Hydrolase-catalyzed kinetic resolutions of chiral alcohols : Mechanistic study on the origin of the enantioselectivity

Mechanistic studies on the enantioselectivity in the hydrolase-catalyzed ki netic resolutions of racemic alcohols are described. Based on kinetic measu rements, molecular orbital calculations and computer modeling with X-ray cr ystal structures of several lipases, we proposed the transition-state model that is consistent with the experimental observations such as ( i) high en antioselectivity, (ii) broad substrate specificity and (iii) an empirical r ule (R-preference for secondary alcohols). A large secondary alcohol having a tetraphenylporphyrin as the substituent was successfully resolved by sev eral lipases, demonstrating the validity of our transition-state model. The S-preference of subtilisins for secondary alcohols was rationalized by app lying the protocol used in the transition-state model for lipases to subtil isins. We also found that the lipase-catalyzed transesterifications of chir al alcohols in organic solvents can proceed even at -40 degrees C. Interest ingly, the E value increased with decreasing temperature, and a linear rela tionship was observed between In E and 1/T, from which the Delta Delta H do uble dagger and Delta Delta S double dagger values were calculated. These t hermodynamic parameters were useful for investigating the mechanism of the enantioselectivity of the hydrolases toward chiral alcohols.