11, respectively, much higher than the previous permethylated β‐cyclodextrin with the linkage at the C6 position.     

Enantioselective separation of chiral vicinal diols in capillary electrophoresis using a mono‐6A‐aminoethylamino‐β‐cyclodextrin as a chiral selector

This paper describes an improved access to mono‐6^A‐aminoethylamino‐β‐CD (β‐CDen), a very efficient cationic chiral selector for CZE in the separation of eight chiral aromatic vicinal diols. The β‐CDen concentration has a strong influence on the efficiency of enantioseparation. The effects of the pH and concentration of the BGE, the capillary temperature, and the applied voltage on the resolution and separation selectivity have been studied. Excellent chiral resolution was achieved under the optimal conditions of β‐CDen 10 mM, pH 10, 200 mM borate buffer at 15 kV and 20°C within 20 min. Moreover, the developed method was successfully applied to the determination of the enantiomeric purity of the catalytic asymmetric dihydroxylation (AD) reaction products.     

Enantioseparation tuned by solvent polarity on a β‐cyclodextrin clicked chiral stationary phase

The efficient enantioseparation of 26 racemates has been achieved with the perphenylcarbamoylated cyclodextrin clicked chiral stationary phase by screening the optimum composition of mobile phase in high‐performance liquid chromatography. The chromatographic results indicate that both the retention and chiral resolution of racemates are closely related to the polarity of the mobile phases and the structures of analytes. The addition of alcohols can significantly tune the enantioseparation in normal‐phase high‐performance liquid chromatography. The addition of methanol and the ratio of ethanol/methanol or isopropanol/methanol played a key role on the resolution of flavonoids in ternary eluent systems. The chiral separation of flavonoids with pure organic solvent as mobile phase indicates the preferential order for chiral resolution is methanol>ethanol>isopropanol>n‐propanol>acetonitrile.     

Capillary GC using pyridyl β‐cyclodextrin stationary phase

A new β‐CD derivative, heptakis [2,6‐di‐O‐pentyl‐3‐O‐(4′‐chloro‐5′‐pyridylmethyl)]‐β‐CD, was synthesized by the selective introduction of a pyridyl group on the 3‐positions of β‐CD. The chromatographic properties of the pyridyl β‐CD derivative were studied by using it as the stationary phase in capillary GC. The polarity of the prepared stationary phase was moderate, and the separation results demonstrated that the prepared stationary phase possessed excellent separation ability and chiral recognition for a wide range of analytes. Not only the aromatic positional isomers, such as o‐, m‐, p‐xylene and α‐, β‐naphthol isomers, but also some compounds with multi‐stereogenic centers, such as n‐(1‐methylpropyl)‐3‐(2,2‐dichloroethenyl)‐2,2‐dimethylcyclopropanecarboxamide and n‐(1‐methylpropyl)‐3‐(2‐chloro‐3,3,3‐trifluoropropenyl)‐2,2‐dimethylcyclopropanecarboxamide with three stereogenic centers including eight configurational isomers, were successfully separated. The results also indicated that the polarity of the β‐CD derivative, and the hydrogen bonding between the β‐CD derivative, and the analytes had a very important effect on separation.     

Effect of the residual silanol group protection on the liquid chromatographic resolution of α‐amino acids and proton pump inhibitors on a ligand exchange chiral stationary phase

A new ligand exchange chiral stationary phase (new CSP) containing residual silanol group‐protecting n‐octyl groups on the silica surface was prepared by treating a ligand exchange CSP (original CSP) based on sodium N‐[(R)‐2‐hydroxy‐1‐phenylethyl]‐N‐undecylaminoacetate bonded to silica gel with excess n‐octyltriethoxysilane. The new and original CSPs containing an identical amount of chiral selector were applied to the resolution of α‐amino acids and proton pump inhibitors (PPIs) including omeprazole, pantoprazole, lansoprazole, and rabeprazole. The separation factors (α) and resolutions (R_S) were greater on the new CSP than on the original CSP except for the resolution of asparagine. The trends of the retention factors (k₁) for the resolution of α‐amino acids on the new and original CSPs with the variation of the organic modifier content in aqueous mobile phase were opposite to those for the resolution of PPIs. Removal of the nonenantioselective interactions between the residual silanol groups and the analytes and the improved lipophilicity of the new CSP were proposed to be responsible for the improved chiral recognition ability of the new CSP and the different retention behaviors of the enantiomers between the new and original CSPs.     

Preparation and HPLC application of chiral stationary phase from 4‐tert‐butylphenylcarbamates of cellulose and amylose immobilized onto silica gel

The 4‐tert‐butylphenylcarbamates of cellulose and amylose bearing a small amount of 3‐(triethoxysilyl)propyl residues were synthesized by a one‐pot process and efficiently immobilized onto a silica gel through intermolecular polycondensation of the triethoxysilyl groups. The obtained chiral packing materials (CPMs) were evaluated by HPLC. The polysaccharide derivatives containing about 1–2% of the 3‐(triethoxysilyl)propyl residue were efficiently immobilized with a high chiral recognition ability. The immobilized CPMs could be used with the eluents containing chloroform and tetrahydrofuran (THF), which cannot be used with the conventional coated‐type CPMs. By using these eluents, the chiral recognition for many racemates was improved.     

Correlation between structure of dihydropyridine calcium antagonists and their retention behavior and enantioseparation on the β-cyclodextrin stationary phase in HPLC

Chromatographic data for five dihydropyridine calcium antagonists (CAs) were obtained on β-cyclodextrin column in the reversed phase HPLC. Interaction energies of solute-cyclodextrin complexes formed by docking of different parts of the CAs' molecules into the cyclodextrin cavity were calculated. Results from the HPLC experiments and molecular modeling were compared. It was found that several types of inclusion complexes are energetically possible and contribute to the retention simultaneously. Only certain inclusion complex are assumed to be responsible for enantioseparation of CAs. Nevertheless molecular modeling studies are a useful tool for explaining the enantioselectivity and predicting the elution order of enantiomers.     

Preparation and characterization of phenyl carbamate derivative β-cyclodextrin bonded phase for chiral HPLC

A partially substituted β-cyclodextrin chiral stationary phase was prepared by the reaction of phenyl isocyanate. The enantiomers of a series of O,O-diethyl(p-methylbenzenesulfonamido)-aryl(or alkyl)-methylphosphonates were studied on the prepared phenyl carbamate derivative β-cyclodextrin bonded phase and a commercial (S)-(+)-1–(1-naphthyl) ethylcarbamate derivative β-cyclodextrin bonded phase on normal phase chromatographic condition. Results show that the prepared phenyl carbamate derivative β-cyclodextrin bonded phase has better enantiomeric selectivity to the series of compounds. A chiral recognition mechanism was suggested for the separation of these novel organic phosphorus enantiomers.