J. Cent. South Univ. Technol. (2007)03-0353-04

DOI: 10.1007/s11771-007-0070-8               

Chiral extraction of ketoprofen enantiomers with chiral selector tartaric esters

ZHOU Dan(周 丹) ^1,2, LIU Jia-jia(刘佳佳)¹, TANG Ke-wen(唐课文)³, HUANG Ke-long(黄可龙)¹

 (1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Pharmarcy Department, Hainan Medical College, Haikou 571101, China;

3. Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology,

Yueyang 414000, China)

Abstract：Distribution behavior of ketoprofen enantiomers was examined in methanol aqueous and organic solvent mixture containing tartaric esters. The influence of length of alkyl chain of tartaric esters, concentration of L-tartaric esters and methanol aqueous, kind of organic solvent on partition ratio and separation factors was investigated. The results show that L-tartaric and D-tartaric esters have different chiral recognition abilities. S-ketoprofen is easily extracted by L-tartaric esters, and R-ketoprofen is easily extracted by D-tartaric esters. L-tartaric esters form more stable diastereomeric complexes with S-enantiomer than that with R-enantiomer. This distribution behavior is consistent with chiral recognition mechanism. With the increase of the concentration of tartaric ester from 0 to 0.3 mol/L, partition coefficient K and separation factor α increase. Also, the kind of organic solvent and the concentration of the methanol aqueous have significant influence on K and α.

Key words: chiral extraction; ketoprofen; enantiomer; tartaric ester; partition coefficient; separation factor

1 Introduction

The question of enantiomeric purity in drug substances is still an academic and industrial issue^[1].  It is not surprising, therefore, that, of 528 chiral synthetic drugs marketed worldwide today, only 61 are available as single pure enantiomers^[2].  As a result, there will be a strong demand for methods to produce optically pure products. To fulfill these requirements, the methods of diastereomeric salt crystallization, asymmetric synthesis, kinetic resolution and chiral separation have been developed^[3]. However, the diastereomeric salt crystallization usually requires many process steps, thus making the process complicated and resulting in considerable losses of product. To avoid these problems associated with diastereomeric salt crystallizations, asymmetric synthesis or kinetic resolution is often considered as viable alternative. However, an appropriate route has to be developed for each individual compound, leading to considerable costs and increased development time^[4]. For many cases, chiral separation still remains the most popular approach because it is relatively inexpensive, simple to carry out and has reasonably low time demand. So it receives more and more attentions^[5-12].

Ketoprofen (2-(3-benzoylphenyl)-propionic acid,KT), one of the most useful NSAIDs, has received attention since the past two decades and its anti-inflammatory effect is approximately 160 times the anti-inflammatory potency of aspirin on a unit mass basis^[13]. Commercially, ketoprofen is formulated as a racemic mixture of ‘‘R’’ and ‘‘S’’ enantiomers, which are equivalent on a unit mass basis. A major reason for the use of mixtures of enantiomers remains the cost of separation of the enantiomers with the potential advantage of a possible increase in the activity. However, S-ketoprofen and R-ketoprofen display significantly different pharmacologic activities and benefits^[14]. S-enantiomer has the therapeutic activity of reducing inflammation and relieving pains, while the R- enantiomer can be used as a toothpaste additive to prevent periodontal disease. Thus, the effort of producing single enantiomer of ketoprofen is important from the view of pharmacological benefits of both enantiomers^[15].

The aim of this study was to investigate the distribution behavior of ketoprofen enantiomers in the separation system containing tartaric esters. The effect of the composition, ratio, concentration, kind of organic solvents of chiral separation system on the partition coefficients(K) and separation factors(α) was studied so as to provide experimental and theoretical foundation for chiral extraction of ketoprofen enantiomers. 

2 Experimental

2.1 Apparatus and reagents

L-tartaric acid and D-tartaric acid were obtained from Tianjin Guangfu Fine Chemical Research Institute, China. L, D-tartaric esters were synthesized according to Ref.[16]. Ketoprofen was obtained by Hubei Wuxue Xunda Pharmaceutical Co. Ltd, China. All other reagents were in analytical grade.

For the high-pressure liquid chromatographic runs, a HP-1100 pump and a UV detector operating at 300 nm were employed. The reversed-phase chromatographic column (Lichrospher C8, 5 μm, 4.6mm(i.d.)×250 mm)^[17]  was operated at 0.6 mL/min mobile phase rate with a injector equipped with a 20 μL loop. The pH setting was realized by a Lei Ci pH meter.

