The aza-Diels-Alder reaction of cyclopentadiene with protonated (S)-phenylethylimine of methyl carboxilate was studied using density functional theory (DFT) at the B3LYP/6-31G(d) level to elucidate the reported stereoselectivity of this reaction. Four independent reaction pathways were found, all of them proceeding through a concerted, asynchronous, mechanism. Inclusion of solvent effects revealed a high exo/endo stereoselectivity that decreases with increasing temperature, in good accordance with the experimental reports. (c) 2009 Published by Elsevier B.V.