DFT method has been used in combination with various basis sets to model the ionic cycloaddition of cationic heteroaromatic diene, 2.3-dimethylisoquinoliniuim ion with cyclopentadiene with a view to understand the factors that influence the stereochernical outcome of the reaction. Calculations show that this reaction is an inverse electron demand type reaction and it passes through highly asynchronous transition states and mainly electrostatic repulsion govern the endo/exo selectivity of the reaction. Endo TS is more destabilized than exo TS due to repulsion between positive charges delocalized over the two reacting partners in the transition state and hence, exo adduct is more preferred than endo adduct in total agreement with experiment. (c) 2005 Elsevier B.V. All rights reserved.