Synthesis of paclitaxel. 2. construction of the ABCD ring and formal synthesis

A formal synthesis of the antitumor diterpenoid paclitaxel (Taxol) is described. The ABC ring of paclitaxel, synthesized starting from 1,3-cyclohexanedione and tri-O-acetyl-d-glucal by SmI₂-mediated cyclization as the key transformation, was successfully converted to Takahashi's tetracyclic oxetane intermediate. A double Chugaev reaction was employed for introduction of the strained bridgehead olefin, and stereoselective formation of the oxetane ring afforded the known synthetic intermediate, completing the formal synthesis of paclitaxel.