Separation of racemic sulfoxides and sulfinate esters on four derivatized cyclodextrin chiral stationary phases using capillary gas chromatography

The separation of 17 chiral sulfoxides and eight chiral sulfinate esters by gas chromatography (GC) on four derivatized cyclodextrin chiral stationary phases (CSPs) (Chiraldex G-TA, G-BP, G-PN, B-DM) is presented. Many of these compounds are structural isomers or part of a homologous series. Differences in enantioselectivity of the methyl phenyl sulfoxide isomers on the derivatized gamma cyclodextrin and the heptakis 2,6-di-O-methyl-beta-cyclodextrin (i.e. B-DM) CSPs are discussed. Under the conditions of this study, the molecular mass cut-off for the GC separation of these compounds was approximately 230. Compounds of higher molecular mass were not eluted from the CSPs at reasonable times and temperatures, but these higher molecular mass enantiomers can be separated by liquid chromatography and capillary electrophoresis. The enantiomeric separation and elution order of a sulfinate ester containing two stereogenic centers as well as 15 chiral sulfoxides is presented. The G-TA and B-DM CSPs generally gave opposite elution orders for most of the compounds studied.     

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.     

Reversed-phase chiral HPLC and LC/MS analysis with tris(chloromethylphenylcarbamate) derivatives of cellulose and amylose as chiral stationary phases

Three polysaccharide-derived chiral stationary phases (CSP) were evaluated for the resolution of more than 200 racemic compounds of pharmaceutical interest in the reversed-phase (RP) separation mode. The population of test probes was carefully evaluated in order to insure that it covers as completely as possible all structural diversity of chiral pharmaceuticals. RP showed the highest potential for successful chiral resolution in HPLC and LC/MS analysis when compared to normal phase and polar organic separation modes. Method development consisted of optimizing mobile phase eluting strength, nature of organic modifier, nature of additive and column temperature. The newer CSPs, cellulose tris(3-chloro-4-methylphenylcarbamate) and amylose tris(2-chloro-5-methylphenylcarbamate), were compared to the commonly used cellulose tris(3,5-dimethylphenylcarbamate) in regards to their ability to provide baseline resolution. Comparable success rates were observed for these three CSPs of quite complimentary chiral recognition ability. The same method development strategy was evaluated for LC/MS analysis. Diethylamine as additive had a negative effect on analyte response with positive ion mode electrospray (ESI⁺) MS(/MS) detection, even at very low concentration levels (e.g., 0.025%). Decreasing the organic modifier (acetonitrile or methanol) content in the mobile phase often improved enantioselectivity. The column temperature had only a limited effect on chiral resolution, and this effect was compound dependent. Ammonium hydrogencarbonate was the preferred buffer salt for chiral LC with ESI⁺ MS detection for the successful separation and detection of most basic pharmaceutical racemic compounds. Ammonium acetate is a viable alternative to ammonium hydrogencarbonate. Aqueous formic acid with acetonitrile or methanol can be successfully used in the separation of acidic and neutral racemates. Cellulose tris(3-chloro-4-methylphenylcarbamate) and amylose tris(2-chloro-5-methylphenylcarbamate) emerge as CSPs of wide applicability in either commonly used separation modes rivaling such well established CSPs as cellulose tris(3,5-dimethylphenylcarbamate). Screening protocols including these two new CSPs in the preferentially screened set of chiral columns have higher success rates in achieving baseline resolution in shorter screening time.     

