Acetic acid, 2-​[(3aS,​5aR,​9aR,​9bR)​-​6-​(carboxymethylene)​-​1,​3a,​4,​5,​5a,​6,​8,​9,​9a,​9b-​decahydro-​3a-​methyl-​7H-​benz[e]​inden-​7-​ylidene]​-