Felkin-anh stereoselectivity in cycloadditions of acetylketene: Evidence for a concerted, pseudopericyclic pathway

The cycloadditions of acetylketene with a-chiral aldehydes and ketones are shown to be diastereoselective, forming a tertiary or quaternary chiral cen ter at an acetal or ketal carbon with good stereocontrol. X-ray crystallogr aphy of a minor product (5b) shows that the major products (e.g., 4b) are t hose predicted by the Felkin-Anh model. Transition states are reported at t he 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 c onformations of the reactants and products are used to rationalize the rela tive energies and geometries of the transition states without the need to i nvoke the Cieplak hypothesis. However, chiral substituents on the alpha -ox oketene show no diastereoselectivity. These experimental and computational results are only consistent with the nearly planar, pseudopericyclic transi tion state previously proposed.