SummaryAnalysis of the formation and repair of structurally modified DNA is of particular interest in the study of carcinogenesis, cancer therapy and aging. The quantification of specific DNA lesions by sensitive immunoanalytical methods requires radiotracers with high specific activity. We describe the synthesis of 3 H-labelled adenine-, cytosine-, guanine-and thymine-alkyl derivatives by nucleophilic N-and O-alkylation using alkyl halides and diazoalkanes: 3-alkyl- to the corresponding 5-methylcytidine; the synthesis of three different 8-oxoguanine tracers; and the generation of thymidine glycol (5,6-dihydroxy-5,6-dihydro-[methyl-3 H]thymidine) from thymidine. All radiotracers were sucessfully employed in competitive radioimmunoassays for the quantification of defined DNA alkylation products in DNA repair analyses.

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loss of isocyanate followed by a four-to a five-membered ring expansion upon insertion of CO. The detailed mechanism of formation of 3 (as well as 2) from aryl isocyanate and Pdo is not evident, although decarbonylation of the 7-membered palladacycle 5, formed by the head to tail coupling of three isocyanates and Pd', is probably the last step. Note. however. that a classical coupling process of two C,H,NCO ligands on Pd' to form a five-membered palladacycle such 2. followed by a further insertion of isocyanate to give 5. can be excluded since 2 does not react with isocyanate.Although the above reactions are not directly implicated in the catalytic carbonylation of nitrobenzene [']. the pervasiveness and stability of the palladacycles 1-4 and their interconversions would offer credence to the intermediacy of analogous cycles in this catalytic process, particularly if the isoelectronic and isolobal nature of certain entities involved. e.g. RNCO and CO,. are considered."" Furthermore, considering the relationship of 2 and 3 to diphenylurea and triphenylbiuret. which are undesirable by-products'21 in the carbonylation process, our results indicate that such metallacycles, however, may indeed play a subsidiary perhaps even detrimental, role in this catalytic reaction. Finally the formation of both l and 2 under the catalytic conditions described above in the absence of a proton source or alcohol would seem to argue against the existence of intermediates such as aniline in the formation of cdrbamates in this palladium

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