The cycloadditions of acetylketene with alpha-chiral aldehydes and ketones are shown to be diastereoselective, forming a tertiary or quaternary chiral center at an acetal or ketal carbon with good stereocontrol. X-ray crystallography of a minor product (5b) shows that the major products (e.g., 4b) are those predicted by the Felkin-Anh model. Transition states are reported at the MP2/6-31G level for the addition of ethanal to formylketene and at the B3LYP/6-31G level for the addition of 2-phenylpropanal. The ground-state conformations of the reactants and products are used to rationalize the relative energies and geometries of the transition states without the need to invoke the Cieplak hypothesis. However, chiral substituents on the alpha-oxoketene show no diastereoselectivity. These experimental and computational results are only consistent with the nearly planar, pseudopericyclic transition state previously proposed.