The doubly bridged substituted biphenyls are prepared and separated into the diastereomers by normal chromatography. While in 5,11-dichloro-4,10-dimethoxy-4,10-dihydrodibenzo[ef,kl]heptalene, 1A, all the three possible diastereomers, i.e.endo–endo; endo–exo; and exo–exo, are formed; in 5,11-dichloro-4,12-dimethoxy-4,12-dihydrodibenzo[ef,kl] heptalene, 2A, only exo–exo and endo–exo diastereomers could be detected. The diethoxy substituted compounds behave like the methoxy ones, but for the propoxy group, two diastereomers for 1C and two diastereomers for 2C are observed. The endo–endo diastereomer in 1A could be converted to the exo–exo
by a mechanism involving rotation around the pivot bond and a double ring inversion but the reverse process is not observed. The barrier to 1Aendo–endo
→
1Aexo–exo conversion is found to be ΔG#403
= 30 ± 0.3 kcal mol−1 with ΔH# of 24.4 ± 0.2 kcal mol−1 and ΔS# of −14 ± 4 e.u. The structures of the 1Aendo–exo and 1Aexo–exo isomers and the structure of 2Aendo–exo were determined by single crystal X-ray crystallography. Allylic coupling constants are derived for all compounds and compared
to the calculated ones. All derivatives having an ethoxy or propoxy group show the CH2 protons attached to oxygen, diastereotopics.