A catalyst-free [4+3]-cycloaddition reaction of N-aryl sulfilimines with cyclobutenones is described, which provides a straightforward protocol for synthesizing 1,5-dihydro-2H-benzo[b]azepin-2-ones under mild reaction conditions. This reaction features a broad substrate scope and good functional group tolerance and does not require catalysts or additives. Moreover, using N-pyridinyl sulfilimine as the substrate, a series of pyridoazepinones have also been prepared.