Synthesis of the heterocyclic core of martinelline and martinellic acid

A Povarov reaction, between an aromatic imine derived from cinnamaldehyde and a cyclic enamide, was employed to rapidly construct the tricyclic core of the alkaloids martinelline and martinellic acid. The cycloaddition was completely regioselective though the exo/endo selectivity was poor. The diastereoisomers were readily separated by flash chromatography and the relative stereochemistry of the exo-isomer confirmed by single crystal X-ray crystallography. This intermediate was converted to the central core of the aforementioned alkaloids in five additional synthetic operations.