Chiral Stationary Phase HPLC Research Articles

A novel homochiral metal–organic framework with an expanded open cage based on (R)-3,3′-bis(6-carboxy-2-naphthyl)-2,2′-dihydroxy-1,1′-binaphthyl: synthesis, X-ray structure and efficient HPLC enantiomer separation

A new homochiral MOF with an expanded open cage was synthesized and utilized as a chiral stationary phase for HPLC.

A Pair of Enantiomeric Bis- seco-abietane Diterpenoids from Cryptomeria fortunei.

(±)-Cryptomeriolide, a pair of racemic bis- seco-abietane diterpenoids, were isolated from the bark of Cryptomeria fortunei. The separation of enantiomers was achieved by using chiral stationary phase HPLC. Their structures including the absolute configuration and conformations in solution and solid state were determined by extensive analysis of spectroscopic data, single-crystal X-ray diffraction, and comparison of calculated and experimental electronic circular dichroism data. A bioinspired one-pot enantiomeric synthesis of 1a and 1b was accomplished via a readily made intermediate orthoquinone from sugiol. All compounds including the synthetic intermediates were assayed for their cytotoxic activities on human cancer cell lines HL-60, A549, and SGC7901.

Evaluation of the Edman degradation product of vancomycin bonded to core-shell particles as a new HPLC chiral stationary phase.

A modified macrocyclic glycopeptide-based chiral stationary phase (CSP), prepared via Edman degradation of vancomycin, was evaluated as a chiral selector for the first time. Its applicability was compared with other macrocyclic glycopeptide-based CSPs: TeicoShell and VancoShell. In addition, another modified macrocyclic glycopeptide-based CSP, NicoShell, was further examined. Initial evaluation was focused on the complementary behavior with these glycopeptides. A screening procedure was used based on previous work for the enantiomeric separation of 50 chiral compounds including amino acids, pesticides, stimulants, and a variety of pharmaceuticals. Fast and efficient chiral separations resulted by using superficially porous (core-shell) particle supports. Overall, the vancomycin Edman degradation product (EDP) resembled TeicoShell with high enantioselectivity for acidic compounds in the polar ionic mode. The simultaneous enantiomeric separation of 5 racemic profens using liquid chromatography-mass spectrometry with EDP was performed in approximately 3minutes. Other highlights include simultaneous liquid chromatography separations of rac-amphetamine and rac-methamphetamine with VancoShell, rac-pseudoephedrine and rac-ephedrine with NicoShell, and rac-dichlorprop and rac-haloxyfop with TeicoShell.

Molecularly imprinted polymer as stationary phase for HPLC separation of phenylalanine enantiomers

l-Phenylalanine molecularly imprinted polymers were synthesized by bulk polymerization. Methacrylic acid and acrylamide were tested as functional monomers. Ethanol and acetonitrile were used as porogenic solvents. Optimal composition of polymerization mixture was methacrylic acid, template, and ethylene glycol dimethacrylate in molar ratio 1:5:26. MIP was applied as HPLC chiral stationary phase. The influences of the mobile phase composition, flow rate, column temperature, and column length on the efficiency of enantioseparation were investigated. The enantioselective separation of phenylalanine was attained in reversed phase mode at 45 °C with acetonitrile/water containing 1.5% acetic acid (90/10, v/v) as mobile phase (resolution value was 1.49, selectivity factor was 1.38). Applicability of polymeric stationary phase prepared for l-phenylalanine was tested for analysis of dietary supplement sample. The UV detection limits for both enantiomers were 1 mg cm−3 (S/N = 3). Good linearity was observed from 1 to 10 mg cm−3.

Preparation and evaluation of a deoxycholic-calix[4]arene hybrid-type receptor as a chiral stationary phase for HPLC.

We report the synthesis and enantioseparation characteristics of two novel covalently immobilized deoxycholic acid derivatives as chiral stationary phases for high-performance liquid chromatography. In the structure of the first stationary phase, the 3-position of deoxycholic acid is substituted with a 3,5-dinitrophenylcarbamoyl group and the second one has an additional calix[4]arene attached to the carboxylic group of the deoxycholic acid. The chromatographic performance of the stationary phases was evaluated with enantioseparation of N-(3,5-dinitrobenzoyl)-dl-leucine, N-(3,5-dinitrobenzoyl)-dl-valine, omeprazole, diclofop-methyl, dl-mandelic acid and (RS)-pregabalin. Comparison of the performance characteristics of the prepared chiral stationary phases provided evidence for the active involvement of the calix[4]arene unit in the chiral recognition process. Both stationary phases are chemically bonded to the silica and can be used in both normal-phase and reversed-phase modes.

