Strong cation exchange chiral stationary phase—A comparative study in high-performance liquid chromatography and subcritical fluid chromatography

Chiral separation and spectroscopic characterization of mefloquine analogues

Mefloquine, a widely used antimalarial agent, has spurred ongoing research into the development of derivatives with enhanced efficacy and reduced side effects. In this investigation, we synthesized two compounds containing N-allyl or N-tert-butylacetamid groups. A chiral liquid chromatography with polysaccharide chiral stationary phase was utilized to separate the enantiomers of both derivatives. We employed spectroscopic chiroptical and non-polarizable methods such as electronic and vibrational circular dichroism, infrared absorption and ultraviolet spectroscopies. Combined with density functional theory calculations, the stable conformers were found in solution and their spectra were subsequently simulated. We elucidated the three-dimensional structure of the enantiomerically pure compounds and assigned the absolute configuration of all prepared derivatives using both experimental and simulated spectra.

A Review of Modifications of Quinoline Antimalarials: Mefloquine and (hydroxy)Chloroquine

Late-stage modification of drug molecules is a fast method to introduce diversity into the already biologically active scaffold. A notable number of analogs of mefloquine, chloroquine, and hydroxychloroquine have been synthesized, starting from the readily available active pharmaceutical ingredient (API). In the current review, all the modifications sites and reactivity types are summarized and provide insight into the chemistry of these molecules. The approaches include the introduction of simple groups and functionalities. Coupling to other drugs, polymers, or carriers afforded hybrid compounds or conjugates with either easily hydrolyzable or more chemically inert bonds. The utility of some of the compounds was tested in antiprotozoal, antibacterial, and antiproliferative assays, as well as in enantiodifferentiation experiments.

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Design and synthesis of naphthalene-based chiral strong cation exchangers and their application for chiral separation of basic drugs

In continuation of our efforts to synthesize a highly dedicated strong cation exchanger, we introduce four chiral stationary phases based on a laterally substituted naphthalene core featuring chiral 2-aminocyclohexansulfonic acid as the chiral cation-exchange site. The selectors were modified with two different terminal units, which enabled immobilization to the silica support by thiol-ene radical reaction or azide-yne click chemistry. The chromatographic parameters of these chiral stationary phases were determined using a set of chiral amines, mainly from the family of β-blocker pharmaceuticals. The chiral stationary phases immobilized by means of click chemistry were found to be superior to those possessing the sulfide linker to the silica support. The chromatographic results and visualization of density functional theory-calculated conformations of the selectors hint at a combination of a steric and electronic effect of the triazole ring in the course of chiral resolution of the target analytes.

Chaotropic Effects in Sub/Supercritical Fluid Chromatography via Ammonium Hydroxide in Water-Rich Modifiers: Enabling Separation of Peptides and Highly Polar Pharmaceuticals at the Preparative Scale

Chromatographic separation, analysis and characterization of complex highly polar analyte mixtures can often be very challenging using conventional separation approaches. Analysis and purification of hydrophilic compounds have been dominated by liquid chromatography (LC) and ion-exchange chromatography (IC), with sub/supercritical fluid chromatography (SFC) moving toward these new applications beyond traditional chiral separations. However, the low polarity of supercritical carbon dioxide (CO₂) has limited the use of SFC for separation and purification in the bioanalytical space, especially at the preparative scale. Reaction mixtures of highly polar species are strongly retained even using polar additives in alcohol modifier/CO₂ based eluents. Herein, we overcome these problems by introducing chaotropic effects in SFC separations using a nontraditional mobile phase mixture consisting of ammonium hydroxide combined with high water concentration in the alcohol modifier and carbon dioxide. The separation mechanism was here elucidated based on extensive IC-CD (IC couple to conductivity detection) analysis of cyclic peptides subjected to the SFC conditions, indicating the in situ formation of a bicarbonate counterion (HCO₃^-). In contrast to other salts, HCO₃^- was found to play a crucial role acting as a chaotropic agent that disrupts undesired H-bonding interactions, which was demonstrated by size-exclusion chromatography coupled with differential hydrogen-deuterium exchange-mass spectrometry experiments (SEC-HDX-MS). In addition, the use of NH₄OH in water-rich MeOH modifiers was compared to other commonly used basic additives (diethylamine, triethylamine, and isobutylamine) showing unmatched chromatographic and MS detection performance in terms of peak shape, retention, selectivity, and ionization as well as a completely different selectivity and retention behavior. Moreover, relative to ammonium formate and ammonium acetate in water-rich methanol modifier, the ammonium hydroxide in water additive showed better chromatographic performance with enhanced sensitivity. Further optimization of NH₄OH and H₂O levels in conjunction with MeOH/CO₂ served to furnish a generic modifier (0.2% NH₄OH, 5% H₂O in MeOH) that enables the widespread transition of SFC to domains that were previously considered out of its scope. This approach is extensively applied to the separation, analysis, and purification of multicomponent reaction mixtures of closely related polar pharmaceuticals using readily available SFC instrumentation. The examples described here cover a broad spectrum of bioanalytical and pharmaceutical applications including analytical and preparative chromatography of organohalogenated species, nucleobases, nucleosides, nucleotides, sulfonamides, and cyclic peptides among other highly polar species.

