Synthesis of Cyclopent[a]azulenes and 1(3)-Methylenecyclopent[a]azulene Derivatives Having an Electron Withdrawing Group at the 9-Position

1H- and 3H-Cyclopent[a]azulenes having an electron withdrawing group, such as a methoxycarbonyl or a cyano group, at the 9-position were synthesized from their dihydro compounds by means of bromination with NBS and succeeding dehydrobromination by refluxing in chloroform. Base-catalyzed condensation of these cyclopent[a]azulenes with carbonyl compounds afforded 1-methylene-1H- and 3-methylene-3H-cyclopent[a]azulene derivatives having an electron withdrawing group at the 9-position. The physical properties of these methylenecyclopent[a]azulenes are also discussed.