2.2 Extraction experiments with chiral solvent

    In the separation system, the aqueous phase was prepared by dissolving 0.5 g ketoprofen in water- methanol(50:50, volume ratio) solution. L-tartaric esters dissolving in 1, 2-dichloroethane formed the organic phase. 5 mL organic phase and 5 mL aqueous phase were added into a 50 mL centrifugal tube, then were shaken for 1 h at room temperature. After the two phases were separated, the concentrations of enantiomers in aqueous conditions(c_W) are detected by HPLC. The concentration of enantiomers in organic phase(c_O) can be given by

c_O=c₀ – c_W                  (1)

where  c₀ is the initial concentration of ketoprofen in aqueous phase. The partition coefficients(K) can be obtained as follows:

K =c_O/c_W                 (2)

So, the partition coefficients of R-ketoprofen enantiomer and S-ketoprofen enantiomer are

K_R =,  K_S =            (3)

The separation factor (α) is described by

α = K_R/K_S                (4)

3 Results and discussion

3.1 Effect of length of alkyl chain of tartaric esters

The results are summarized in Table 1. It can be seen that an increase of length of alkyl chain of tartaric esters leads to the increase of K_R and K_S. The data reflect the difference in enantioselectivity of L and D tartaric esters. It is indicated that the R-enantiomer forms a more stable complex with D-tartaric esters than that with L-tartaric esters(K_R>K_S). And, S-enantiomer forms a more stable complex with L-tartaric esters(K_S>K_R). As found in the experiment result, S-enantiomer is easily extracted by L-tartaric esters and R-enantiomer is easily extracted by D-tartaric esters. The distribution characteristic is consistent with that reported in Ref.[9].

Table 1 Effect of length of alkyl chain of tartaric esters on K and α

This can be explained by the chiral recognition mechanism. We postulate the intermediate complex structures of L-n-butyl-L-tartaric ester with R, S-ketoprofen enantiomer, as shown in Fig.1.

Fig.1 Structures of L-n-butyl-L-tartaric ester and R, S-ketoprofen

The interactions between L-n-butyl-L-tartaric ester and R, S-ketoprifen enantiomers are shown in Fig.2. When L-n-butyl-L-tartaric ester acts with S-ketoprofen, there is little steric hindrance because benzene ring of S-ketoprofen is far away from L-n-butyl-L-tartaric ester. However, when it acts with R-ketoprofen, the steric hindrance of benzene ring has great influence on the stability of the complex. So, L-n-butyl-L-tartaric ester can form more stable complex with S-ketoprofen than with R-ketoprofen. During chiral extraction, more S-ketoprofen enters into the organic phase. This results in the excess of S-ketoprofen in organic phase and excess of R-ketoprofen in aqueous phase. As a result, R, S-ketoprofen enantiomers are separated by chiral extraction.

3.2 Effect of concentration of L-n-butyl-L-tartaric ester 

The effect of the concentration of L-n-butyl-L- tartaric ester on K and α was investigated. The results are shown in Figs.3 and 4, respectively. It can be seen that the increase of concentration of L-n-butyl-L-tartaric leads to increase of partition coefficients K and separation factors α. The partition coefficient of ketoprofen enantiomer K_S is bigger than K_R and the separation factor α is always over 1.0. It can be explained that L-n-butyl-L-tartaric ester forms a more stable complex with S-enantiomer than with R-enantiomer.

Fig.2 Interaction of L-n-butyl-L-tartaric ester and   R, S-ketoprofen enantiomers

3.3 Effect of organic solvents

Table 2 displays the results of the effect of organic solvents on K and α. It can be seen that, distribution of ketoprofen enantiomers in these six kinds of organic solvents is different, and that in solvent 1,2- dichloroethane is the biggest. The increase of length of alkyl chain leads to the increase of α. In these organic solvents, the distribution of R-ketoprofen enantiomer is always bigger than that of S-ketoprofen enantiomer. Maybe, that is because, during the extraction, L-tartaric esters form complexes with R, S enantiomers as the existence of action force of molecules. With the difference of the two complexes, the two enantiomers can be separated by chiral extraction. So, the property and especially the polarity affect the stability of two complexes, thus affecting the separation.

Fig.3 Effect of concentration of L-n-butyl-L-tartaric ester on K

Fig.4 Effect of concentration of L-n-butyl-L-tartaric ester on α

Table 2 Effect of organic solvents on K and α

3.4 Effect of content of methanol

Ketoprofen has little solubility in the water. In this experiment, ketoprofen is dissolved in the methanol-water(50:50,volume ratio). It is found that the content of methanol has great influence on the separation. The results are shown in Figs.5 and 6.

Fig.5 Effect of content of methanol on partition coefficients K

Fig.6 Effect of content of methanol on separation factor α

With the increase of content of methanol, partition coefficient K and separation factor α decrease. It can be explained that, as the content of methanol increases, more and more methanol forms hydrogen bond with ketoprofen, leading to the possibility of complexes formed by ketoprofen with chiral selector L-tartaric esters. So, the best content of methanol aqueous is 50%.

4 Conclusions

1) L,D-tartaric esters have different chiral recognition abilities. S-ketoprofen is easily extracted by L-tartaric esters, and R-ketoprofen is easily extracted by D-tartaric esters.

2) The concentration of L-n-butyl-L-tartaric ester has visible effect on the chiral separation. With the increase of concentration of tartaric ester, the partition coefficient and separation factor increase.

3) The kind of organic solvents and the content of methanol have influence on the chiral separation.

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(Edited by YANG Bing)

Foundation item: Project(20376085) supportecd by the National Natural Science Foundation of China

Received date: 2006-08-24; Accepted date: 2006-10-27

Corresponding author: LIU Jia-jia, Professor; Tel: +86-13974870621; E-mail: Liujj0903@163.net