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Synthesis of 2-oxo-4-phenylbutanoic acid: Parameter optimization using response surface methodology
Chemical Engineering Journal 171 (2011) 640–645
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Synthesis of 2-oxo-4-phenylbutanoic acid: Parameter optimization using response surface methodology A.L. Ahmad∗ , P.C. Oh, S.R. Abd Shukor School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 26 November 2010 Received in revised form 31 March 2011 Accepted 6 April 2011 Keywords: Synthesis Design of experiment Optimization Response surface methodology
a b s t r a c t A simple approach for the formation of 2-oxo-4-phenylbutanoic acid (OPBA), which might find important use as substrate for production of angiotensin-converting enzyme (ACE) inhibitor drug precursor, was studied. With the appropriate choice of solvent and base, it was possible to produce pure OPBA in good yields. In this study, optimization of OPBA synthesis was investigated. Response surface methodology (RSM) and 4-factor central-composite rotatable design (CCRD) were employed to evaluate the effects of synthesis parameters, such as reaction temperature (40–70 ◦ C), reaction time (5–13 h), choice of solvent (dried tetrahydrofuran or diglyme) and base (sodium methoxide or sodium hydride) on product yield. On the basis of numerical optimization, the optimum conditions for synthesis were: reaction time 6 h, reaction temperature 65 ◦ C, using dried tetrahydrofuran as solvent and sodium methoxide as the base. The yield obtained was 98%. Comparison of predicted and experimental values revealed good correspondence, implying that empirical models derived from RSM can be used to adequately describe the relationship between the factors and response in OPBA synthesis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 2-Oxo-4-phenylbutanoic acid (OPBA) is the common substrate used to produce various angiotensin-converting enzyme (ACE) inhibitor drug precursors, including (S)-2-amino-4phenylbutanoic acid and (R)-2-hydroxy-4-phenylbutanoic acid. ACE inhibitors are a class of antihypertensive drug which has shown to reduce morbidity and mortality in chronic heart failure. The corresponding ACE inhibitor drug precursors are generally produced from their keto acid counterparts [1]. To date, several methodologies have been developed for chemical synthesis of the ␣-keto acid esters, including preparation from 2-phenylethylmagnesium bromide and diethyl oxalate in the presence of mild base [2]. However, a more convenient and straightforward synthetic method for the synthesis of 2-oxo-4-phenylbutanoic acid is needed. In this work, OPBA was prepared to be used as the starting material for producing ACE inhibitor drug precursors. The reaction time, temperature, type of solvent and base used in the synthesis of the title compound were varied, in order to produce OPBA in sufficient yield. The synthesized OPBA was subsequently characterized using NMR. This study also aimed to better understand the relationships between parameters (reaction time, temperature, choice of solvent and base) and the response result (yield); and to determine the
∗ Corresponding author. Tel.: +60 4 6533090; fax: +60 4 6584149. E-mail addresses: [email protected], [email protected] (A.L. Ahmad). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.015
optimal conditions for OPBA synthesis using a central-composite rotatable design (CCRD) and response surface methodology (RSM) analysis. The classical method of optimization for chemical synthesis involves varying one parameter at a time and keeping the others constant. Nevertheless, this method is inefficient as it fails to understand relationships between the variables and the response [3,4]. RSM is an effective statistical technique to quantify the relationships between the response and the input factors in which the objective is to find a desirable location in the design space. The main advantage of RSM is the reduced number of experimental runs needed to provide sufficient information for statistically acceptable result. It is a faster and less expensive method for gathering results than the classical method [5–8], and can be applied for a wide range of chemical reactions involving more than one parameter or response. 2. Materials and methods 2.1. Chemicals 3-Phenylpropionic acid, sulphuric acid, magnesium sulphate, tetrahydrofuran, sodium, benzophenone and hydrochloric acid were purchased from Sigma–Aldrich. Methanol, ethyl acetate, sodium hydrogen carbonate, sodium hydride, calcium chloride anhydrous powder, 2-propanol, dimethyl oxalate, absolute methanol, diethyl ether, ethanol gradient grade, deuterated
A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
chloroform (0.03 vol% TMS), petroleum ether (boiling range 50–70 ◦ C), acetone, silica gel 60 (0.040–0.063 mm) for column chromatography (230–400 mesh ASTM), and trifluoroacetic acid for spectroscopy were obtained from Merck. Purified nitrogen and hydrogen were obtained from Gas Pantai Timur.
641
O
O H3C
OH
OH
O CH3
H2SO 4
3-phenylpropanoic acid
methyl 3-phenylpropanoate
2.2. Experimental design
Scheme 1. Esterification reaction structure.
