The syntheses of the oligoribonucleotides GpApGpC, GpCpUpC, and ApGpCpUpC by an improved phosphotriester method are described. These sequences are found in the double-stranded region adjacent to the dihydrouridine loop of several transfer RNAs. Of special significance is the improvement in yields that the activating agent, mesitylenesulfonyl triazolide, provides in coupling reactions involving purine residues.