Sinudenoids A-E, C 19 -Norcembranoid Diterpenes with Unusual Scaffolds from the Soft Coral Sinularia densa .

Five new furanobutenolide-derived C ₁₉ -norcembranoid diterpenes, sinudenoids A-E ( 1-5 , respectively), were isolated from the soft coral Sinularia densa . Sinudenoid A ( 1 ) possesses an uncommon 5/5/11-fused tricyclic ring system. Sinudenoids B-D ( 2-4 , respectively) share the same tetracyclic 5/5/6/6 ring system but represent two kinds of new skeletons. Sinudenoid E ( 5 ) is the second compound with the rare 8/8 bicyclic carbon core. A plausible biosynthesis pathway for compounds 1-6 is proposed. Compound 5 exhibits strong anti-inflammatory activity in the zebrafish model.