The four transition structures (TS＇s) of the reaction between butadiene and acrolein, both uncatalyzed and catalyzed by BF3, have been theoretically studied taking into account electron correlation effects by means of DFT (B3LYP) calculations. In both the uncatalyzed and catalyzed reactions, the endo s-cis is the most stable of the four possible transition structures. In the case of the catalyzed reaction, the inclusion of electron correlation in the search for this transition structure indicates the classical [4+2] reaction path, instead of that corresponding to a [2+4] inverse electron, demands hetero-Diels-Alder reaction, as happens when the Hartree-Fock level of theory is used. The B3LYP/6-31G(d) calculations lead to activation energy values close to those experimentally found. The origin of the endo/exo selectivity, both in the presence and in the absence of the catalyst, is discussed.