LC Separation of Enantiomers on Silica-Bonded Thiostrepton Derivatives

The macrocyclic antibiotic thiostrepton has been structurally modified with achiral reagents containing aromatic groups and isocyanate or isothiocyanate-terminated linkages to improve its selectivity in the LC resolution of aromatic enantiomers in polar–organic mode. The enantiomer resolutions achieved could not be reproduced either with underivatized thiostrepton or with the rifamycin SV stationary phase prepared by use of the same reaction procedures. Maximum enantioselectivity was achieved by optimizing the number of achiral derivatizing groups attached to thiostrepton. Chromatographic data for resolution of enantiomers containing aromatic groups suggest that the enhanced resolution is highly dependent on the π–π interaction between the analyte and the chiral selector. A mechanistic study entailing resolution of amino acid derivatives also indicated that analyte structure affects the strength of the π–π interaction and is crucial for chiral recognition to be possible. Other effects, for example steric hindrance, dipole–dipole interaction, and hydrogen bonding with the isocyanate and isothiocyanate functional groups of reagents also facilitate resolution.