Enantioseparation of novel chiral sulfoxides on chlorinated polysaccharide stationary phases in supercritical fluid chromatography

Enantioselectivity of polysaccharide-based chiral stationary phases in supercritical fluid chromatography using methanol-containing carbon dioxide mobile phases

Characterization of retention and separation mechanisms with Pirkle-type enantioselective stationary phases in supercritical fluid chromatography.

Exploration of the enantiorecognition mechanism of protected amino acids on amylose tris (3,5-dimethylphenylcarbamate) stationary phase in supercritical fluid chromatography: Thermodynamic considerations and molecular docking study.

Supercritical fluid chromatography (SFC)-based chiral separation has been considered as one of the green, highly efficient, and precise methods for the resolution of new chiral pharmaceuticals. Herein, a comparative study of chiral separation of afoxolaner on polysaccharide-based chiral stationary phases (CSPs) by SFC and high-performance liquid chromatography (HPLC) has been conducted. First, effects of chromatographic conditions on chiral separation of afoxolaner by SFC, including CSPs, modifier types and ratios, flow rate, column temperature, and back pressure, have been discussed in detail. Cellulose tris(4-methylbenzoate)-coated CSP demonstrated the best separation performance for afoxolaner by SFC with the resolution of 2.37 among five CSPs. Afoxolaner enantiomers were eluted at 3.53 min and 4.54 min under the optimized SFC conditions, respectively, and the total analysis time was less than 6 min, much shorter than that by HPLC. Subsequently, molecular docking studies revealed that hydrogen bonds and halogen bonds formed by afoxolaner and CSPs dominated the selective interaction for the separation of afoxolaner. Additionally, hydrophobic effects and π-π stacks between afoxolaner and chiral selectors enhanced the resolution of afoxolaner. Moreover, quantitative determination results of afoxolaner by SFC showed good linearity relationships (R2 > 0.999) between concentration of enantiomers and the corresponding chromatographic peak area in the range from 0.025 to 0.800 mg/mL, and the limit of quantification of enantiomers was 0.025 mg/mL. In brief, SFC separation would offer a green alternative to overcome potential technical bottlenecks in the resolution of new chiral pharmaceuticals.

High performance liquid chromatography (HPLC) employing chiral stationary phases (CSPs) is the most popular and effective method for the separation of enantiomers. In this dissertation, the first chapter is an overview of chiral stationary phases for HPLC, which includes the structures, separation mechanisms, and applications of a variety of chiral stationary phases. The use of some chiral stationary phases in SFC also is discussed. The next three chapters present the enantiomeric separations of chiral furans, isochromenes, and polycycles on cyclodextrin-based chiral stationary phases. The performance of chiral stationary phases for the separation of these analytes was compared. The effect of the mobile phase compositions and structures of the analytes on the chiral recognitions were discussed. Chapter 5 through chapter 7 focuses mainly on the development and evaluation of new synthetic polymeric chiral stationary phases. First, the enantiomeric separation abilities of a new polymeric chiral stationary phase based on the monomer N,N’-(1S,2S)-1,2cyclohexanediyl-bis-2-propenamide was screened with 200 racemic samples. The enantiomeric separations obtained were optimized. The mobile phase compositions and a mobile phase additive (trifluoroacetic acid) were evaluated and the chiral recognition mechanism was discussed. The new CSP showed high sample loading capacity. Then, we developed two new synthetic polymeric CSPs with two other monomers, which are polymerible derivatives of trans-1,2-diphenylethylenediamine and trans-9,10-dihydro-9,10ethanoanthracene-(11S,12S)-11,12-dicarboxylic acid. The two new CSPs also showed enantiomeric selectivities for a variety of chiral compounds and high sample loading capacity. The three new synthetic polymeric CSPs are complementary to each other. Chapter 8 is a study on the use of the new synthetic polymeric CSPs with supercritical fluid eluents. The new CSPs also can separate many compounds using supercritical fluid chromatography (SFC). They showed high stabilities under SFC conditions. Compared with HPLC, SFC provides much faster separations due to the high flow rates. For some analytes, better enantiomeric separations were observed with SFC due to the better separation efficiencies.

Pharmaceutical-enantiomers resolution using immobilized polysaccharide-based chiral stationary phases in supercritical fluid chromatography

Systematic evaluation of new chiral stationary phases for supercritical fluid chromatography using a standard racemate library

The enantioselective potential of two polysaccharide-based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5μm silica particles were tris-(3,5-dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose-based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose-based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose-based chiral stationary phase were achieved particularly with propane-2-ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO2 , respectively. Methanol and basic additive isopropylamine were preferred on amylose-based chiral stationary phase. The complementary enantioselectivity of the cellulose- and amylose-based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.

Insights into chiral recognition mechanism in supercritical fluid chromatography III. Non-halogenated polysaccharide stationary phases

Cyclofructan-based chiral stationary phases were previously shown as a promising possibility for separation of chiral compounds in high performance liquid chromatography. In this work retention and enantiodiscrimination properties of the 3,5-dimethylphenyl carbamate cyclofructan 7 chiral stationary phase are described in supercritical fluid chromatography. The results obtained in both of the separation methods were compared. A set of compounds with axial or central chirality was used as analytes. The effect of mobile phase composition, that is, addition of different alcohol modifiers and/or trifluoroacetic acid to carbon dioxide, was examined in the supercritical system. Similarly, mobile phases composed of hexane modified with propan-2-ol and/or trifluoracetic acid were used in liquid chromatography. A linear free energy relationship model was utilized for characterization of interactions that are decisive for retention and separation in both techniques. Dispersion interactions showed similar negative values using both methods. The main contribution of hydrogen bond acidity was also comparable for both methods. The propensity to interact with n- and/or π-electron pairs of solutes was significant only in the supercritical system.

