The development of novel, high-yielding, and selective methodologies for the asymmetric synthesis of stereocenters is at the forefront of modern synthetic chemistry research. Organocatalysis can now be viewed as a viable alternative to the use of the sometimes toxic transition-metal catalysts. In this experiment, the simple synthesis of an achiral imidazolidinone organocatalyst is described and its performance is compared to that of a related commercially available chiral catalyst in a Diels–Alder cycloaddition. The organocatalyst operated via an iminium ion intermediate and the product of the Diels–Alder cycloaddition was purified by silica plug filtration (no flash column chromatography required). The diastereoselectivity was readily measured with 1H NMR spectroscopy and optionally, the enantioselectivity of the chiral catalyst can be assessed with chiral GC. The experiment can be performed over three laboratory sessions and is suitable for a third- or fourth-year undergraduate laboratory curriculum. The experiment demonstrated not only the synthesis of an organocatalyst but also the effect and importance of catalyst design on the stereochemical outcome of a Diels–Alder cycloaddition.