Biomimetic Studies Using Artificial Systems. II. Enantioselective Thiolysis of D- or L-α-Amino Acid Ester Salts by Thiol-Bearing Chiral Crown Ethers as an Enzyme Model

Abstract

The rates of transacylation were studied between thiol-bearing chiral crown ethers (1-10) and α-amino acid p-nitrophenyl ester salts. Enantioselectivities, k_D/k_L ratios, of 6.5 for valine ester salt, 8.7 for phenylalanine ester salt, and 7.7 for valine ester salt were achieved by 1, 5, and 8, respectively. Saturation phenomena of rate acceleration depending on crown concentration were observed and analysis of these data revealed that the chiral recognition occurs in the step of liberation of p-nitrophenol by intra-complex thiolysis, not in the complex-forming step. A possible mechanism for the enantioselectivity is proposed on the basis of the kinetic data.