Practical approach for the stereoselective introduction of beta-arabinofuranosides

A practical approach for the stereoselective introduction of beta-arabinofuranosides has been developed on the basis of locking an arabinosyl donor in a conformation in which nucleophilic attack from the beta face is favored. The new glycosyl donor was designed by analyzing optimized geometries of low-energy conformers of the arabinofuranosyl oxacarbenium ion. The Newman projection of the E(3) conformer indicated that nucleophilic attack from the alpha face is disfavored because an eclipsed H-2 will be encountered. On the other hand, an approach from the beta face was expected to be more favorable, because it will experience only staggered substituents. The arabinofuranosyl oxacarbenium ion could be locked in the E(3) conformation by employing a 3,5-O-di-tert-butylsilane protecting group, which places C-5 and O-3 in a pseudoequatorial orientation, resulting in a perfect chair conformation of the protecting group. The new glycosyl donor gave excellent beta selectivities in a range of glycosylations with glycosyl acceptors having primary and secondary alcohols. The attractiveness of the new methodology was demonstrated by the chemical synthesis of a fragment of arabinogalactan, which is an important constituent of the primary plant cell wall.