CHEMICAL-MODEL STUDIES ON THE MUTAGENESIS OF BENZOFURAN DIOXETANES IN THE AMES TEST - EVIDENCE FOR THE BENZOFURAN EPOXIDE AS ULTIMATE MUTAGEN

The synthesis of the first benzofuran epoxide 3a was achieved by epoxidation of the benzofuran 1a with dimethyldioxirane and alternatively by deoxygenation of the benzofuran dioxetane 2a with sulfides. This labile epoxide formed with nucleophiles such as water, methanol, thiophenol, and imidazole the corresponding adducts 13a-16a. In contrast to epoxide 3a, the dioxetanes 2 required acid catalysis (CF3CO2H) for the addition of water, methanol, and azide ion to give the corresponding adducts 9-11; in the absence of nucleophiles the allylic hydroperoxides 8 were formed. The decomposition of benzofuran dioxetanes 2 in the polar, protic solvents water and methanol afforded not only expected cleavage products 4 but also the 1,3-dioxols 5, the spiroepoxide dimer 6a, and the 1,4-dioxines 7. An intramolecular electron-transfer mechanism is postulated for the formation of the spiroepoxide, which subsequently dimerizes to 6a or rearranges into 5 and 7. Only the benzofuran epoxide 3a, besides the benzofuran dioxetanes 2, was mutagenic in the Salmonella typhimurium strain TA100. Therefore, we implicate the epoxide 3a as the ultimate mutagen responsible for the high mutagenic activity observed with dioxetane 2a in the Ames test. We postulate that in the oxidative metabolism of polycyclic arenes and heteroarenes the corresponding epoxides are generated from the intermediary dioxetanes by deoxygenation with sulfides.