D-Allose, a rare sugar. Synthesis of D-allopyranosyl acceptors from glucose, and their regioselectivity in glycosidation reactions.

Although D-allose (D-All) is a sugar with low natural abundance, it has great pharmacological and alimentary potential due to its biological properties. However, its chemistry, regarding the regioselectivity in protective reactions and glycosidations, has been scarcely explored. Glycobiological studies require appreciable quantities of carbohydrates with defined structures and high purity. Thus, the development of efficient strategies for their synthesis is crucial. In this frame, the knowledge of the regioselectivity between different hydroxyl groups of glycosyl acceptors is valuable because it allows minimizing the use of protecting groups. We have long been interested in the relative reactivity of OH-3 and OH-4 of glycosyl acceptors in glycosidation reactions. In this paper we synthesized D-allose glycopyranosyl acceptors with free OH-3 and OH-4 from D-Glc precursors. We assessed glycosidations with galactose trichloroacetimidates as donors and the experimental results were compared with those obtained by molecular modeling. Axial O-3 was the preferred site of glycosylation for α-anomers, whereas equatorial O-4 was the preferred site for a β-anomer. A good correlation between the experimental and modeling results was observed using atomic charges and cationic intermediates, although Fukui indices did not predict adequately the experimental results. The achieved regioselectivities are useful for the efficient design of oligosaccharide synthesis containing D-All moieties.