Chiral Stationary Phase Research Articles

Novel Mixed-Mode Adsorbents for HPLC Based on Different Substrates Modified with Eremomycin

New adsorbents based on silica and polystyrene–divinylbenzene (PS–DVB) for hydrophilic interaction liquid chromatography (HILIC) with eremomycin in functional layers were obtained. The chromatographic properties of the new adsorbents were assessed using the Tanaka test for hydrophilic stationary phases and by studying the retention of substances of various classes in HILIC, chiral, and reversed-phase chromatography modes. It was shown that the use of eremomycin to create functional layers leads to an increase in the hydrophilicity of the adsorbents on different types of substrates and ensures the shielding of their charge. Eleven nitrogenous bases, nucleosides with an efficiency of up to 25 000 tp/m, or seven vitamins with an efficiency of up to 40 000 tp/m can be separated on a modified sorbent based on aminopropyl silica, and three different HPLC modes can be implemented on the sorbent with eremomycin based on PS–DVB.

A chiral metal-organic framework synthesized by the mixture of chiral and non-chiral organic ligands for enantioseparation of drugs by open-tubular capillary electrochromatography

A chiral metal-organic framework L-Histidine-Zeolitic imidazolate framework-67 (L-His-ZIF-67) was synthesized by the mixture of chiral organic ligand L-histidine and non-chiral organic ligand 2-methylimidazole directly, and to the author's knowledge, the chiral L-His-ZIF-67 coated capillary column we prepared has still not been reported to date in the field of capillary electrophoresis. This chiral metal-organic frameworks material was used as the chiral stationary phase for enantioseparation of drugs by open-tubular capillary electrochromatography. The separation conditions such as pH value, buffer concentration and proportion of organic modifier were optimized. Under optimal conditions, the established enantioseparation system achieved good separation effect, and the resolution of five chiral drugs: esmolol (7.93), nefopam (3.03), salbutamol (2.42), scopolamine (1.08) and sotalol (0.81). In addition, the chiral recognition mechanism of L-His-ZIF-67 was elucidated by a series of mechanism experiments, and the specific interaction force was preliminarily speculated.

Chiral separation of sesquineolignans from the stems and leaves of Neoshirakia japonica

Twelve pairs of sesquineolignan enantiomers (1a/1b−6a/6b and 1c/1d−6c/6d), including twenty one undescribed and three known (2b, 3b, and 4b) sesquineolignans were isolated from an ethanol extract of the stems and leaves of Neoshirakia japonica (Euphorbiaceae). The successful separation of twelve pairs of enantiomers with mirror image-like electronic circular dichroism (ECD) curves and opposite specific rotation values, as one of the most important steps in compound isolation, was carried out by chiral HPLC columns. The absolute configurations of all undescribed sesquineolignans were elucidated by comprehensive analysis of their experimental ECD spectra. The effects of all the isolates on antineuroinflammatory and radical scavenging activity were evaluated. Compared with the positive control minocycline (IC50 = 1.2 μM), compounds 1a/1b/1c/1d−6a/6b/6c/6d with IC50 values being greater than 50 μM displayed almost no effect on the inhibition of NO production in LPS-induced BV-2 microglial cells. The results of DPPH-radical scavenging activity for them showed that compound 3c had moderate radical scavenging ability (EC50 = 48.47 μM), while the EC50 value of positive control vitamin C was 18.21 μM.

Eugenilones A−N: sesquiterpenoids from the fruits of Eugenia uniflora

(+) and (−)-Eugenilones A−K, 11 pairs of undescribed enantiomeric sesquiterpenoids, together with three undescribed biogenetically related members eugenilones L−N, were discovered from the fruits of Eugenia uniflora Linn. (Myrtaceae). Structurally, eugenilones A−D were four caged sesquiterpenoids featuring 9,10-dioxatricyclo [6.2.2.02,7]dodecane, 11-oxatricyclo [5.3.1.03,8]undecane, and tricyclo [4.4.0.02,8]decane cores, respectively. Eugenilones E−K were eudesmane-type sesquiterpenoids, while eugenilones L−N were epoxy germacrane-type sesquiterpenoids. Notably, eugenilones A−K were efficiently resolved by chiral HPLC to give 11 pairs of optically pure enantiomers. The structures and absolute configurations of eugenilones A−N were determined through spectroscopic analyses, X-ray crystallography, and ECD calculations. The putative biosynthetic pathways for these undescribed isolates were proposed. Moreover, eugenilones A and E exhibited significant anti-inflammatory effects by inhibiting LPS-stimulated NO overproduction in RAW264.7 cells (IC50 values of 4.89 ± 0.37 μM and 20.89 ± 1.49 μM, respectively) and TNF-α-induced NF-κB activation in HEK293 cells (IC50 values of 10.97 ± 1.03 μM and 28.63 ± 1.59 μM, respectively).

