Discovery of Ircinianin Lactones B and C—Two New Cyclic Sesterterpenes from the Marine Sponge Ircinia wistarii

Two new ircinianin-type sesterterpenoids, ircinianin lactone B and ircinianin lactone C (7 and 8), together with five known entities from the ircinianin compound family (1, 3–6) were isolated from the marine sponge Ircinia wistarii. Ircinianin lactones B and C (7 and 8) represent new ircinianin terpenoids with a modified oxidation pattern. Despite their labile nature, the structures could be established using a combination of spectroscopic data, including HRESIMS and 1D/2D NMR techniques, as well as computational chemistry and quantum-mechanical calculations. In a broad screening approach for biological activity, the class-defining compound ircinianin (1) showed moderate antiprotozoal activity against Plasmodium falciparum (IC50 25.4 μM) and Leishmania donovani (IC50 16.6 μM).