Direct resolution of the enantiomers of new diphosphine and diphosphine oxide ligands by high-performance liquid chromatography

Summary Resolution of the enantiomers of new racemic diphosphines, which are very useful ligands for stereoselective catalysts, and of the corresponding phosphine oxides has been investigated by high-performance liquid chromatography (HPLC) on four different chiral stationary phases (CSP)—Chiralpak AD, Whelk-01, and Supelcosil naphthylurea and phenylurea columns. The mobile phases were optimized to achieve separation of the enantiomers. α andR _S values ranged from 1.05 to 5.17 and from 0.37 to 6.57, respectively, for the Chiralpak AD and (R,R)-Whelk-01 columns. For the Supelcosil LC-(R)-phenylurea and Supelcosil LC-(S)-naphthylurea columns α values ranged from 1.05 to 1.62 andR _S from 0.35 to 3.61.     

Stereogenic properties of spiranes combined with four equivalent conventional centres of chirality

Derivatives of the tri-spirane pentaerythritoxy-cyclophosphazene compound, 1, have been used to investigate the stereogenic properties of spiranes combined with four equivalent conventional centres of chirality. In compound 1 the two inner rings are carbocyclic and symmetrical and the two outer rings are cyclotriphosphazenes substituted in different positions to provide the conventional centres of chirality. The case of combining spiranes with four equivalent centres of chirality has been investigated by the reaction of 1 with dimethylamine in a 1 : 8 molar ratio to give four diastereoisomeric products, in which the two cyclophosphazene rings are non-geminally di-substituted in either cis or trans configurations; viz. the cis-cis (2a), cis-trans (2b) and two trans-trans substituted dimethylamine derivatives 2c and 2d. The structure and stereogenic properties of compound 2a (C2 symmetry) have been characterized previously by both X-ray crystallography and 31P NMR spectroscopy on addition of a chiral solvating agent (CSA) and the structures and stereogenic properties of compounds 2b (C1) and 2d (C2) have been similarly characterized in this work. The structure of compound 2c has been characterised by X-ray crystallography and found to be meso with the relatively rare S4 symmetry. The four diastereoisomeric products 2a-2d represent all the possible stereochemical isomers of a spirane combined with four equivalent conventional centres of chirality.     

Synthesis, resolution, and absolute configuration of difunctionalized Tröger's base derivatives

Two racemic derivatives of Tr?ger's base, the 2,8-diboronic acid ester 6 and the 3,9-dibromo-substituted derivative 5, were synthesized and successfully resolved by HPLC on a chiral stationary Whelk-01 phase on a semipreparative scale, thereby giving rise to both enantiomers in a pure form. These functionalized C(2)-symmetric building blocks are valuable precursors for a variety of further applications. Their absolute configurations were determined by comparison of their quantum chemically calculated CD and UV/Vis spectra with the experimental ones and were independently confirmed by X-ray diffraction analysis.     

手性HPLC法测定布洛芬立体选择性酯化的转化率和对映体过量

以脂肪酶在正己烷中催化消旋布洛芬与正丁醇的立体选择性酯化反应为例,研究了用手性ＨＰＬＣ法进行分析时产物及底物的峰面积变化与转化率的关系,确定了布洛芬丁酯与未反应布洛芬的峰面积－浓度响应因子的比值,从而可通过一次进样用峰面积直接测定布洛芬的转化率和对映体过量。     

Determination of the absolute configuration of the enantiomers of dihydroquinolines, isolated by chiral chromatography, by non empirical analysis of circular dichroism spectra and X-ray analysis

The enantiomers of racemic 3,4-dihydroquinolines with an acetal or thioacetal spiro ring and a quaternary stereogenic carbon have been isolated through semipreparative chiral chromatography using a polysaccharide-derived chiral stationary phase (Chiralpak AD) and n-hexane/ethanol as a mobile phase. The absolute configurations of the enantiomers of four compounds have been determined by a comparison of density functional theory (DFT) calculations of their electronic circular dichroism (ECD) spectra with the experimental ECD data. A detailed conformer population search to achieve a conformational average of these compounds was crucial, due to the flexibility of these molecules. The conformer distribution was evaluated by spartan 02 and the structure of each of the conformers found within 4 kcal/mol energy range was optimized with DFT. The final calculated ECD spectrum obtained after Boltzmann averaging was compared with the ECD spectrum of the less well retained enantiomer and the correlation (R)/(−) was established for all compounds. The monocrystals of both enantiomers of one compound were obtained from the HPLC eluates. Their absolute configurations were determined by X-ray crystallographic analysis and confirmed by ECD analysis. In all cases, the second-eluted enantiomer in chiral HPLC exhibits an (R)-configuration.     