Enantiomeric separation using temperature-responsive chiral polymers composed of L-valine diamide derivatives in aqueous liquid chromatography

This paper describes enantiomer separation by aqueous liquid chromatography using chiral stationary phases (CSPs) in which temperature-responsive polymers derived from acryloyl-L-valine N-methylamide (1) and its N,N-dimethylamide analogue (2) were bound on silica gel supports. The linear polymers composed of monomer 1 and monomer 2 are temperature-responsive in solution and their aggregation and extension states related to water solubility are reversible at particular critical temperatures. During chromatography, enantioselectivity and retentivity for solute enantiomers were controlled by column temperature, which changes the aggregation and extension states of the chiral polymers depending upon their interior hydrophobic nature. Two different types of CSPs were made: a temperature-responsive linear polymer derived from 3-mercaptopropyl silica gel, and another polymer cross-linked with ethylene dimethacrylate from 3-methacryloyloxypropyl silica gel. The former CSP could separate racemic N-(3,5-dinitrobenzoyl(DNB))amino acid isopropyl esters. Retention of the amino acid derivatives was prolonged with an increase in column temperature. Enantioselectivity was also enhanced with temperature increase until the particular critical temperature. The latter, cross-linked CSP could not provide enantioselectivity for the amino acid derivatives in aqueous media, although the chiral valine diamide moieties were effective for enantiomer separation in non-aqueous media. The degree of hydrophobicity and volume of the bonded phase formed by the polymers on the support surface was determined by measuring the fluorescence of pyrene.     

Enantiomeric separation of chiral pesticides by high-performance liquid chromatography on an amylose tris-(S)-1-phenylethylcarbamate chiral stationary phase

Amylose tris-(S)-1-phenylethylcarbamate chiral stationary phase (CSP) was prepared. The direct enantiomeric separation of chiral pesticides on this CSP had been studied by HPLC. The mobile phase was n-hexane-isopropanol at a flow rate of 1.0 mL/min. The effects of isopropanol content and column temperature on retention and enantioselectivity were investigated. Thirty-two samples were tested, of which ten interacted enantioselectively with the CSP. Five samples were completely resolved and another five underwent near-baseline or partial resolution. The enantiomers were identified by a circular dichroism detector. Linear van't Hoff plots were established and the thermodynamic parameters were thus calculated.     

Monosubstituted positively charged cyclodextrins: Synthesis and applications in chiral separation

Several series of novel structurally well-defined positively charged CDs, applicable to alpha-, beta- and gamma-CDs for chiral separation using CE and chromatographic techniques have been developed. The chiral resolution capabilities of different series CDs towards amino acids and anionic analytes in CE are systematically investigated by considering all separation parameters including CD type, alkyl chain length of the cations attached to the CD rim, CD concentration, buffer pH, separation temperature and organic solvent. Typical results are demonstrated in the context. Examples of chiral separation with HPLC and supercritical fluid chromatography are first demonstrated by using coated chiral stationary phases (CSPs). Optimum CD loading content on the coated CSPs was explored in the chiral separation of neutral analytes.     

大环抗生素手性固定相拆分盐酸马布特罗对映体

基于三种大环抗生素类手性固定相Chirobiotic V,T和R,利用高效液相色谱法对盐酸马布特罗对映体进行了拆分;考察了洗脱模式、流动相组成、柱温等因素对分离的影响,对分离结果进行了比较,对分离机制进行了探讨.结果表明,盐酸马布特罗对映体在Chirobiotic R手性固定相上不能实现分离,在Chirobiotic V和T手性固定相上均可实现较好的基线分离.最佳色谱条件为:新极性有机相模式,流动相甲醇-冰醋酸-三乙胺(100∶0.01∶0.01,V/V/V),流速1.0mL/min,柱温20℃;相应的分离度分别可达3.08和3.73.与此同时,盐酸马布特罗对映体与大环抗生素类固定相之间的离子相互作用是实现对映体分离的最主要分离机制.     

Synthesis and characterization of homologous series with chiral (S)-1-methylpropyl terminal substituent

Homologous series of liquid crystalline azoesters and azomethine esters consisting of a (S)-1-methylpropyl group attached in one of the terminal positions have been synthesized and thermally characterized. All twenty-four derivatives from both series, namely, the 4-(4-n-alkoxybenzoyloxy)-4'-1-(S)-methylpropylazobenzenes and 4-(4-n-alkoxybenzoyloxy)benzylidene-4'-1-(S)-methylpropylanilines exhibit mesomorphism. The lower members of the homologous series show a chiral nematic phase while the higher members show smectic C*, smectic A as well as chiral nematic mesophases. The homologues have been characterized using IR, NMR and UV-Visible, spectroscopies, X-ray diffraction and DSC. Their mesomorphic properties are compared with those of structurally related homologous series.     

