Synthesis of 5-Selenium-Substituted Uracil Derivatives. Inhibition of Thymidylate Synthetase by 5-Hydroseleno-2ʹ-deoxyuridylate

5-Selenium-substituted derivatives (diselenides) of uracil, 2ʹ-deoxyuridine, and 2ʹ-deoxyuridylic acid were synthesized via the addition of methyl hypobromite to the 5, 6 double bond, followed by reaction of the adducts with sodium diselenide. The physical and chemical properties of these compounds (including their facile reduction by dithiothreitol and rapid reoxidation) were similar to those of the corresponding 5-sulfur analogues. 5-Hydroseleno-2ʹ-deoxyuridylic acid was as potent as 5-mercapto-2ʹ-deoxyuridylate in inhibiting thymidylate synthetase from L. casei (κ_i≈ 6 X 10^-8M) but the nucleoside III was considerably less active than 5-mercapto-2ʹ-deoxyuridine in the inhibition of growth of the leukemia L1210 cell in culture.