The results presented here provide experimental support for a hypothesis made by us to rationalize literature observations on intramolecular Diels-Alder reactions (IMDA) and our own observations on IMDA with a furan diene (IMDAF) regarding the quantity (catalytic or stoichiometric) of Lewis acid required to facilitate reaction. Evidence suggests that the reactions can be divided into two classes::those that proceed with catalytic quantities of Lewis acid (herein defined as type A) and those that require a stoichiometric quantity of Lewis acid (type B). We believe that the relative basicity of the controlling functional groups in addend and adduct can be critical in determining the quantity of Lewis acid required. The relative Lewis basicity has been studied using competitive complexation studies using low-temperature NMR experiments to study the coordination of methylaluminum dichloride (MAC)and BF3.Et(2)O with model oxygen Lewis bases and IMDAF addends and adducts.