Synthesis of 1,1′‐ and 2,2′‐Bicarbazole Alkaloids by Iron(III)‐Catalyzed Oxidative Coupling of 2‐ and 1‐Hydroxycarbazoles

We describe the synthesis of 1,1′- and 2,2′-bicarbazoles by oxidative homo-coupling of 2- and 1-hydroxycarbazoles. The oxidative coupling using catalytic amounts of F16PcFe can be applied to both groups of substrates. While it generally provides the best yields for the synthesis of 1,1′-bicarbazoles, di-tert-butyl peroxide affords better results for the 2,2′-bicarbazoles. In our study, we have achieved the first syntheses of the biscarbalexines A-C, bisglybomine B, 2,2′-dihydroxy-7,7′-dimethoxy-3,3′-dimethyl-1,1′-bicarbazole, bispyrayafoline C, and bisisomahanine. The iron-catalyzed coupling of koenigine led to an improved synthesis of 8,8″-biskoenigine and afforded an unprecedented decacylic product. Oxidative coupling of 1-hydroxycarbazoles led to bisclausenol and to the first total syntheses of bismurrayafoline B and D.