(1aR,5aα,8aβ,8bα)-1aβ,3,3a,4,5,5a,6,8a,8b,8c-Decahydro-3α-hydroxy-3aβ,8cβ-dimethyl-6-methyleneoxireno[7,8]naphtho[1,2-b]furan-7(2H)-one