Catalytic Asymmetric Induction with Proline Derivatives

2 Catalytic Asymmetric Induction with Proline Derivatives 

R = Ph, /0-Me-C6H4, o-MeO-C6H4, /a-MeO-C6H4, 3,5-Cp3-C6H3, /BU-C6H4, /0-(C5Hii)-C6H4, /7-pent, 1-cyclohexenyl, 

---

Asymmetric reduction of ketimines to sec-aminesf Of the various hydride reagents found to achieve high enantioselective reduction of ketones, the oxazaborolidine 1 of Itsuno, prepared from BH3 and (S)-(—)-2-amino-3-methyl-I,l-diphenylbutane-l-ol, derived from (S)-valine, (12,31), is the most effective in terms of asymmetric induction. Like Corey s oxazaborolidines derived from (S)-proline, 1 can also be used in catalytic amounts. The highest enantioselectivities obtain in reduction of N-phenylimines of aromatic ketones (as high as 88% ee). The enantioselectivities are lower in the case of N-t-butylimines of aryl ketones (80% ee). Reduction of N-phenylimines of prochiral dialkyl ketones with 1 results in 10-25% ees. 

---