Decachloro-4-allylidenecyclopentene (6) was synthesized by the reaction of pentachlorocyclopentadiene with hexachloropropene in the presence of anhydrous aluminium chloride. Hexachloro-2-allylidene-4-cyclopentene-1, 3-dione (11), prepared from 6 by treatment with coned nitric acid, cyclized to hexachloro-2, 5-dihydrocyclopenta [b] pyran- 5-one (14) on heating. In a solvent such as acetone and acetonitrile, 4 rearranged to ...