methyl (3bR,5aR,6R,10aR,10bR)-3b,10a-dimethyl-9-oxo-3b,4,5,5a,6,7,9,10,10a,10b,11,12-dodecahydrofuro[3',4':5,6]naphtho[1,2-d]oxepine-6-carboxylate