Enantiospecific Synthesis of (+)- and (−)-Ferruginine from l -Glutamic Acid. Synthesis of Tropanes via Intramolecular Iminium Ion Cyclization

Iminium ions, generated by decarbonylation of N -benzyl-5-[1-(methoxycarbonyl)-4-oxopentyl]prolines, undergo intramolecular cyclization to afford 2,4-disubstituted tropanes in good yields. This transformation is also shown to be a stereospecific reaction. The value of these substituted tropanes has been demonstrated by functional group manipulation, leading to the enantiospecific synthesis of (+)-ferruginine, an alkaloid isolated from Darlinga ferruginea, and its unnatural enantiomer, (−)-ferruginine.