Enzymatic hydrolytic resolution of (R,S)-tropic acid esters and (R,S)-ethyl α-methoxyphenyl acetate in biphasic media

A thermally stable esterase from Klebsiella oxytoca is explored as an excellent enantioselective biocatalyst (E > 100) for the hydrolytic resolution of (R,S)-tropic acid esters and (R,S)-ethyl α-methoxyphenyl acetate in biphasic media. An expanded Michaelis-Menten mechanism for the enzymatic acylation step is adopted for the kinetic analysis, where the structure-enantioselectivity correlations in terms of the logarithms of specificity constants varied with the inductive parameter of leaving alcohol for (R,S)-tropic acid esters can be employed for interpreting the reaction mechanism and rationalizing the optimal enantioselectivity at the methyl ester. The pH effects on changing the relative specific constants k_2R/K_mR and k_2S/K_mS are further applied for estimating the intrinsic specificity constants for both enantiomers. A kinetic analysis among (R,S)-tropic acid ethyl ester, (R,S)-ethyl α-methoxyphenyl acetate, (R,S)-ethyl α-methylphenyl acetate, (R,S)-ethyl mandelate and (R,S)-ethyl α-chlorophenyl acetate indicates that the α-substituent has profound influence on the enzyme activity and enantioselectivity, i.e. good (100 > E > 50) to excellent (E > 100). Crown