By finely tuning the cavity volume of a coordination cage, the Diels-Alder reactivity of aromatic compounds was enhanced. Even unsubstituted naphthalene could be made to undergo the Diels Alder reaction with N-tert-butylmaleimide, with perfectly controlled syn-selectivity. M6L4 cages with standard and contracted cavities had opposite effects on the reaction: the former favored larger Diels Alder substrates, whereas the latter favored smaller ones.