4-Factor CCRD was employed in this study, requiring 52 experiments. The experiments contained 1 replicate of factorial points, 1 replicate of axial point and 5 center points. To avoid bias, the runs were performed in a random order. Preliminary studies were performed prior to this study in order to determine the most suitable range for the numerical factors. The numerical and categorical factors selected for the study of OPBA synthesis were: reaction temperature (40–70 ◦ C), reaction time (5–13 h), choice of solvent (dried tetrahydrofuran or diglyme) and base (sodium methoxide or sodium hydride). All experiments were performed in dried inert
system. Table 1 shows the process factors and actual experimental design in terms of coded and uncoded. 2.3. Preparation of reaction solution and synthesis 3-Phenylpropionic acid was dissolved in dry methanol, followed by addition of concentrated sulphuric acid (Scheme 1). Boiling chips were subsequently added and the clear solution was heated to
Table 1 Central-composite rotatable quadratic polynomial model, experimental data, actual and predicted values for four-factor response surface analysis. Treatment
T (◦ C), A
Time (h), B
Base, C
Solvent, D
Actual yield (%)
Pred. yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
45(−1) 55(0) 45(−1) 55(0) 70(1.414) 55(0) 40(−1.414) 40(−1.414) 70(1.414) 55(0) 55(0) 55(0) 55(0) 65(1) 65(1) 55(0) 55(0) 40(−1.414) 55(0) 45(−1) 70(1.414) 55(0) 45(−1) 65(1) 55(0) 55(0) 65(1) 55(0) 65(1) 45(−1) 55(0) 55(0) 45(−1) 55(0) 55(0) 45(−1) 55(0) 55(0) 55(0) 55(0) 55(0) 55(0) 55(0) 65(1) 65(1) 55(0) 65(1) 70(1.414) 55(0) 40(−1.414) 55(0) 45(−1)
12(1) 9(0) 6(−1) 13(1.414) 9(0) 5(−1.414) 9(0) 9(0) 9(0) 9(0) 5(−1.414) 9(0) 9(0) 6(−1) 6(−1) 9(0) 9(0) 9(0) 9(0) 6(−1) 9(0) 13(1.414) 6(−1) 12(1) 9(0) 5(−1.414) 12(1) 9(0) 12(1) 12(1) 9(0) 9(0) 6(−1) 9(0) 9(0) 12(1) 9(0) 9(0) 9(0) 9(0) 9(0) 9(0) 13(1.414) 12(1) 6(−1) 13(1.414) 6(−1) 9(0) 5(−1.414) 9(0) 9(0) 12(1)
Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1)
Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1)
52.2 88.7 35.6 70.1 58.1 86.8 74.1 47.7 53.6 52.7 49.5 51.2 61.3 63.2 46.2 50.4 48.5 45.2 88.7 45.2 54.7 87.6 75.8 96.3 62 37.5 53.9 88.1 75.5 79.5 62 63.8 42.2 63.3 48.6 47.9 48 50 89.3 51.8 87.5 52 56.7 56.7 57.2 50.3 98 55.8 49.5 42.9 46.8 59.7
51.9 88.6 35.4 70.4 57.3 86.2 74.2 48 53.7 51.8 50 51.8 61.9 64.2 46.1 51.8 48.7 45.1 88.6 44.9 53.7 87.5 75.8 96.2 61.9 37.3 53.5 88.6 76 79.6 61.9 61.9 42.6 61.9 48.7 47.8 48.7 48.7 88.6 51.8 88.6 51.8 56.7 56.9 56.3 50 98.1 57.3 49.8 42.6 48.7 60
642
A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
O
O
C O CH3
methyl 3-phenylpropanoate
-
C
D
O O
CH3
H3C
+
H
O O
CH3 O
1-(methoxycarbonyl)-2-phenylethanide
dimethyl oxalate
O O CH3 O O
O CH3
dimethyl 2-benzyl-3-oxobutanedioate CH3O
-
O O
-
CH3
C
O O
O CH3
1,2-bis(methoxycarbonyl)-1-oxo-3-phenylpropan-2-ide Dilute HCl
O OH O O
OH
2-benzyl-3-oxobutanedioic acid CO2 O OH O
2-oxo-4-phenylbutanoic acid Scheme 2. Reaction route for production of 2-oxo-4-phenylbutanoic acid from methyl 3-phenylpropanoate, a light yellow liquid.
reflux for 8 h. The resulting solution was concentrated in vacuum via rotary evaporator. The concentrate was diluted with ethyl acetate, washed with saturated sodium hydrogen carbonate, and dried over magnesium sulphate. The resulting solution was then filtered and concentrated in vacuum to yield a light yellow liquid. The light yellow liquid was then mixed with C and dissolved in D. Solution was stirred for 10 min, for deprotonation. Following this procedure, dimethyl oxalate was added to the mixture and the solution was stirred at A ◦ C for B h after the system was purged with nitrogen gas. After cooling to room temperature, tetrahydrofuran solvent was evaporated using rotary evaporator, and hydrochloric acid was subsequently added to the cooled solution. The solution was dissolved in diethyl ether where the upper layer of the resulting solution was separated and washed with saturated sodium hydrogen carbonate, dried over magnesium sulphate, then filtered and
concentrated in vacuum to yield an oil compound. Hydrochloric acid was then added to the oil compound and the mixture was refluxed for 6 h at 100 ◦ C. The resulting solution was cooled at 4 ◦ C and filtered to collect the 2-oxo-4-phenylbutanoic acid precipitate (Scheme 2).
