Cyclopenta[4,5]pentaleno[1,6a-b]oxirene-4- carboxylic acid,1a,2,3,3a,3b,6,6a,7-octahydro- 3a,5-dimethyl-3-methylene-2-oxo-,(1aS,- 3aR,3bR,6aR,7aS)-