Achiral cycledextrin analogues

The synthesis of a new family of cycledextrin (CD) analogues is described. This family consists of novel cyclic oligosaccharides built from monosaccharides that possess the same relative but opposite absolute (D- and L-) configurations. The alternation of such D- and L-residues-specifically, D- and L-rhamnose or D- and L-mannose-in a macrocyclic structure results in S(n)-type symmetry and, consequently, optical inactivity. The synthesis of these cyclic oligosaccharides was achieved by an economical polycondensation/cycloglycosylation approach that relies on an appropriately-derivatized disaccharide monomer and that avoids the time-consuming, and often low-yielding, stepwise growth of long linear oligosaccharide precursors. In the cases reported, the key precursors are the disaccharide menumars 1-RR and 1-MM, which bear both a glycosyl donor (cyanoethylidene function) and a glycosyl acceptor (trity1oxy group). These compounds are able to undergo Tr⁺-catalyzed polycondensation which, under appropriate dilution conditions, can be terminated by cycloglycosylation. Thus, compound 1-RR was converted into a range of protected cyclic rhamnooligosaccharides 15-19 in 64% overall yield. All these products, including the unique cyclic dodeca- and tetradecasaccharides 18 and 19, have been isolated by preparative HPLC. Unexpectedly, treatment of the manno analogue of thedisaccharide 1-RR (compound 1-MM) under the same conditions produced only the cyclic hexasaccharide 28 and numerous apparently linear oligomers. Removal of the protecting groups from 16-19 afforded the free cyclic oligosaccharides 2124, which exhibited the predicted zero optical rotation and very simple NMR spectra, indicating highly symmetrical structures. X-ray crystallography reveals that in the solid state the cyclooctaoside 21 possesses a C₂ symmetric structure, on account of a slight deformation of its cylindrical shape. The channel-type crystal packing of molecules of 21 forms nanotubes with an internal diameter of around 1 nm. Conversely, the cyclic hexasaccharide 29 possesses a C symmetric solid-state structure and its molecules pack to form a parquet-like superstructure.