11,16,18,19-Tetraoxapentacyclo(12.2.2.16,9.01,15.010,12)nonadeca-6,8-diene-4-acetic acid, 2,5-bis(acetyloxy)-7,12-dimethyl-alpha-methylene-17-oxo-, methyl ester, (1S-(1R*,2R*,4S*,5S*,10R*,12R*,14S*,15S*))-