Lewis‐acid Promoted Chemoselective Condensation of 2‐Aminobenzimidazoles or 3‐Aminoindazoles with 3‐Ethoxycyclobutanones to Construct Fused Nitrogen heterocycles

A Lewis‐acid promoted chemoselective condensation of 2‐aminobenzimidazoles or 3‐aminoindazoles with 3‐ethoxycyclobutanones is presented. Diverse fused heterocycles benzo[4,5]‐imidazo[1,2‐a]pyrimidine and pyrimido[1,2‐b]‐indazole derivatives were obtained in moderate to high yields under mild conditions, the reaction mechanism of which was in sharp contrast to previous [3+3] annulation reaction of 3‐ethoxycyclobutanones.