Deacylation of Various Phenyl Esters Catalyzed by Stereoselective Micelles with N-Acyl-L-histidine

Abstract

In deacylation of 3-nitro-4-(acyloxy)benzoic acids (S₂^--S₁₆^-), p-nitrophenyl carboxylate (S₂-S₁₆) and N-acylphenylalanine-p-nitrophenyl esters (DL-, D- or L-S_n : n=2-16) with N-acyl-L-histidine (AcetHis, OctHis, LauHis, and PalHis) in the presence of cationic micelles (CTAB or CBzAC), the increase in acyl chain length of the nucleophiles enhanced the catalytic activity of the micelles. The selective deacylation of substrates (S₁₂^-, S₁₀, DL-S₁₀, D- or L-S₁₀, etc.) with appropriate acyl chain length was also enhanced by approximating the substrates toward the nucleophiles through hydrophobic interaction between the substrate and the cationic micelles. The deacylation rate of DL-, D-, or L-S_n(n=2-16) was remarkably enhanced by the above micellar catalysts as compared with that of S_n^- or S_n (n=2-16). This fact is attributable to approximation effect induced by hydrophobic interaction between the phenyl of amino acid ester (substrate) and imidazolyl (nuccleophile) as well as by the bonding (-C=O···H-N-) between the amide bonds of the both species. It is noteworthy that the N-acyl-L-histidine and CTAB system resulted in the maximum deacylation rate for the D- and L-S₁₀ enantiomers involving same acyl chain lengths (L-S₁₀ was changed into L-S₁₂ by the PalHis-CTAB system) with the largest enantiomer rate ratio. Especially, in the deacylation of D- or L-S_n (n=2-6) with the comicelles of LauHis- or PalHis-CTAB, the deacylation rate of L-S₁₀ (or L-S₁₂) was found to be 2.6-2.8 times as higher as that of D-S₁₀ or D-S₁₂. These experimental findings might lead to a conclusion that the stereoselection of a specific enantiomer such as L-S₁₀ in the deacylation of D- and L-S_n (n=2-16) is mainly due to the following factors : (a) the selective incorporation of the substrate with an appropriate acyl chain length by the micellar catalyst, and (b) the predominant approximating effects of the hydrophobic substrate-nucleophile interaction, which was mentioned above, and the hydrogen bonding for one of substrate enantiomers.