New scalarane-based sesterterpenes from the sponge Phyllospongia madagascarensis.

Two new natural scalaranes, 16beta-acetoxy-20,24-dimethyl-12,24-dioxo-25-norscalarane and 12beta-hydroxy-20,24-dimethyl-13,18-oxa-25-norscalarane, were isolated from the Madagascaran sponge Phyllospongiamadagascarensis, together with three previously decribed tetracyclic sesterterpenoids, phyllofolactones F, G, and B, and a new artifactual ethyl ether of scalaradisin B. Structure determinations were based on NMR and other spectral data. Compounds 1 and 2 represent new examples of 20,24-dimethyl-25-norscalarane sesterterpenes, and compound 2 has a seven-membered oxacycle that makes its skeletal system unique.