Glycosyl Imidates, 75. Synthesis of the Hexasaccharide Moiety of Globo H (Human Breast Cancer) Antigen

The hexasaccharide moiety 1c of the globo H antigen 1a was synthesized based on a highly efficient strategy. To this end galactosyl trichloroacetimidate 2 was employed for the glycosylation of azidogalactose derivative 3 as acceptor to afford β(1→3)‐linked disaccharide 7 in high yield. Deacetylation and regioselective benzoylation and then fucosylation with trichloroacetimidate 5 gave trisaccharide 10 which was readily transformed into trichloroacetimidate 12 as glycosyl donor. Thus, with galactose derivatives 4a, b as acceptors tetrasaccharides 13a, b were obtained in overall seven steps in high yields. Transformation of 13 into a glycosyl donor was best performed with 13b via 2‐O‐benzylation, 1‐O‐deallylation and then transformation into trichloroacetimidates 20α,β. With 6 as acceptor the desired hexasaccharide 21 was obtained which could be fully structurally assigned. Hydrogenolytic debenzylation, debenzylidenation, and azido group reduction in one step and then acetylation concluded the synthesis of hexasaccharide 1c.