text: Capnellene has been a popular target for synthesis due to its molecular architecture, its role in the defense mechanism of soft corals, and the challenge posed by the high degree of stereochemical sophistication and the complexity of the undecane skeleton. In 1981, the first stereocontrolled synthesis of (±)-Δ-capnellene was performed in nine steps, with an overall yield of 60%. Their starting reagent was a dimethylated cyclopentenyl carboxaldehyde and the overall synthesis took the form of a series of pentane ring annulations. The second pentane ring was formed by condensation of the aldehyde by vinylmagnesium bromide, followed by Nazarov cyclization of the dienone.  A regiospecific [3+2] cyclopentannulation, using ozonolysis and an intramolecular aldol condensation, formed the third ring and a simple dehydration reaction yielded the target capnellene.
  Since the first synthesis, many investigators have successfully assembled capnellene and its derivatives. Approaches to this synthesis are diverse, and include central steps such as annululation, olefin metathesis, radical cyclization, and trapping reactions. The most heavily cited synthesis in the literature involves two key intermediates formed by a Stille reaction, the palladium-catalyzed coupling of vinyl triflate with vinyl stannane. The readily prepared trimethylcyclopentanone can be converted into vinyl triflate, which is coupled with vinylstannane in the first palladium-catalyzed step to yield the desired divinyl ketone. The second 5-membered ring is formed via Nazarov cyclization, and the product is prepared for a second palladium-catalyzed coupling. This step yields another divinyl ketone, which can be cyclized to an enone, hydrated, and converted to an alkene via olefination to yield capnellene.