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

Sarcotragusolides A-D (-), four new butenolide sesterterpenes featuring a rare methyl-transferred 6/6/6-tricyclic fused ring system with a butyrolactone moiety, and echinohalimane B (), an unprecedented monocyclic diterpenoid featuring a 2,7-ring-opened halimane-type skeleton, were isolated from the sponge sp. A γ-hydroxybutenolide sesterterpene derivative (), a new scalarane sesterterpene (), a new subersin-type diterpenoid (), and two known terpenoids were also isolated and identified. The discovery of sarcotragusolides C and D ( and ) 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 , , and presented modest cytotoxic activities against several human cancer cell lines.