Hybridation of different chiral separation techniques with ICP-MS detection for the separation and determination of selenomethionine enantiomers: chiral speciation of selenized yeast

Enantioseparation and determination of selenomethionine enantiomers in selenized yeast was investigated using chiral separation techniques based on different principles, coupled on-line to inductively coupled plasma mass spectrometry (ICP-MS) for selenium-specific detection. High performance liquid chromatography (HPLC) on a beta-cyclodestrin (beta-CD) column, cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC), gas chromatography (GC) on a Chirasil-L-Val column, and HPLC on a Chirobiotic T column have been investigated as the chiral separation techniques. For HPLC separation on the beta-CD column, and also for CD-MEKC, selenomethionine enantiomers were derivatized with NDA/CN(-). For chiral separation by GC, selenomethionine enantiomers were converted into their N-trifluoroacetyl (TFA)-O-alkyl esters. The developed hybridation methodologies are compared with respect to enantioselectivity, sensitivity and analysis time. The usefulness of the best-suited method [HPLC (Chirobiotic T)-ICP-MS] was demonstrated by its application to the successful chiral speciation of selenium and D-and L-selenomethionine content determination in selenized yeast.     

Asymmetric hydroformylation of styrene using rhodium and platinum complexes of diphosphites containing chiral chelate backbones and chiral 1,3,2-dioxaphosphorinane moieties

Several chiral diphosphite ligands containing six stereogenic centres were synthesised and tested in order to study chiral cooperativity in the Rh- and Pt-catalysed asymmetric hydroformylation of styrene. The ligands were prepared either by the reaction of 2,4-pentanediol enantiomers with (4R,6R)-4,6-dimethyl-2-chloro-1,3,2-dioxaphosphorinane or that of (1S,3S)-1,3-diphenyl-1,3-propanediol with 4,6-dimethyl-2-chloro-1,3,2-dioxaphosphorinane enantiomers. Thus the chirality was varied both in the chelate backbone and in the terminal groups of the ligands. In case of Pt-catalysed hydroformylation, the stereogenic elements in the bridge have been found to be determinate for the product configuration with a cooperative effect from the terminal groups when the constellations are matched with 40% e.e. maximum enantioselectivity. Some coordination chemistry and the crystal structure determination of these ligands are also reported.     

Enantiomeric separation of some pharmaceutical intermediates and reversal of elution orders by high-performance liquid chromatography using cellulose and amylose tris(3,5-dimethylphenylcarbamate) derivatives as stationary phases

Several pairs of enantiomers of pharmaceutical intermediates were separated by HPLC directly on cellulose and amylose tris(3,5-dimethylphenylcarbamate) derivatives (Chiralcel OD and Chiralpak AD) using hexane as mobile phase with 2-propanol or ethanol as modifier. The separation and elution order of the enantiomers on the two columns using different alcohol modifiers were compared. Reversal of the elution order of some enantiomeric pairs associated with increased retention of many of these solutes upon changing the mobile phase modifier from 2-propanol to ethanol was observed. The effect of structural variation of two pairs of enantiomers on their k' and separation factor alpha was noted. Chiralcel OD and Chiralpak AD columns provided different retention, separation and elution order of some of the enantiomeric pairs.     

