An unusual glycosylation product from a partially protected fucosyl donor under silver triflate activation conditions.

Partially protected glycosyl donors are extremely useful reagents for oligosaccharide synthesis allowing more facile deprotection and enhanced activity due to lower steric restraints. A partially protected fucosyl donor containing tert-butyldimethylsilyl (TBDMS) protecting groups was activated under bromine-silver triflate conditions in the presence of primary alcohols and found to give difucoside products exclusively, in good yield with excellent diastereoselectivity. The dimerisation reaction appears to require a conformational relaxation of steric crowding, induced upon activation of the glycosyl donor. The scope and limitations of this unusual glycosylation methodology are reported.