Alternatively, the transition state 249 is encoun-
tered in which the nucleophile might be delivered
internally, requiring the axial position of the carbonyl
group to lead to 250. These two models have been
proposed by Evans87,91,92 for hydride reduction of
â-hydroxyketones (Schemes 75, 76) along with the
successful development of reagents which would
effectively differentiate between the two possible
reaction modes.