Electrolyte and additive effects on enantiomer separation of peptides by nonaqueous ion-pair capillary electrophoresis using tert.-butylcarbamoylquinine as chiral counterion

Nonaqueous ion-pair capillary electrophoresis separations of N-protected (all-R)/(all-S) alanine peptide enantiomers with up to six amino acid residues using tert.-butylcarbamoylquinine as selector and employing the partial filling technique are presented. The effects of various conditional parameters on separation were studied, namely chemical nature of the capillary wall, solvent composition of the background electrolyte (BGE), acid-base-ratio (equivalent to apparent pH), ionic strength and selector concentration. The influence of the solvent composition (methanol-ethanol ratios) on resolution turned out to be rather complex. The separation of the peptide enantiomers was strongly altered by small changes in pH and ionic strength. An increase of the selector concentration was found to offer an easy way for enhancing enantioselectivity, although some drawbacks, e.g., elongation of run times, have to be considered. A method was developed that allowed the separation of N-3,5-dinitrobenzoyl oligoalanine enantiomers containing 1-6 amino acid residues in one run. Like in a recent high-performance liquid chromatography (HPLC) study, separation selectivity thereby decreased from 1.541 (Ala), 1.340 (Ala(2)), 1.054 (Ala(3)), 1.029 (Ala(4)), 1.024 (Ala(5)) to 1.020 (Ala(6)). In addition, all four stereoisomers of N-2,4-dinitrophenyl- and N-3,5-dinitrobenzyloxycarbonyl-protected alanylalanine could be baseline-resolved.     

Optimization of a capillary electrophoresis method to determine the chiral purity of a drug

Capillary electrophoresis (CE) using hydroxypropyl-β-cyclodextrin (HP-β-CD) in the separation buffer was investigated to determine the overall chiral purity of a drug containing a single stereogenic center. The effects of primary factors —pH, buffer components, buffer concentration, cyclodextrin concentration and sample amount (concentration and injection volume) — on the resolution of the enantiomers were investigated. Secondary factors such as the HP-β -CD source, lot and degree of substitution that were expected to affect the robustness of the assay were investigated also. The linearity, quantitation limit for the trace enantiomer and the precision of the measurements were determined. This study shows that understanding and optimizing the assay conditions leads to a chiral CE separation that is comparable to that obtained by chiral HPLC. However, chiral CE separations achieved with buffer additives have the advantages of shorter run times, higher numbers of theoretical plates (greater resolution), smaller amounts of chiral additive (less cost) and greater ruggedness (separation virtually independent of column properties unlike HPLC).     

Retention mechanism of high-performance liquid chromatographic enantioseparation on macrocyclic glycopeptide-based chiral stationary phases

The development of methods for the separation of enantiomers has attracted great interest in the past 20 years, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of high-performance liquid chromatography (HPLC), thin-layer chromatography and electrophoresis. The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these macrocyclic glycopeptide antibiotics and, through their application, endeavors to demonstrate the mechanism of separation on macrocyclic glycopeptides. The sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.     

A Validated Chiral HPLC Method for the Determination of Enantiomeric Purity of R-β-amino-β-(4-methoxyphenyl) Propionic Acid

A normal phase chiral LC method for chiral purity evaluation of β-amino-β-(4-methoxyphenyl) propionic acid was developed on donor–acceptor (pirkle) column. The chiral stationary phase used was a 250 × 4.6 mm (R, R) Whelk-01 with 5 μm particle size, which was accompanied with a 1 cm long guard column. The “hybrid” pi-electron donor–acceptor based stationary phase (R, R) Whelk-01 was found to be enantiomeric selective for (R) and (S) enantiomers of β-amino-β-(4-methoxyphenyl) propionic acid with a resolution >2.5. The concentration of 2-propanol and TFA in the mobile phase plays an important role on the chrmatographic efficiency and resolution between the enantiomers. The limit of detection and limit of quantification of (S) enantiomer was 0.3 and 1.0 μg mL^-1 for 20 μL injection volume. The percentage RSD of the peak area of six replicate injections of (S) enantiomer at LOQ concentration was 4.5. The percentage recovery of (S) enantiomer from (R) enantiomer samples ranged from 92 to 100. The test solution was observed to be stable up to 24 h after the preparation. The developed method was also checked by different analysts and on different lots of columns, reagents and it was proved to be rugged. The developed normal phase chiral LC method can be used for the determination of the enantiomeric purity of R-β-amino-β-(4-methoxyphenyl) propionic acid.     

