| Abstract
| - The role of ortho esters in the formation of 2,3,4-tri-O-benzoyl-β-xylopyranosides from various donor/promoter pairs has been investigated. It is concluded that for the activation of sulfoxides withTf2O, thioglycosides with PhSOTf, and bromides with AgOTf the anomeric configuration of thedonor is of no consequence on the outcome of the reaction. In all methods studied, the presence orabsence of a non-nucleophilic, hindered base is of crucial importance with ortho esters only beingdiscernible in its presence. S-Phenyl 2,3,4-tri-O-benzoyl-1-deoxy-1-thia-β-d-xylopyranoside wassynthesized enriched with 13C at each of the three carbonyl carbons. Activation of this thioglycosidewith PhSOTf in CD2Cl2 at −78 °C with or without the base permits, for the first time, the observationby 13C NMR spectroscopy of a bridging dioxalenium ion as an intermediate in a neighboring groupdirected glycosylation. Quenching of this cation in the presence of the base leads to the ortho ester,whereas in the absence of the base the glycosides are the only products detected.
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