Synthesis of [2-14C,5-3H]cytosine and [2-14C,5-3H]uracil via bromination and catalytic bromine-tritium gas exchange

In micro‐scale experiments, [2‐14C,5‐3H]cytosine and [2‐14C,5‐3H] uracil were synthesized via bromination and catalytic Br‐3H exchange reaction with the use of [2‐14C]‐cytosine and‐uracil and tritium gas. The double labelling percentages of these products were 70 and 26, respectively. It was assumed that [2‐14C,5‐Br] uracil was subjected to reaction with hydrogen atom originally adsorbed on a palladium catalyst. This is to a lesser extent valid for [2‐14C,5‐Br]cytosine. The percentages of 3H labelling at 5 position of pyrimidine ring of cytosine and uracil were proved to be 96 and 73, respectively. For the analysis and purification of products, the HPLC eluting conditions using C18 reverse column and NaH2PO4 aqueous solution or H2O/methanol mixture as eluent were studied. Unreacted tritium gas was recovered with the use of adsorbents such as active charcoal and Zr‐V‐Fe getter.