Synthesis, NMR, and Conformational Studies of Cyclic Oligo-(1→6)-β-D-Glucosamines

The first synthesis of a series of homologous cyclic oligo-(1 -> 6)-beta-D-glucosamines consisting of two to seven residues and representing a new type of functionalized cyclic oligosaccharides is reported Remarkably high yields of the studied macrocyclization reaction irrespective of the length of the acyclic precursors were observed. In the case of compounds constituted of foul to seven glucosamine units alpha-stereoisomers formed as side products despite the presence of a strongly participating 2-N-phthaloyl group to control beta-glycosylation Both phenomena may be accounted for by conformational features of the linear bifunctional precursors According to computer modeling and NMR conformational studies, the described linear (1 -> 6)-beta-linked oligoglucosamines exist in a right-handed helix-like conformation, in which the glycosyl donor and acceptor moieties are pie-arranged in a way that facilitates intramolecular glycosylation from the alpha-side Prepared cyclo-oligoglucosamines differ in their conformational flexibilities, as illustrated by their spectral characteristics and calculated asphericity distributions Moreover, the obtained compounds do not possess a distinct hydrophobic cavity, which is in contrast to the well-known cyclodextrins All these characteristics provide an excellent basis for the use of these novel cyclic oligosaccharides as scaffolds for the construction of biomolecular conjugates.