Structurally diversified ent-kaurane and abietane diterpenoids from the stems of Tripterygium wilfordii and their anti-inflammatory activity

Four undescribed ent-kaurane diterpenoids, wilkaunoids A - D (1-4), and three undescribed abietane diterpenoids, wilabinoids A - C (13-15), along with thirteen known ones (5-12 and 16-20), were isolated from Tripterygium wilfordii. Their structures were elucidated by extensive spectroscopic methods, electronic circular dichroism calculation, and X-ray diffraction analysis. Compounds 1 and 2 were a pair of C-19 epimers of ent-kaurane diterpenoids, featuring a rare 19,20-epoxy-19,20-dimethoxy-kaurane fragment. Compound 3 possessed a rare naturally occurring 1,3-dioxacyclohexane moiety. Compounds 13 and 15 represented the first example of abietane diterpenoids with an isovalerate substitution from the genus of Tripterygium. The possible biosynthetic pathways of 1-3 were postulated. The effect of 1-20 on nitric oxide production was examined in lipopolysaccharide-stimulated RAW 264.7 cells. Abietane diterpenoid quinones 7-13 (IC50: 1.9-10.2 mu M) exhibited the significant activity to inhibit nitric oxide production versus positive control (NG-monomethyl-L-arginine acetate salt, IC50 = 24.9 mu M). The structure activity relationship of 7-13 in inhibiting nitric oxide production was then discussed. The most potent 7 and 8 were found to significantly suppress the expression of cyclooxygenase-2 and inducible nitric oxide synthase proteins, showing a good anti-inflammatory potential. The findings provided some valuable insights for the discovery and structural modification of abietane diterpenoids towards anti-inflammatory lead compounds.