Enzyme‐Catalyzed Efficient Synthesis of Enantiomerically Pure ( S )‐ and ( R )‐n‐Butylphthalide

3‐n‐Butylphthalide (NBP) is an effective commercial drug for the treatment of acute ischemic stroke, with its S‐enantiomer, ( S )‐NBP, demonstrating clinical superiority over ( R )‐NBP. However, the stereoselective synthesis of both enantiomers with high enantiomeric excess (ee) presents significant challenges. Herein, a novel enzymatic strategy for the efficient and highly stereoselective synthesis of ( S )‐ and ( R )‐NBPs under mild reaction conditions is presented. Specifically, two carbonyl reductases, Sm CR K6 and Ss CR K1 , are engineered to facilitate the asymmetric reduction of the prochiral aromatic ketone, 2‐pentanoyl benzonitrile ( 1 ), followed by intramolecular cyclization to produce chiral NBPs. Sm CR K6 exhibits a catalytic activity of 0.46 Umg − 1 protein, which is 23‐fold greater than that of its parent enzyme, Sm CR V4 , and the ee value of ( S )‐NBP increases from 2% to 94% ( S ). The catalytic activity of Ss CR K1 (9.46 U mg −1 protein) is fourfold higher than that of its parent, Ss CR, and the ee of ( R )‐NBP reachs 99%. In the preparative synthesis, ( S )‐NBP and ( R )‐NBP are generated continuously in a 3D microfluidic reactor, achieving space‐time yields that are 9‐fold and 30‐fold higher, respectively, than those obtained in batch reactions. This continuous‐flow enzymatic process has the potential for future scale‐up for the industrial production of these important chiral drugs and similar derivatives.