Occurrence, Biogenesis, and Synthesis of Biologically Active Carbazole Alkaloids

A study was conducted to investigate the occurrence, biogenesis, and synthesis of biologically active carbazole alkaloids. Biologically active carbazole alkaloids were isolated from diverse natural sources and exhibited a broad range of different frameworks and functional groups. A large number of carbazole alkaloids were prepared via the Fischer-Borsche synthesis, while condensation of cyclohexanone (1.1) with phenylhydrazines 1.2 afforded the arylhydrazones 1.3. Fischer-Borsche cyclization of the arylhydrazones 1.3 formed an indole moiety and led to the 1,2,3,4-tetrahydrocarbazoles 1.4. An alternative method for benzannulation was developed to overcome the limitations of the electrocyclic ring closure of divinylindoles in which cyclization of 2,3-difunctionalized indoles was achieved via an allene intermediate 6.2 as the actual starting point for the electrocyclic reaction. It was found that cyclodehydrogenation of diarylamines is the most versatile and practical method for the synthesis of carbazoles.