ent-6,7-Secokaurane diterpenoids from Rabdosia serra and their cytotoxic activities

Investigation of the hydrophobic extract of Rabdosia serra (Lamiaceae) led to the isolation of seven ent-6,7-secokaurane diterpenoids rabdosins E-K, along with twelve known ent-kaurane diterpenoids. Their structures were established by extensive spectroscopic analysis. The absolute configurations of rabdosins E, F, H and I were determined by single-crystal X-ray diffraction, with rabdosin E having a 3,20-epoxy group. These diterpenoids could be divided into two types according to their skeletons: rabdosins E-J are ent-6,20-epoxy-6,7-secokaur-1,7-olides, and rabdosin K is an ent-6,7-secokaur-7,20-olide. Cytotoxicity evaluation showed that serrin B, serrin A, isodocarpin and lushanrubescensin J exhibited weak to moderate cytotoxic activities (IC50<10μM) against two human cancer cell lines.