Polyoxygenated diterpenes from the sponge Phorbas sp.

Ten new polyoxygenated diterpenes (7-16) along with six known gagunin compounds (1-6) were isolated from the sponge Phorbas sp. collected in the Korean Sea. On the basis of a combination of NMR and mass spectroscopic analyses, the molecular structures of these diterpenes, designated as gagunins H-Q, were determined to be penta- or hexa-oxygenated diterpenes of the 10,13-bis-epi-homoverrucosane class. A new diterpene acid (17) of a bisabolane-related skeletal class was also isolated and structurally defined by the spectroscopic analyses. These compounds exhibited moderate to significant cytotoxicity against the K-562 cell line as well as weak inhibitory activity against isocitrate lyase (ICL).