Bromination of 11-acetoxydibenzo[a,e]cycloocten-5(6H)-one (6) gave the 6-bromo derivative (8), which on further bromination gave a hydrogen bromide adduct (12 or 13) of 6,6,12-tribromodibenzo[a,e]cyclooctene-5,11(6H,12H)-dione (14). Treatment of this adduct with aqueous-ethanolic ammonia followed by sublimation gave 11-oxoindeno[1,2-c]isocoumarin (4), not dibenzo[a,e]cyclooctene-5,6,11(12H)-trione (3) as earlier suggested. The conversion of 12 or 13 to 4 has been shown to involve the formation of 3-bromo-2-(o-carboethoxyphenyl)indone (17), which gave 4 and ethyl bromide on being heated at 180°. Oxidation of dibenzo[a,e]cyclooctene-5,11(6H,12H)-dione with selenium dioxide gave a bimolecular product 25 considered to be derived by loss of carbon monoxide from the dimer of dibenzo[b,ƒ]pentalene-5,10-dione (27), a selenium-containing product, 28, and the hydrate 29 of 3.