Surface-up constructed tandem-inverted bilayer cyclodextrins for enhanced enantioseparation and adsorption

Surface-up construction of quinine bridged functional cyclodextrin for single-column versatile enantioseparation

Seeking for single-column versatile chiral separation methodology is the ultimate goal for analysts engrossed in enantioseparation. However, the versatility and selectivity are always contradictory due to negative influence among the recognition domains and the relatively low surface concentrations in a limited support surface area. Herein, we reported a novel series of quinine (QN) bridged cyclodextrin (CD) chiral stationary phases (CSPs) with satisfied surface concentration of both selectors, prepared via a facile surface-up 'thiol-ene click' approach, where QN and CD can not only nicely exhibit their individual resolution capability but also afford possible synergism in resolving difficult-to-separate analytes. QN bridged phenylcarbamoylated CD CSP exhibits powerful resolution ability by positive combining the resolving ability of QN and functional CD and achieves the resolution of almost a double number of racemates over QN or CD CSPs. Meanwhile, it exhibits comparable and even better chiral selectivity over the widely used chemical-bonded chiral column (CHIRALPAK® IA, CHIRALPAK® IB and CHIRALPAK® IC of Daicel) for the studied analytes. This work thus advances the duplex QN-CD structure as a relatively versatile platform for chiral resolution and commendably promotes the design of functional CSPs with chiral molecular bridge.

Preparation and chiral resolution properties of bridged bis(cyclodextrin)s hybrid spheres for high performance liquid chromatography

Selenium-bridged bis(β-cyclodextrin)s organic-inorganic hybrid silica material with regular spherical shape as new type of chiral stationary phase was directly synthesized under the one-pot hydrothermal synthesis method, and the chiral stationary phase was further characterized by infrared spectroscopy, scanning electron microscopy, thermogravimetry, and elemental analysis. The results of chiral separation showed that eight chiral compounds including various types of chiral alcohols and flavanone were successfully separated in the reversed-phase separation mode by high performance liquid chromatography, which showed the better chiral resolution effect than that on the C2 position of single β-cyclodextrin. The mechanism of chiral separation was likely due to multiple interactions such as inclusion, hydrogen bonding, electrostatic interaction, dipole-dipole interaction, and the synergistic effect of two cyclodextrins during the chiral resolution process. The synergy of the two cyclodextrins has great potential for development in chiral resolution.

Preparation of a novel bridged bis(β-cyclodextrin) chiral stationary phase by thiol-ene click chemistry for enhanced enantioseparation in HPLC

A bridged bis(β-cyclodextrin) ligand was firstly synthesized via a thiol–ene click chemistry reaction between allyl-ureido-β-cyclodextrin and 4-4′-thiobisthiophenol, which was then bonded onto a 5 μm spherical silica gel to obtain a novel bridged bis(β-cyclodextrin) chiral stationary phase (HTCDP). The structures of HTCDP and the bridged bis(β-cyclodextrin) ligand were characterized by the ¹H nuclear magnetic resonance (¹H NMR), solid state ¹³C nuclear magnetic resonance (¹³C NMR) spectra spectrum, scanning electron microscope, elemental analysis, mass spectrometry, infrared spectrometry and thermogravimetric analysis. The performance of HTCDP in enantioseparation was systematically examined by separating 21 chiral compounds, including 8 flavanones, 8 triazole pesticides and 5 other common chiral drugs (benzoin, praziquantel, 1-1′-bi-2-naphthol, Tröger's base and bicalutamide) in the reversed-phase chromatographic mode. By optimizing the chromatographic conditions such as formic acid content, mobile phase composition, pH values and column temperature, 19 analytes were completely separated with high resolution (1.50–4.48), in which the enantiomeric resolution of silymarin, 4-hydroxyflavanone, 2-hydroxyflavanone and flavanone were up to 4.34, 4.48, 3.89 and 3.06 within 35 min, respectively. Compared to the native β-CD chiral stationary phase (CDCSP), HTCDP had superior enantiomer separation and chiral recognition abilities. For example, HTCDP completely separated 5 other common chiral drugs, 2 flavanones and 3 triazole pesticides that CDCSP failed to separate. Unlike CDCSP, which has a small cavity (0.65 nm), the two cavities in HTCDP joined by the aryl connector could synergistically accommodate relatively bulky chiral analytes. Thus, HTCDP may have a broader prospect in enantiomeric separation, analysis and detection.

Separation of chiral compounds on HTCDP.

HPLC Enantioseparation on Cyclodextrin-Based Chiral Stationary Phases

As one of the commonly used chiral separation materials, cyclodextrin-based chiral stationary phases (CD-CSP) have been developed rapidly in the past 30 years. A large number of CD-CSPs have been designed and applied for enantioseparation in high-performance liquid chromatography (HPLC). The development of novel CD-CSPs focuses on two aspects: the immobilization chemistry and the functionalization of the CD skeleton. Although such studies are not regarded as the prime research topic in analytical chemistry, there are still many recent works pushing this research forward tardily. In this chapter, the fabrication procedure of a triazole-bridged duplex CD-CSP and its application to HPLC enantioseparations is described.

A cationic cyclodextrin clicked bilayer chiral stationary phase for versatile chiral separation in HPLC

A cationic cyclodextrin (CD) clicked bilayer chiral stationary phase (CSP) was developed via click chemistry for chiral separations in multimode high-performance chromatography (HPLC).