Flash vacuum pyrolysis of 9-methyl-1,3-dihydronaphtho[1,2-c]furan-1,3-dione (8) at 750-880� gave exclusively 1H-cyclopent[ cd ]indene (9) and of 8-methyl-1,2-dihydrocyclobuta[a]naphthalene-1.2-dione (10) at 600-840� gave a mixture of (9) and acenaphthylene. Acenaphthylen-4-ol (12) was synthesized and found to be stable to flash vacuum pyrolysis over thetemperature range 600-900�. These findings are discussed in relation to the rearrangement described in the title.