Magnesiate Addition/Ring-Expansion Strategy To Access the 6–7–6 Tricyclic Core of Hetisine-Type C₂₀-Diterpenoid Alkaloids

A synthetic strategy to access the fused 6–7–6 tricyclic core of hetisine-type C₂₀-diterpenoid alkaloids is reported. This strategy employs a Diels–Alder cycloaddition to assemble a fused bicyclic anhydride intermediate, which is elaborated to a vinyl lactone-acetal bearing an aromatic ring in five steps. Aromatic iodination is followed by magnesium–halogen exchange with a trialkyl magnesiate species, which undergoes intramolecular cyclization. Subsequent oxidation provides the desired 6–7–6 tricyclic diketoaldehyde, with carbonyl groups at all three positions for eventual C–N bond formation and subsequent elaboration.