Asymmetric Total Syntheses of Hetidine-Type C ₂₀ -Diterpenoid Alkaloids: Spirasines V and VI, Spiradine D, and the Proposed Structures of Spirafines II and III

Herein we describe a divergent strategy toward the asymmetric total syntheses of five hetidine-type C₂₀-diterpenoid alkaloids, namely, spirasines V and VI, spiradine D, and the proposed structures of spirafines II and III. Crucial transformations include an Enders asymmetric addition, an oxidative dearomatization induced Diels-Alder cycloaddition, and a MHAT-initiated transannular radical cyclization. The heterocyclic rings were assembled by an efficient reductive cyclization sequence at a late stage. Our approach has enabled the total syntheses of these natural products in 17-20 steps from commercially available starting materials with high enantio- and diastereocontrol.