Transition states for the Diels-Alder reactions of butadiene and cyclopentadiene with methyl acrylate coordinated to BH3 have been located with ab initio molecular orbital calculations and the 3-21G basis set; the activation energies were also evaluated by single point calculations with the 6-31G* basis set. In addition a solvent cavity (SCRF) calculation at the RHF/3-21G level was performed for the reaction of cyclopentadiene and methyl acrylate coordinated to BH3 in a medium with a dielectric constant of 9.08. These data have been used to develop a modified MM2 force field applicable to the prediction of stereochemistries of Lewis acid catalyzed reactions of chiral acrylates to dienes. This force field has been shown to reproduce the selectivities of a number of literature examples. It has also been applied to the predictions of stereoselectivities of new reactions. Insights into the controlling elements in the stereoselectivity are obtained.