Synthesis of methyl 2-acetamido-2,6-dideoxy-α- and β-d-xylo-hexopyranosid-4-ulose, a keto sugar which misled the analytical chemists

To understand the contradictory results on the structure of the lipopolysaccharide isolated from a Yersinia enterocolitica O:3, both anomers of methyl 2-acetamido-2,6-dideoxy-d-xylo-hexopyranosid-4-ulose were prepared. The key steps of the synthetic pathway were the selective acetylation of the methyl 2-acetamido-2,6-dideoxy-α,β-d-glucopyranosides, the oxidation of the 4-position to form the keto-sugars, and deacetylation to provide the target compound. Surprisingly, the last step was accompanied by a disproportionation to give methyl 2-acetamido-2,6-dideoxy-α- and β-d-glucopyranosides and N-(5-hydroxy-6-methyl-4-oxo-4H-pyran-3-yl)acetamide as side-products.