Application of a new chiral stationary phase containing the glycopeptide antibiotic A-40,926 in the direct chromatographic resolution of β-amino acids

A new enantioselective HPLC procedure for the direct resolution of β-amino acids is described, based on the use of a new chiral stationary phase (CSP) containing the macrocyclic glycopeptide antibiotic A-40,926, structurally related to teicoplanin, covalently bonded to silica gel microparticles. The new CSP shows higher enantioselectivity and broader applicability in this field compared to the parent teicoplanin phase. The potential for semi-preparative separations on the A-40,926-CSP is demonstrated for a selected cyclic β-amino acid.     

High-performance liquid chromatographic enantioseparation of cyclic β-aminohydroxamic acids on zwitterionic chiral stationary phases based on Cinchona alkaloids

Cyclic β-aminohydroxamic acid enantiomer pairs were stereoselectively separated by high-performance liquid chromatography on the recently developed Cinchona alkaloid-based zwitterionic chiral stationary phases Chiralpak ZWIX(+)™, ZWIX(−)™, ZWIX(+A) and ZWIX(−A). The results of variation of the applied chromatographic conditions, such as the bulk solvent composition, the concentrations and ratio of the acid and base additives, the presence of water as mobile phase additive and the counter-ion concentration furnished a better understanding of the retention mechanism. A thermodynamic study in the temperature range 5–50 °C revealed enthalpy-controlled enantiodiscrimination in all cases. The structure–selectivity relationships clearly indicated the importance of the strereochemistry of the functional groups. From an enantiorecognition aspect, the diexo position of the functional groups always proved more favorable than the diendo position. The elution sequence was determined in all cases and was found to reversed when ZWIX(+)™ was changed to ZWIX(−)™ or ZWIX(+A) to ZWIX(−A).     

Enantioseparation of α-amino acids on an 18-crown-6-tetracarboxylic acid-bonded silica by capillary electrochromatography

(−)-(18-Crown-6)-2,3,11,12-tetracarboxylic acid-bonded silica was used as the chiral stationary phase in capillary electrochromatography (CEC) for enantioseparation of some α-amino acids. Separation data in CEC were measured in mobile phases of varying pH, and composition of methanol and buffer, and compared with those in capillary liquid chromatography (CLC). In CEC better enantioseparation was generally obtained in the eluent of lower pH, higher buffer concentration and intermediate MeOH content, usually at the expense of analysis time. CEC showed generally better enantioselectivity and resolutions than CLC for the amino acids investigated.     

New gas chromatographic method for the enantioseparation of β-amino acids by a rapid double derivatization technique

A novel gas chromatographic method was developed for the enantioseparation of valuable acyclic and carbocyclic cis- and trans-β-amino acids, including cispentacin and a number of its analogues and homologues. Excellent (in most cases baseline) separation was achieved for the racemates of these β-amino acids on CP-Chirasil-Dex CB or CP-Chirasil L-Val columns after a simple and rapid double derivatization (esterification followed by N-acylation). The elution sequences were determined in all cases.     

Direct high-performance liquid chromatographic enantioseparation of free α-, β- and γ-aminophosphonic acids employing cinchona-based chiral zwitterionic ion exchangers

We report a chiral high-performance liquid chromatographic enantioseparation method for free α-aminophosphonic, β-aminophosphonic, and γ-aminophosphonic acids, aminohydroxyphosphonic acids, and aromatic aminophosphinic acids with different substitution patterns. Enantioseparation of these synthons was achieved by means of high-performance liquid chromatography on CHIRALPAK ZWIX(+) and ZWIX(-) (cinchona-based chiral zwitterionic ion exchangers) under polar organic chromatographic elution conditions. Mobile phase characteristics such as acid-to-base ratio, type of counterion, and solvent composition were systematically varied in order to investigate their effect on the separation performance and to achieve optimal separation conditions for the set of analytes. Under the optimized conditions, 32 of 37 racemic aminophosphonic acids studied reached baseline separation when we employed a single generic mass-spectrometry-compatible mobile phase, with reversal of the elution order when we used (+) and (-) versions of the chiral stationary phase.

Preparation and chromatographic evaluation of a chiral stationary phase based on carboxymethyl-β-cyclodextrin for high-performance liquid chromatography

A novel chiral stationary phase (CSP) was prepared by chemically bonding carboxymethyl-β-cyclodextrin (CM-β-CD) onto 3-aminopropyl silica gel and showed excellent enantioseparation abilities for a broad range of chiral compounds and drugs.     

