methyl (1'R,4a'R,4b'R,7'S,9a'R,10'S,10a'R)-7'-hydroxy-1'-methyl-13'-oxo-1',4',4b',5',6',7',10',10a'-octahydro-9'H-spiro[[1,3]dioxolane-2,8'-[4a,1](epoxymethano)[7,9a]methanobenzo[a]azulene]-10'-carboxylate