One-pot synthesis of pyrrolo[2,3-b]indoles from 3-formylindoles

Pyrrole is a scaffold implicated in a variety of pharmacologically active compounds, exhibiting antimicrobial, anticancer, and neuroprotective effects.¹ Although several methods for the synthesis of pyrrolo[2,3-b]indoles have been reported in the literature,^2-4 photochemical approaches remain largely unexplored, with only a single UV light-mediated method described to date.⁵ In this work, we developed a novel one-pot method for the synthesis of pyrrolo[2,3-b]indoles from α-azidoketones and 3-formylindoles. The protocol combines a Knoevenagel condensation with an intramolecular photocyclization under blue LED irradiation, providing efficient access to this heterocycle under mild conditions. Using the optimized conditions, 13 pyrrolo[2,3-b]indole derivatives were prepared with overall yields in the range of 10–67%. Furthermore, the methodology was successfully translated to continuous-flow regime, demonstrating its potential for reaction scale-up. For R_1, R₂ = H, transposition to continuous flow afforded 42% yield with a 20 min residence time (RT), compared with 49% yield after 1.5 h of irradiation in batch.