Abietane Diterpenoids from the Roots of Clerodendrum trichotomum 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 IC 50 values of 5.6–16.1 μM. The structure–activity relationships of the isolated diterpenoids are also estimated.