Novel strong cation-exchange type chiral stationary phase for the enantiomer separation of chiral amines by high-performance liquid chromatography

The preparation of novel brush-type chiral cation-exchange materials based on de novo designed synthetic low molecular mass selectors (SOs) and their evaluation for enantioselective separation of chiral amines by HPLC are presented. The SO as the functional unit for enantioselectivity contains a beta-aminocyclohexanesulfonic acid moiety and is readily accessible via straightforward synthesis in both enantiomeric forms yielding chiral stationary phases (CSPs) with opposite configurations, CSPs 1 and 2, and reversed elution orders. For the evaluation of these novel CSPs by HPLC a sound set of chiral amines, mainly amino-alcohol type drug molecules, was selected. The chromatographic evaluations were carried out using polar organic mobile phase conditions. All of the analytes could be baseline separated, compared to common CSPs in parts with excellent peak efficiencies (up to 70000 theoretical plates per meter for the second eluted enantiomer). A number of experimental parameters have been varied to look at and prove the underlying ion-exchange process on CSPs 1 and 2, and to reveal suitable conditions for their operation. In this context, the influence of proton activity in the mobile phase and the effects of varying concentration and type of the counterion as well as type of co-ion and of bulk solvent components were thoroughly investigated.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid with a methyleneoxy linking group

Three homologous series of chiral materials derived from (S)-2-(6-hydroxy-2-naphthly)propionic acid with a methyleneoxy linking group were synthesized for the investigation of mesomorphic properties. All the materials displayed enantiotropic SmA* and SmC* phases. The spontaneous polarization (P _s) and optical tilt angles in the SmC* phase of the chiral materials were measured. The maximum spontaneous polarizations are in the range 12.3-19.1 nC cm^-2. No significant difference could be found in the P _s values with respect to the various alkyl chiral chain lengths, n. The maximum optical tilt angles are in the range 25-30°. The optical tilt angles decrease with the increase in alkyl chain length, n.     

Enantiomeric separation of chiral theophylline derivatives by liquid chromatography on cellulose-based sorbents

The enantiomeric resolution of seven closely related theophylline racemates by high-performance liquid chromatography on cellulose-based sorbents, in particular Chiralcel-OD, -OC and -OJ, is described. Although all chiral stationary phases (CSPs) are suitable for the enantioseparation of all racemates investigated, it is obvious that method development is different for each stationary phase. The screenings with the above-mentioned CSP included variation of mobile phase and temperature. It turned out that Chiralcel-OD should be used with 2-propanol in n-hexane as the mobile phase at higher temperatures, whereas Chiralcel-OC performed best with methanol at ambient temperature. Improved enantioseparations were observed for Chiralcel-OJ with increased modifier concentrations in n-hexane at increased temperature.     

巴拉苏酰胺对映体的色谱分离研究

利用手性HPLC法对天然产物(+)-巴拉苏酰胺(balasubramide)及其对映体进行分离和光学纯度测定。在手性分离过程中,考察了两种不同的手性固定相和不同比例的流动相(正己烷和异丙醇),以进行手性分离方法的优化。结果表明:正己烷和异丙醇(70/30,V/V)在手性柱Chiralpak AD-H上获得最佳分离。光学活性的巴拉苏酰胺的对映体过量值高于98%,其分离因子(α)和分离度为2.15和21.80。本研究为光学活性的巴拉苏酰胺及其后续衍生物光学纯度控制提供了方法学基础。     

Unusual resolution of N-(3,5-dinitrobenzoyl)-alpha-amino acids on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

