(1R)-1,2,3,3a,5aβ,6,10b,10cβ-Octahydro-1,6α-dihydroxy-3aβ,10bα-dimethyl-7-isopropyl-4H,9H-furo[2,3,4:4,5]naphtho[2,1-c]pyran-4,9-dione