Novel [12+2] Cycloaddition of 2H-4,9-Methanocycloundeca[b]furan-2-ones with Enamines: Synthesis and Properties of 4,9-Methanocyclopentacycloundecenes and a Benzo-annulated Derivative

Novel cycloaddition reactions of 2H-4,9-methanocycloundeca[b]furan-2-one and 3-chloro-substituted derivative with enamines underwent novel [12 + 2] cycloaddition reaction followed by decarboxylation and elimination of amine to give 4,9-methanocyclopentacycloundecene derivatives in modest yields. Upon treatment of DDQ, cyclohexane-annulated 5-chloro-4,9-methanocyclopentacycloundecene was dehydrogenated easily to give the first example of a benzo-annulated 4,9-methanocyclopentacycloundecene ring system. Chemical as well as spectroscopic properties of them were studied, and it was clarified that benzoannulation causes an appreciable degree of bond alternation and reduced ring current.