Optically active distorted cyclic triptycenes: chiral stationary phases for HPLC.

A pair of optically active triptycene derivatives ((R,R)- and (S,S)-8) with a distorted cyclic structure were synthesized via an intramolecular etherification and evaluated as a novel chiral selector for high-performance liquid chromatography. The (R,R)- and (S,S)-8-based chiral stationary phases (CSPs) were found to be particularly effective in the resolution of axially chiral biaryl compounds. The importance of the distorted cyclic structure present in 8 for chiral recognition was demonstrated by comparisons with the corresponding non-cyclic model compound ((R,R)-9), which did not display enantioselectivity in the separation of the test racemates.

Immobilized strychnine as a new chiral stationary phase for HPLC.

A new ion-exchanger type chiral stationary phase for high-performance liquid chromatography was prepared. The synthetic protocol is based on derivatization of silica with (3-iodopropyl)trimethoxysilane in the first step followed by immobilization of strychnine via quaternization of nitrogen atom of the alkaloid strychnine. The synthesized chiral stationary phase was chromatographically characterized. The main effort was headed towards the evaluation of the enantioselectivity of the novel sorbent. For that purpose a set of suitable chiral probes, specifically, binaphthyl derivatives, was employed. The influence of methanol content, concentration of aqueous ammonium acetate buffer, and its pH on retention factors, separation selectivity, and resolution of the atropoisomers of the mentioned chiral solutes was studied in detail. It was demonstrated that the new chiral stationary phase was capable to separate atropoisomers of four out of seven testing compounds. Despite the strong influence of the above mentioned variables on retention, their impact on selectivity and resolution was rather moderate. Concerning retention mechanism, it seems that electrostatic interaction between the positively charged quaternary nitrogen of the chiral stationary phase and anionic solute participates significantly in the retention process.

Laboratory-Scale Preparative Enantioseparations of Pharmaceutically Relevant Compounds on Commercially Available Chiral Stationary Phases for HPLC.

In response to the outburst of research in the field of synthetic medicinal chemistry, enantioselective chromatography methods based on the use of chiral stationary phases (CSPs) found immediate acceptance as the elective choice for the analytical determinations of the enantiomeric purity of synthetic compounds. In contrast to an initial scepticism, also the preparative-scale applications are gaining increasing recognition as a powerful alternative to enantioselective synthesis for the supply of pure enantiomers of bioactive compounds. The increasing success of liquid chromatography methods has been made possible thanks to the development of highly efficient CSPs allowing the enantioresolution of practically all the chemical classes of chiral compounds. However, only few CSPs are really suitable for preparative- scale applications, being the loading capacity is the major concern for preparativescale enantioseparations. The cellulose- and amylose-based CSPs present the highest loading capacity and enantiodiscrimination power, which makes these CSPs the most versatile and applicable for preparative-scale applications in all the applicable elution modes (reversedphase, normal-phase, and with polar-organic or polar-ionic eluents). However, also other types of CSPs have been successfully employed at this regard (brush-type phases, polyacrylamide and cross-linked di-allyltartardiamide phases as well as cyclodextrin, and glycopeptide containing phases). Several instrumental methods exist for the determination of the absolute configuration of organic compounds in absence of known enantiopure reference standards. The most widely known are X-ray crystallography, followed by chirooptical methods [e.g., electronic and vibrational circular dichroism (ECD and VCD, respectively)] and nuclear magnetic resonance (NMR) spectroscopy. All these aspects will be treated in the review.

Chiral Recognition Ability of an Optically Active Poly(diphenylacetylene) as a Chiral Stationary Phase for HPLC

An optically active poly(diphenylacetylene) with a pendant amide functional group was synthesized by a macromolecular reaction of the optically inactive poly(diphenylacetylene) bearing carboxy grou...

Chlorinated aromatic derivatives of cyclofructan 6 as HPLC chiral stationary phases

ABSTRACTCyclofructans (CF) are cyclic oligosaccharides consisting of a crown ether core with pendent fructofuranose units. These unique macrocycles were reported recently to be powerful chiral selectors. Derivatized and bonded to silica, CFs make excellent chiral selectors for HPLC. In this study, several new derivatives of cyclofructan 6 (CF6) were prepared by introducing aromatic moieties with electron-withdrawing chloro and nitro groups and an electron-donating methyl group. Their enantioselectivities were evaluated in the normal phase mode in comparison to the commercially available cyclofructan columns (LARIHC CF6-P, LARIHC CF6-RN, and LARIHC CF7-DMP). In several cases, the columns prepared in this work showed improved enantioselectivity over the existing commercially available stationary phases. Furthermore, an evaluation of the number and position of chloro and methyl groups on the phenyl substituents of CF6 is discussed in terms of their ability to alter enantioselectivity.

