ent-Abietane-Type and Related Seco-/Nor-diterpenoids from the Rare Chloranthaceae Plant Chloranthus sessilifolius and Their Antineuroinflammatory Activities

Fourteen new ent-abietane-type diterpenoids, sessilifols A–N (1–14), and three related new norditerpenoids (15–17) were isolated from Chloranthus sessilifolius. The absolute configurations were determined by single-crystal X-ray diffraction analysis, the modified Mosher's method, and/or the observed Cotton effects in their electronic circular dichroism spectra. Sessilifols A (1) and B (2) possess an uncommon five-membered C-ring rearranged by oxidative cleavage of the C-13/C-14 bond in abieta-7,13-diene followed by the formation of a new C–C bond between C-12 and C-14. Sessilifol C (3) is a rare 7,8-seco-9-spiro-fused ent-abietane, whereas sessilifol O (15) represents the first example of a naturally occurring 14-norabietane-type diterpenoid. Compounds 6 and 9 were found to have moderate antineuroinflammatory activities by inhibiting the nitric oxide production in lipopolysaccharide-stimulated murine BV-2 microglial cells, with IC50 values of 8.3 and 7.4 μM, respectively.