Scheme 5.

Lewis acid-promoted diastereoselective additions of silyl enol ethers to a chiral oxazolinium ion and computational stereochemical models for the observed diastereoselectivity. Red dotted lines indicate attractive cation–π interactions, while the solid red curve represents the dihedral angle leading to repulsive steric interactions. Black and grey dotted lines show the breaking and forming bonds. The differential energies of activation are shown in kcal/mol (M06–2X/6-311+G(d,p)//B3LYP/6-31G(d)). TMS = trimethylsilyl,