(3aS)-6β,6aβ-Epoxy-3,3a,5aβ,6,6a,7,10b,10cβ-octahydro-3a,10bα-dimethyl-7β-isopropyl-4H,9H-furo[2,3,4:4,5]naphtho[2,1-c]pyran-4,9-dione