While HPLC chiral stationary phases (CSPs) based on chiral crown ethers have been known useful for the resolution of only racemic primary amino compounds or some secondary amino compounds, in this study, we first demonstrated that the CSP based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid is also useful for the resolution of N-benzoyl-alpha-amino acids, which do not contain a primary or secondary amino group. Especially, N-(3,5-dinitrobenzoyl)-alpha-amino acids were resolved better than corresponding N-(3-nitrobenzoyl)- or N-benzoyl-alpha-amino acids, the separation (alpha) and the resolution factors (R(S)) for the resolution of eight N-(3,5-dinitrobenzoyl)-alpha-amino acids being in the range of 1.06-1.81 and 0.54-2.81, respectively. The optimum mobile phase condition was the mixture of acetic acid-triethylamine-acetonitrile with the ratio of 0.05/0.25/100 (v/v/v).     

Enantiomeric separations of chiral polychlorinated biphenyls on three polysaccharide-type chiral stationary phases by supercritical fluid chromatography

Enantiomeric separations of 18 chiral polychlorinated biphenyls (PCBs) were investigated on three polysaccharide-type chiral stationary phases (CSPs; Sino-Chiral OJ, Chiralpak IB, and Chiralcel OD) by supercritical fluid chromatography (SFC). With these commonly used polysaccharide CSPs, 17 PCBs except PCB 135 (R(S) = 0.81) were well resolved (R(S) > 1.5) under appropriate mobile phases and temperatures. Using Sino-Chiral OJ, 14 PCBs could be baseline-separated, while only one and nine PCBs could be completely separated using Chiralpak IB and Chiralcel OD, respectively. The influence of column temperature was studied for the optimization of resolution, as well as for the type and percentage of organic modifier in the mobile phase. The resolution decreased as the temperature increased in the range of 26-40 °C in which the enantiomeric separations were an enthalpy-driven process. The addition of modifiers in the mobile phase decreased the resolution of the PCB enantiomers, but it clearly shortened their retention time. These separation results indicate that SFC is a promising chromatographic technique for chiral separation and enantiopure standard preparation.     

(S)-(+)-2-octanol as a chiral oil core for the microemulsion electrokinetic chromatographic separation of chiral basic drugs.

The enantiomeric separation of basic drugs was successfully demonstrated by using a novel chiral microemulsion electrokinetic chromatography (MEEKC). An interesting finding was that the chiral oil core ((S)-(+)-2-octanol) within the microemulsion droplets appeared to play an important role in the chiral separation mechanism. In addition, the enantioselectivity of the analyte-selector complex could be influenced by methanol, through an interaction with the complex. The chiral resolution (R(s)) and partition coefficient were strongly influenced by the concentration of methanol, pH, the concentration of chiral oil and the concentration of a cosurfactant. Under the optimized microemulsion conditions, the baseline separation of (+/-)-ephedrine (R(s) = 2.7), and the partial separations of (+/-)-norephedrine (R(s) = 1.3), (+/-)-synephrine (R(s) = 1.4) and (+/-)-propranolol (R(s) = 1.3), could be achieved.     

The solute molecule—Rigid or partially flexible? calculation of benzodiazepineR F values as an example

Summary The liquid-chromatographic separation of the enantiomers of amino acid esters as benzophenone Schiff-base derivatives on polysaccharide-derived chiral stationary phases (CSPs) is described. The performance of Chiralcel OF was superior to that of the other CSPs for resolution of benzophenone imine derivatives of amino acid ethyl and methyl esters. The enantiomers of most of the amino acid esters examined as their benzophenone imine derivatives were resolved to baseline on Chiralcel OF. The L-(−) enantiomers of all the analytes were preferentially retained on Chiralcel OF. The resolution of several imine derivatives of amino acid esters was investigated, as was the effect of eluent composition on the resolution of amino acid ethyl esters as their benzophenone imine derivatives.     

