Benzocarbazole-6,11-quinones

Most of the early applications of palladium to indole chemistry involved oxidative coupling or cyclization using stoichiometric Pd(II). Akermark first reported the efficient oxidative coupling of diphenyl amines to carbazoles 37 with Pd(OAc)2 in refluxing acetic acid [45]. The reaction is applicable to several ring-substituted carbazoles (Br, Cl, OMe, Me, NO2), and 20 years later Akermark and colleagues made this reaction catalytic in the conversion of arylaminoquinones 38 to carbazole-l,4-quinones 39 [46]. This oxidative cyclization is particularly useful for the synthesis of benzocarbazole-6,11-quinones (e.g., 40). 

In the past decade, a few examples of benzoannulated carbazole ring systems were found in nature as marine products. In 1993, Chan et al. reported a novel marine benzocarbazole alkaloid, purpurone (281) from the marine sponge lotrochota sp. in its racemic form. Purpurone, as indicated by its name, is purple in color. This represents the first example of a benzocarbazole alkaloid with a biphenylene quinone methide functionality. The isolate showed ATP-citrate lyase (ACL) inhibitory activity (247). 

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