Macroporous monolithic chiral stationary phases for capillary electrochromatography: New chiral monomer derived from cinchona alkaloid with enhanced enantioselectivity.

A new chiral monomer derived from cinchona alkaloid, namely O-9-(tert-butylcarbamoyl)-11-[2-(methacryloyloxy)ethylthio]-10,11-dihydroquinine 1, was employed for the preparation of enantioselective monolithic capillary columns by an in situ copolymerization with 2-hydroxyethyl methacrylate 2 (HEMA), ethylene dimethacrylate 3 (EDMA) in the presence of cyclohexanol and 1-dodecanol as porogens (UV or thermal initiation of azobisisobutyronitrile (AIBN) as radical initiator). The porous properties and the electrochromatographic behavior of the new chiral monoliths were comparatively evaluated with previously described analogs obtained from O-9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 4 as chiral monomer. Despite close structural and physicochemical similarities of the both chiral monomers, the pore distribution profiles of the resulting monoliths were shifted typically towards larger pore diameters with the new monomer 1. Once more, it was confirmed that a low cross-linking (10 wt% related to total monomers) and a pore diameter of about 1 microm in the dry state provides the best electrochromatographic efficiency as a result of lower resistance to mass transfer (smaller C-term contribution to peak broadening) and more homogeneous flow profile (smaller A-term). Most importantly, as expected the new poly(1-co-HEMA-co-EDMA) monoliths showed enhanced enantioselectivities and in addition faster separations as compared to poly(4-co-HEMA-co-EDMA) analogs, which represents a significant improvement. Further, the elution order was reversed owing to the pseudoenantiomeric behavior of quinine- and quinidine-derived monomers. Fluorescence-labeled 9-fluorenylmethoxycarbonyl (FMOC), dansyl (DNS), 7-dimethylaminosulfonyl-1,3,2-benzoxadiazol-4-yl (DBD), carbazole-9-carbonyl (CC) amino acids could be separated with resolution values between 2 and 4 (with efficiencies typically between 100,000 and 200,000 plates/m) and fluorescence detection (variable wavelength fluorescence detector in-line with UV) yielding routinely a gain in detection sensitivities up to two orders of magnitude without specific optimization of the conditions with regards to fluorescence efficiency.

Low-molecular-weight chiral cation exchangers: novel chiral stationary phases and their application for enantioseparation of chiral bases by nonaqueous capillary electrochromatography.

Cation exchange type chiral stationary phases (CSPs) based on 3,5-dichlorobenzoyl amino acid and amino phosphonic acid derivatives as chiral selectors (SOs) and silica as chromatographic support were developed and applied to enantiomer separations of chiral bases by nonaqueous capillary electrochromatography (NA-CEC). As a rationale for efficient CSP development we adopted the combined use of the "reciprocity principle of chiral recognition" and nonaqueous ion-pair CE as screening assay. Thus, (S)-atenolol was employed as chiral counter-ion added to the BGE in CE and a series of N-derivatized amino acids and amino phosphonic acids were screened to derive reciprocally information on their chiral recognition abilities for atenolol enantiomers. Two SO candidates, namely N-(3,5-dichlorobenzoyl)-O-allyl-tyrosine and N-(4-allyloxy-3,5-dichlorobenzoyl)-1-amino-3-methylbutane phosphonic acid that have been identified as potential SOs in the CE screening were, after immobilization on thiol-modified silica, evaluated in cation-exchange NA-CEC. The strong chiral cation exchanger with the free phosphonic acid group exhibited enhanced enantioselectivity compared to the weak chiral cation exchanger with the carboxylic acid group. A wide variety of chiral bases could be successfully resolved on the strong chiral cation exchanger with alpha-values up to 2.2 and efficiencies up to 375000 m-1 including beta-blockers and other amino alcohols, local anesthetics like etidocaine, antimalarial agents like mefloquine, Tröger's base, phenothiazines like promethazine, and antihistaminics. The influence of several experimental parameters (electrolyte concentration, acid-base ratio and acetonitrile-methanol ratio) was evaluated.