Common Strategy for the Synthesis of Some Strychnos Indole Alkaloids

Indole alkaloids are important natural compounds with interesting bio-activities that can be found in various species belonging to the Amaryllidaceae, Apocynaceae, or Strychnaceae families. Although these compounds have different connections, substituents, and functionalities, their main core can be produced via the formation of a common functionalized tetracyclic subunit, which is rapidly obtained by an oxidative de-aromatization process mediated by a hypervalent iodine reagent from an inexpensive phenol containing a lactate moiety as the chiral auxiliary. A subsequent stereoselective aza-Michael addition and an intramolecular Heck-type reaction lead to the formation of a common key intermediate. This approach provides a solid foundation for the further elaborations of several natural products. The asymmetric syntheses of (−)-strychnopivotine and the polycyclic main cores of (−)-strychnosplendine, (+)-isosplendine, and (+)-malagashanol, three other indole alkaloids, are reported.