Conformational Analyses of Alkylated β-Cyclodextrins by NMR Spectroscopy

Abstract

β-Cyclodextrin was derivatized with 1-bromoalkanes of different chain lengths. The yield as well as the purity of the resulting cyclodextrin derivatives were improved by using solvent mixtures instead of pure solvents. The heptakis(2,6-di-O-alkyl)- and heptakis(2,3,6-tri-O-alkyl)-β-cyclodextrins were studied by means of NMR spectroscopy employing one-dimensional as well as two-dimensional NMR techniques. Analysis of the NMR data showed that only in completely derivatized β-cyclodextrins the cyclodextrin torus undergoes a conformational change. Best separation factors were found for the cyclodextrin derivative with the highest internal mobility as determined from the corresponding NT1-values, the validity of which was confirmed by NOE determinations. Therefore, 13C relaxation times could be used as a measure of the efficiency and usefulness of a given cyclodextrin derivative as an enantioselective stationary phase.