Enantioseparation of new axially chiral carboxylic acids on polysaccharide-based chiral stationary phases under normal phase elution conditions

In the last decade, the availability of new and versatile synthetic strategies for the preparation of substituted 4,4’-bipyridyl derivatives based on chemo- and regioselective functionalization of the 4,4’-bipyridine core has encouraged studies for exploring the bioactivity of these compounds in the fields of drug discovery and medicinal chemistry. In substituted 4,4’-bipyridines, chirality may emerge from restricted rotation induced by sterically hindered atoms or functional groups located around the 4,4’-biaryl bond (chiral axis). The first atropisomeric substituted 4,4’-bipyridine was prepared in 2008, and no asymmetric synthesis to produce pure atropisomers of chiral 4,4’-bipyridine derivatives has been available so far. Thus, in the last few years, our groups developed methods to separate atropisomers of a wide series of 4,4’-derivatives by high-performance liquid chromatography (HPLC) using polysaccharide-based chiral stationary phases (CSPs). In the frame of our interest in this field, we reported herein the synthesis of two new chiral carboxylic acids containing an axially chiral 4,4’-bipyridyl unit as source of chirality, and their HPLC enantioseparation on polysaccharide-based CSPs. In particular, the impact of analyte and CSP structures on the enantioseparation outcomes as well as mechanisms and noncovalent interactions underlying the enantioseparation were explored by using electrostatic potential analysis and molecular dynamics (MD) simulations.

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S0-S1 transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (BCPL) up to 66.5 M-1 cm-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (gabs) and luminescence dissymmetry (glum) factor.

Enantioseparation of a-substituted proline analogs with macrocyclic glycopeptide-based chiral stationary phases immobilized on superficially porous particles of silica applying liquid chromatography with ultraviolet and mass spectrometric detection

In this study, the liquid chromatography-based direct enantioseparation of the stereoisomers of α-substituted proline analogs has been investigated utilizing chiral stationary phases with UV and/or mass spectrometric (MS) detection. Macrocyclic antibiotics, such as vancomycin, teicoplanin, modified teicoplanin, and teicoplanin aglycone, all covalently immobilized to 2.7 μm superficially porous silica particles have been applied as stationary phases. Mobile phases utilizing mixtures of methanol and acetonitrile with different additives (polar-ionic mode) were optimized during method development. Best separations were achieved with mobile phases of 100% MeOH containing either 20mM acetic acid or 20mM triethylammonium acetate. Special attention was given to the applicability of MS-compatible mobile phases. Acetic acid was found to be advantageous as a mobile phase additive for MS detection. Enantioselective chromatographic behaviors are interpreted based on the explored correlations between the analytes' structural features and those of the applied chiral stationary phases. For the thermodynamic characterization, separations were studied in the temperature range of 5-50°C. Generally, retention and selectivity decreased with increasing temperature, and in most cases, enthalpy-driven enantiorecognition was observed, but entropic contributions also were present. Unexpectedly, unusual shapes for the van Deemter curves were registered in the kinetic evaluations. General trends could be observed in the enantiomeric elution orders: S < R on VancoShell and NicoShell, and opposite R < S on TeicoShell and TagShell columns.

Cyclodextrin-NH-MIL-53 open tubular stationary phase for capillary electrochromatography enantioseparation.

A novel chiral group functionalized metal-organic framework, Cyclodextrin-NH-MIL-53, was synthesized and modified on the inner wall of a capillary column via a post-synthetic process. The prepared chiral metal-organic framework was utilized as a chiral capillary stationary phase and used in an open-tubular capillary electrochromatography method to enantioseparate several racemic amino acids. Excellent enantioseparation of five pairs of enantiomers was obtained in this chiral separation system (Resolutions of D/L-Alanine = 16.844, D/L-Cysteine = 3.617, D/L-Histidine = 9.513, D/L-Phenylalanine = 8.133, and D/L-Tryptophan = 2.778). The prepared Cyclodextrin-NH-MIL-53 and the Cyclodextrin-NH-MIL-53-based capillary columns were characterized by scanning electron microscopy, X-ray diffraction, Fourie-transform infrared spectroscopy, and circular dichroism. The chiral capillary electrochromatography conditions, such as separation conditions, amount of Cyclodextrin-NH-MIL-53, and electroosmotic flow, were optimized. This research is estimated to present a novel insight and method for the design and use of metal-organic framework-based capillaries for enantioseparation.