Comparative study on the enantiomer separation of 1,1'-binaphthyl-2,2'diyl hydrogenphosphate and 1,1'-bi-2-naphthol by liquid chromatography and capillary electrophoresis using single and combined chiral selector systems

The chiral recognition ability of single and dual selectors, that were used as additives, have been investigated by HPLC and CE. Native beta- and gamma-cyclodextrins, permethylated beta-cyclodextrin, hydroxypropyl-beta-cyclodextrin, cholic acid and taurodeoxycholic acid sodium salts were applied as chiral selectors, whereas the atropisomers of 1,1'-binaphthyl-2,2'-diyl hydrogenphosphate, and 1,1'-bi-2-naphthol served as model compounds. It was found that all investigated selectors, except for gamma-cyclodextrin, display the same affinity pattern for binaphthyl enantiomers, i.e., binding the S more strongly than the R enantiomer. However, the differences in the phase distribution of chiral selectors led to the opposit elution order of enantiomers: with cyclodextrins, the first eluted is S enantiomer, while R is the first eluted for bile salts. Under the conditions studied, cyclodextrins (except gamma-cyclodextrin), as well as cholic acid sodium salts acting singly, enable the separation of 1,1'-binaphthyl-2,2'-diyl hydrogenphosphate enantiomers both by HPLC and CE methods, while 1,1'-bi-2-naphthol enantiomers were resolved only under CE conditions with permethylated cyclodextrin or bile salts. In both techniques the application of dual systems could improve resolution or make it worse (oreven cancel), depending on the sign of enantioselectivity of particular selectors, their concentrations and localization: mobile or stationary phase. It has been found that the mechanism of separation as well as interactions occurring between two selectors may be followed by using combined HPLC and CE methods. The obtained results proved that, as well as beta-CD, TM-beta-D and gamma-CD also form inclusion complexes with cholic acid sodium salts. The reversal of elution order may be realized by two procedures: changing a single selector, i.e., cyclodextrin on cholic acid sodium salt or vice versa, and by changing the proportion of selectors in the combined bile salt-cyclodextrin system.     

Capillary electrophoretic separation of the enantiomers of organophosphates with a phosphorus stereogenic center using the sodium salt of octakis(2,3-diacetyl-6-sulfo)-gamma-cyclodextrin as resolving agent

The sodium salt of the single-isomer, chiral resolving agent, octakis(2,3-diacetyl-6-sulfo)-gamma-cyclodextrin (ODAS-gammaCD) has been used for the capillary electrophoretic separation of the enantiomers of alkylarylphosphates which carry a phosphorus-based stereogenic center. The effective mobilities and separation selectivities were measured at different ODAS-gammaCD and methanol concentrations to find the conditions under which the minor enantiomers could be adequately quantitated in samples obtained by chemical resolution of the racemic mixtures. This work extends the utility of ODAS-gammaCD to a hitherto unexplored field, the capillary electrophoretic separation of the enantiomers of organophosphorus compounds.     

On the capillary gas chromatographic separation of enantiomers of N-trifluoroacetyl-O-alkyl esters of selected amino acids on 2,3-di-O-pentyl-6-O-acyl cyclodextrins

In this work, the separation of enantiomers of N-TFA-O-alkyl amino acids on the 2,3-di-O-pentyl-6-O-acyl alpha-, beta- and gamma-cyclodextrin stationary phases has been studied. The influence of structure differences in the alkyl substituents bonded to the stereogenic carbon atom (R1), as well as in the ester group (R2) of the selected amino acid derivatives, and the selectivity of modified alpha-, beta- and gamma-cyclodextrin phases in gas chromatographic separation of derivatized amino acid enantiomers was studied in detail. A model set of N-TFA-alkyl esters of four amino acids was separated on five columns. The separation of enantiomers was evaluated in terms of the interactions of the alkyl substituents bonded to the stereogenic carbon (R1) and/or the ester group (R2) of the N-TFA-O-alkyl amino acid derivatives as well as the nature of the 3-O-acyl group in the 2,6-di-O-pentyl-3-O-acyl alpha-, beta- and gamma-cyclodextrins. It was shown that the variation in the enantiomeric separation with temperature and the retention order of enantiomers on a given cyclodextrin capillary column depends both on the nature of the bonded R1 and R2 alkyl groups. It was found that the temperature dependencies of selectivity factors, ln alpha on 1/T, were mostly non-linear. The thermodynamic data [delta(deltaS) and [delta(deltaH)] which characterize the chiral recognition were used to gain more insight into the mechanistic aspects of enantio separation of the N-TFA-O-alkyl amino acid derivatives on 2,6-di-O-pentyl-3-O-acyl-alpha-, beta- and gamma-cyclodextrins.     

