The typically non-chelatable Lewis acid BF3•OEt2 also resulted in anti selectivity (Table , entries 3 and 7). The major anti product observed with BF3•OEt2 suggests that the dipolar mechanism proposed previously by Evans does not adequately explain the analogous reaction with N-Ts imine (Figure ). Where BF3 would preferentially bind Z to the H substituent on the C=O bond of 3 (Figure ), BF3 is forced to bind Z to the alkyl substituent of imine 12 due to the presence of the N-tosyl group. To achieve the minimization of dipoles in the analogous aldehyde substrate, one would have to rotate the C-O bond of the β-alkoxy bond 180 °C from its orientation in 3a (Figure ). This would result in nucleophilic attack that avoids steric interactions with the β-methyl substituent, leading to syn-14 instead of the anti-14 product observed. (Figure ).