Novel 4H-pyrimido[2,1-b]benzothiazoles derivatives: Camphorsulphonic acid catalyzed enantioselective synthesis, optimization, and biological study

An efficient, highly convergent route for the synthesis of novel fourteen single isomer derivatives of ethyl-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate have been developed by one-pot, three-component reaction between pyridine 2-aldehyde, β-ketoester and various derivatives of 2-amino benzothiazole in the presence of D-(+)-10-camphorsulphonic acid (D-(+)-10-CSA) as an effective chiral acid catalyst. Under the optimized conditions with 20 mol% catalyst loading, a wide range of optically active 4H-pyrimido[2,1-b]benzothiazole derivatives were obtained in good yields (up to 82%) with good to excellent enantioselectivities (84-99%ee). Their structures were elucidated by various spectroscopic methods such as 1H-NMR, 13C-NMR, Chiral HPLC, mass and elemental analysis. The attractive feature of this approach is one-pot synthesis, good yield, environment benign, mild reaction condition, simple, efficient, excellent stereoselectivity, water soluble catalyst and easy workup. All the final scaffolds have been screened for antibacterial activity. Among them, 4a’, 4c’, 4d’, 4e’, 4f’, 4g’, 4j’,4m’ and 4n’ were proven to possess enhanced antibacterial properties as compared to the standard drug.

Preparation of Chiral Porous Organic Cage Clicked Chiral Stationary Phase for HPLC Enantioseparation.

Porous organic cages (POCs) are a new subclass of porous materials, which are constructed from discrete cage molecules with permanent cavities via weak intermolecular forces. In this study, a novel chiral stationary phase (CSP) has been prepared by chemically binding a [4 + 6]-type chiral POC (C120H96N12O4) with thiol-functionalized silica gel using a thiol-ene click reaction and applied to HPLC separations. The column packed with this CSP presented good separation capability for chiral compounds and positional isomers. Thirteen racemates have been enantioseparated on this column, including alcohols, diols, ketones, amines, epoxides, and organic acids. Upon comparison with a previously reported chiral POC NC1-R-based column, commercial Chiralpak AD-H, and Chiralcel OD-H columns, this column is complementary to these three columns in terms of its enantiomeric separation; and can also separate some racemic compounds that cannot be separated by the three columns. In addition, eight positional isomers (iodoaniline, bromoaniline, chloroaniline, dibromobenzene, dichlorobenzene, toluidine, nitrobromobenzene, and nitroaniline) have also been separated. The influences of the injection weight and column temperature on separation have been explored. After the column has undergone multiple injections, the relative standard deviations (RSDs) for the retention time and selectivity were below 1.0 and 1.5%, respectively, indicating the good reproducibility and stability of the column for separation. This work demonstrates that POCs are promising materials for HPLC separation.

Surface-up click access to allylimidazolium bridged cyclodextrin dimer phase for efficient enantioseparation.

In this work, a novel allylimidazolium-bridged bis(β-cyclodextrin) chiral stationary phase was fabricated via a surface-up thiol-ene click chemistry reaction between 7-SH-β-cyclodextrin and 1-allylimidazole-β-cyclodextrin bonded on a silica surface. The structure of the allylimidazolium-bridged bis(β-cyclodextrin) chiral stationary phase was characterized by Fourier transform infrared spectra, 13 C nuclear magnetic resonance, thermogravimetric analysis, and elemental analysis. Its chiral chromatographic performances were systematically evaluated by separating 35 racemic analytes including isoxazolines, dansyl-amino acids, and flavanones under reversed-phase high-performance liquid chromatography. Compared with the corresponding bottom and top layer of the β-cyclodextrin stationary phase, the allylimidazolium-bridged bis(β-cyclodextrin) chiral stationary phase afforded significantly accentuated chiral recognition ability due to its abundant hydrogen bond sites, electrostatic interactions, and synergistic inclusion. Furthermore, the allylimidazolium-bridged bis(β-cyclodextrin) chiral stationary phase showed better enantioseparation ability compared to other reported bridged cyclodextrin stationary phases. In particular, Ar-Phs and dansyl-amino acid could be completely separated by allylimidazolium-bridged bis(β-mono-6A -deoxy-6-allylimidazolium-β-cyclodextrin chiral stationary phase) chiral stationary phase with high resolutions of 1.14-7.20 and 3.16-5.82, respectively. Molecular docking reveals that good enantioseparation ability arises from the different interaction modes and the synergistic effect of allylimidazolium-bridged bis(β-cyclodextrin) chiral stationary phase.

