title: 2,3-Anhydro sugars in glycoside bond synthesis. Highly stereoselective syntheses of oligosaccharides containing alpha- and beta-arabinofuranosyl linkages.
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.
authors.affiliation: Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
authors.forename: Rajendrakumar Reddy
authors.initials: RR
authors.lastname: Gadikota
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Callam
authors.forename: Timothy
authors.initials: T
authors.lastname: Wagner
authors.forename: Brian
authors.initials: B
authors.lastname: Del Fraino
authors.forename: Todd L
authors.initials: TL
authors.lastname: Lowary
mesh_terms: D001089:Arabinose / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry*; D002236:Carbohydrate Conformation; D004852:Epoxy Compounds / Q000737:chemistry; D006027:Glycosides / Q000138:chemical synthesis* / Q000737:chemistry; D009844:Oligosaccharides / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism
chemical_list: D004852:Epoxy Compounds; D006027:Glycosides; D009844:Oligosaccharides; C431714:arabinofuranose; D001089:Arabinose