Concise Total Syntheses of Pyrido[4,3-b]carbazole Alkaloids Using Copper-Mediated 6π-Electrocyclization

Concise syntheses of 9-methoxyellipticine, 3,4-dihydroellipticine (μ-alkaloid D), 1,2,3,4-tetrahydroellipticine, 2-methyl-1,2,3,4-tetrahydroellipticine, olivacine, 3,4-dihydroolivacine, (±)-guatambuine, and (±)-janetine were developed starting from hexatriene intermediates readily obtained by Pd-catalyzed tandem cyclization/cross-coupling reaction of indolylborates. The route enables the facile construction of pyrido[4,3-b]carbazoles by Cu-catalyzed 6π-electrocyclization and subsequent transformation of the pyridocarbazole intermediates into pyrido[4,3-b]carbazole alkaloids. Concise total syntheses of pyrido[4,3-b]carbazole alkaloids were accomplished using Cu-catalyzed 6π-electrocyclization of hexatriene as a key step.