Chiral separation of N‐imidazole derivatives, aromatase inhibitors, by cyclodextrin‐capillary zone electrophoresis. Mechanism of enantioselective recognition

Abstract

Baseline separation of ten new, substituted [1-(imidazo-1-yl)-1-phenylmethyl)] benzothiazolinone and benzoxazolinone derivatives with one chiral center was achieved using cyclodextrin-capillary zone electrophoresis (CD-CZE). A method for the enantiomeric resolution of these compounds was developed using neutral CDs (native alpha-, beta-, gamma-CDs or alpha-, beta-, gamma-hydroxypropyl (HP)-CDs) as chiral selectors. Operational parameters including the nature and concentration of the chiral selectors, pH, ionic strength, organic modifiers, temperature, and applied voltage were investigated. The use of neutral CDs provides enantiomeric resolution by inclusion of compounds in the CD cavity. The HP-alpha-CD and HP-beta-CD were found to be the most effective complexing agents and allowed efficient enantiomeric resolutions. Optimal separation of N-imidazole derivatives was obtained using 50 mM phosphate buffer at pH 2.5 containing either HP-alpha-CD or HP-beta-CD (7.5-12.5 mM) at 25 degrees C, with an applied field of 0.50 kV.cm(-1) giving resolution factors Rs superior to 1.70 with migration times of the second enantiomer less than 13 min. The same enantiomer migration order observed for all molecules can be related to a close interaction mechanism with CDs. The influence of structural features of the solutes on Rs and tm was studied. The lipophilic character (log kw) of the solutes and the apparent and averaged association constants of inclusion complexes for four compounds with the six different CDs led us to rationalize the enantioseparation mechanisms. The conclusions were corroborated with reversed-phase high-performance liquid chromatography (HPLC) on chiral stationary phases (CSPs) based on CDs.

Enantioselective separation of azole compounds by EKC. Reversal of migration order of enantiomers with CD concentration

“…As it can be observed in Table 1, the highest analyte-CD interactions (i.e., the highest apparent binding constant values) were not related to the highest enantioselectivities, fact that has already been reported for other azole derivatives with neutral and highly sulfated CDs [32,33]. Thus, TM-b-CD was the best chiral selector for the separation of the enantiomers of ketoconazole, terconazole, and bifonazole.…”

“…This supposition was supported by previous works where the relation between the structure and enantioseparation of different N-imidazole derivatives was studied by using neutral CDs or anionic highly sulfated CDs [32][33][34]. In order to calculate the apparent analyte-CD binding constants for the six compounds studied in this work with three neutral b-CDs (b-CD, HP-b-CD, and TM-b-CD), the mobilities of the enantiomers of each compound were measured at different concentrations of chiral selector.…”

An observation of unusual temperature effects for enantioselective CZE employing highly sulfated‐β‐cyclodextrin

“…Below the concentration, which results in the maximal mobility difference, the separation should decrease with increasing temperature as normally observed (Fig. 1B) [12][13][14][15].…”

“…Temperature reductions are normally expected to increase the enantioselectivity in CE separations [12][13][14][15]. An investigation of how the temperature influences the enantioselectivity for different racemic aminoalcohols (terbutaline, salbutamol and bambuterol) was performed by varying the temperature between 157C and 557C for a number of commonly used chiral selectors.…”

Chiral capillary electrophoretic determination of the enantiomeric purity of homocamptothecin derivatives, promising antitumor topoisomerase I inhibitors, using highly sulfated CDs and fluorescence detection

“…The large increase in migration times associated to a decrease in resolution when isopropanol is used can be explained by its hydrophobicity leading to a greater solvation of the analytes and competition between analytes and solvent for the CD cavities. The difference of the effect with ACN with respect to alcohol on the separation (small increase in the migration times and increase in resolution) may be due to its aprotic solvent nature and its weak influence on the viscosity of the BGE [39]. Because ACN leads to the best R s /t, it was logically chosen for the next step of the optimization procedure.…”