Evaluation of the chiral recognition properties as well as the column performance of four chiral stationary phases based on cellulose (3,5-dimethylphenylcarbamate) by parallel HPLC and SFC

The performance of four commercially available cellulose tris(3,5-dimethylphenylcarbamate) based chiral stationary phases (CSPs) was evaluated with parallel high performance liquid chromatography (HPLC) and super critical fluid chromatography (SFC). Retention, enantioselectivity, resolution and efficiency were compared for a set of neutral, basic and acidic compounds having different physico-chemical properties by using different mobile phase conditions. Although the chiral selector is the same in all the four CSPs, a large difference in the ability to retain and resolve enantiomers was observed under the same chromatographic conditions. We believe that this is mainly due to differences in the silica matrix and immobilization techniques used by the different vendors. An extended study of metoprolol and structure analogues gave a deeper understanding of the accessibility of the chiral discriminating interactions and its impact on the resolution of the racemic compounds on the four CSPs studied. Also, a clear difference in enantioselectivity is observed between SFC and LC mode, hydrogen bonding was found to play an important role in the differential binding of the enantiomers to the CSPs.     

Preparation of a new chiral stationary phase for HPLC based on the (R)- 1-phenyl-2-(4-methylphenyl)ethylamine amide derivative of (S)-valine and 2-chloro-3,5-dinitrobenzoic acid: enantioseparation of amino acid derivatives and pyrethroid insecticides

A novel chiral stationary phase (CSP) for HPLC was prepared by bonding (R)-1-phenyl-2-(4-methylphenyl)ethylamine amide derivative of (S)-valine to aminopropyl silica gel through a 2-amino-3,5-dinitro-1-carboxamido-benzene unit. The CSP was used for the separation of some amino acid derivatives and pyrethroid insecticides by chiral HPLC. Satisfactory baseline separation required optimization of the variables of mobile phase composition. Use of dichloromethane as modifier in the mobile phase gave baseline separations of amino acid derivatives. The two enantiomers of fenpropathrin and four stereoisomers of fenvalerate were baseline separated using hexane-dichloromethane-ethanol as mobile phase. The results show that the enantioselectivity of the new CSP is better than Pirkle type 1-A column for these compounds. Only partial separations were observed for the stereoisomers of cypermethrin and cyfluthrin, which gave even and eight peaks, respectively.     

Study of chiral recognition mechanism of O,O-diethyl (p-methylbenzenesulfonamindo)-aryl(alkyl)-methylphosphonates by HPLC with a series of CSPs

Five chiral stationary phases (CSPs) were used to separate the enantiomers of a series of O,O-diethyl (p-methyl-benzenesulfonamindo)- aryl(alkyl)-methylphosphonates. A chiral recognition mechanism was presented to explain the resolution of these compounds. Results show that CSP with strong π-acceptor 3,5-dinitrobenzoyl group and high steric hindrance has the best resolution ability in chiral separation of O,O-diethyi (p-methyl-benzenesulfonamindo)- aryl(alkyl)-methylphosphonates. When a CSP has just a strong π-acceptor 3,5-dinitrobenzoyl or high steric hindrance it does not have good chiral resolution ability. The chiral recognition is more difficult when the CSP has more than one asymmetric center.     

Study of the enantiomeric separation of the anticholinergic drugs on two immobilized polysaccharide‐based chiral stationary phases by HPLC and the possible chiral recognition mechanisms

In this work, the enantiomeric separation of ten anticholinergic drugs was first examined on two derivative polysaccharide chiral stationary phases (CSPs), i.e., Chiralpak ID and Chiralpak IA in the normal phase mode. Except for scopolamine hydrobromide, the remaining nine analytes could be completely separated with good resolutions using both columns under the optimized mobile phase conditions. And the enantiomeric discrimination ability of the studied CSPs towards nine analytes was in the order of Chiralpak ID > Chiralpak IA. The influences of organic modifier types, alcohol content, and base/acid additives on the enantiomeric separation were evaluated and optimized. According to the experimental results, the effect of the structures of analytes on enantiomeric separation was discussed. Additionally, the chiral recognition mechanisms were proposed based on the thermodynamic analysis of the experimental data.