Substituents exhibiting an electron-withdrawing effect are regarded as L independent of their steric bulk.
In this way, the low-lying ó*C-L orbital is aligned parallel with theð- and ð*-orbital of the carbonyl
group, allowing delocalization of electron density by hyperconjugation from the reaction center toward L
(Figure 2). Moreover, the reaction pathway is advantageous compared to the one starting from the Cram
conformation since it leads directly to a staggered
conformation in the product.