Total Synthesis of Carbazole Alkaloids – Ekeberginine, Harmandianamine A, and Furanoclausamine B
The anionic [4 + 2] annulation of 3-substituted furoindolones with dimethyl maleate provides an expedient route for the synthesis of the title carbazole alkaloids. The employment of this strategy in combination with a Krapcho reaction/retro-Kolbe–Schmitt reaction has culminated in the first total synthesis of the lactonic carbazoles harmandianamine A and furanoclausamine B and a brief total synthesis of ekeberginine from a prenylated furoindolone. An unusual fragmentation of an ester function during the annulation is also reported.