A highly enantioselective, moderately anti-selective aldol reaction using a novel hydrazone moiety as stereo director

The use of novel hydrazones as stereo directors with a view to develop a highly enantioselective, anti-diastereoselective aldol addition procedure has been investigated. A number of proline-derived hydrazones were produced and their effectiveness in directing simple alkylation of aza-enolates investigated. The most promising of these hydrazones were then used in the aldol reaction. The substituent on the oxygen of the proline had a profound effect on both the magnitude and the sense of asymmetric induction. The optimum hydrazone for the formal aldol reaction between pentanone and propionaldehyde gave a diastereoselectivity of 37% in favour of the anti-isomer while both isomers had an ee of 83-84%.