A nucleophile-induced cyclization of octadehydrodibenzoannulene derivative 1b was investigated. Treatment of 1b with BuLi gave fused 5-9-membered ring products, dibenzobicyclo[7.3.0]tetraenediynes 2a and 2b, formed via a single transannular cyclization reaction. The reason why double cyclization did not occur is ascribed to the structure of a plausible anionic intermediate in which a negative charge locates apart from the remaining triple bond carbons. The second cyclization did not take place even when once-isolated product 2b was treated with BuLi because of the higher reactivity toward a nucleophile of the fulvene moiety compared to the triple bonds. On the other hand, heating a chlorobenzene solution of 2b containing cyclohexa-1,4-diene gave products 3a-c possessing a benzonaphthopentalene framework via a Bergman cyclization pathway albeit in low yields after inter- or intramolecular hydrogen abstraction. Moreover, iodine-induced transannular cyclization of 2b afforded the corresponding diiodide 3d.
- Transannular cyclization