Axial Chirality about Boron-Carbon Bond: Atropisomeric Azaborines.

The preparation of atropisomeric 2,1-borazaronaphthalenes is described. Resolution of the atropisomeric pair was achieved by preparative Chiral Stationary Phase HPLC (CSP-HPLC). The absolute configuration of the stereogenic axis was derived from Time-Dependent DFT (TD-DFT) simulation of the Electronic Circular Dichroism spectra (ECD). X-ray diffraction and Dynamic NMR data allowed structural and dynamic comparison with the analogue isosteric carbon compounds.

Chiral separation on various modified amino alcohol-derived HPLC chiral stationary phases.

3,5-Dinitrobenzoyl chloride was previously used for the preparation of (R)-phenylglycinol- and (S)-leucinol-derived chiral stationary phases. In this study, 3,5-bis(trifluoromethyl)benzoyl chloride, 2-furoyl chloride, 2-theonyl chloride, 10,11-dihydro-5H-dibenzo[b,f]azepine-5-carbonyl chloride, diphenylcarbamoyl chloride, and 1-adamantanecarbonyl chloride were used to prepare six new phenylglycinol-derived chiral stationary phases (CSPs) and five new leucinol-derived CSPs. Using these 11 CSPs, chiral separation of nine π-acidic amino acid derivatives and five π-basic compounds was performed, and the separation results were compared. An adamantyl-derived CSP showed good separation.

HPLC enantioseparation on a homochiral MOF–silica composite as a novel chiral stationary phase

Using enantiopure (R)-2,2′-dihydroxy-1,1′-binaphthalene-6,6′-dicarboxylic acid as a starting material, we prepared three homochiral MOFs that were further used as chiral stationary phases for HPLC to separate the enantiomers.

Reinvestigation of a Catalytic, Enantioselective Alkene Dibromination and Chlorohydroxylation.

Attempts to reproduce eight, putative, enantioselective dibromination and chlorohydroxylation reactions from oft-cited literature studies are described. The reactions were performed with full fidelity to the original report wherever possible. Analysis of the enantiomeric composition was performed by chiral stationary phase HPLC or SFC (CSP-HPLC or CSP-SFC), as opposed to the original report, which used chiral shift reagent NMR spectroscopy. After careful study, the reported levels of enantioselectivity were found to be incorrect. Possible explanations for the false positive results are discussed.

High efficiency, narrow particle size distribution, sub-2 μm based macrocyclic glycopeptide chiral stationary phases in HPLC and SFC

State of the art chiral chromatography still employs 3–5 μm bonded or immobilized chiral selectors in 10–25 cm columns. With the availability of 1.9 μm narrow particle size distribution (NPSD) silica, it is now possible to make ever shorter, high efficiency columns practical for sub-minute chiral separations. Three macrocyclic glycopeptides (teicoplanin, teicoplanin aglycone, and vancomycin) were bonded onto 1.9 μm NPSD particles. Such packed columns had ∼80% lower backpressure as compared to polydisperse (PD) 1.7 μm silica materials when using the same mobile phase. The decreased backpressure allowed for diminution of frictional heating and allowed for the use of the 1.9 μm NPSD particle based columns at high flow rates. The 1.9 μm NPSD particle based columns showed up to 190,000 plates m−1 for chiral molecules and 210,000 plates m−1 for achiral probes. Representative enantiomeric separations are shown for wide classes of compounds, including different types of amino acids, β-blockers, and pharmaceutically important heterocyclic compounds such as oxazolidinones. Applications in three liquid chromatography modes, namely, reversed phase, polar organic mode and normal phase chiral separations were shown with resolution values ranging from 1.5 to 5.7. Additionally, the same columns were used with supercritical fluid chromatography (SFC) for ultrafast separations.

Resolution and Determination of the Absolute Configuration of a Twisted Bis-Lactam Analogue of Tröger's Base: A Comparative Spectroscopic and Computational Study.

