New Type of Aldol Condensations Catalyzed by Metal(II) Complexes of α-Amino Acid Esters and that with Cyclodextrin System

The aldol condensations of p-nitrobenzaldehyde with acetone catalyzed by metal complexes of α-amino acid esters proceeded under mild and neutral conditions to afford an enantiomeric excess aldol-type product, 4-hydroxy-4-(4-nitrophenyl)-2-butanone, along with some dehydrated products without any by-product. The most effective catalyst system was a zinc(II) complex of the tyrosine ethyl ester, ZnII–(TyrOEt)2 in MeOH. Complexes of the second-row transition metals were inadequate for the reactions. Reactions at 30–40 °C for 24 h were favorable regarding the asymmetric induction. An H2O solvent was the best for aldol-type product formations without asymmetric induction. An assistant effect of cyclodextrins(especially, β-CD) on the catalysis of the ZnII–TyrOEt complex were thought to be observed.