FeCl₃-catalyzed AB₂ three-component [3+3] annulation: an efficient access to functionalized indolo[3,2-b]carbazoles

An efficient FeCl3-catalyzed AB(2) three-component reaction of indole-2-carbaldehydes and 5-aminopyrazoles was established for the direct synthesis of highly valuable indolo[3,2-b]carbazoles. This high atom- and step-economical [3 + 3] annulation process generates three new C-C bonds in a single synthetic operation. The use of the inexpensive FeCl3 catalyst, no pre-functionalization of the starting materials, wide substrate scope and gram-scale synthesis are the salient features of this approach. The proposed mechanistic pathway involves two consecutive nucleophilic addition reactions followed by intramolecular Friedel-Crafts cyclization and dehydrative aromatization steps.