Separation of triphenyl atropisomers of a pharmaceutical compound on a novel mixed mode stationary phase: a case study involving dynamic chromatography, dynamic NMR and molecular modeling.

Analysis of atropisomers is of considerable interest in the pharmaceutical industry. For complex chiral molecules with several chiral centers hindered axial rotation can lead to formation of interconverting diastereomers that should be separable on achiral stationary phases. However, achieving the actual separation may be difficult as the on-column separation speed must match or be faster then the rate of isomer interconversion. Often, this requirement can be satisfied by using low-temperature conditions and by improving selectivity via use of chiral stationary phases. In the current study, we present an alternative approach utilizing an Obelisc R column, a novel mixed mode stationary phase that provided acceptable separation of triphenyl atropisomers inside a conventional HPLC temperature range. The separation was investigated under various chromatographic conditions. The interconversion chromatograms exhibited classic peak-plateau-peak behavior indicating the simultaneous atropisomer separation and interconversion. The elution profiles were integrated in order to deconvolute the peak areas of the "pure" (non-exchanged) and interconverted species; these data were used to obtain kinetic information. Analysis of retention data rendered thermodynamic information on the mechanism of retention and selectivity. Chromatographic kinetic data were complemented with variable-temperature NMR and molecular modeling studies, which provided additional support and insights into the energetics of the interconversion process.