Electrostatic attraction-repulsion model with Cinchona alkaloid-based zwitterionic chiral stationary phases exemplified for zwitterionic analytes

In the present paper, we demonstrated that Cinchona alkaloid cyclohexyl sulfonic acid-based zwitterionic chiral selectors (SOs) and the respective chiral stationary phases (CSPs) can be successfully employed for the enantioseparation of underivatized thus zwitterionic amino acids (the selectands, SAs) even in the absence of ionic additives in the eluent, generally used as displacer counter-ions in ion exchange chromatography. Therefore, we provided evidence that cooperative "intramolecular and intermolecular counter-ion effects" of the zwitterionic SO moiety and the zwitterionic SAs can be sufficient to modulate alone the retention characteristics without a loss of stereoselectivity. Four fully constrained β-amino acids were used as target compounds for this study. The analyses were carried out with either neat methanol, acetonitrile, water or their binary hydro-organic mixtures. A U-shaped retention profile was observed both with methanol- and acetonitrile-based eluents. Except a few cases, enantioselectivity experienced a remarkable amelioration at the "balanced region" of a buffer free hydroorganic mobile phase composition. At "the bottom" of the U-shaped curve, high α- and resolution values could be reached with most of the screened mobile phases. An electrostatically driven "attraction-repulsion model" was postulated to explain the very favourable characteristic of the two studied zwitterion-type CSPs for the retention and enantiomer separation of zwitterionic analytes.

Determination of Optical Purity of Lactic Acid-Based Chiral Liquid Crystals and Corresponding Building Blocks by Chiral High-Performance Liquid Chromatography and Supercritical Fluid Chromatography

Liquid crystals (LCs) are among the most prominent materials of the current information age, mainly due to their well-known application in liquid crystal displays (LCDs). Their unique electro-optical properties stem from their ability to form organised structures (mesophases) on the transition from solid state to isotropic liquid. Molecules of LCs in a mesophase still maintain the anisotropy of solid crystals, while simultaneously exhibiting the fluidity of liquids, which gives the system the ability to react immediately to external stimuli such as electric or magnetic fields, light, mechanical stress, pressure and, of course, temperature. For the proper function of LC-based devices, not only chemical, but also optical purity of materials is strongly desirable, since any impurity could be detrimental to the self-assembly of the molecules. Therefore, in this study we aimed to verify synthetic methods published in the literature, which are used nowadays to prepare chiral building blocks based on lactic acid, for their enantioselectivity. Moreover, we have focused on the development of an analytical chiral separation method for target liquid crystalline materials. Using a chiral polysaccharide-based column operated in liquid chromatography mode, we show that not all published methods of LC synthesis are enantioselective, which could lead to significant differences in the properties of the resulting materials. We show that high-performance liquid chromatography with UV detection and supercritical fluid chromatography with UV and mass spectrometry detection enable full control over the chemical and optical purity of the target LCs and the corresponding chiral building blocks. For the first time, we utilise supercritical fluid chromatography with mass detection for the direct chiral analysis of liquid crystalline materials and impurities formed during the synthesis.

A Comparative Study of Enantioseparations of Nα-Fmoc Proteinogenic Amino Acids on Quinine-Based Zwitterionic and Anion Exchanger-Type Chiral Stationary Phases under Hydro-Organic Liquid and Subcritical Fluid Chromatographic Conditions

The focus of this contribution is a comparative investigation of enantioseparations of 19 N^α-Fmoc proteinogenic amino acids on Quinine-based zwitterionic and anion-exchanger type chiral stationary phases employing hydro-organic and polar-ionic liquid and subcritical fluid chromatographic conditions. Effects of mobile phase composition (including additives, e.g., water, basis and acids) and nature of chiral selectors on the chromatographic performances were studied at different chromatographic modes. Thermodynamic parameters of the temperature dependent enantioseparation results were calculated in the temperature range 5–50 °C applying plots of lnα versus 1/T. The differences in standard enthalpy and standard entropy for a given pair of enantiomers were calculated and served as a basis for comparisons. Elution sequence in all cases was determined, where a general rule could be observed, both in liquid and subcritical fluid chromatographic mode the d-enantiomers eluted before the L ones. In both modes, the principles of ion exchange chromatography apply.