Regioselective synthesis of 3,4-diarylpyrimido[1,2- b ]indazole derivatives enabled by iron-catalyzed ring-opening of styrene oxides.

The first synthesis of 3,4-diarylpyrimido[1,2- b ]indazole derivatives from 3-aminoindazoles has been realized. The FeCl ₃ -catalyzed intermolecular epoxide ring-opening reaction altered the order of annulation, with the free primary NH ₂ groups in 3-aminoindazoles preferentially reacting with styrene oxides instead of aromatic aldehydes. This protocol is further highlighted by its broad substrate compatibility, high chemo- and regioselectivities, and the late-stage modifications of bioactive molecules. Without aromatic aldehydes, the synthesis of 3-aryl-4-acylpyrimido[1,2- b ]indazole derivatives can also be accomplished using alternative reaction conditions.