Indirect synchronous fluorescence spectroscopy and direct high‐performance thin‐layer chromatographic methods for enantioseperation of zopiclone and determination of chiral‐switching eszopiclone: Evaluation of thermodynamic quantities of chromatographic separation

Abstract

Economic and enantioselective synchronous fluorescence spectroscopy and high-performance thin-layer chromatography methods have been developed and validated as per ICH guidelines for the separation of zopiclone enantiomers using L-(+)-tartaric acid as a chiral selector, followed by determination of the chiral-switching eszopiclone. Synchronous fluorescence spectroscopy was successfully applied for chiral recognition of R & S enantiomers of zopiclone at ∆λ=110nm based on creating of diastereomeric complexes with 0.06M tartaric acid in an aqueous medium containing 0.2M disodium hydrogen orthophosphate. Synchronous fluorescence intensities of eszopiclone were recorded at 296nm in concentration range 0.2- to 4-μg/mL eszopiclone. High-performance thin-layer chromatography method depends on resolution of zopiclone enantiomers on achiral HPTLC silica-gel plates using acetonitrile:methanol:water (8:2:0.25, v/v/v) containing L-(+)-tartaric acid as a chiral mobile-phase additive followed by densitometric measurements at 304nm in concentration range of 1 to 10μg/band of eszopiclone. The effect of chiral-selector concentration, pH, and temperature on the resolution have been studied and optimized for the proposed methods. The cited procedures were successfully applied to determine eszopiclone in commercial tablets of pure and racemic forms. Enantiomeric excess was evaluated using optical purity test and integrated peak area to describe the enantiomeric ratio. Thermodynamics of chromatographic separation, enthalpy, and entropy were evaluated using the Van't Hoff equation. The proposed methods were found to be selective for identification and determination of the eutomer in drug substances and products.