ISSN:
0947-6539
Keywords:
carbohydrates
;
cyclodextrin analogues
;
cyclooligomerizations
;
glycosylations
;
nanostructures
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The 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.
Additional Material:
18 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.19970030818
Permalink