2,3-Anhydro Sugars in Glycoside Bond Synthesis. Highly Stereoselective Syntheses of Oligosaccharides Containing α- and β-Arabinofuranosyl Linkages

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Abstract
The ever-increasing discovery of biologically important events mediated by carbohydrates has generated great interest in the synthesis of oligosaccharides and the development of new methods for glycosidic bond formation. In this paper, we report that 2,3-anhydrofuranose thioglycosides (1, 5) and glycosyl sulfoxides (2, 6), in which the hydroxyl groups C-2 and C-3 are "protected" as an epoxide, glycosylate alcohols with an exceptionally high degree of stereocontrol. The predominant or exclusive product of reactions with this fundamentally new class of glycosylating agent is that in which the newly formed glycosidic bond is cis to the epoxide moiety. We further demonstrate that subsequent nucleophilic opening of the epoxide moiety proceeds under basic conditions to give products in high yield and with good to excellent regioselectivity. The major ring-opened products possess the arabino stereochemistry, and thus this methodology constitutes a new approach for the synthesis of arabinofuranosides. In the epoxide opening reactions of glycosides with the 2,3-anhydro-beta-D-lyxo stereochemistry (e.g., 73), the addition of (-)-sparteine (78) to the reaction mixture dramatically enhanced the regioselectivity in favor of the arabino product. This represents the first example of the use of 78 to influence the regioselectivity of an epoxide ring opening reaction with a non-carbon nucleophile. We have demonstrated the utility of this methodology through the efficient synthesis of an arabinofuranosyl hexasaccharide, 7, which is a key structural motif in two mycobacterial cell wall polysaccharides.