The geometry and the electronic structure of 6-oxa-heptacyclo [126.96.36.199(3,9).1(13,16).0(2,10).0(4,8).0(12,17)] docosan-14,18,20-triene-12,17-dicarboxylic anhydride (HDTD) have been investigated by DFT methods. The study shows that the norborneyl double bond of the HDTD molecule is endo pyramidalized and the central and bicyclo[2,2,2]octenyl double bonds are exo pyramidalized. The structure and stability of the cation intermediates and products of the reaction have been investigated by DFT methods. The most stable cation intermediate is U-N-type ion and the reaction takes place over this cation. The cation centre of U-N-type ion mutually interacts with the unshared electron pair of the oxygen atom of the tetrahydrofuran ring on the exo face and is sterically hindered by the tetrahydrofuran ring. The nucleophilic attack of bromide anion (Br-) on the cation centre of U-N-type ion occurs on the endo face and as a result the exo,endo-dibromide isomer of U-N-type product is obtained, which is 6.913 kcal mol(-1) more stable than the exo,exo-isomer.