Preparation of heptakis(2,6-di-O-ethyl)-β-cyclodextrin and its nuclear magnetic resonance spectroscopic characterization

Heptakis(2,6-di-O-ethyl)-β-cyclodextrin (DE-β-CyD) was prepared and its 1H and 13C nuclear magnetic resonance (NMR) signals in DMSO-d6 were unequivocally assigned by two-dimensional COSY and ROESY. The results on 1H coupling constants indicated that all ethylated glucose units are in a 4C1 chair conformation. The average spin-lattice relaxation times (T1) of ring carbons of DE-β-CyD were only slightly shorter, and their standard deviations from the mean Tl value were larger, than those of β-cyclodextrin (β-CyD) and heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CyD), suggesting the presence of slightly irregular internal motion in the ethylated glucose units. The temperature dependence of chemical shift of DE-β-CyD in DMSO-d6 suggested that the C3 hydroxyl protons may participate as proton donor in the intramolecular hydrogen bond to the C2 ethoxyl groups of neighboring glucose, and the intramolecular hydrogen bond of DE- and DM-β-CyDs is much stronger than that of β-CyD, suggesting the stable macrocyclic ring structure of DE-β-CyD.