2.4. Analytical methods 2.4.1. Characterization of synthesized 2-oxo-4-phenylbutanoic acid After chemical synthesis of 2-oxo-4-phenylbutanoic acid, the compound was characterized to ascertain its chemical structure and purity. Characterization was performed via Nuclear Magnetic Resonance (NMR) Spectroscopy and High Performance Liquid Chromatography (HPLC).
A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
643
Table 2 Analysis of variance (ANOVA) for synthetic variables pertaining to response percent yield. Source
Degree of freedom
Mean square
F-value
Prob > F
Model Temperature (A) Time (B) Base (C) Solvent (D) A2 B2 AB AC AD BC BD CD Residual Lack of fit Pure error
12 1 1 1 1 1 1 1 1 1 1 1 1 39 21 18
1138.19 1354.97 426.67 2862.35 7634.28 2.04 20.94 32.21 1.07 198.33 374.85 1.66 1775.09 0.64 0.35 0.98
1775.96 2114.20 665.75 4466.21 11,911.99 3.18 32.68 50.25 1.67 309.46 584.89 2.58 2769.73
<0.0001a <0.0001a <0.0001a <0.0001a <0.0001a 0.0822b <0.0001a <0.0001a 0.2041b <0.0001a <0.0001a 0.1160b <0.0001a
a b
0.35
0.9882
Significant at “Prob > F” less than 0.05. Insignificant at “Prob > F” more than 0.05.
b 75.9511
98.2916
67.6145
92.6685
Yield (%)
Yield (%)
a
59.2779 50.9412
87.0454 81.4223
42.6046
75.7992
12.00
12.00 65.00
65.00
10.50
10.50
60.00 9.00
B: Time (h)
60.00 9.00
55.00 7.50
B: Time (h)
50.00
A: Temperature (Deg C)
50.00
A: Temperature (Deg C)
45.00
6.00
c
d
56.368
56.9304
53.5037
51.5443
Yield (%)
Yield (%)
6.00
55.00 7.50
50.6393 47.7749
45.00
46.1582 40.7721 35.386
44.9105
12.00
12.00 65.00 10.50
65.00 10.50 60.00
60.00 9.00
9.00
B: Time (h)
55.00
55.00 7.50
50.00 6.00
45.00
B: Time (h)
7.50
50.00
A: Temperature (Deg C) 6.00
A: Temperature (Deg C)
45.00
Fig. 1. Response surface plots showing the effect of reaction time, temperature and their mutual effect on the synthesis of OPBA. Other variables are constant: (a) base, sodium hydride and solvent, tetrahydrofuran; (b) base, sodium methoxide and solvent, tetrahydrofuran; (c) base, sodium methoxide and solvent, diglyme; (d) base, sodium hydride and solvent, diglyme.
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A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
Table 3 Solutions of optimum conditions. Experiment
Temperature (◦ C)
Time (h)
Base
Solvent
Predicted yield (%)
Actual yield (%)
1 2 3 4 5
65.00 65.00 65.00 65.00 65.00
6.00 6.00 6.00 6.68 6.61
Sodium methoxide Sodium hydride Sodium methoxide Sodium hydride Sodium hydride
Tetrahydrofuran Tetrahydrofuran Diglyme Diglyme Diglyme
98.1 64.2 56.3 47.7 47.5
98.0 63.2 57.2 47.0 46.5
2.4.2. Statistical analysis The data from the experiments performed were analyzed using Design-ExpertTM , Version 6.0.6 and then interpreted [9]. Three main analytical steps: analysis of variance (ANOVA), a regression analysis and the plotting of response surface were performed to establish an optimum condition for the synthesis of OPBA. 3. Results and discussion
temperature, the yield was 67.6%. A reaction with sodium methoxide as base and tetrahydrofuran as solvent led to a yield of ∼98%, as shown in Fig. 1b. A yield of ∼56% can be obtained at high temperature and short reaction time using sodium methoxide as base and diglyme as solvent, whereas reaction with sodium hydride as base and diglyme as solvent led to a maximum yield of ∼57% at high temperature and longest time, as shown in Fig. 1c and d, respectively. At any given solvent and base, the yield increases with increasing temperature and time.
