Total synthesis of (±)-methyl atis-16-en-19-oate via homoallyl-homoallyl radical rearrangement

Total synthesis of (+/-)-methyl atis-16-en-19-oate (5c), a tetracyclic diterpenoid possessing a bicyclo-[2.2.2]octane ring system, was accomplished. Intramolecular Diels-Alder reaction of tetraene 14 was employed in a construction of kaurene skeleton 13. The pivotal step involved a homoallyl-homoallyl radical rearrangement process of (+/-)-methyl 12-hydroxykaur-16-en-19-oate monothioimidazolide 12, which led to 5c in good yield. Interestingly, treatment of methyl 12-oxo-kaur-16-en-19-oate 30 with hydrazine monohydrate in the presence of KOH in bis(ethylene glycol) at 200 degrees C resulted in cyclopropanation to furnish, directly, trachyloban-19-oic acid (4b), together with kaur-16-en-l9-oic acid (6b).