Protecting-Group-Free Total Synthesis of (E) and (Z)-Alstoscholarine

Looking for hidden symmetry: The first asymmetric total synthesis of pentacyclic (E)- and (Z)-alstoscholarines is accomplished starting from cyclic meso-anhydride 9. The absolute configuration was set by an organocatalytic desymmetrization of 9. Other key steps involved a regioselective hemiaminal formation to differentiate two aldehydes with concurrent creation of the D-ring, a chemoselective palladium-catalyzed indole synthesis and a Takeda olefination to install the ethylidene moiety. Each step of this eight-stage synthesis led to the constructive C-C, C-N or C-O bond formation. The synthesis is protecting-group-free, with high synthetic economy, and confirms the absolute configuration assigned to the natural products.