Non-conventional glycosidic linkages: Syntheses and structures of thiooligosaccharides and carbohydrates with three-bond glycosidic connections

Thiooligosaccharides display inhibitory activity against glycoside hydrolases and they constitute a valuable tool for structural biology. The construction of the interglycosidic linkage, in which the oxygen atom is replaced by sulfur, usually involves one of the following reactions: i) nucleophilic displacement of good leaving groups in a carbohydrate moiety by a sugar thiol, ii) Michael and Michael-type additions of sugar thiols to unsaturated acceptors, iii) ring-opening of aziridines and oxiranes by thiosugars, iv) enzyme-catalyzed couplings of thiosugar moieties. The application of these reactions for the syntheses of thiooligosaccharides are surveyed, and the biological activities of the resulting products briefly described. Carbohydrate structures incorporating three-bond glycosidic linkages with two heteroatoms are uncommon in Nature. The -N-O- interglycosidic bond in the oligosaccharide part of enediyne antibiotics is partly responsible for their potent antitumor activities. The disulfide bond which plays an essential role in proteins has recently been introduced as an interglycosidic connecting motif. The sulfenamide functionality is the sulfur analog of the hydroxylamine type glycosidic linkages in calicheamicins. Novel glycosylation strategies have recently been developed by taking advantage of unconventional, three-bond glycosidic linkages to construct neoglycoproteins and other glycoconjugates which are increasingly important tools in glycobiology and drug discovery. To explore conformational preferences and molecular flexibility in these structures NMR spectroscopy, chiroptical methods and X-ray crystallography are being used, supplemented by molecular modelling calculations. The structural features will be briefly discussed with relevance to biological interactions such as enzyme binding.