The asymmetric hetero-Diels-Alder reaction of benzaldehyde with 1-dimethylamino-3- tert-butyldimethylsiloxy butadiene catalyzed by ( R, R)-1-Np-TADDOL was studied using computational methods. A theoretical rationale was developed through the combined use of molecular mechanics and ONIOM(B3LYP/6-31G(d):AM1) calculations. The origins of stereoselection in this process were identified, and excellent correlation between experiment and theory was found.