Convergent Route to ent-Kaurane Diterpenoids: Total Synthesis of Lungshengenin D and 1α,6α-Diacetoxy-ent-kaura-9(11),16-dien-12,15-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 1α,6α-diacetoxy-ent-kaura-9­(11),16-dien-12,15-dione.