New cyclic metalloporphyrin hosts, 6 and 7, have been prepared. At 0.33 mM in dichloromethane at 25 °C, they accelerate 65-fold and 840-fold respectively the reaction of diene 1 and dienophile 2 and also bind the hetero Diels−Alder product 3 very strongly. More importantly, small single crystals of solvated 6, 7, and the 6.3 complex were grown and their structures were determined. As the Diels−Alder product resembles the Diels−Alder transition state, the structures of the product-free host 6 and the 6·3 host−product complex allow, for the first time for synthetic receptors, a detailed structural analysis of the geometrical changes imposed on an accelerating agent on binding of a Diels−Alder product. Comparison of these structures reveals that when the Diels−Alder product 3 is bound within the cavity, it induces significant structural changes in 6. This provides the first crystallographic structural evidence that accelerated product formation can be accompanied by substantial host distortion. Desolvation o...