The reaction mechanism of the Diels-Alder reaction between 2-methylacrolein and cyclopentadiene catalyzed by a cationic oxazaborolidine Lewis acid has been characterized using density functional theory calculations. The solvent effect on the studied reaction is taken into account by optimizing all stationary points in CH(2)Cl(2) using the polarizable-continuum model. Our calculations show that the presence of this cationic oxazaborolidine catalyst changes the molecular mechanism of the studied reaction from concerted to stepwise, and lowers the activation barrier by more than 10.0 kcal/mol. A comparison of the results obtained for four different reaction channels reveals that the studied reaction is energetically favorable to occur along the exo/s-cis channel, which reasonably explains the stereoselectivity of the title reaction observed in related experiments. The electronic effect of this cationic oxazaborolidine complex in catalyzing the title reaction is found to be very similar to that of the commonly used Lewis acid BF(3).