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A reaction of octa-1,7and deca-1,9-dienes with an excess of bromofluorocarbene proceeds predominantly with the formation of a monoadduct, whereas the bis-adduct can be prepared by repeated cyclopropanation. The [1+2] cycloaddition of dichlorocarbene to the monoadduct of bromofluorocarbene with deca-1,9-diene gave a new bis(cyclopropane) containing gem-dichloro- and gem-bromofluorocyclopropane fragments in one molecule. The bis-adducts obtained were subjected to the heterocyclization with nitronium triflate: in the case of bis( gem-bromofluorocyclopropane) the reaction proceeded with retention of one dihalocyclopropane fragment and gave mononuclear pyrimidine N-oxide. [ABSTRACT FROM AUTHOR]