Abstract The C5–C8–C5 fused ring compounds (the title compound, 11) constituting the carbon skeletal ring of ophiobolane sesterterpenes have been synthesized from one of the photoadducts, 1-acetoxytricyclo[5.4.0.02,6] undecan-8-one (4a) of cyclopentene and 3-acetoxy-2-cyclohexen-1-one. The photoadduct 4a was converted via bromination and retro-aldol cleavage into 5-bromobicyclo[6.3.0]undecane-2,6-dione (8). The allylation of 8 with allyltributyltin and oxidation of the allyl group with palladium(II) chloride-copper(I) chloride yielded 5-acetonylbicyclo[6.3.0]undecane-2,6-dione (10a). Under basic conditions, the intramolecular cyclization of 10a afforded the title compound 11, and was accompanied by tetracyclo[9.3.01,5.05,8]tetradecane-2,7-dione (12). Under acidic conditions, 10a cyclized to 2-methyl-4,5,6a,7,8,9,9a,10-octahydro-6H-cyclopenta[6,7]cycloocta[1,2-b]furan-6-one (13). The structures of 8, 11, 12, and 13 were determined by X-ray analyses.