metal-free protocol for the synthesis of indenoquinolinones and 2-substituted quinolinesvia [4 + 2] cycloaddition reaction using readily available 2-aminobenzaldehydes and ketones as starting materials. Different quinoline derivatives can be selectively synthesized by changing the type of ketones. O2 and dimethyl sulfoxide (DMSO) as co-oxidants play an important role in the synthesis of indenoquinolinones