2-Butenoic acid, 2-methyl-, 2,3,3a,5,6,9,9a,9b-octahydro-5-hydroxy-6,9-dimethyl-3-methylene-2-oxo-4H-6a,9-ethenofuro(2',3':3,4)cyclohepta(1,2-c)(1,2)dioxol-6-yl ester, (3aS-(3aalpha,5alpha,6alpha(E),6abeta,9beta,9aalpha,9bbeta))-