Total Synthesis of the Neuronal Cell‐Protecting Carbazole Alkaloids Carbazomadurin A and (S)‐(+)‐Carbazomadurin B

Abstract

AbstractThe total syntheses of the neuronal cell‐protecting carbazole alkaloids carbazomadurin A and (S)‐(+)‐carbazomadurin B were achieved. The key step of the synthesis of the polysubstituted carbazole rings included an allene‐mediated electrocyclic reaction of the 6π‐electron system that involved the indole 2,3‐bond. The cleavage of the alkoxy groups of the resulting 3‐ethoxy‐8‐isopropoxycarbazole successfully gave the 3,8‐dihydroxycarbazole, which was converted into the 3,8‐bis(OSEM)‐carbazole (SEM = 2‐trimethylsilylethoxymethyl). A Suzuki–Miyaura cross‐coupling reaction of the 3,8‐bis(OSEM)‐carbazole with the corresponding alkenyl pinacol borates afforded the 1‐alkenylcarbazoles, which were treated with tetra‐n‐butylammonium fluoride (TBAF) followed by reduction with NaBH4 to provide carbazomadurin A and (S)‐(+)‐carbazomadurin B, respectively.