Efficient synthesis of novel cytotoxic cis-fused α-methylene γ-lactones from 7,14-dihydroxy-ent-kaurenes by transformation under Mitsunobu reaction conditions

Facile transformation of 7,14-dihydroxy-ent-kaurenes such as excisanin A (8), kamebanin (9), and kamebakaurin (10), which are abundant in plants of the genus Rabdosia species (Labiatae), to ent-abietanes was accomplished under the Mitsunobu reaction conditions. The δ, ε-unsaturated cis-fused α-methylene γ-lactones (17, 18, and 25) thus prepared showed a moderate cytotoxic activity on P388 murine leukemia cells.