(3aR,4R,4aS,7aS,8S,9aS)-4a,8-dimethyl-3-methylidene-3,3a,4,4a,7a,8,9,9a-octahydrofuro[2',3':4,5]cyclohepta[1,2-c][1,2]dioxol-4-yl 2-methylpropanoate