Rationally designed syntheses of high-mannose and complex type undecasaccharides

Synthetic routes are described to a high-mannose type triantenary undecasaccharide 1 and a completely protected form 39 of the complex type biantenary undecasaccharide 2 carrying alpha-(2-->3)-linked sialic acid residues. Starting from a previously reported trisaccharide 4, the core pentasaccharides 15 and 37 were synthesized through regio- and/or stereo-selective mannosylation with a suitably protected mannosyl donor. Chain elongation of 15 by stepwise addition of a mannose residue afforded an undecasaccharide 20 that was eventually deprotected to give 1. On the other hand, coupling of 38 and a trisaccharide glycosyl donor 36 afforded 39.