3.1. Model fitting and ANOVA 3.3. Optimization of reaction The synthesized OPBA had the following physical properties: 1 H NMR (CDCl ): 2.97 ppm (t, 2H), 3.28 ppm (t, 2H), 7.20–7.35 ppm 3
(m, 5H) NMR (CDCl3 ): 29.2, 40.7, 126.9, 128.8, 129.1, 140.4, 162.2, 195.8 ppm melting point, 41–44 ◦ C
13 C
Experimental data for OPBA synthesis are given in Table 1. The predicted values were obtained from model fitting technique using Design-ExpertTM , Version 6.0.6 and were seen to be sufficiently correlated to the observed values. Fitting of the data to various models (linear, two factorial, quadratic and cubic) and their subsequent ANOVA showed that the reaction was most suitably described with quadratic polynomial model. From the Design-Expert Software, the quadratic polynomial is given below: yield (%) = +62.73 + 6.83A + 3.65B + 7.44C − 12.48D − 0.28A2 − 0.87B2 − 1.42AB + 0.18AC − 2.72AD − 3.42BC − 0.23BD − 5.92CD where A is the temperature; B the time; C the type of base; D the type of solvent. The computed Model F-value of 1775.96 was large, which implies that the model is significant. There is only a 0.01% chance that the Model F-value this large could occur due to noise. Values of Prob > F less than 0.0500 indicate model terms were significant. In this case A, B, C, D, B2 , AB, AD, BC and CD were significant model terms. The lack of fit F-value of 0.35 implied the lack of fit was not significant relative to the pure error. The pure error was also very low, indicating good reproducibility of the data obtained. With very small P-value (0.0001) from the analysis of ANOVA and a suitable coefficient of determination (R2 = 0.998), the quadratic polynomial model was highly significant and sufficient to represent the actual relationship between the response (% yield) and the significant variables. All variables were significant in the process (Table 2). 3.2. Mutual effect of parameters Reaction temperature and reaction time were investigated in the range of 40–70 ◦ C and 5–13 h, respectively. Fig. 1a shows the effect of reaction temperature and reaction time and their mutual effect on OPBA synthesis using sodium hydride as base and tetrahydrofuran as solvent. At the shortest reaction time, with highest
The optimal conditions for OPBA synthesis were predicted using the optimization function of Design-Expert Software. These are presented in Table 3 along with their predicted and actual values, when time was minimized and yield was maximized. Time factor should be minimized in order to complete the synthesis in the shortest time without concern for cost, whereas the highest yield will give the highest desirability. Among the various optimum conditions, the highest % yield was from experiment 1. This was chosen as the optimum condition in order to produce high % yield of OPBA. 4. Conclusions The optimum reaction condition was studied by centralcomposite rotatable design and response surface methodology. As the experiment was designed to utilize reaction time 6 h, reaction temperature 65 ◦ C, using dried tetrahydrofuran as solvent and sodium methoxide as base, maximum yield of 98% was obtained. Comparison of predicted and experimental values revealed good correspondence between them, implying that empirical models derived from RSM can be used to adequately describe the relationship between the factors and response in OPBA synthesis. These models can then be used to predict OPBA yield under any given conditions within the experimental range. The optimum synthesis of OPBA can be successfully predicted by RSM. Acknowledgements This work was supported by Universiti Sains Malaysia under the Research University Postgraduate Research Grant Scheme No. 8041013, and Research University Grant No. 814002. P.C. Oh gratefully acknowledges the National Science Fellowship (NSF) received from the Malaysian Ministry of Science, Technology and Innovation (MOSTI) for her Ph.D. program. The authors also thank Dr. K.C. Wong for his helpful discussion. References [1] E. Schmidt, O. Ghisalba, D. Gygax, G. Sedelmeier, Optimization of a process for the production of (R)-2-hydroxy-4-phenylbutyric acid: an intermediate for inhibitors of angiotensin converting enzyme, J. Biotechnol. 24 (1992) 315–327. [2] C.W. Bradshaw, C.H. Wong, W. Hummel, M.R. Kula, Enzyme-catalyzed asymmetric-synthesis of (S)-2-amino-4-phenylbutanoic acid and (R)-2hydroxy-4-phenylbutanoic acid, Bioorg. Chem. 19 (1991) 29–39. [3] D.R. Hamsaveni, S.G. Prapulla, S. Divakar, Response surface methodological approach for the synthesis of isobutyl isobutyrate, Process Biochem. 36 (2001) 1103–1109.
A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645 [4] E.L. Soo, A.B. Salleh, M. Basri, R.N.Z.A. Rahman, K. Kamaruddin, Response surface methodological study on lipase-catalyzed synthesis of amino acid surfactants, Process Biochem. 39 (2004) 1511–1518. [5] E.R. Gunawan, M. Basri, M.B.A. Rahman, A.B. Salleh, R.N.Z.A. Rahman, Study on response surface methodology (RSM) of lipase-catalyzed synthesis of palmbased wax ester, Enzyme Microb. Technol. 37 (2005) 739–744. [6] H.C. Chen, H.Y. Ju, Y.K. Twu, J.H. Chen, C.M.J. Chang, Y.C. Liu, C. Chang, C.J. Shieh, Optimized enzymatic synthesis of caffeic acid phenethyl ester by RSM, New Biotechnol. 27 (2010) 89–93.
