Comparison of cyclofructan-, cyclodextrin-, and polysaccharide-based chiral stationary phases for the separation of pharmaceuticals

In this study, cyclofructan (CF)-, cyclodextrin (CD)-, and polysaccharide-based chiral stationary phases (CSPs) were exploited in high-performance liquid chromatography (HPLC) for the chiral separations of different clinically and pharmaceutically important compounds. In particular, R-naphthylethyl carbamate CF6 (RN-CF6), 3,5-dimethylphenyl carbamate CF7 (DMP-CF7), neutral beta cyclodextrin (β-CD), 3,5-dimethylphenyl carbamate β-CD (DMP-β-CD), and cellulose tris-(3,5-dimethylphenylcarbamate) (Cellulose-Tris DMP) columns were utilized under isocratic elution. The performance of these CSPs as chiral separation media was evaluated by use of nine analytes: acidic, basic, and amphiprotic. A possible correlation between the functional groups of these analytes and the chiral-recognition ability of each chiral column was also examined. The enantioseparations were optimized by varying different parameters, such as mobile phase additives, column temperature, and flow rate. Finally, a comparison was made between all CSPs, and it was expressed in terms of resolution (R_S), efficiency (N), selectivity (α), retention factors (k₁', k₂') and analysis time (t_R1, t_R2). It was observed that RN-CF6 was the most suitable and efficient CSP for the chiral separation of various types of analytes, including acids, primary and tertiary amines, alcohols, and many neutral compounds. It was the only CSP that provided baseline enantioseparation of thyroxine (R_S = 1.6) and cetirizine (R_S = 2.0).

A fully derivatized 4-chlorophenylcarbamate-β-cyclodextrin bonded chiral stationary phase for enhanced enantioseparation in HPLC

This paper reports an effective approach for the fabrication of a per-4-chlorophenylcarbamate-β-cyclodextrin (β-CD) bonded chiral stationary phase (CPCDP) in high-performance liquid chromatography. The morphology and structure of the ligand and the chiral stationary phase (CSP) were characterized by scanning electron microscopy, transmission electron microscopy, solid state ¹³C nuclear magnetic resonance spectra, fourier transform infrared spectra, elemental analysis and thermogravimetric analysis. Because CPCDP was a kind of multimode enantioseparation materials, the enantioseparation of chiral compounds including twelve azole antifungal agents, five proton pump inhibitors and five dihydropyridine calcium antagonists were studied in both reversed-phase and normal-phase chromatography. All analytes were obtained enantiomeric separation. Especially, the resolution of azoles was excellent. The selectivity and resolution of voriconazole reached 15.41 and 16.80, which was an exciting achievement for the enantioseparations by β-CD based chiral stationary phases. Compared with the commercial 3,5-dimethylphenyl carbamate-β-CD based chiral stationary phase (DMP), enhanced enantioselectivities for all the above compounds (except ilaprazole) were obtained on CPCDP column, which indicated that the 4-chlorophenylcarbamate group was conducive to the chiral recognition. Chromatographic studies elucidated that enhancement of analyte-chiral substrate interactions were attributed to the inclusion complexation, π-π stacking interaction, hydrogen-bonding, dipole-dipole interaction and steric hindrance. For further study, we also prepared semi-preparative chromatographic columns to obtain a single enantiomer. In addition to excellent chromatographic performance, the prepared CD-based column is stable and much cheaper than commercial columns, which can reduce the cost of test and has a good application prospect in chiral drug analysis.

Liquid chromatographic enantioseparation of carbocyclic β-amino acids possessing limonene skeleton on macrocyclic glycopeptide-based chiral stationary phases

Polar-ionic and reversed-phase high-performance liquid chromatographic separations of limonene-based cyclic β-amino acid enantiomers were carried out by using macrocyclic glycopeptide-based chiral selectors applying Chirobiotic T, TAG and R columns. The effects of additives, concentration of the co- and counter-ions and the temperature in polar-ionic mobile phase systems were studied. The influence of pH, MeOH content and alcohol additives were investigated in the reversed-phase mode. The difference in the change in standard enthalpy Δ(ΔH°), entropy Δ(ΔS°), and free energy Δ(ΔG°) was calculated from the linear van't Hoff plots derived from the ln α vs 1/T curves in the temperature range 5-40°C. Unusual temperature behavior was observed on Chirobiotic TAG for most of the analytes: decreased retention times were accompanied with increased separation factors with increasing temperature, and separation was entropically-driven. For two of the studied analytes enthalpically-driven enantioseparations were observed. The elution sequence was determined in all cases, but no general rule could be established.