Study of the Endocyclic versus Exocyclic C–O Bond Cleavage Pathways of α- and β‑Methyl Furanosides

The activation and ring-opening of methyl furanosides in the four natural sugar scaffolds (ribo, lyxo, arabino, and xylo) efficiently afforded acyclic thioacetals with high SN2-like selectivity at the acetal center in the presence of Me2BBr and thiophenol. The stereochemical outcome of these reactions provides important mechanistic insights into the activation pathway of five-membered semicyclic acetals. The thioacetal products should find applications in oligosaccharides synthesis and allow further development of acyclic strategies for the synthesis of novel nucleoside analogues.