The ionic chlorination of 9,10-dihydro-9,10-ethenoanthracene was shown to yield exo-and endo-4-syn-8dichlorodibenzobicyclo[3.2.1 ]octadiene as the sole reaction products. The less stable of these isomers was the preponderant product. The structure proofs of these addition-rearrangement reaction products and some of their chemistry are discussed. Ionic chlorination of 9,10-dihydro-9,10-ethenoanthracene (I) in carbon tetrachloride gave two isomeric dichlorides, A and B, neither of which were cisor trans-11,12-dichlorodibenzobicy clo [2.2.2 ]octadiene (IV) and (V), which had been previously prepared in this laboratory. 2 Both isomers gave an immediate precipitate of silver chloride when treated with silver nitrate in acetone. Compounds IV and V were unreactive to this reagent under these conditions.Addition rearrangements during the halogenation of bicyclic olefins have been previously reported,3-5 and this type of reaction seemed to be the best explanation for the formation of compounds A and B during the chlorination reaction. The formation of four isomeric dichlorinated hydrocarbons, II, III, VI, and VII, from the addition-rearrangement to I is possible.Catalytic hydrogenation of each of the dichlorinated isomers with palladium on barium sulfate yielded the same monochlorinated product (C), which might have any of the structures VIII to XI.Since the steric arrangement of one of the chlorine atoms in each isomer was thus shown to be identical with that in the other, it then follows that the isomeric pair (A and B) could have only the following combination