B3LYP/6-31 G(d) density functional theory has been used to study Diels-Alder reactions of cyclopentadiene with alpha,beta-unsaturated aldehydes and ketones organocatalyzed by MacMillan's chiral imidazolidinones. Preferred conformations of transition structures and reaction intermediates have been located. The dramatically different reactivities and enantioselectivities exhibited by two similar chiral imidazolidinones are rationalized.