(1aR,10aβ,10bα)-2β-Isopropyl-5bα,8aβ-dimethyl-6,7,8,8bβ,10a,10b-hexahydro-4H,9H-furo[2,3,4:4,5]oxireno[2,3]naphtho[2,1-c]pyran-4,9-dione