A Total Synthesis of Galanthamine Involving De Novo Construction of the Aromatic C-Ring

The tetracyclic alkaloid galanthamine is used clinically in a number of countries for the symptomatic treatment of mild to moderate forms of Alzheimer's disease, and this feature coupled with its novel molecular architecture has prompted an extensive focus on its synthesis. The present study reports a new and distinct synthesis of galanthamine wherein the AB-ring substructure and associated quaternary carbon centre are constructed by using a palladium-catalyzed intramolecular Alder-ene reaction. The product of this process is engaged in a Tsuji-Trost-type reaction to generate a semicyclic diene that participates in a normal-electron-demand Diels-Alder reaction to generate, after oxidation of the initially formed adduct, the aromatic C-ring of the target alkaloid. Modified Bischler-Napieralski chemistry is then deployed to construct the seven-membered D-ring and thereby furnishing narwedine, an established precursor to both (+)- and (-)-galanthamine.