Synthesis and evaluation of a chiral stationary phase based on quinine: enantioresolution of dinitrophenyl derivatives of α-amino acids

The natural alkaloid quinine (QN) was immobilized on porous silica particles, and part of the material was subsequently endcapped with n-hexyl hydrocarbon chains. Two synthetic strategies for silanization of the support were first compared. These columns were thoroughly evaluated in order to study the influence of endcapping in the enantiorecognition features. Enantioseparations of twenty N-derivatized 2,4-dinitrophenyl α-amino acids (DNP-amino acids) were studied by changing mobile phase pH, buffer concentration, type of organic solvent in the mobile phase, and column temperature. Maximum retention factors were observed at pH ≈6, at this intermediate pH the tertiary amine of the quinine is protonated to a high degree and therefore available for strong electrostatic interactions with unprotonated anionic DNP-amino acids. The enantioselectivity factors, however, increased as the pH did in the range between 5 and 7. The increase in ionic strength had influence on retention, but not on enantioselectivity, allowing the use of this variable for optimization of retention factors. Finally, the thermodynamic transfer parameters of the enantiomers from the mobile to both CSPs (with and without endcapping, QN-CSP(EC) and QN-CSP, respectively) were estimated from van't Hoff plots within the range of 10-40 °C. Thus, the differences in the transfer enthalpy, Δ(ΔH°), and transfer entropy, Δ(ΔS°), enabled an investigation of the origin of the differences in interaction energies.     

Enantiomeric resolution of (±)-licarin A by high-performance liquid-chromatography using a chiral stationary phase

(±)-Licarin A (1), a neolignan obtained by the oxidative coupling reaction of isoeugenol, had in this study its enantiomers resolved. A novel, quick and efficient enantiomeric resolution of 1 was directly performed by chiral high-performance liquid chromatography (HPLC-PDA) protocol (CHIRALPACK^® AD column; 9:1 (v/v) n-hexane:2-propanol; 1.0 mL/min). This method provided a chromatogram profile with a well-resolved peak separation. After isolation of each enantiomer with ee > 99.9%, they were analysed in a polarimeter. Compound 2, which showed a retention time (t_r) of 12.13 min, was the (+)-enantiomer and compound 3 (t_r = 18.90 min) was the (−)-enantiomer.     

Preparation and enantioseparation characteristics of three chiral stationary phases based on modified β-cyclodextrin for liquid chromatography

In order to study the effect of the nature and the length of the spacer, three mixed 10-undecenoate/phenylcarbamate derivatives of β-cyclodextrin have been prepared and linked to allylsilica gel by means of a radical reaction. The chiral recognition ability of the resulting materials, when used as liquid chromatography chiral stationary phases (CSPs), was evaluated using heptane and either 2-propanol or chloroform as organic mobile-phase modifiers. A large variety of racemic compounds have been separated successfully on these CSPs (mainly pharmaceuticals and herbicides). Optimization of these separations was discussed in terms of mobile-phase composition and structural patterns of the injected analytes. The efficiencies of the three prepared materials were compared to those of previously described perphenylated-β-cyclodextrin column and to analogous cellulose derivative-based CSPs. Schematic illustration of the b-cyclodextrin/mandelic acid inclusion complex     

Cu(II) mediated selective transport of α-amino acids across a bulk liquid membrane using a chiral lipophilic ligand as a carrier

The chiral ligand (R)-1 acts as a carrier for the Cu(II) mediated membrane transport of α-amino acids coupled with the counter-transport of protons.     

Chromatographic enantioseparation of racemic α-(1-naphthyl)ethylammonium perchlorate by a Merrifield resin-bound enantiomerically pure chiral dimethylpyridino-18-crown-6 ligand

Three novel chiral pyridino-18-crown-6 ligands (S,S)-1, (S,S)-2 and (S,S)-3 were prepared and (S,S)-1 was attached to a Merrifield resin. The resulting adsorbent (S,S)-5 was used as a chiral stationary phase in the chromatographic enantioseparation of racemic α-(1-naphthyl)ethylammonium perchlorate. Also, a new chiral pyridono-18-crown-6 ligand (S,S)-6, used for the synthesis of (S,S)-1 and (S,S)-2, was prepared in two different ways.     

Micellar liquid chromatography for enantioseparation of β-adrenolytics using (S)-ketoprofen-based reagents

Micellar liquid chromatographic method has been developed for enantioseparation of four β-adrenolytics, namely, (RS)-salbutamol, (RS)-carvedilol, (RS)-bisoprolol, and (RS)-betaxolol. Both sodium docyl sulfate and Brij-35 were used as the surfactants in water as the mobile phase. Advantages for using both the surfactants in combination were investigated. Two (S)-ketoprofen-based activated esters were synthesized by activating its carboxylic group with N-hydroxybenzotriazole and N-hydroxysuccinimide, respectively. The esters were characterized by UV, IR, ¹H-NMR, HRMS, and elemental analyses. These reagents were used for the synthesis of diastereomeric derivatives of the chosen β-adrenolytics. These diastereomeric derivatives were enantioseparated on C₁₈ column by high-performance liquid chromatography. Chromatographic conditions were optimized by varying concentration of surfactant and buffer, and pH. The method was validated according to International Conference of Harmonization guideline and the retention factor (k), selectivity factor (α), resolution factor (R_S), and limit of detection and limit of quantification were calculated.     

