Enantioselective synthesis of (S)-Naproxen ester prodrug and dynamic kinetic resolution of (S)-Naproxen by lipases in organic solvent

• Main Authors: Chang, Chun-Sheng, 張春生
• Other Authors: Tsai Shau-Wei
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/89849190485685966533

博士 === 國立成功大學 === 化學工程學系 === 86 === A facile enzymatic process for the direct synthesis of chiral 4-morpholinoethyl(S)-Naproxen ester prodrug from the racemic Naproxen has been developed byusing lipases as the biocatalyst in the organic solvent. By screening thelipases from different sources and the organic solvents from consideringNaproxen solubility, enzyme enantioselectivity and activity, Lipase MY andcyclohexane were selected as the best lipase and the reaction medium,respectively. The apparent fit of the initial rate for (S)-Naproxen and thetime-course conversion for each enantiomer supported the proposed Ping-PongBi Bi reaction mechanism with competitive inhibition by the alcohol. Moreover,different deactivation constants for both enantiomers were elucidated from theeffect of organic solvent on the enzyme conformation change. The surfactant effect on the lipase-catalyzed enantioselective synthesis of(S)-Naproxen ester prodrug at 37 ℃ in organic solvents was investigated.Naproxen solubility in isooctane or cyclohexane is improved by adding AOT asthe surfactant. The low enzyme activity observed due to the surfactantinhibition is relaxed by adding a small amount of buffer solution. Theunfavorable surfactant inhibition on the enzyme activity is compensated by theenhancement of Naproxen solubility and results in an increase of (S)-Naproxenester productivity without any loss of enantiomeric excess of the product. In order to further enhance the production of the desired (S)-Naproxen esterprodrug, 2,2,2-trifluoroethyl Naproxen ester, with the saturated solubilityten thousand times higher than that of Naproxen in isooctane, was empolyed asthe substrate in the enantioselective transesterification. Although Candidarugosa lipase shows good reactivity and enantioselectivity, this enzymaticprocess, like other conventional resolution processes, is subject to thecontrain of maximum yield of 50% for the desired product. Dynamic kineticresolution which allows in situ racemization of the undesired substrate priorto the reaction has the advantage that it can in principle bring aboutquantitative conversion of starting material into the desired singleenantiomer. When activated thioesters containing an electro-withdrawing moietysuch as benzyl or trifuoroethyl group were used, great improvements in theenzyme activity were found. Among the used thioesters, (S)-Naproxentrifluoroethyl thioester was found to have the highest racemization rate whichincreases with the trioctylamine concentration in isooctane. Using the highalcohol concentration has the benefit of inhibiting hydrolysis side- reactionin thiotransesterification. However, owing to the water content containing inthe crude lipase and the aggregation of the enzyme in using high alcoholconcentration, we have employed an immobilized lipase to overcome thosedeficiencies and obtained more than 50% yield for the desired (S)-ester productwith high eeP,M value.The apparent fit of the initial rate of racemization for (S)-Naproxentrifluoroethyl thioester and the time-course eeS data supported the proposedpseudo-first order kinetics for racemization. Moreover, by combining theproposed Michaelis Menten reaction mechanism with the former racemization model, theoretical and experimental agreements of the initial rate for (S)-Naproxentrifluoroethyl thioester and the time-course conversion for each enantiomerwere found. This indicates that very high conversion of the desired(S)-Naproxen with good optical purity is obtained by using dynamic kineticresolution for the enantioselective hydrolysis of racemic Naproxentrifluoroethyl thioester.