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

Twelve new abietane diterpenoids (<b>1</b>–<b>12</b>) and 31 known analogues (<b>13</b>–<b>43</b>) were isolated from a medicinal Chinese herb, <i>Clerodendrum trichotomum</i> Thunberg. The absolute configurations of <b>1</b>–<b>3</b> were established on the basis of ECD and X-ray crystallography data, whereas that of <b>4</b> was elucidated by comparison of experimental and calculated ECD data. Eight diterpenoids, 15,16-dehydroteuvincenone G (<b>1</b>), trichotomin A (<b>4</b>), 2α-hydrocaryopincaolide F (<b>7</b>), villosin C (<b>20</b>), 15-dehydro-17-hydroxycyrtophyllone A (<b>22</b>), demethylcryptojaponol (<b>38</b>), 6β-hydroxydemethylcryptojaponol (<b>39</b>), and trichotomone (<b>43</b>), 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