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

The tricyclic core of martinelline and martinellic acid was rapidly assembl ed utilising an imino Diels-Alder reaction of an imine derived from cinnama ldehyde with a cyclic enamide. The cycloaddition was completely regioselect ive though the exo endo selectivity was poor. These diastercoisomers were r eadily separated by flash chromatography and the relative stereochemistry o f the exo-isomer confirmed by single crystal X-ray crystallography. This in termediate was converted to the central core of the aforementioned alkaloid s in five additional synthetic operations. (C) 2001 Elsevier Science Ltd. A ll rights reserved.