One-step syntheses of the pyrrolo[3,4-b]indole and pyrrolo[2,3-b]indole ring systems from 3-nitroindoles

The reaction of 1-ethoxycarbonyl-3-nitroindole with ethyl isocyanoacetate in the presence of DBU gives ethyl 4-ethoxycarbonyl-2,4-dihydropyrrolo[3,4-b]indole-3-carboxylate, averting a novel rearrangement that we previously reported with 3-nitro-1-phenylsulfonylindole that yielded ethyl 8-phenylsulfonyl-1,8-dihydropyrrolo[2,3-b]indole-2-carboxylate.