Intramolecular Diels–Alder reactions of cyclopentadiene systems: Model studies for the total synthesis of the capnellenes

Model studies for a general intramolecular Diels–Alder approach to the tricyclo[6.3.0.02,6]undecene skeleton common to the capnellene marine sesquiterpenes are described. Suitably functionalized cyclopentadiene systems 14 and 21 were synthesized and their cycloaddition reactivity examined. Triene 14 cyclized to the tricyclic adducts 15 and 16. In contrast, the anticipated product 22 from intramolecular cycloaddition of triene 21 was not formed in isolatable yield. Instead the minor adduct (10% yield) possessed an exocyclic double bond and was assigned structure 30 based on its relative stability, as determined by MM2 type calculations, and nuclear magnetic resonance data. X-ray analysis established that the major adduct (40% yield) was a tetracyclic[6.3.1.01,6,01,10]dodecene system corresponding to 32, and which arose from competitive isomerization of the initial diene and cyclization via an exo transition state.