Synthesis of S-linked thiooligosaccharide analogues of nodulation factors.: 2. Synthesis of an intermediate thiotrisaccharide

An S-linked thiotrisaccharide analogue of chitin that carries different N-protecting groups at the reducing and nonreducing end glucosamine residues was prepared as an intermediate for the synthesis of thioanalogues of nodulation factors. 1,6-Anhydro-2-azido-3-O-benzoyl-2-deoxy-beta-D-glucopyranose (3) was prepared either through the nucleophilic displacement of the 4-triflate galacto analogue 1 with potassium chloroacetate or via the selective acylation of the analogous gluco diol 4. Condensation of 3 with the N-phthalimido trichloroacetimidate 9 led to the disaccharide 10, which was converted in four steps to the glucosyl bromide 15. Nucleophilic displacement of the anomeric bromide by a 4-thiolate derivative of glucosamine (16) bearing a trichloroethoxycarbamate at C-2 was performed in anhydrous oxygen-free THF and led to the desired thiotrisaccharide precursor of thioanalogues of nodulation factors. Alternatively, the disaccharide 10 was prepared by regioselective glycosylation of the 1,6-anhydro diol 4 followed by benzoylation of the remaining free hydroxyl group.