Preparation of chiral oxazolinyl-functionalized β-cyclodextrin-bonded stationary phases and their enantioseparation performance in high-performance liquid chromatography.

Abstract

A simple procedure for the synthesis of three new oxazolinyl-substituted β-cyclodextrins (6-deoxy-6-R-(-)-4-phenyl-4,5-dihydrooxazolinyl-β-cyclodextrin, 6-deoxy-6-S-(-)-4-phenyl-4,5-dihydrooxazolinyl-β-cyclodextrin, and 6-deoxy-6-S-(-)-(4-pyridin-1-ium-4-methyl-benzenesulphonate)-4,5-dihy-drooxazolinyl-β-cyclodextrin) and their covalent bonding to silica are reported. The ability of these chiral stationary phase columns for separating compounds is also presented and discussed. Twenty-eight compounds were examined in the polar-organic mobile phase mode, and 11 β-nitroethanols were tested in the reversed-phase mode. Excellent enantioseparations were achieved for most of the analytes, even for several challenging compounds. The rigid and flexible structures of mono-substituted chiral groups and the fragments around the rim of the β-cyclodextrin cavity played an important role in the separation process. Factors such as π-π stacking, dipole-dipole interactions, ion-pairing, and steric hindrance effects were found to affect the chromatographic performance. Moreover, the buffer composition, and percentages of organic modifiers in the mobile phase, were investigated and compared. The mechanisms involved in the separation were postulated based on the chromatographic data.