The first reported twisted bis-lactam, a racemic Tröger's base (TB) analogue (2), was resolved into its enantiomers on a chiral stationary phase HPLC column. The absolute configuration of (+)-2 was determined to be (R,R)-2 by comparing experimental and calculated vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectra. The absolute configuration of (-)-2 was determined by comparing experimental and calculated electronic circular dichroism (ECD) spectra. The corresponding theoretical spectra were calculated using the lowest energy conformation of (R,R)-2 and (S,S)-2 at the B3LYP/6-31G(d,p) level of theory. The absolute configuration of (+)-2 was also determined to (R,R)-2 by anomalous X-ray diffraction (AXRD) in a chiral space group P212121 using Cu-irradiation resulting in a very low Flack parameter of -0.06(3), despite the heaviest element being an oxygen atom, thus unambiguously confirming the results from the spectroscopic studies. We conclude that, for the Tröger's base (TB) analogue (2), we may rank the reliability of the individual methods for AC determination as AXRD ≫ VCD > ECD, while the synergy of all three methods provides very strong confidence in the assigned ACs of (+)-(R,R)-2 and (-)-(S,S)-2.

Development of HPLC Chiral Stationary Phases Based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic Acid and Their Applications.

Crown ether-based chiral stationary phases (CSPs) have been known to be useful for the resolution of racemic primary amino compounds. In particular, CSPs based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid have been reported to be useful for the resolution of secondary amino compounds as well as primary amino compounds. In this article, the process of developing various CSPs based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid to improve the chiral recognition efficiency and/or the stability of the CSPs and their applications to the resolution of various primary and nonprimary amino compounds are reviewed.

Preparation and Evaluation of HPLC Chiral Stationary Phases Based on Cationic/Basic Derivatives of Cyclofructan 6

The cyclofructan 6 (CF6) macrocyclic-oligosaccharide was derivatized with five different substituents able to bear positive charges: propyl imidazole, methyl benzimidazole, dimethyl aminopropyl, pyridine, and dimethyl aminophenyl. The derivatized cyclofructans were reacted with triethoxysilyl-propylisocyanate as a linker to bond them to 5 µm spherical silica gel particles and then used to prepare liquid chromatography columns. The bonded silica particles were analyzed to establish the bonding densities. A set of 34 chiral compounds including acids, neutral compounds, and bases was tested with nine different mobile phase compositions including two reverse phase (RP) acetonitrile/pH 4 buffer, three polar organic (PO) acetonitrile/methanol, and four normal phase (NP) heptane/ethanol mobile phases. No compounds could be separated in the RP mode. Eight compounds only could be enantioseparated in the PO mode and 21 compounds in the NP mode. The most effective chiral stationary phase was the propyl imidazole derivatized CF6 phase, provided no more than six imidazole substituents and two linkers are attached per CF6 unit.

Spherical β-cyclodextrin-silica hybrid materials for multifunctional chiral stationary phases

Spherical β-CD-silica hybrid materials have been prepared successfully by simple one-pot polymerization, which provide a new strategy to construct new type of HPLC chiral stationary phases. Various β-CD, ethane, triazinyl and 3,5-dimethylphenyl functional groups that can provide multiple interactions were introduced into the pore channels and pore wall framework of mesoporous materials, respectively. The materials towards some chiral, acidic, anilines and phenols compounds showed multiple chromatographic separation functions including racemic resolution, anion exchange and achiral separations with a typical feature of normal/reversed phase chromatography. Multi-tasking including racemic resolution and achiral separations for selected compounds were performed simultaneously on a chiral chromatographic column. The multifunctional character of materials arises from the multiple interactions including hydrophobic interaction, π–π interaction, anion exchange, inclusion interaction and hydrogen bonding interaction.

ChemInform Abstract: Regioselective Internal Carbonylation of the 2‐Aza‐21‐carbaporphyrin: Access to Configurationally Stable Chiral Porphyrinoids.

Reaction of paraformaldehyde with meso-tetraaryl-2-aza-21-carbaporphyrins (NCP) in the presence of a basic catalyst afforded fused lactam derivatives comprising a >C═O bridge linking the internal carbon C21 with one of the internal nitrogens. The isomer 2 with C21-C(O)-N24 bridge is formed with about 9-fold molar excess over that with C21-C(O)-N22 bridge (3). The (1)H NMR and UV-vis spectral characteristics indicate aromatic character of the derivatives. For meso-tetrakis(3',5'-dimethoxy)- and meso-tetrakis(3',4',5'-trimethoxy)-21,24-carbonyl-NCP an efficient external ring fusion by linking C3, i.e., the external carbon of the confused pyrrole, with an ortho carbon of the adjacent aryl was observed under acidic conditions, yielding derivatives 4c and 4d, comprising a linear system of five fused rings. The chirality and configurational stability of these carbonylated systems were established by a chiral stationary phase HPLC and circular dichroism. The interaction of 2 with chiral acids and alcohols leading to the formation of diastereomers was observed by (1)H NMR. Slow racemization of 2 under acidic conditions was established by HPLC and (1)H NMR and a mechanism for this process was proposed.