Sinusiaetone A, an Anti-inflammatory Norditerpenoid with a Bicyclo[11.3.0]hexadecane Nucleus from the Hainan Soft Coral Sinularia siaesensis

A novel norditerpenoid, sinusiaetone A (1), featuring an uncommon bicyclo[11.3.0]hexadecane carbon skeleton, and four polyoxygenated cembranoids (2–5) were isolated from the Hainan soft coral Sinularia siaesensis. Their structures were established by spectroscopic analysis, X-ray diffraction, quantum chemical computational approaches, and/or a modified Mosher’s method. A plausible biosynthetic pathway of 1 and its biogenetic relationship with 2–5 were proposed. New compounds 1–3 displayed an interesting inhibitory activity against lipopolysaccharide-induced inflammation in BV-2 microglial cells.