Gombaspiroketals A–C, Sesterterpenes from the Sponge Clathria gombawuiensis

Gombaspiroketals A-C (1-3), tetracyclic sesterterpenes of a novel skeletal class, were isolated from the Korean marine sponge Clathria gombawuiensis. On the basis of the combined spectroscopic analyses, the structures of these compounds were determined to be highly rearranged sesterterpene spiroketal methoxyacetals (1 and 2) and a corresponding hemiacetal (3). The relative and absolute configurations were assigned by NOESY analysis and ECD calculations, respectively. These compounds exhibited moderate cytotoxicities and antibacterial activities.