Chemo-Enzymic Synthesis Of (R)-2-hydroxy-4-phenylbutyric Acid Ethyl Ester

To synthesis of (R) -2-hydroxy-4-phenylbutyric acid ethyl ester ((R)-HPBE), chemo-enzymic method is used in this paper. Some key steps those are related to production and transformation of chiral molecules are catalyzed by microbe or enzyme, the other common steps are made by conventional chemical catalyzers. In this way, enantiomerically pure compounds can be produced of high yield. (R)-HPBE is a versatile key intermediate for the synthesis of a variety of angiotensin converting enzyme (ACE) inhibitors. The technology of synthesis of ethyl 2-oxo-4-phenylbutyrate (OPBE) from benzaldehyde was studied. Then the asymmetric reduction from OPBE to (R)-HPBE catalyzed by Baker Yeast under the optimum conditions in water or nonaqueous phase was investigated in the paper. The main points were as follows:1. The technology of synthesis of ethyl 2-oxo-4-phenylbutyrate (OPBE) from benzaldehyde was studied. Cinnamic acid was firstly prepared by condensation of benzaldehyde with acetic anhydride catalyzed by anhydrous potassium acetate, which was then hydrogenated with molecular hydrogen and esterified with ethanol to form ethyl 3-phenylpropionate. Diethyl 2-oxo-3-benzyl-succinate intermediate was the condensation product of ethyl 3-phenylpropionate with diethyl oxalate by suitable molar ratio of sodium alcoholate: ethyl 3-phenylpropionate being 1:1, and prochiral intermediate of 2-oxo-4-phenylbutyric acid(OPBA) was finally synthesized with the above intermediate undergoing hydrolysis by w(H2SO4)=20% aqueous sulfuric acid. Orthogonalizing process optimized the key steps of condensation and hydrolysis in the synthesis of OPBA. Yield of 69.5% of 2-oxo-4-phenylbutyric acid was obtained in such following optimized conditions as the molar ratio of ethyl 3-phenylpropionate: diethyl oxalate being 1:3~1: 2, duration and temperature of condensation being 1.5 hours and 60, and period of the hydrolysis maintaining 15 hours. The conversion of OPBA esterified with ethanol was 94.1%.2. The asymmetric reduction of OPBE catalyzed by Baker Yeast in water medium to synthesis HPBE was investigated. The suitable temperature of chiral column of GC chromatogram to separate (R)-HPBE and (5) -HPBE is 160癈, and the compounds were determined by observing retention times in GC chromatogram. Baker Yeast named of "Rainbow" was selected from 6 kinds of Baker Yeast sold on the marketfor the equilibrium time, yield of HPBE, ee% being 1.25h, 72.2%, -70-80%.The production asymmetrically catalyzed by Baker Yeast was (S)-HPBE in water. The enhancement of temperature can boost the speed of the reaction and reducethe selectivity of Baker Yeast. The addition of glucose and Tween-80 has little effect on the reaction. Tween-80 can enhance the conversion from 72.15% to 75. 06% and also does not influence ee%. The time course and mechanism of the decomposition of OPBE and HPBE to form by-product of phenylpropanol was also investigated.3. The asymmetric reduction of OPBE catalyzed by Baker Yeast in nonaqueous phase to synthesis HPBE was investigated. The selectivity of Baker Yeast was totally changed in different nonaqueous phase. Diethyl ether was selected from several organic solvents those were different in hydrophobility and the production was (R)-HPBE. Effects of factors on ee%, yield of HPBE and conversion of OPBE were focused on and the optimum reaction conditions were as follows: 30; initial water content 30g/L; concentration of substrate 5mmol/L; dosage of Baker Yeast 50g/L; reaction time 24h. Under the optimum conditions, ee% being 41.78%, conversion of OPBE being 98. 15% and yield of HPBE being 77. 77% were obtained. The decomposition of HPBE can be inhibited in nonaqueous phase, but OPBE cannot. The change of alkoxy group of substrate from ethyl to isopropyl reduced the activity and selectivity of Baker Yeast. The pre-disposal of Baker Yeast by addition of inhibitors whose structures are similar to the substrate such as benzaldehyde ,benzyl alcohol and benzene carbonic acid can enhance the selectivity of Baker yeast remarkably to ee% from 41. 7...