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[7] Y.K. Twu, I.L. Shih, Y.H. Yen, Y.F. Ling, C.J. Shieh, Optimization of lipase-catalyzed synthesis of octyl hydroxyphenylpropionate by response surface methodology, J. Agric. Food Chem. 53 (2005) 1012–1016. [8] H.C. Chen, C.C. Chang, C.X. Chen, Optimization of arachidonic acid production by Mortierella alpina Wuji-H4 isolate, J. Am. Oil Chem. Soc. 74 (1997) 569–578. [9] Design-Expert, Version 6.0.6, User’s Guide, Stat-Ease Inc. USA.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Synthesis of 2-oxo-4-phenylbutanoic acid: Parameter optimization using response surface methodology A.L. Ahmad∗ , P.C. Oh, S.R. Abd Shukor School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 26 November 2010 Received in revised form 31 March 2011 Accepted 6 April 2011 Keywords: Synthesis Design of experiment Optimization Response surface methodology
a b s t r a c t A simple approach for the formation of 2-oxo-4-phenylbutanoic acid (OPBA), which might find important use as substrate for production of angiotensin-converting enzyme (ACE) inhibitor drug precursor, was studied. With the appropriate choice of solvent and base, it was possible to produce pure OPBA in good yields. In this study, optimization of OPBA synthesis was investigated. Response surface methodology (RSM) and 4-factor central-composite rotatable design (CCRD) were employed to evaluate the effects of synthesis parameters, such as reaction temperature (40–70 ◦ C), reaction time (5–13 h), choice of solvent (dried tetrahydrofuran or diglyme) and base (sodium methoxide or sodium hydride) on product yield. On the basis of numerical optimization, the optimum conditions for synthesis were: reaction time 6 h, reaction temperature 65 ◦ C, using dried tetrahydrofuran as solvent and sodium methoxide as the base. The yield obtained was 98%. Comparison of predicted and experimental values revealed good correspondence, implying that empirical models derived from RSM can be used to adequately describe the relationship between the factors and response in OPBA synthesis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 2-Oxo-4-phenylbutanoic acid (OPBA) is the common substrate used to produce various angiotensin-converting enzyme (ACE) inhibitor drug precursors, including (S)-2-amino-4phenylbutanoic acid and (R)-2-hydroxy-4-phenylbutanoic acid. ACE inhibitors are a class of antihypertensive drug which has shown to reduce morbidity and mortality in chronic heart failure. The corresponding ACE inhibitor drug precursors are generally produced from their keto acid counterparts [1]. To date, several methodologies have been developed for chemical synthesis of the ␣-keto acid esters, including preparation from 2-phenylethylmagnesium bromide and diethyl oxalate in the presence of mild base [2]. However, a more convenient and straightforward synthetic method for the synthesis of 2-oxo-4-phenylbutanoic acid is needed. In this work, OPBA was prepared to be used as the starting material for producing ACE inhibitor drug precursors. The reaction time, temperature, type of solvent and base used in the synthesis of the title compound were varied, in order to produce OPBA in sufficient yield. The synthesized OPBA was subsequently characterized using NMR. This study also aimed to better understand the relationships between parameters (reaction time, temperature, choice of solvent and base) and the response result (yield); and to determine the
∗ Corresponding author. Tel.: +60 4 6533090; fax: +60 4 6584149. E-mail addresses: [email protected], [email protected] (A.L. Ahmad). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.015
optimal conditions for OPBA synthesis using a central-composite rotatable design (CCRD) and response surface methodology (RSM) analysis. The classical method of optimization for chemical synthesis involves varying one parameter at a time and keeping the others constant. Nevertheless, this method is inefficient as it fails to understand relationships between the variables and the response [3,4]. RSM is an effective statistical technique to quantify the relationships between the response and the input factors in which the objective is to find a desirable location in the design space. The main advantage of RSM is the reduced number of experimental runs needed to provide sufficient information for statistically acceptable result. It is a faster and less expensive method for gathering results than the classical method [5–8], and can be applied for a wide range of chemical reactions involving more than one parameter or response. 2. Materials and methods 2.1. Chemicals 3-Phenylpropionic acid, sulphuric acid, magnesium sulphate, tetrahydrofuran, sodium, benzophenone and hydrochloric acid were purchased from Sigma–Aldrich. Methanol, ethyl acetate, sodium hydrogen carbonate, sodium hydride, calcium chloride anhydrous powder, 2-propanol, dimethyl oxalate, absolute methanol, diethyl ether, ethanol gradient grade, deuterated
A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
chloroform (0.03 vol% TMS), petroleum ether (boiling range 50–70 ◦ C), acetone, silica gel 60 (0.040–0.063 mm) for column chromatography (230–400 mesh ASTM), and trifluoroacetic acid for spectroscopy were obtained from Merck. Purified nitrogen and hydrogen were obtained from Gas Pantai Timur.
