Abietane Diterpenoids from the Roots of <i>Clerodendrum trichotomum</i> and Their Nitric Oxide Inhibitory Activities

Twelve new abietane diterpenoids (1–12) and 31 known analogues (13–43) were isolated from a medicinal Chinese herb, Clerodendrum trichotomum Thunberg. The absolute configurations of 1–3 were established on the basis of ECD and X-ray crystallography data, whereas that of 4 was elucidated by comparison of experimental and calculated ECD data. Eight diterpenoids, 15,16-dehydroteuvincenone G (1), trichotomin A (4), 2α-hydrocaryopincaolide F (7), villosin C (20), 15-dehydro-17-hydroxycyrtophyllone A (22), demethylcryptojaponol (38), 6β-hydroxydemethylcryptojaponol (39), and trichotomone (43), exerted inhibitory effects against NO production with IC50 values of 5.6–16.1 μM. The structure–activity relationships of the isolated diterpenoids are also estimated.