Achiral Cyclodextrin Analogues

Abstract

AbstractThe synthesis of a new family of cyclodextrin (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 Sn‐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 disac‐charide 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 monomers 1‐RR and 1‐MM, which bear both a glycosyl donor (cyanoethylidene function) and a glycosyl acceptor (trity‐loxy group). These compounds are able to undergo Tr+‐catalyzed polycondensation which, under appropriate dilution conditions, can be terminated by cycloglycosyl‐ation. 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 the disaccharide 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 21–24, 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 Ci symmetric solid‐state structure and its molecules pack to form a parquet‐like superstructure.