[en] The synthesis of the title compound (3) is described. Treatment of a solution of triethyl [14C]orthoformate in dry chloroform with cis-[4-(2,5-diamino-6-chloro-4-pyrimidinyl)-amino]-2-cyclopentenyl]carbinol (1) afforded crude 2. Hydrolysis of crude 2 with 2 N sodium hydroxide gave 3 in 34% overall radiochemical yield with specific activity of 19.6 mCi/ mmol. (author)