Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 82, Issue 2, Pages 848-868Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.joc.6b02593
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- Netherlands Organization for Scientific Research (NWO)
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The synthesis of complex oligosaccharides is often hindered by a lack of knowledge on the reactivity and selectivity of their constituent building blocks. We investigated the reactivity and selectivity of 2-azidofucosyl (FucN(3)) donors, valuable synthons in the synthesis of 2-acetamido-2-deoxyfucose (FucNAc) containing oligosaccharides. Six FucN3 donors, bearing benzyl, benzoyl, or tert-butyldimethylsilyl protecting groups at the C3-O and C4-O positions, were synthesized, and their reactivity was assessed in a series of glycosylations using acceptors of varying nucleophilicity and size. It was found that more reactive nucleophiles and electron-withdrawing benzoyl groups on the donor favor the formation of beta-glycosides, while poorly reactive nucleophiles and electron-donating protecting groups on the donor favor a-glycosidic bond formation. Low-temperature NMR activation studies of Bn- and Bz-protected donors revealed the formation of covalent FucN(3) triflates and oxosulfonium triflates. From these results, a mechanistic explanation is offered in which more reactive acceptors preferentially react via an S(N)2-like pathway, while less reactive acceptors react via an S(N)1-like pathway. The knowledge obtained in this reactivity study was then applied in the construction of alpha-FucN(3) linkages relevant to bacterial saccharides. Finally, a modular synthesis of the Staphylococcus aureus type 5 capsular polysaccharide repeating unit, a trisaccharide consisting of two FucNAc units, is described.
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