Terpenoids from the Sponge Sarcotragus sp. Collected in the South China Sea.

Sarcotragusolides A-D ( 1 - 4 ), four new butenolide sesterterpenes featuring a rare methyl-transferred 6/6/6-tricyclic fused ring system with a butyrolactone moiety, and echinohalimane B ( 8 ), an unprecedented monocyclic diterpenoid featuring a 2,7-ring-opened halimane-type skeleton, were isolated from the sponge Sarcotragus sp. A γ-hydroxybutenolide sesterterpene derivative ( 5 ), a new scalarane sesterterpene ( 7 ), a new subersin-type diterpenoid ( 10 ), and two known terpenoids were also isolated and identified. The discovery of sarcotragusolides C and D ( 3 and 4 ) with an unprecedented inversion of configuration implied a distinct biosynthetic pathway. The structures of these compounds were elucidated based on their spectroscopic data, single-crystal X-ray diffraction, chemical derivatization, and quantum chemical calculations. Compounds 1a , 1b , and 2 presented modest cytotoxic activities against several human cancer cell lines.