The dinucleoside monophosphates d (TpA)(lla) and d (CpA)(19a)-both chirally labeled with'80 at phosphorus and representing cleavage sites in a number of restriction endonuclease recognition sequences-have been synthesized via the fully proteded phosphate triesters 8a/b and 16a/b. The latter were obtained from condensation of the pyrimidine phosphoramidites 3a/b and 5a/b with an appropriately protected adenosine ...