Selective alkylation of pyrimidyldianions: synthesis and purification of 11C labeled thymidine for tumor visualization using positron emission tomography

Reaction of 5-bromo-2'-deoxyuridine and dihydropyran in THF gave a tetrahydropyranyl ether derivative which, after treatment with n-butyllithium, was selectively alkylated in situ by [11C]methyl iodide to give [methyl-11C]thymidine and unlabeled 2'-deoxyuridine. The labeled compound can be isolated by HPLC on a reversed phase column in 12 min. The ether derivative is stable which makes it possible to prepare 11C labeled thymidine by organic synthesis in 17-25% radiochemical yield in 30-45 min with radiopurities greater than 99%. In a synthesis with no-carrier-added, a radiochemical yield of 20% was obtained. Carrier added syntheses gave 2.4-4.7 mCi of [11C]thymidine with specific activities of 250-300 mCi/mmol.