Chiral allyl cations are captured by furan with 100% stereoselectivity: synthesis of enantiopure 2-alkoxy-8-oxabicyclo

A low-temperature (-95 degrees C) protocol for intermolecular cycloadditions of furan to chiral silyloxyallyl cations in dichloromethane is described. Key precursors are open-chain, mixed a-ketoacetals, which are chiral. The resulting [4+3] cycloadducts are densely functionalized and are isolated as single enantiomers in high chemical yield. The yield of the cycloadducts increases with increasing dilution. Three and four stereogenic centres are created in one single step.     

纤维素-三（3,5-二甲苯基氨基甲酸酯）手性固定相拆分氨鲁米特对映体

以纤维素-三（3,5-二甲苯基氨基甲酸酯）为手性固定相（Chiralcel OD-H）在高效液相色谱上拆分了氨鲁米特对映体。通过测定氨鲁米特在正己烷/乙醇和正己烷/异丙醇中的溶解度,优选了对样品溶解度大的流动相体系,并考察了流动相添加剂乙醇胺对拆分效果的影响。在此基础上进一步研究了流动相中乙醇含量、柱温和进样量对分离因子、分离度、不对称因子和理论板数的影响,从而确定了最佳的拆分条件：固定相为Chiralcel OD-H,流动相为正己烷/乙醇/乙醇胺（体积比为30：70：0.1）,柱温25℃。本文所得结果可为工业放大提供基础数据。     

Synthesis of new chiral PNAs bearing a dipeptide-mimic monomer with two lysine-derived stereogenic centres

The synthesis of new chiral PNA analogues based on lysine is reported. In particular, l- and/or d-lysine-based PNA submonomers bearing two lysine side chains exactly spaced as in the dipeptide Lys-Lys were synthesized and incorporated in the middle of decameric PNA strands, obtaining four diastereomeric (LD, DL, LL and DD) lysine-based chiral PNAs. The hybridization with their complementary antiparallel DNA strand was studied by melting temperature determination and compared with the analogue achiral PNA and chiral PNAs bearing one residue with either of the two lysine enantiomers. The binding abilities were shown to be strongly dependent on the configuration of the stereogenic centres.     

Analytical and Preparative Chiral Separation of β-Carboline Derivatives, LDL Oxidation Inhibitors, Using HPLC and CE Methodologies: Determination of Enantiomeric Purity

In the present study, baseline separation of the enantiomers of 16 β-carboline derivatives was successfully achieved using both capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC) techniques in short run times (<15?min) and thus permit the determination of enantiomeric excess. In HPLC methodology, cellulose chiral stationary phase (Chiralcel OD-H) was used with a binary mobile phase constituted of n-hexane/ethanol 85/15 leading to a resolution factor of 12.6 in 15?min. Preparative HPLC allowed to obtain pure enantiomers of two compounds. In CE, chiral selectivity was developed with an in-capillary stacking strategy using anionic (highly sulfated-γ) cyclodextrins 5% (w/v) as chiral selectors and a 60?mM phosphate buffer (pH 2.5) resulting in a resolution of 10.26 in 14?min of analysis. The analytical characteristics of the two developed methods were studied in terms of repeatability, limits of detection and limits of quantification showing their suitability to be extended to all the other molecules.     

Conformational studies by dynamic NMR. 100. Enantiomerization process of stereolabile atropisomers in pyridine-substituted adamantane derivatives

The barriers for interconverting the conformational enantiomers (stereolabile atropisomers) of pyridine-substituted adamantane derivatives have been determined by dynamic 13C NMR spectroscopy. The trend of these values parallels that anticipated by MM calculations. In at least one case, the computed structure was found to agree with that obtained by single-crystal X-ray diffraction. In addition, it has been possible to achieve a physical separation of a pair of these stereolabile atropisomers at -60 degrees C by means of the enantioselective cryogenic HPLC technique.