Tris(4-bromophenyl)aminium hexachloroantimonate-mediated glycosylations of selenoglycosides and thioglycosides. Evidence for single electron transfer?

Radical cation-initiated glycosylation reactions of phenyl selenoglycosides are described. Glycosylations of phenyl selenoglycosides effected by the single-electron-transfer (SET) reagent, tris(4-bromophenyl)aminium hexachloroantimonate (BAHA), are examined with primary and secondary hydroxyl acceptors. The corresponding reaction of an ethyl thioglycoside with a primary hydroxyl acceptor is also examined. Reactions are performed in the presence of the SET quenching reagent, 1,2,4,5-tetramethoxybenzene, to assess whether BAHA-mediated glycosylation reactions involve SET. These experiments indicate that the reactions are completely quenched in dichloromethane but only partially in acetonitrile. The results provide support for the SET mechanism but an alternative mechanism involving electrophilic activation cannot be discounted. The oxidation potentials of various selenoglycosides are determined by cyclic voltammetry.