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organic compounds
A sponge from the new genus Petrosaspongia has yielded two new isomeric sesterterpene lactones. We report here the structure of one of these lactones, [4aS(4a,4 b,6a,7,8a,11a,12,13a,13b,15a)]-12-acetoxyoctadecahydro- 1,1,4a,13b-tetramethyl-7,11a-epoxy- 2H-furo[2,3-b]phenanthro[1,2-f]oxocin-10(11H)-one, C27H40O6.