Dibenzo [a, e] cyclooctene-5, 6-dione (18), a derivative of 1, 2-cyclooctatetraenoquinone (4), has been synthesized by selenium dioxide oxidation of dibenzo [a, e] cycloocten-5 (6 H)-one (26), itself obtained from the reaction of 5, 11-dibromo-5, 6, 11, 12-tetrahydrodibenzo [a, e] cyclooctene (20) with dimethyl sulfoxide and collidine. Infrared and proton magnetic resonance spectroscopic studies have shown that the 8-membered ring of 18 is not ...