Merging a Negatively Curved Nanographene and a Carbon Nanoring

Two molecular nanocarbons result from merging a negatively curved nanographene and a carbon nanoring in two constitutional isomers of D2 and C2v symmetry, respectively. They were synthesized by attachment of C-shaped paraphenylene precursors to 2,11,18,27-tetrabromooctabenzo[8]circulene and the subsequent intramolecular Yamamoto coupling and reductive aromatization reactions. The flexible nature of octabenzo[8]circulene enabled two different ways of connection in the Yamamoto coupling reactions, leading to the two constitutional isomers. The D2 isomer is shaped like a figure eight, as revealed by X-ray crystallography, and is resolved into two enantiomers by chiral HPLC. The synthesis of the C2v isomer is regarded as a further step toward precision synthesis of carbon schwarzites through a bottom-up approach.

N-acetyldopamine derivatives from the egg cases of the insect Tenodera sinensis Saussure with COX-2 inhibitory activity

Mantidisamides A–D (1–4), four new N-acetyldopamine derivatives were isolated from the egg cases of Tenodera sinensis Saussure. Structures of the isolated compounds were confirmed by spectroscopic and computational methods. In particular, compound 4 is an unusual adduct composed of a N-acetyldopamine and a 4-quinolone moiety. Besides, 4 is a racemic mixture submitted to chiral HPLC to afford the enantiomers 4a and 4b. Biological activities of COX-2 inhibition and anti-renal fibrosis toward all the new compounds were evaluated.

Synthesis and Characterization of Phenylalanine Amides Active against Mycobacterium abscessus and Other Mycobacteria.

Nα-2-thiophenoyl-d-phenylalanine-2-morpholinoanilide [MMV688845, Pathogen Box; Medicines for Malaria Venture; IUPAC: (2R)-N-(1-((2-morpholinophenyl)amino)-1-oxo-3-phenylpropan-2-yl)thiophene-2-carboxamide)] is a hit compound, which shows activity against Mycobacterium abscessus (MIC90 6.25-12.5 μM) and other mycobacteria. This work describes derivatization of MMV688845 by introducing a thiomorpholine moiety and the preparation of the corresponding sulfones and sulfoxides. The molecular structures of three analogs are confirmed by X-ray crystallography. Conservation of the essential R configuration during synthesis is proven by chiral HPLC for an exemplary compound. All analogs were characterized in a MIC assay against M. abscessus, Mycobacterium intracellulare, Mycobacterium smegmatis, and Mycobacterium tuberculosis. The sulfone derivatives exhibit lower MIC90 values (M. abscessus: 0.78 μM), and the sulfoxides show higher aqueous solubility than the hit compound. The most potent derivatives possess bactericidal activity (99% inactivation of M. abscessus at 12.5 μM), while they are not cytotoxic against mammalian cell lines.

Screw sense excess and reversals of helical polymers in solution

The helix reversal is a structural motif found in helical polymers in the solid state, but whose existence is elusive in solution. Herein, we have shown how the photochemical electrocyclization (PEC) of poly(phenylacetylene)s (PPAs) can be used to determine not only the presence of helix reversals in polymer solution, but also to estimate the screw sense excess. To perform these studies, we used a library of well folded PPAs and different copolymers series made by enantiomeric comonomers that show chiral conflict effect. The results obtained indicate that the PEC of a PPA will depend on the helical scaffold adopted by the PPA backbone and on its folding degree. Then, from these studies it is possible to determine the screw sense excess of a PPA, highly important in applications such as chiral stationary phases in HPLC or asymmetric synthesis.

Modelling the enantiorecognition of structurally diverse pharmaceuticals on O-substituted polysaccharide-based stationary phases

This study aims to develop models to predict the retention, separation and elution sequence of the enantiomers of structurally diverse pharmaceuticals. More specifically, Quantitative Structure Retention Relationships (QSRR) models are built that describe the relationship between molecular descriptors and retention. Eighteen structurally diverse chiral mixtures, each consisting of a pair of enantiomers, were analyzed on two polysaccharide chiral stationary phases, Chiralcel OD-RH (cellulose tris(3,5-dimethylphenylcarbamate)) and Lux amylose-2 (amylose tris(5-chloro-2-methylphenylcarbamate)), applying either a basic or an acidic mobile phase, and their retention factor and elution sequence were determined. Both achiral and, in-house defined, chiral descriptors were used as descriptive variables to build the models. Linear regression techniques, i.e. stepwise multiple linear regression (sMLR) and partial least squares (PLS) regression, were applied to model the retention or separation as a function of the descriptors. In a first step, models were built with only achiral descriptors to model the global retention of both enantiomers of a chiral molecule. Subsequently, models were built with only chiral descriptors to predict the enantioseparation and elution sequence, and finally, models were considered with both descriptor types to predict the retention, the separation and the elution sequence of the enantiomers. The global retention was predicted well by the sMLR models with only achiral descriptors. The models with only chiral descriptors were not found suitable to predict the enantioseparation and elution sequence. Finally, the models containing both chiral and achiral descriptors allowed predicting the retention well, but their ability to predict the elution sequence and separation of the enantiomers differed widely for the chromatographic systems considered.

