Rapid assembly of gp120 oligosaccharide moieties via one-pot glycosidation-deprotection sequences

Mannosyl trihaloacetimidate donors equipped with a 2-O-Fmoc group can be effectively activated by catalytic Bi(OTf)3 in glycosidations. Despite the expected participating effect of the Fmoc group, the reaction solvent was found to be decisive for obtaining highly selective α-mannosylations. The Fmoc 2-O-protecting group can be then simply removed from the obtained di-oligosaccharide in the same vessel where the glycosidation is conducted. The resulting oligosaccharide can thus be directly employed as a glycosyl acceptor for further elongation. The preparation of biologically important linear and branched oligomannoses incorporated into HIV gp120 demonstrates that iteration of this one-pot sequence leads to very straightforward oligosaccharide assembly. As an additional result, a rapid approach has been disclosed for accessing a 3,6-OH mannose building-block to be incorporated in branched structures. This relies on a double reductive opening of a di-O-benzylidene mannose intermediate whose regioselectivity appears to be independent of the configuration of the five-membered benzylidene.