The use of compressed (dense) gases and supercritical fluids as chromatographic mobile phases in conjunction with liquid chromatographic (LC)-type packed columns was first reported by Klesper et al. in 1962. During its relatively short history, supercritical fluid chromatography (SFC) has become an attractive alternative to GC and LC in certain industrially important applications. SFC gives the advantage of high efficiency and allows the analysis of nonvolatile or thermally labile mixtures. The density of the mobile phase in SFC is about 200-500 times that in gas chromatography. The effect of shorter intermolecular distances and the resulting increase in molecular interactions is an enhanced solubilizing capability of the solvent towards various solutes. Compounds with much higher molecular weights than gas chromatography normally allows can therefore be chromatographed. However, the most commonly used mobile phases in SFC are all relatively non-polar fluids. Carbon dioxide, the most widely used fluid, is no more polar than hexane, even at high densities. Solute polarity should be between that of the stationary phase and the mobile phase in order to have a well behaved separation. Few real samples contain only nonpolar solutes, so a major objective of research in SFC has been directed toward increasing the range of solute polarity that can be handled by the technique. To bring the SFC technique into routine use, mobile phases that are more polar than the commonly used carbon dioxide are necessary. The solvent strength of supercritical CO2, even at high density, is not sufficient for the elution of polar solutes. Polar mobile phase such as NH3 exhibit useful properties, but a more practical way to extend the range of compounds separable by SFC is to use a mixed mobile phase. The solubility of the solute in the supercritical phase can be influenced considerably by adding modifiers to the mobile phase. The use of modifiers has been reported by Jentoft and Gouw and by Novotny et al. The latter group showed that adding 0.1% 2-propanol to pentane as the mobile phase decreases the observed partition coefficient (K) values for many polynuclear aromatic hydrocarbons by 20-35%. Thus, the addition of modifiers (generally organic solvents) to a supercritical mobile phase changes the polarity of the mobile phase and also leads to deactivation of the column packing material. In capillary SFC, most separations are made with pure CO2 because of its compatibility with an flame ionization detection (FID); except for formic acid and water, the addition of any common modifier precludes the use of FID. Modifiers are essential in packed-column SFC for the elution of polar compounds and are extensively used. Several workers have reported the influence of modifiers on peak shape, selectivity and retention time in capillary and packed-column SFC. A simple and effective way to add modifiers to a supercritical fluid mobile phase is to use a saturator column, which is usually a silica column saturated with polar alcohols. In this work, water was used as a polar modifier and a μporasil column as a saturator column. The μ-porasil column was inserted between the pump outlet and the injection valve. During the passage of the supercritical fluid mobile phase through the silica column, a polar modifier (water) can be dissolved in the pressurized supercritical fluid. Dimethylpolysiloxane polymer has been known as more polar polymer than polystyrene polymer. Dimethylpolysiloxane polymer has never been separated using water modified mobile phase. In this paper, using a μ-porasil column as a saturator column, excellent supercritical fluid chromatograms of dimethylpolysiloxane oligomers were obtained.

Chiral separations of cathinone and amphetamine-derivatives: Comparative study between capillary electrochromatography, supercritical fluid chromatography and three liquid chromatographic modes

Characterization of three macrocyclic glycopeptide stationary phases in supercritical fluid chromatography

In-depth characterization of six cellulose tris-(3,5-dimethylphenylcarbamate) chiral stationary phases in supercritical fluid chromatography

ABSTRACTIn the last decade, by integrating experimental and computational analyses, it was demonstrated that halogen bond (HaB) may contribute to binding and enantiorecognition mechanisms underlying the HPLC enantioseparation of halogenated chiral analytes by using cellulose tris(3,5‐dimethylphenylcarbamate) (CDMPC)‐based chiral columns and n‐hexane‐based mixtures as mobile phases. When used as a pivotal component of the mobile phase in supercritical fluid chromatography (SFC), carbon dioxide is often considered as an n‐hexane‐like nonpolar solvent because of its low dielectric constant and zero molecular dipole moment. On the other hand, carbon dioxide may also serve as hydrogen bond (HB) and HaB acceptor due to the presence of nonbonding electrons on the two oxygen atoms, interacting with analyte enantiomers, chiral selectors, and co‐solvents. On this basis, we report herein the results of a study aiming at evaluating the impact of using carbon dioxide in SFC in place of n‐hexane in HPLC on halogen‐dependent enantioseparations by using atropisomeric halogenated 4,4′‐bipyridines as analytes and Lux Cellulose‐1 as CDMPC‐based chiral column. The experimental investigation was complemented by a computational study performed using (a) quantum mechanics (QM) calculations to map and quantify noncovalent interactions possibly underlying the contact of the analytes with carbon dioxide and with the distinctive pendant groups of the CDMPC and (b) molecular dynamics (MD) simulations to visualize noncovalent interactions acting in the analyte 1/CDMPC chromatographic system in different media. The use of MD simulations to model enantioseparations performed in carbon dioxide‐based media was not reported in the literature so far.