Synthesis of [2- ^<14>C,5- ^3H]Cytosine with use of Bromine and Tritium Gas

A small scale synthesis of [2-14C, 5-3H] cytosine was attemped by the bromination of [2-14C] cytosine followed by the catalytic exchange reaction with tritium gas. The radioactive materials were always in the presence of those nonradioactive as carriers. Bromination were compared for the solutions of CCl4 and CCl4/H2O. In the former, bromocytosine was formed without any byproduct, but the reaction rate was very slow. In the latter, the reaction occurred quite rapidly and bromocytosine was obtained in a relatively high yield (41～70%), though undesirable bromouracil was also found. The double labeled cytosine was easily formed by a Br-3H exchange between [2-14C, 5-Br] cytosine and 3H2 gas using a catalyst of 10% Pd/CaCO3. This paper also presents the experimental results for the adsorption of unreacted 3H2 gas by active charcoal at 77K. The amount of 3H2 gas remaining in the reaction vessel was determined with a tritium monitor. The radioactiveity was 13 μCi, which was 1/4000 that of the 3H2 gas initially used.