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 alpha and beta anomers. The mixture was separated and the final Compounds were obtained by deacetylation in basic conditions. (C) 2008 Elsevier Ltd. All rights reserved.