Convergent Route to ent-Kaurane Diterpenoids: Total Synthesis of Lungshengenin D and 1α,6α-Diacetoxy-ent-kaura-9(11),16-dien12,15-dione

The Hoppe's homoaldol reaction of a cyclo-hexenyl carbamate with an aldehyde followed by an unprecedented BF3.OEt2 mediated intramolecular Mukaiyama-Michael-type reaction affords the tetracyclic core structure of ent-kaurane diterpenoids. The usage of this convergent approach for assembling these natural products is demonstrated by the first asymmetric total syntheses of two highly oxidized ent-kaurane diterpenoids: Lungshengenin D and l alpha,6 alpha-diacetoxy-ent-kaura-9(11),16dien-12,15-dione.