641
O
O H3C
OH
OH
O CH3
H2SO 4
3-phenylpropanoic acid
methyl 3-phenylpropanoate
2.2. Experimental design
Scheme 1. Esterification reaction structure.
4-Factor CCRD was employed in this study, requiring 52 experiments. The experiments contained 1 replicate of factorial points, 1 replicate of axial point and 5 center points. To avoid bias, the runs were performed in a random order. Preliminary studies were performed prior to this study in order to determine the most suitable range for the numerical factors. The numerical and categorical factors selected for the study of OPBA synthesis were: reaction temperature (40–70 ◦ C), reaction time (5–13 h), choice of solvent (dried tetrahydrofuran or diglyme) and base (sodium methoxide or sodium hydride). All experiments were performed in dried inert
system. Table 1 shows the process factors and actual experimental design in terms of coded and uncoded. 2.3. Preparation of reaction solution and synthesis 3-Phenylpropionic acid was dissolved in dry methanol, followed by addition of concentrated sulphuric acid (Scheme 1). Boiling chips were subsequently added and the clear solution was heated to
Table 1 Central-composite rotatable quadratic polynomial model, experimental data, actual and predicted values for four-factor response surface analysis. Treatment
T (◦ C), A
Time (h), B
Base, C
Solvent, D
Actual yield (%)
Pred. yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
45(−1) 55(0) 45(−1) 55(0) 70(1.414) 55(0) 40(−1.414) 40(−1.414) 70(1.414) 55(0) 55(0) 55(0) 55(0) 65(1) 65(1) 55(0) 55(0) 40(−1.414) 55(0) 45(−1) 70(1.414) 55(0) 45(−1) 65(1) 55(0) 55(0) 65(1) 55(0) 65(1) 45(−1) 55(0) 55(0) 45(−1) 55(0) 55(0) 45(−1) 55(0) 55(0) 55(0) 55(0) 55(0) 55(0) 55(0) 65(1) 65(1) 55(0) 65(1) 70(1.414) 55(0) 40(−1.414) 55(0) 45(−1)
12(1) 9(0) 6(−1) 13(1.414) 9(0) 5(−1.414) 9(0) 9(0) 9(0) 9(0) 5(−1.414) 9(0) 9(0) 6(−1) 6(−1) 9(0) 9(0) 9(0) 9(0) 6(−1) 9(0) 13(1.414) 6(−1) 12(1) 9(0) 5(−1.414) 12(1) 9(0) 12(1) 12(1) 9(0) 9(0) 6(−1) 9(0) 9(0) 12(1) 9(0) 9(0) 9(0) 9(0) 9(0) 9(0) 13(1.414) 12(1) 6(−1) 13(1.414) 6(−1) 9(0) 5(−1.414) 9(0) 9(0) 12(1)
Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium methoxide(1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1) Sodium hydride(−1)
Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Tetrahydrofuran(−1) Diglyme(1) Diglyme(1) Tetrahydrofuran(−1)
52.2 88.7 35.6 70.1 58.1 86.8 74.1 47.7 53.6 52.7 49.5 51.2 61.3 63.2 46.2 50.4 48.5 45.2 88.7 45.2 54.7 87.6 75.8 96.3 62 37.5 53.9 88.1 75.5 79.5 62 63.8 42.2 63.3 48.6 47.9 48 50 89.3 51.8 87.5 52 56.7 56.7 57.2 50.3 98 55.8 49.5 42.9 46.8 59.7
51.9 88.6 35.4 70.4 57.3 86.2 74.2 48 53.7 51.8 50 51.8 61.9 64.2 46.1 51.8 48.7 45.1 88.6 44.9 53.7 87.5 75.8 96.2 61.9 37.3 53.5 88.6 76 79.6 61.9 61.9 42.6 61.9 48.7 47.8 48.7 48.7 88.6 51.8 88.6 51.8 56.7 56.9 56.3 50 98.1 57.3 49.8 42.6 48.7 60
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A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
O
O
C O CH3
methyl 3-phenylpropanoate
-
C
D
O O
CH3
H3C
+
H
O O
CH3 O
1-(methoxycarbonyl)-2-phenylethanide
dimethyl oxalate
O O CH3 O O
O CH3
dimethyl 2-benzyl-3-oxobutanedioate CH3O
-
O O
-
CH3
C
O O
O CH3
1,2-bis(methoxycarbonyl)-1-oxo-3-phenylpropan-2-ide Dilute HCl
O OH O O
OH
2-benzyl-3-oxobutanedioic acid CO2 O OH O
2-oxo-4-phenylbutanoic acid Scheme 2. Reaction route for production of 2-oxo-4-phenylbutanoic acid from methyl 3-phenylpropanoate, a light yellow liquid.
