Synthesis of the heterocyclic core of the alkaloids martinelline and martinellic acid

The tricyclic core of martinelline and martinellic acid was rapidly assembled utilising an imino Diels–Alder reaction of an imine derived from cinnamaldehyde with a cyclic enamide. The cycloaddition was completely regioselective though the exo–endo selectivity was poor. These 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.