Klyflaccilides A and B, Diterpenoids with 6/5/8/3 Fused Tetracyclic Carbon Skeleton from the Hainan Soft Coral Klyxum flaccidum

Two novel polyoxygenated diterpenoids, klyflaccilides A (1) and B (2), featuring an uncommon 6/5/8/3 tetracyclic ring system, were isolated from the Hainan soft coral Klyxum flaccidum. Their structures were established by extensive spectroscopic analysis, X-ray diffraction analysis, and/or a quantum chemical computation method. A plausible biosynthetic relationship of 1 and 2 with their potential precursor 3 was further proposed. Compounds 1 and 3 exhibited moderate anti-inflammatory activity by the inhibition of LPS-induced TNF-alpha protein release.