A total synthesis of Δ⁹⁽¹²⁾-capnellene, a cis-anti-cis-tricyclo[6.3.0.0^2,6]undecane sesquiterpene of marine origin, is described. Starting with bicyclic enone 5, which was prepared by a Nazarov cyclization, the remaining portion of the molecule is elaborated by means of a new and relatively efficient cyclopentenone annulation scheme. The relative stereochemistries of the chiral centers are properly incorporated by taking advantage of prevailing steric factors.