Sinuscalide A: An Antiviral Norcembranoid with an 8/8-Fused Carbon Scaffold from the South China Sea Soft Coral Sinularia scabra .

Sinuscalide A ( 1 ), featuring an uncommon 8/8-fused carbon scaffold, three new norditerpenes, sinuscalides B-D ( 2 - 4 ), and sinuscatone A ( 5 ), with a 5/4/9 tricyclic carbon ring system, along with four known compounds were isolated from the South China Sea soft coral Sinularia scabra . The structures of the new compounds were established by extensive spectroscopic analysis, X-ray diffraction, and electronic circular dichroism (ECD). A plausible biosynthetic pathway for 1 was proposed. In a bioassay, compound 1 showed antiviral activity against human enterovirus EV71 (IC 50 = 5.0 μM) and an inhibitory effect against RANKL-induced osteoclastogenesis (92.3% inhibition at 10 μM). Compound 5 exhibited mild inhibition against Streptococcus pneumoniae and Salmonella paratyph (MIC 16 μg/mL).