Synthesis of Hexahydropyrrolo[2,3-b]indole Alkaloids Based on the Aza-Pauson−Khand-Type Reaction of Alkynecarbodiimides

Upon treatment with 30 mol % of Co₂(CO)₈ and 30 mol % of TMTU in toluene at 70 °C, benzene-bridged alkynecarbodiimides efficiently underwent a ring-closing reaction to give the pyrrolo[2,3-b]indol-2-ones in good yields. These conditions could nearly suppress the formation of the urea derivatives, which were consistently observed when 10 mol % of Co₂(CO)₈ and 60 mol % of TMTU in benzene were used. The synthesis of the eight hexahydropyrrolo[2,3-b]indole alkaloids was accomplished from the resulting pyrrolo[2,3-b]indol-2-ones via the introduction of an angular substituent at the C_3a-position by treatment with NaBH₄/alkyl bromide as the crucial step.