(10bS)-1,2,3,3aβ,6,7,8,9,10,10aα,13,13aβ-Dodecahydro-6α-acetoxy-1,1-dimethyl-7-methylene-5H-10β,13β-epoxy-5aβ,8β-methanocyclohepta[c]furo[3,4-e][1]benzopyran-5-one