Preparation and chromatographic characteristics of a novel 2,6-dimethyl-β-CD bonded HPLC chiral stationary phase

A novel 2,6-dimethyl-b-CD bonded and silica based HPLC chiral stationary phase（CSP） was prepared.The diphenylmethane diisocyanate（MDI） was applied for the first time in the immobilization of 2,6-dimethyl-b-CD onto the surface of aminized silica gel under mild conditions. The composite materials obtained were used as the CSP for chiral separation processes and this kind of CSP exhibits good enantioselectivity for a variety of chiral compounds under reversed-phase conditions.     

Enantioseparation of a novel “click” chemistry derived native β-cyclodextrin chiral stationary phase for high-performance liquid chromatography

A novel native β-cyclodextrin chiral stationary phase was prepared via “click” chemistry with cuprous iodide–triphenylphosphine complex as the catalyst and applied for enantioseparation of Dns-amino acids, substituted phenyl and phenoxy group modified propionic acids, flavonoids, and some pharmaceutical compounds such as nimodipine, propranolol, brompheniramine and bendroflumethiazide in reversed-phase high-performance liquid chromatography. The studied analytes could be resolved under different separation conditions. The resolution of Dns-DL-Leu could reach 5.08 using a mobile phase consisting of 1% (w/w) triethylammonium acetate buffer (pH 4.11) and methanol (50:50 v/v). The effects of buffer pH and the content of organic modifier on enantioseparation of Dns-amino acids by this novel chiral phase were being investigated. The separation results demonstrate that click chemistry, a versatile reaction, affords a facile approach towards the preparation of stable chiral stationary phases.     

Capillary electrophoresis and molecular modeling of the chiral separation of aromatic amino acids using α/β-cyclodextrin and 18-crown-6

In this work, chiral separation of enantiomers of three amino acids was achieved using capillary electrophoresis technique with α-cyclodextrin (αCD) as a running buffer additive. Only tryptophan has exhibited baseline separation in the presence of αCD, while the enantiomers of the other two amino acids, phenylalanine and tyrosine, were only partially separated. The addition of 18-crown-6 (18C6) as a second additive imparted only slight improvement to the separation of all enantiomers. On the other hand, all three racemic amino acid mixtures demonstrated no indication of separation when the larger cavity cyclodextrin members, β- and γCD, are used as running buffer chiral additives. However, remarkable improvements in the separation of the enantiomers of phenylalanine and tyrosine were obtained when 18C6 is used together with βCD as a running buffer additive. Surprisingly, tryptophan enantiomers were not separated by the dual additive system of cyclodextrin and crown ether. Using electrospray ionization mass spectrometry (ESI-MS), all amino acids were found to form stable binary complexes with individual hosts as well as ternary compounds involving the crown ether and the cyclodextrin. Furthermore, we used molecular dynamics (MD) simulations to build a clear picture about the interaction between the guest and the hosts. Most of these complexes remained stable throughout the simulation times, and the molecular dynamics study allowed better understanding of these supramolecular assemblies.     

Ternary enantioselective complexes from α-amino acids, 18-crown-6 ether and a macrocyclic xanthone-based receptor

A macrocyclic receptor shaped by two xanthone units bonded to binaphthyl is able to extract zwitterionic amino acids enantioselectively from water to chloroform in the presence of 18-crown-6 ether. Based on ¹H NMR and circular dichroism spectra, the most stable associate of receptor 1 with alanine is assigned to the (M,S) configuration while the opposite configuration in phenylglycine yields the strong associate (M,R).     

Permethylated β-cyclodextrin in liquid poly(oxyethylene) as a stationary phase for capillary gas chromatography

This paper reports the study of poly(oxyethylene) as a solvent for heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (PM-β-CD) and the potential of these mixtures for constructing enantioselective gas chromatography columns. Enantioseparations of volatile racemic mixtures using capillaries coated with 10–50% PM-β-CD diluted in Carbowax 20M were evaluated. The influence of the polymer type on retention, separation and reproducibility over a period of time is critically discussed. The poly(oxyethylene)-based columns were also compared to columns prepared by dilution of PM-β-CD in two polysiloxanes, SE-30 and SE-54, at several temperatures. Finally, we evaluated a new stationary phase prepared by crosslinking the chiral selector to the poly(oxyethylene) matrix.