A C–N insertion of β-lactam to benzyne: unusual formation of acridone

Intermolecular insertion of benzyne into the C–N bond of a β-lactam is described. This σ-insertion is followed by ring expansion that produces dihydroquinolinone, which rapidly reacts with an additional benzyne unit to afford an acridone through intramolecular C–C bond formation to the carbonyl group and rapid elimination of ethylene.