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 k2R/KmR and k2S/KmS 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).