Micro‐scale preparation of 4‐thiouracil labelled with 14C or 14C + 35S

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Abstract
A selective and quantitative method was developed for a microscale preparation of 4‐thiouracil‐2‐14C by the direct thiation of uracil‐2‐14C. From pyridine, dioxane, and tetralin as the thiation solvents, the best yield was obtained in dioxane (96.6% of 4‐thiouracil‐2‐14C). The simultaneous labelling with 14C + 35S was obtained by isotope exchange with elemental sulfur‐35S on refluxing in dimethylformamide. The present procedure may be used as a general method for the preparation of 14C or 14C + 35S labelled bases of nucleic acids of both the pyrimidine and purine type as well as the corresponding nucleosides and nucleotides.