Abstract
Lewis acids affect reactivity, selectivity, and mechanism in the carbonyl‐ene reaction. The diastereoselectivity in the glyoxylate‐ene reaction depends on Lewis acids. While the SnCl~4~‐promoted reaction can be achieved with a high level of anti‐selectivity, the use of Al reagents leads to a high syn‐selectivity. The origin of the Lewis acid dependency of the diastereoselectivity in the carbonylene reaction of (E)‐but‐2‐ene with glyoxylate was theoretically studied (HF/6‐31G*) from the point of view of differences and similarities between the ene and the Diels–Alder reactions. Though it has been widely accepted that the endo‐preference would be less obvious in the ene reaction than in the Diels–Alder reaction, our ab initio molecular studies showed that the electrostatic interaction between carbonyl O‐atom lone pair and cationic allylic central C‐atom of ene component exists in the Lewis acid‐promoted carbonyl–ene reaction to affect the transition‐state conformation. It is illustrated that such an electrostatic interaction is essential to control the exo/endo‐selectivity, which provides the diastereoselectivity of the product in the transition state of the Lewis acid promoted carbonylene reaction.