Synthesis of hexahydropyrrolo[2,3-b]indole alkaloids based on the aza-Pauson-Khand-type reaction of alkynecarbodiimides

Article abstract of DOI:10.1021/jo071137b

(Chemical Equation Presented) Upon treatment with 30 mol % of Co ₂(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.