Facile Ring Enlargement of Azulene to Give the Cyclopentacyclononene System by Dipolar Cycloaddition

The cyclopentacyclononene derivative (3). the first compound to contain this ring system and also the first stable cyclononatetraene not to have an annelated benzene ring, has been synthesized from the “ethanoazulene” (1). This synthesis is possible because azulene reacts readily with the ynamine (2), to give the tricyclic species (4). Although 4,6,8-methylazulene initially gives the desired ring system (5), immediate transannular bonding occurs. The bracket in (1) prevents this reaction.