Iron-Catalyzed One-Pot Synthesis of Indole-Tethered Tetrasubstituted Pyrroles and Their Transformations to Indolizino[8,7-b]indole Derivatives

We have developed an efficient iron-catalyzed one-pot reaction of tryptamines, ynones and nitroolefins, affording indole-tethered tetrasubstituted pyrroles in acceptable to good yields. Other aromatic and aliphatic amines can also be utilized in this process, delivering the corresponding highly functionalized tetrasubstituted pyrroles. Indolizino[8,7-b]indole derivatives could be obtained through TFA-, TfOH- or Fe(OTf)3-mediated cyclizations via dearomatization of indole. Unexpected dibrominated products, 7,9-dibromo-6,11-dihydro-5H-indolizino[8,7-b]indoles, were formed when trimethylphenylammonium tribromide (PTAP) was employed as electrophilic cyclization promoter.