Intramolecular [2 + 2]-photocycloaddition/thermal fragmentation approach toward 5-8-5 ring systems

[reaction: see text]. An intramolecular [2 + 2]-photocycloaddition is used to provide a photoadduct, which upon fragmentation, lactone cleavage, and subsequent Cope rearrangement provides a dicyclopenta[a,d]cyclooctene ring system with substituents in place (e.g., C3 and C11) to access several 5-8-5 diterpene and sesterterpene natural products.