REARRANGEMENT OF A BICYCLO [2.2.2]OCTADIENOL TO A DIHYDROPENTALENE

Either of the epimeric alcohols 1 and 2 (1,3,3,4,7,8-hexamethyl-5,6-benzobicyclo[2.2.2]octa-5,7-dien-2-ols), synthesized by addition of benzyne to hexamethyl-2,4-cyclohexadienone followed by hydride reduction, rearranged on dehydration with strong acid to the crystalline benzodihydropentalene 14. The structure of 14 was deduced from its spectra and chemical transformations. In particular, 14 is hydrogenated preferentially at the “central” double bond, forms Diels-Alder adducts 21 and 22 with tetracyanoethylene and N-phenyImaleimide, and undergoes exclusive exchange in 80 % D2SO4-D2O at the C-3 methyl group. Treatment of 1 and 2 with strong acid for shorter periods and lower temperatures gave reduced yields of 14 and permitted the isolation of three isomeric methylenebicyclo[3.2.1 Joctadienes (5, 7 or 8, and 25), each of which with further acid treatment gave 14. Reactions with deuterium-labeled precursors permitted the various methyl groups to be traced. A mechanism is suggested which involves rapid epimerization of 1 and 2 and the formation of equilibrating ions 37, 38, and 39 which may, by proton loss, furnish hydrocarbons 24, 7 (or 8), and 5, respectively. Alternatively, prolonged treatment with acid permits opening of the bicyclo[3.2.1] system in 37-39, leading to the bicyclo[3.3.0] system in 14. © 1968, American Chemical Society. All rights reserved.