Approach to the synthesis of antitumor quassinoids from labdane diterpenes: an efficient synthesis of a picrasane-related intermediate.

[structure: see text]. The tetracyclic ketal 24, a suitable intermediate for the synthesis of antitumor pentacyclic quassinoids, has been efficiently prepared from communic acids (5a-c), via methyl ketone 9. The synthetic sequence from 9 to 24 consists of 15 steps in 12% overall yield.