Synthesis of N-methyl-d-ribopyranuronamide nucleosides

The synthesis of N-methyl-d-ribopyranuronamide nucleosides is described. The key route is the rearrangement of a 1,2-O-isopropylidene protected furanose sugar with a carboxamide function in the 4-position to a ribopyranuronamide ring. The Lewis acid catalyzed condensation of adenine and thymine nucleobases with the per-O-acetylated N-methyl-d-ribopyranuronamide sugar is used to give the target nucleosides as a mixture of the α and β anomers. The mixture was separated and the final compounds were obtained by deacetylation in basic conditions.