Suitably protected dithymidine H-phosphonates were quantitatively converted into the corresponding dinucleoside 4-pyridylphosphonates by treatment with 1, 8-diazabicylo [5.4. 0] undec-7-ene (DBU) in the presence of trityl chloride in pyridine. The reaction was found to be stereospecific and proceeded, most likely, with retention of configuration at the phosphorus centre.
