Natural diterpenoid ludongnin, octa­hydro‐5′‐hydroxy‐7a‐methyl‐8′‐methyl­ene­spiro­[isobenzo­furan‐4(1H),4′(3′H)‐[1H‐7,9a]­methano­cyclo­hepta­[c]­pyran]‐1′,3,9′(3aH,4′aH)‐trione