Synthesis of Tri-, Hexa-, and Nonasaccharide Subunits of the Atypical O-Antigen Polysaccharide of the Lipopolysaccharide from Danish Helicobacter pylori Strains

Synthesis of trisaccharide repeating unit, →3)-α-d-Rhap-(1→2)-α-d-Manp3CMe-(1→3)-α-l-Rha p-(1→, and its dimeric hexa- and trimeric nonasaccharide subunits of the atypical O-antigen polysaccharide of the lipopolysaccharide from Danish H. pylori strains D1, D3, and D6 has been accomplished. Successful synthesis of the hexasaccharide and the nonasaccharide was possible by dimerization and trimerization of the suitably protected trisaccharide repeating unit, in which three monosaccharide moieties were arranged in a proper order by placing the sterically demanding 3-C-methyl-d-mannose moiety in between d- and l-rhamnoses. Key steps include the coupling of three monosaccharide moieties and dimerization and trimerization of the trisaccharide unit by glycosylations employing the 2‘-carboxybenzyl glycoside method. Also presented is a method for the synthesis of the novel branched sugar, 3-C-methyl-d-mannose moiety by elaboration of its equatorial hydroxyl and axial methyl groups at C-3‘ in the disaccharide stage.