Subtilisins enantiopreference

The enantiopreference of the protease subtilisin in the acylalion of chiral alcohols is known to be opposite to that observed with lipases, providing for access to both enantiomers with DKR, depending on the enzyme used [137, 138, 139]. Acylation using 2,2,2-trifluoroethyl butyrate as the acyl donor was combined with in situ racemization, affording the corresponding esters in high yield and [135]. 

Dynamic resolution of various sec-alcohols was achieved by coupling a Candida antarctica lipase-catalyzed acyl transfer to in-situ racemization based on a second-generation transition metal complex (Scheme 3.17) [237]. In accordance with the Kazlauskas rule (Scheme 2.49) (/ )-acetate esters were obtained in excellent optical purity and chemical yields were far beyond the 50% limit set for classical kinetic resolution. This strategy is highly flexible and is also applicable to mixtures of functional scc-alcohols [238-241] and rac- and mcso-diols [242, 243]. In order to access products of opposite configuration, the protease subtilisin, which shows opposite enantiopreference to that of lipases (Fig. 2.12), was employed in a dynamic transition-metal-protease combo-catalysis [244, 245]. 

In water, the polarity of the substituents determined the enantiopreference [Ij. Placing a nonpolar substituent in water is unfavorable. Reactions in water involving methyl and nonpolar aryl substituents will favor the nonpolar aryl substituent in the S/ pocket, opposite to that predicted based on size alone. Thus, subtilisin favors the (S)-enantiomer of l-(p-tolyl)ethanol in organic solvents (p-tolyl is larger than methyl), but the (R)-enantiomer in water (methyl is more polar than p-tolyl). On the other hand, with a polar aryl group such as (4-pyridine N-oxide), the (S)-enantiomer is favored both in water, where solvation of the pyridine N-oxide is favorable, and in organic solvent, where placing the pyridine N-oxide in the solvent avoids steric interactions in the S/ pocket. 

R. J. Kazlauskas, A.N.E. Weissfloch, A structure-based rationalization of the enantiopreference of subtilisin toward secondary alcohols and isosteric primary amines, 1. Mol. Catal. B Enzymatic 3 (1997) 65-72. 

---