Total syntheses of strychnos alkaloids
Syntheses of dihydromossambine (1), lagunamine (a, echitamidine (a and strychnine (4) were achieved by two alternative synthetic pathways for construction of the pentacyclic framework of these alkaloids. Path A is based on an intramolecular Diels-Alder reaction of indoloacrylate and enamine functions while in path B a new electrocylic rearrangement and cyclization provides the tetracyclic core for further elaboration. For illustration and comparison, complete reaction schemes are shown for echitamidine (9) and strychnine (4).