Stereoselective preparation of four 3-C-mannosylated d- and l-glucals from a single starting compound

The corresponding oxadiene, prepared from the starting perbenzylated α-d-mannopyranosylethanal was subjected to stereoselective cycloaddition reactions with R and S methyl (ethenyloxy)(phenyl)acetates. From the two obtained diastereoisomeric cycloadducts, 3-C-α-d-mannosylated 1,2-d-glucal and 3-C-α-d-mannosylated 1,2-l-glucal were prepared. A simple epimerisation of the starting α-d-mannopyranosylethanal afforded perbenzylated β-d-mannopyranosylethanal, which was converted to 3-C-β-d-mannosylated 1,2-d-glucal or to 3-C-β-d-mannosylated 1,2-l-glucal by the same procedure. The structure of the obtained 3-C-α-d-mannosylated 1,2-d-glucal has been confirmed independently by its transformation to the known peracetylated methyl α-C-(1→3)-mannobioside. The prepared glucals are suitable precursors for the synthesis of stable glycoconjugates with non-hydrolyzable mannose-containing epitopes.