New Chiral Catalysts ContainingN,O-Heterocycles Derived from Chiral Amino Alcohols

The enantioselectivity exerted by a new series of chiral catalysts containing N,O-heterocycles of different sizes has been checked in the addition of diethylzinc to benzaldehyde, which was used as a model reaction. The catalysts were derived from natural amino acids, following a relatively simple procedure, and in several cases excellent ee values were obtained. The results were complementary since ee's ranged from 98% (R) to 94% (S) excesses of the final 1-phenylpropan-1-ol. Molecular mechanics calculations suggested that the production of the R alcohol may be explained by a mechanism similar to that described by Noyori, in which ZnEt2 interacts solely with the N–C–C–OH fragment, whereas the formation of the S enantiomer needed the direct participation of the lateral chain of the parent amino acid and the N,O-heterocycle.