Divergent Diazo Approach toward Alkyl 5/4-Hydroxy-3-benzo[]indole-4/5-carboxylates.

A divergent diazo approach toward alkyl 5/4-hydroxy-3-benzo[]indole-4/5-carboxylates has been developed. The reaction of 1,3-diketones with alkyl 2-diazo-3-oxo-3-(2-azirin-2-yl)propanoates catalyzed by Co(acac) or Ni(acac) gives various alkyl 3-(1-pyrrol-2-yl)-2-diazo-3-oxopropanoate in good yields. The latter undergo Wolff rearrangement followed by the 6π-cyclization of transient ketene to form alkyl 5-hydroxy-3-benzo[]indole-4-carboxylates bearing various substituents in positions 1, 2, 7, and 8, as well as derivatives of methyl 4-hydroxy-6-thieno[2,3-]indole-5-carboxylates and methyl 5-hydroxy-7-benzo[]carbazole-6-carboxylate under thermolysis or Rh(OAc) catalysis. Isomeric benzoindoles, alkyl 4-hydroxy-3-benzo[]indole-5-carboxylates, have been prepared by Boc-protection of the pyrrole nitrogen of alkyl 3-(1-pyrrol-2-yl)-2-diazo-3-oxopropanoates followed by an intramolecular formal carbene insertion into the aromatic C-H bond catalyzed by Cu(OTf). The hydroxyl group of alkyl 5/4-hydroxy-3-benzo[]indole-4/5-carboxylates, through the formation of the corresponding triflates, allows the introduction of various substituents into the 5/4 position of benzo[]indoles using the cross-coupling reaction and even form a new heterocyclic backbone, benzo[]pyrrolo[2,3-]phenanthridine, via a tandem Suzuki reaction/nucleophilic acyl substitution.