X-ray crystallography and solution NMR spectroscopy characterization of heptakis(2,3-di- O-acetyl-6-bromo-6-deoxy)cyclomaltoheptaose

Abstract

Heptakis(2,3-di- O-acetyl-6-bromo-6-deoxy)cyclomaltoheptaose ( 2 ) has been characterized in aqueous solution by 1D and 2D NMR spectroscopy and in the solid state by X-ray crystallography. In methanol solution, the acetyl groups were found to interact with both inward and outward-pointing protons. This and the strong deshielding of the bridging carbons, relative to the nonacetylated precursor, indicate macrocyclic flexibility. In the crystalline state the macrocycle exists as a methanol complex. It exhibits elliptical distortion, all glucose residues been tilted with their primary side toward the cavity. The existing strain due to the congestion of 14 acetyl groups at the secondary site is relieved by two glucose rings acquiring the rarely observed skew-boat conformation, 0 S 2, by the increased tilting of two glucose residues, as well as by minor variations of the torsion angles of the acetyl groups. The seven bromine atoms are quite accessible to nucleophiles.