Multimodal cation exchange-type tyrosine-based chiral stationary phases: Synthesis and applications in high-performance liquid chromatography

Background

High-performance liquid chromatography (HPLC) using chiral stationary phases (CSPs) is among the most prevalent techniques for the separation of enantiomers. Among the most widespread CSPs, those containing ion-exchangers as chiral selectors (SOs) have emerged as powerful tools for the separation of polar and polarizable compounds. In addition to well-established commercial materials, such as Cinchona alkaloid-based chiral weak anion exchangers (WAX) and zwitterionic ion-exchange phases (ZWIX), chiral cation-exchange CSPs represent a valuable alternative. These materials have demonstrated broad applicability in the enantioseparation of racemic amines, encompassing a wide spectrum of pharmaceutical compounds. This study presents the design, synthesis, and chromatographic evaluation of novel multimodal chiral cation exchangers for HPLC.

Results

These CSPs innovatively combine the principles of both donor-acceptor and cation-exchange interactions by implementing a 3,5-dinitrobenzoyl tyrosine core with either a sulfonic acid (i.e., strong cation exchanger, SCX-type, CSP I and II) or a carboxylic acid moiety (i.e., weak cation exchanger, WCX-type, CSP III) as the respective ion exchange sites. A key feature of the synthetic strategy was the efficient covalent immobilization of the chiral selectors onto the silica support via a copper(I)-catalyzed azide-alkyne cycloaddition reaction (i.e., click chemistry). The chromatographic performance of the CSPs was systematically investigated under polar organic (PO) mode conditions for the enantioseparation of various chiral basic analytes. The influence of the mobile phase composition – including solvent polarity and the nature and concentration of acidic and basic additives – on retention as well as chemo- and enantioselectivity was thoroughly studied. Furthermore, the successful enantioseparation of uncharged analytes under normal phase (NP) conditions could be demonstrated.

Significance

This work introduces improved tyrosine-based CSPs that effectively integrate donor-acceptor and cation-exchange functionalities, rendering them a versatile and powerful addition to the available toolkit for challenging chiral separations.