Preparation and chromatographic performance of chiral peptide-based stationary phases for enantiomeric separation.

This study presents the development of three new chiral stationary phases. They are based on silica modified with peptides containing phenylalanine and proline. Successful analyses and characterizations were conducted using Fourier transform infrared spectra, elemental analysis, and thermogravimetric analysis. After this, the enantioselective performance of the three chiral peptide-based columns was evaluated. The evaluation used 11 racemic compounds under normal-phase high performance liquid chromatography mode. Optimized enantiomeric separation conditions were established. Under these conditions, the enantiomers of flurbiprofen and naproxen were successfully separated on CSP-1 column: the separation factor of these was 1.27 and 1.21, respectively. In addition, the reproducibility of the CSP-1 column was also investigated. The results of the investigation illustrated that the stationary phases have good reproducibility (RSD = 0.73%, n = 5).

A Cross‐Shaped Monomer as Building Block for Molecular Textiles

AbstractThe exploration of new materials is timeless. Especially 2D‐materials have gotten much interest in the last decades. This work proposes a new route towards a fascinating class of 2D materials: molecular textiles. The suggested bottom‐up approach focuses on the 2D self‐assembly of a cross‐shaped monomer at the water/air interface. A 3D cross‐shaped motive was designed, synthesized, and characterized, which exhibits the required structural features, i. e., static and dynamic control. Analysis of the cross‐shaped motive by 1H‐NMR spectroscopy, X‐ray structure, and chiral stationary phase HPLC proved the rigidity and stability of the system, and thus also its potential for the here suggested new strategy towards molecular textiles. Three variants of a Schiff‐base precursor pair functionalized monomer were synthesized and characterized by 1H‐NMR spectroscopy, 13C‐NMR spectroscopy, and mass spectrometry. Finally, the network formation of the monomer is shown to be triggered by deprotonation of its ammonium salt, corroborated with FT‐IR analysis.

A chiral metal-organic framework {(HQA)(ZnCl2)(2.5H2O)}n for the enantioseparation of chiral amino acids and drugs

Chiral metal-organic frameworks (CMOFs) with enantiomeric subunits have been employed in chiral chemistry. In this study, a CMOF formed from 6-methoxyl-(8S,9R)-cinchonan-9-ol-3-carboxylic acid (HQA) and ZnCl2, {(HQA)(ZnCl2)(2.5H2O)}n, was constructed as a chiral stationary phase (CSP) via an in situ fabrication approach and used for chiral amino acid and drug analyses for the first time. The {(HQA)(ZnCl2)(2.5H2O)}n nanocrystal and the corresponding chiral stationary phase were systematically characterised using a series of analytical techniques including scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, circular dichroism, X-ray photoelectron spectroscopy, thermogravimetric analysis, and Brunauer-Emmett-Teller surface area measurements. In open-tubular capillary electrochromatography (CEC), the novel chiral column exhibited strong and broad enantioselectivity toward a variety of chiral analytes, including 19 racemic dansyl amino acids and several model chiral drugs (both acidic and basic). The chiral CEC conditions were optimised, and the enantioseparation mechanisms are discussed. This study not only introduces a new high-efficiency member of the MOF-type CSP family but also demonstrates the potential of improving the enantioselectivities of traditional chiral recognition reagents by fully using the inherent characteristics of porous organic frameworks.

Facile Synthesis and Chiral Resolution of Expanded Helicenes with up to 35 cata-Fused Benzene Rings.

Expanded helicenes are expected to show enhanced chiroptical properties as compared to the classical helicenes but the synthesis is very challenging. Herein, we report the facile synthesis of a series of expanded helicenes Hn (n=1-4) containing 11, 19, 27 and 35 cata-fused benzene rings through Suzuki coupling-based oligomerization followed by Bi(OTf)3 -mediated regioselective cyclization of vinyl ethers. Their structures were determined by X-ray crystallographic analysis. Enantiopure H2, H3, and H4 can be isolated by chiral HPLC and they all exhibit strong chiroptical responses with high absorption dissymmetry factor (|gabs |) values (0.020 for H2, 0.021 for H3, and 0.021-0.024 for H4).