Bicyclo(2.2.2)oct-5-ene-2,3-dicarboxylic Anhydride6708-37-833 % similar 1,2,3,4-Butanetetracarboxylic dianhydride4534-73-031 % similar Artemisinic acid80286-58-431 % similar exo-5,6-Dehydronorcantharidin6118-51-031 % similar Isobutyric anhydride97-72-330 % similar Dihydroartemisinic acid85031-59-030 % similar