Bojarski J, Oxelbark J, Andersson C, Allenmark S
Department of Organic Chemistry, University of Göteborg, Sweden.
Chirality. 1993;5(3):154-8. doi: 10.1002/chir.530050309.
In order to make a preliminary study of substituent effects on the rate and enantioselectivity obtained in esterolytic reactions catalyzed by a lipase from Candida rugosa, a series of racemic esters, derived from some alpha-alkyl and alpha-halo phenylacetic acids, were prepared. The reactions were studied at pH 6.0 and 50 degrees C under which conditions uncatalyzed hydrolysis was relatively slow. Reaction samples were studied at different points of time by means of analytical chiral reversed-phase liquid chromatography, which permitted the simultaneous determination of product enantiomeric excess and of the degree of total ester hydrolysis. These data were then used to calculate initial rates as well as enantioselectivity. An increase of the steric bulk of the alpha-substituent was found to highly decrease the rate of the reaction. On the other hand, rates were higher for the p-nitrophenyl esters than for the corresponding 2-chloroethyl esters. Consistently, the enantioselectivity was found to be higher for the latter type of ester. The esters of the alpha-halo (bromo and chloro) phenylacetic acids gave mandelic acid as the final product. This was caused by a rapid solvolysis of the alpha-halo phenylacetic acid initially formed.
为了初步研究取代基对皱褶假丝酵母脂肪酶催化酯水解反应速率和对映选择性的影响，制备了一系列由某些α-烷基和α-卤代苯乙酸衍生的外消旋酯。在pH 6.0和50℃下研究这些反应，在此条件下非催化水解相对较慢。通过分析型手性反相液相色谱法在不同时间点研究反应样品，该方法可以同时测定产物对映体过量和总酯水解程度。然后利用这些数据计算初始速率和对映选择性。发现α-取代基空间位阻的增加会显著降低反应速率。另一方面，对硝基苯酯的反应速率高于相应的2-氯乙酯。一致地，发现后一种类型的酯的对映选择性更高。α-卤代（溴和氯）苯乙酸的酯以扁桃酸作为最终产物。这是由于最初形成的α-卤代苯乙酸的快速溶剂解所致。
