Studies of hexahelicene bonded phases for the HPLC resolution of enantiomers
The thesis presents a review of the literature on the HPLC resolution of enantiomers using chiral stationary phases and discusses the mechanisms of separation and the utility of these phases. The synthesis and chemical, chiroptical and spectroscopic properties of helicenes are also reviewed and the potential utility of helicene-based for the HPLC resolution of enantiomers is discussed. The work carried out involved the preparation of a hexahelicene-based chiral stationary phase and demonstrations of its utility for the resolution of chiral analytes. This phase was chemically bonded rather than physically coated, in order to make it stable to hydrolysis and solvent stripping, and therefore to permit its employment with a wide range of mobile phases. This thesis describes the synthesis of hexahelicen-7-ylacetic acid methyl ester, including confirmation of the structure of the key synthetic intermediates by spectroscopic analysis, and the investigation of several proceduresfor the resolution of hexahelicen-7-ylacetic acid methyl ester into its enantiomers, one of which enabled around 100 mg amounts of each of (+) and (-) enantiomers of hexahelicen-7-ylacetic acid to be obtained in highly chemically and optically pure form. The thesis also gives an account of several synthetic approaches to covalently bonding of the chiral selector (hexahelicen-7-ylacetic acid) to a modified silica stationary phase. Chiral stationary phases were prepared from each of the enantiomers in sufficient amount to permit the packing of analytical columns of these phases. Chiral solutes containing nitroaryl functionalities were synthesised to investigate the chiral resolving power of these novel phases. Adequate separations were obtained with both chiral stationary phases and, as anticipated, the eluting order of chiral analytes was reversed between the (+) and (-) stationary phases. The work demonstrated the utility of these columns for the resolution of nitroaryl-containing chiral analytes.