Structural and Experimental Evidence for the Enantiomeric Recognition toward a Bulky sec-Alcohol by Candida antarctica Lipase B

Abstract

Candida antarctica lipase B (CAL-B) exhibits remarkable enantioselectivity for various chiral sec-alcohols, and the enantioselectivity is structurally well-understood. Two substituents at the chiral center of a sec-alcohol separately bind two pockets, namely, large and medium binding pockets. It has been believed that the medium pocket is too small to accommodate a large substituent (larger than an ethyl group), and thus, bulky sec alcohols bearing two large substituents have been regarded as a poor substrate for CAL-B. However, we found that CAL-B can catalyze the transesterification of N-Boc-protected rac-2-amino-1-phenylethanol (1a) enantioselectively with a moderate reaction rate. X-ray crystallography and computer modeling revealed that the rotation of the Leu278 side chain creates a space to accept the N-Boc-aminomethylene group of la. Moreover, a sec-alcohol substrate with less than one hydrogen atom at the gamma-position from the hydroxyl group is required to achieve a moderate reaction rate. On the basis of this observation, we diversified bulky N-Boc-protected rac-2-amino-1-arylethanols for the transesterifications with high enantioselectivities (E > 200).