methyl (1aS,1bR,2R,3R,4R,5R,5aR,6aR)-3,5a-dihydroxy-1b-methyl-7-oxo-4-(prop-1-en-2-yl)octahydro-1aH-2,5-(epoxymethano)indeno[1,2-b]oxirene-1a-carboxylate