Mannolides A–C with an Intact Diterpenoid Skeleton Providing Insights on the Biosynthesis of Antitumor Cephalotaxus Troponoids

Three new diterpenoids, mannolides A–C (1–3), and two new Cephalotaxus troponoids, 4 and 5, were isolated from Cephalotaxus mannii and structurally characterized by spectroscopic data and X-ray crystallography. The discovery of compounds 1–3 featuring a new intact carbon skeleton, proposed as cephalotane, sheds new light on the biogenesis of Cephalotaxus troponoids, a rare class of antitumor C19 norditerpenoids. Antitumor tests showed that the tropone motif is essential for the activity.