On the Role of Neighboring Group Participation and Ortho Esters in β-Xylosylation:  ¹³C NMR Observation of a Bridging 2-Phenyl-1,3-dioxalenium Ion

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 with Tf₂O, thioglycosides with PhSOTf, and bromides with AgOTf the anomeric configuration of the donor is of no consequence on the outcome of the reaction. In all methods studied, the presence or absence of a non-nucleophilic, hindered base is of crucial importance with ortho esters only being discernible in its presence. S-Phenyl 2,3,4-tri-O-benzoyl-1-deoxy-1-thia-β-d-xylopyranoside was synthesized enriched with ¹³C at each of the three carbonyl carbons. Activation of this thioglycoside with PhSOTf in CD₂Cl₂ at -78 °C with or without the base permits, for the first time, the observation by ¹³C NMR spectroscopy of a bridging dioxalenium ion as an intermediate in a neighboring group directed 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.