Sinudenoids A–E, C19-Norcembranoid Diterpenes with Unusual Scaffolds from the Soft Coral Sinularia densa

Five new furanobutenolide-derived C19-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 1–6is proposed. Compound 5exhibits strong anti-inflammatory activity in the zebrafish model.