reflux for 8 h. The resulting solution was concentrated in vacuum via rotary evaporator. The concentrate was diluted with ethyl acetate, washed with saturated sodium hydrogen carbonate, and dried over magnesium sulphate. The resulting solution was then filtered and concentrated in vacuum to yield a light yellow liquid. The light yellow liquid was then mixed with C and dissolved in D. Solution was stirred for 10 min, for deprotonation. Following this procedure, dimethyl oxalate was added to the mixture and the solution was stirred at A ◦ C for B h after the system was purged with nitrogen gas. After cooling to room temperature, tetrahydrofuran solvent was evaporated using rotary evaporator, and hydrochloric acid was subsequently added to the cooled solution. The solution was dissolved in diethyl ether where the upper layer of the resulting solution was separated and washed with saturated sodium hydrogen carbonate, dried over magnesium sulphate, then filtered and
concentrated in vacuum to yield an oil compound. Hydrochloric acid was then added to the oil compound and the mixture was refluxed for 6 h at 100 ◦ C. The resulting solution was cooled at 4 ◦ C and filtered to collect the 2-oxo-4-phenylbutanoic acid precipitate (Scheme 2).
2.4. Analytical methods 2.4.1. Characterization of synthesized 2-oxo-4-phenylbutanoic acid After chemical synthesis of 2-oxo-4-phenylbutanoic acid, the compound was characterized to ascertain its chemical structure and purity. Characterization was performed via Nuclear Magnetic Resonance (NMR) Spectroscopy and High Performance Liquid Chromatography (HPLC).
A.L. Ahmad et al. / Chemical Engineering Journal 171 (2011) 640–645
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Table 2 Analysis of variance (ANOVA) for synthetic variables pertaining to response percent yield. Source
Degree of freedom
Mean square
F-value
Prob > F
Model Temperature (A) Time (B) Base (C) Solvent (D) A2 B2 AB AC AD BC BD CD Residual Lack of fit Pure error
12 1 1 1 1 1 1 1 1 1 1 1 1 39 21 18
1138.19 1354.97 426.67 2862.35 7634.28 2.04 20.94 32.21 1.07 198.33 374.85 1.66 1775.09 0.64 0.35 0.98
1775.96 2114.20 665.75 4466.21 11,911.99 3.18 32.68 50.25 1.67 309.46 584.89 2.58 2769.73
<0.0001a <0.0001a <0.0001a <0.0001a <0.0001a 0.0822b <0.0001a <0.0001a 0.2041b <0.0001a <0.0001a 0.1160b <0.0001a
a b
0.35
0.9882
Significant at “Prob > F” less than 0.05. Insignificant at “Prob > F” more than 0.05.
b 75.9511
98.2916
67.6145
92.6685
Yield (%)
Yield (%)
a
59.2779 50.9412
87.0454 81.4223
42.6046
75.7992
12.00
12.00 65.00
65.00
10.50
10.50
60.00 9.00
B: Time (h)
60.00 9.00
55.00 7.50
B: Time (h)
50.00
A: Temperature (Deg C)
50.00
A: Temperature (Deg C)
45.00
6.00
c
d
56.368
56.9304
53.5037
51.5443
Yield (%)
Yield (%)
6.00
55.00 7.50
50.6393 47.7749
45.00
46.1582 40.7721 35.386
44.9105
12.00
12.00 65.00 10.50
65.00 10.50 60.00
60.00 9.00
9.00
B: Time (h)
55.00
55.00 7.50
50.00 6.00
45.00
B: Time (h)
7.50
50.00
A: Temperature (Deg C) 6.00
A: Temperature (Deg C)
45.00
Fig. 1. Response surface plots showing the effect of reaction time, temperature and their mutual effect on the synthesis of OPBA. Other variables are constant: (a) base, sodium hydride and solvent, tetrahydrofuran; (b) base, sodium methoxide and solvent, tetrahydrofuran; (c) base, sodium methoxide and solvent, diglyme; (d) base, sodium hydride and solvent, diglyme.
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Table 3 Solutions of optimum conditions. Experiment
Temperature (◦ C)
Time (h)
Base
Solvent
Predicted yield (%)
Actual yield (%)
1 2 3 4 5
65.00 65.00 65.00 65.00 65.00
6.00 6.00 6.00 6.68 6.61
Sodium methoxide Sodium hydride Sodium methoxide Sodium hydride Sodium hydride
Tetrahydrofuran Tetrahydrofuran Diglyme Diglyme Diglyme
98.1 64.2 56.3 47.7 47.5
98.0 63.2 57.2 47.0 46.5
2.4.2. Statistical analysis The data from the experiments performed were analyzed using Design-ExpertTM , Version 6.0.6 and then interpreted [9]. Three main analytical steps: analysis of variance (ANOVA), a regression analysis and the plotting of response surface were performed to establish an optimum condition for the synthesis of OPBA. 3. Results and discussion
temperature, the yield was 67.6%. A reaction with sodium methoxide as base and tetrahydrofuran as solvent led to a yield of ∼98%, as shown in Fig. 1b. A yield of ∼56% can be obtained at high temperature and short reaction time using sodium methoxide as base and diglyme as solvent, whereas reaction with sodium hydride as base and diglyme as solvent led to a maximum yield of ∼57% at high temperature and longest time, as shown in Fig. 1c and d, respectively. At any given solvent and base, the yield increases with increasing temperature and time.
