Asymmetric Direct Aldol Reaction Using Modified Hydroxyproline (Hyp) Derivatives

Coupling of 4–hydroxy–D and L–prolines with D– and L–prolines as well D– and L–valines via esterification of the –OH group is carried out. The resulting diamino acid derivatives are purified and used as enantioselective catalysts. For a model reaction, an asymmetric aldol reaction of p–nitrobenzaldehyde with acetone is applied. The catalytic properties of the produced catalysts are investigated to determine the effect of the auxiliary amino acids (proline and valine) on the enantiomeric ratio (er). A virtually quantitative conversion into the aldol product with traces of dehydration product was observed. The prepared catalysts show enantiomeric ratio (er) values of 91: 9 % to 70: 30% of R/S–isomers. All reactions are carried out with 20 % mol of the catalyst.