Forsyditerpenes A-O, CC-type clerodane and aromatic abietane diterpenoids with anti-inflammatory activities from the seeds of Forsythia suspensa.

Diterpenoids, the bioactive terpenoids of many medicinal plants, have not been reported in the seeds of Forsythia suspensa. In this study, twelve undescribed clerodane (1-12) and three new aromatic abietane (13-15) diterpenoids, named forsyditerpenes A-O, were isolated from the seeds of Forsythia suspensa. Among them, forsyditerpenes A-L (1-12) have rear CC-type clerodane skeleton, which possess the cis configuration of the A/B ring junction and the α-oriented C-17 and C-20. Furthermore, forsyditerpenes N (14) and O (15) are rearranged ent-abietane diterpenoids. Structural characterization was achieved by a combination of NMR, HR-ESI-MS, and X-ray diffraction. Forsyditerpenes C (3) and D (4) showed inhibitory effects of LPS-induced NO release with IC50 values of 26.2 and 12.8 μM, respectively, which should be partly achieved through inhibiting iNOS protein expression in RAW264.7 cells.