A novel 5-thioglycosylation method with 1,5-dithioglycosyl donors: relevance to exo- versus endocyclic activation

5-Thioglucosylation of a 1,6-anhydro-2-azido-2-deoxy-β-d-glucopyranose derivative was carried out with various 1,5-dithioglucosyl donors. The use of per-O-acetyl donors resulted in poor yields of an α-disaccharide. On the other hand, per-O-benzyl donors selectively gave an α-disaccharide in good to excellent yields regardless of the anomeric configuration of the donors. To study the relevance of the glycosylation results to the pre-equilibrium of exo- versus endocyclic sulfide activation, the relative nucleophilicities of exo- and endocyclic sulfides were estimated from the regioselectivities in the electrophilic oxidation of the per-O-benzyl donors with m-chloroperbenzoic acid. The results obeyed stereoelectronic effects and the endocyclic sulfides were more nucleophilic than the exocyclic sulfides for both anomers of per-O-benzyl-1,5-dithioglucosyl donors, the relative nucleophilicities of the endocyclic over exocyclic sulfides being 67% and 100%, respectively, for the α- and β-anomers. The results of the glycosidation and oxidation suggest that the glycosidation proceeded through an exocyclic cleavage mechanism despite the preferential endocyclic activation of 1,5-dithioglucosides.

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