Syntheses of lewisx and dimeric lewisx:: Construction of branched oligosaccharides by a combination of preactivation and reactivity based chemoselective one-pot Glycosylations

[GRAPHICS] Two asymmetrically branched oligosaccharides, Lewis' and dimeric Lewis(X), were assembled in one pot with high yields and exclusive regio- and stereoselectivities. p-Tolyl thiogly-cosides were utilized as the sole type of building blocks, thus simplifying the overall synthetic design. The reactivity-independent nature of the preactivation based method allows modular assembly of the dimeric Lewis(X) octasaccharide without the need for tedious protective group manipulation to achieve exact anomeric reactivities.