The bromination of highly halogenated chrysanthemic acid analogues

(Z)-cis- and (Z)-trans-3-(2-chloro-3,3,3-trifluoroprop-1-enyl)-2,2-dimethylcyclo-propanecarboxylic acids react with bromine under radical conditions predominantly to open the cyclopropane ring; the stereochemistry and reactions of the dibromides thus formed are described.