Polycyclic imidazo[1,2-a]pyridine analogs - synthesis via oxidative intramolecular C-H amination and optical properties

A novel and straightforward approach to synthesize 5H-pyrido[2',1':2,3]imidazo[4,5-b]indoles has been developed. The key step is C-H amination of easily available 2-(2'-aminophenyl)imidazo[1,2-a]pyridines by the use of copper(II) triflate, trifluoroacetic acid and (diacetoxyiodo) benzene. The entire strategy consists of just four steps, starting from 2-aminopyridines and acetophenones, giving the target compounds in an overall yield of 20-35%. The optical properties of a library of pi-expanded imidazo[1,2-a]pyridines were for the first time fully characterized, showing that these ladder-type compounds strongly absorb UV radiation and exhibit fluorescence in the 415-461 nm region.