Formation of trisaccharide nucleosides during disaccharide nucleoside synthesis

The unexpected formation of trisaccharide nucleosides during synthesis of disaccharide nucleosides with a purine base moiety has been observed. The occurrence of this side reaction can be explained in terms of the instability of the protected purine nucleosides in the presence of Lewis acids. The formation of these by-products can be avoided by using the 2,3-O-isopropylidene group instead of acyl groups for the blocking of the secondary hydroxyl groups of the nucleoside.