A novel C3-symmetric prolinol-squaramide catalyst for the asymmetric reduction of ketones by borane

A novel C₃-symmetric prolinol-squaramide has been developed for the asymmetric reduction of ketones by borane. By using only 5 mol % catalyst 1a for the reaction, high yields and excellent enantioselectivities (up to 95% yield, 93% ee) were obtained. Moreover, 1a can be easily recovered by simple precipitation and re-used for four cycles without losing the selectivity.