2-​Butenoic acid, 2-​methyl-​, (2aR,​4S,​4aS,​6R,​9S,​11S,​12S,​12aR,​12bS)​-​12b-​(acetyloxy)​-​2a,​3,​4,​4a,​5,​6,​9,​10,​11,​12,​12a,​12b-​dodecahydro-​4,​6,​9,​11-​tetrahydroxy-​4a,​8,​13,​13-​tetramethyl-​5-​oxo-​7,​11-​methano-​1H-​cyclodeca[3,​4]​b