Unusual [2+4] and [2+2] cross-cycloadditions of arenes were efficiently promoted within a self-assembled coordination cage. For example, triphenylene underwent the Diels-Alder reaction with N-cyclohexylmaleimide in good yield in the confined cavity of the cage, despite its inertness under ordinary conditions. Syn-stereoselective photodimerization of pyrene and N-cyclohexylmaleimide was also carried out quantitatively within the cage. The structures of these cross adducts were confirmed by NMR and X-ray crystallographic analysis.