Abstract: A facile synthesis of adenosine 3', 5'-cyclic methylphosphohate (1) and 3', 5'-cyclic methylphosphonothioate (2) was accomplished by treatment of 2'-protectedadenosine with O,O-bis (1-benzotriazolyl) methylphosphonate or O,O-bis (1-benzotriazolyl) methylphosphonothioate under mild conditions, respectively. The ability of the compounds to inhibit DNA synthesis in mice hepatoma was tested. A pair of diastereomers of 2'-protected (1) was separated and the configuration at the phosphorus atom was identified.