Construction of the tetracyclic ring system of diterpene alkaloids via cationic [5 + 2] cycloaddition.

The ABCD ring system of C18/C19 diterpene alkaloids was constructed via cationic [5 + 2] cycloaddition to forge a bicyclo[3.2.1]octane, an intramolecular aldol reaction to form a seven-membered ring, oxidation of a phenol at the para-position, and introduction of a one-carbon unit via Stille coupling followed by oxidative cleavage of a furan ring.