3.1. Model fitting and ANOVA 3.3. Optimization of reaction The synthesized OPBA had the following physical properties: 1 H NMR (CDCl ): 2.97 ppm (t, 2H), 3.28 ppm (t, 2H), 7.20–7.35 ppm 3
(m, 5H) NMR (CDCl3 ): 29.2, 40.7, 126.9, 128.8, 129.1, 140.4, 162.2, 195.8 ppm melting point, 41–44 ◦ C
13 C
Experimental data for OPBA synthesis are given in Table 1. The predicted values were obtained from model fitting technique using Design-ExpertTM , Version 6.0.6 and were seen to be sufficiently correlated to the observed values. Fitting of the data to various models (linear, two factorial, quadratic and cubic) and their subsequent ANOVA showed that the reaction was most suitably described with quadratic polynomial model. From the Design-Expert Software, the quadratic polynomial is given below: yield (%) = +62.73 + 6.83A + 3.65B + 7.44C − 12.48D − 0.28A2 − 0.87B2 − 1.42AB + 0.18AC − 2.72AD − 3.42BC − 0.23BD − 5.92CD where A is the temperature; B the time; C the type of base; D the type of solvent. The computed Model F-value of 1775.96 was large, which implies that the model is significant. There is only a 0.01% chance that the Model F-value this large could occur due to noise. Values of Prob > F less than 0.0500 indicate model terms were significant. In this case A, B, C, D, B2 , AB, AD, BC and CD were significant model terms. The lack of fit F-value of 0.35 implied the lack of fit was not significant relative to the pure error. The pure error was also very low, indicating good reproducibility of the data obtained. With very small P-value (0.0001) from the analysis of ANOVA and a suitable coefficient of determination (R2 = 0.998), the quadratic polynomial model was highly significant and sufficient to represent the actual relationship between the response (% yield) and the significant variables. All variables were significant in the process (Table 2). 3.2. Mutual effect of parameters Reaction temperature and reaction time were investigated in the range of 40–70 ◦ C and 5–13 h, respectively. Fig. 1a shows the effect of reaction temperature and reaction time and their mutual effect on OPBA synthesis using sodium hydride as base and tetrahydrofuran as solvent. At the shortest reaction time, with highest
The optimal conditions for OPBA synthesis were predicted using the optimization function of Design-Expert Software. These are presented in Table 3 along with their predicted and actual values, when time was minimized and yield was maximized. Time factor should be minimized in order to complete the synthesis in the shortest time without concern for cost, whereas the highest yield will give the highest desirability. Among the various optimum conditions, the highest % yield was from experiment 1. This was chosen as the optimum condition in order to produce high % yield of OPBA. 4. Conclusions The optimum reaction condition was studied by centralcomposite rotatable design and response surface methodology. As the experiment was designed to utilize reaction time 6 h, reaction temperature 65 ◦ C, using dried tetrahydrofuran as solvent and sodium methoxide as base, maximum yield of 98% was obtained. Comparison of predicted and experimental values revealed good correspondence between them, implying that empirical models derived from RSM can be used to adequately describe the relationship between the factors and response in OPBA synthesis. These models can then be used to predict OPBA yield under any given conditions within the experimental range. The optimum synthesis of OPBA can be successfully predicted by RSM. Acknowledgements This work was supported by Universiti Sains Malaysia under the Research University Postgraduate Research Grant Scheme No. 8041013, and Research University Grant No. 814002. P.C. Oh gratefully acknowledges the National Science Fellowship (NSF) received from the Malaysian Ministry of Science, Technology and Innovation (MOSTI) for her Ph.D. program. The authors also thank Dr. K.C. Wong for his helpful discussion. References [1] E. Schmidt, O. Ghisalba, D. Gygax, G. Sedelmeier, Optimization of a process for the production of (R)-2-hydroxy-4-phenylbutyric acid: an intermediate for inhibitors of angiotensin converting enzyme, J. Biotechnol. 24 (1992) 315–327. [2] C.W. Bradshaw, C.H. Wong, W. Hummel, M.R. Kula, Enzyme-catalyzed asymmetric-synthesis of (S)-2-amino-4-phenylbutanoic acid and (R)-2hydroxy-4-phenylbutanoic acid, Bioorg. Chem. 19 (1991) 29–39. [3] D.R. Hamsaveni, S.G. Prapulla, S. Divakar, Response surface methodological approach for the synthesis of isobutyl isobutyrate, Process Biochem. 36 (2001) 1103–1109.
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