Lewis tetrasaccharide epitope

Figure 3 A) Repeating hexasaccharide subunit of the capsular polysaccharide of V. cholerae 0139. Colitose is 3,6 di-deoxy-L-galactose. QuiNAc, N-Acetyl quinovosamine is 6-deoxy N-Acetyl glucosamine. B) The Lewis tetrasaccharide epitope. C) The repeating heptasaccharide subunit of the cell wall polysaccharide of S. mitis J22...

A similar problem was encountered during the synthesis of the glycans of glyco-dellin-S [36]. The synthesis of Lewis epitope 26 required bis-a-fucosylation of lac-tosamine moiety 24 (Scheme 6). The tri-O-benzylated fucosyl donor 25 provided the tetrasaccharide 26 with acceptable selectivity but unfortunately as an inseparable mixture. Recourse to a bromide donor under halide ion catalysis (as in the previous example) was not successful because the deactivation was so great it made glyco-sylation of the hindered hydroxyls prohibitively slow. Deactivating the donor species by acetylating the 3- and 4-positions (27) [45], however, increased the a-selectivity to the point that the other anomer was unobservable. 

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