Patent Publication Number: US-5296618-A

Title: Method for the manufacture of the derivatives of propionic acid

Description:
The object of this invention is a method for the manufacture of optically pure derivatives of epoxypropionic acid. The derivatives of epoxypropionic acid are important intermediates in the manufacture of some drug substances, 1,5-benzothiazepine, for example. The general structure of the derivatives of epoxypropionic acid is: ##STR1## in which R1 and R2 are either H, alkylgroup or phenylgroup as such or substituted. 
     The derivatives of epoxypropionic acid can appear as different optically active space structures. 
     Usually only one of the optically active isomers of the final drug substance exhibits the desired effect. The other optical isomers are either ineffective or they may have harmful side effects. For that reason it is appropriate to aim to use and manufacture the drug substances as optically pure isomers. 
     The Patent Publication EP 342 903 comprises the separation of the enantiomers of 3-(4-methoxyphenyl)-epoxypropionic acid from each other by hydrolyzing first the ester of the racemic raw material and bringing the deglycidate thus obtained in touch with an optically active amine. The obtained diastereomeric salt is crystallized as optically pure, converted to an alkalic metal salt and alkylated to an optically pure ester. Also the Patent Publications EP 386 654, JP 61-145159 and JP 61-145160 comprise the corresponding resolution methods based on the formation of diastereomeric salt. These methods contain several phases, they last for many hours and various reagents and solvents are needed in them. They produce plenty of waste solutions, which are expensive to purify and which finally need to be destroyed. 
     The Patent Publication WO 89/10350 comprises the synthesis of the desired optically active derivative of epoxypropionic acid through sulfonate ester intermediate phase. Also this method contains numerous reagents and great amounts of different solvents, which need to be purified for reuse and the destruction of which has to be taken care for. 
     The Patent Publication EP 365 029 comprises the synthesis of the optically active epoxypropionic acids from the racemic dihalopropionic acid or chlorolactic acid by using as catalyst dehalogenase enzyme, that is separated from organism population grown in Pseudomonas geneva culture. The optically pure synthesis product is finally crystallized from the reaction mixture in several phases using many solvents. The enzymatically catalyzed synthesis requires a long reaction time, at least 12 hours, so the manufacturing rate of the method is very low. 
     The Patent Publications EP 362 556, EP 343 714 and WO 90/04643 comprise the separation of the enantiomers of the mixture of racemic epoxypropionic acid from each other by hydrolyzing their esters stereospecifically using enzymes, particularly lipases, as catalysts. Also these methods require vary long reaction times, even 48 hours. Many very dilute solutions, the purification, regeneration, destruction and handling of the waste water of which are expensive, are used in the methods for separation of enantiomers based on the enzymatic hydrolysis. 
     Finnish Patent Publication No. 911,264 discloses a technique for separating enantiomers wherein a solution containing a mixture of isomers is seeded with a crystal of the desired compound, crystals are allowed to form in the solution, and the resulting crystals are isolated. A preferred solvent is t-butylmethylether. 
     It is also known to synthesize optically active epoxypropionic acids or their derivatives by using L-aminoacid as starting material (JP 62212329) or other optically pure starting materials (JP 60013776, JP 60-13775). The method for synthesis contains many phases and many crystallizations from different solvents. Several reagents are used and lots of waste solutions are produced. 
     Surprisingly we have noticed, that the enantiomers of the derivatives of epoxypropionic acid can simply be separated from each other by dissolving the racemic mixture to be separated in carbon dioxide and leading the obtained solution through a chromatographic column. 
     The separation of enantiomers of the derivatives of epoxypropionic acid chromatographically by using carbon dioxide as mobile phase enables a simple, fast and in occupational and environmental safety points of view clean industrial manufacturing process for optically pure enantiomers of epoxypropionic acid. 
     One of the advantages of this invention is that the separation of enantiomers of the derivatives of epoxypropionic acid can be performed stepwise continuously so, that the time needed for separating one batch is short. The next separable batch of substance mixture can be charged to the chromatographic column soon after the previous one so that the outcoming, optically pure enantiomers will not get mixed to it. The separable batches of substance mixture can thus be typically charged to the chromatographic column at intervals of some minutes. This speeds up the purification process considerably in comparison with known methods. The benefit from large production rate is the drastic reduction of equipment sizes. 
     One of the advantages of this invention is also that the optical purity of the obtained enantiomers can be almost freely chosen and it can be optimized according to the requirements of the quality of the product and the economy. 
     Moreover it is an advantage, that the whole manufacturing process is simple. It contains only three main phases: the dissolving of the racemic mixture in carbon dioxide, the chromatographic separation of the enantiomers and the separation of the optically pure product from carbon dioxide. No reagents are needed and the whole purifying process can be performed by using only one solvent, carbon dioxide. Using only one solvent results the significant reduction of producing costs. 
     One advantage of the method is that carbon dioxide used as a solvent can very simply be recirculated for reuse. By reducing the pressure of carbon dioxide the optically pure derivatives of epoxypropionic acid precipitate and they can be simply separated from carbon dioxide by allowing carbon dioxide to evaporate under atmospheric pressure. 
     One advantage of the method is also that carbon dioxide does not leave any residues in the final product, as there always happens when using the previously known methods. Therefore the method according to this invention also advantageously improves the quality of the product. 
     Carbon dioxide separated from the product is evaporated and led after elevating the pressure and adjusting the temperature, to the dissolving of a new racemic, raw material batch. Due to the low heat of evaporation of carbon dioxide, the energy needed for recycling of carbon dioxide is only a fraction of the energy needed for redistillation of organic solvents. Therefore the method according to this invention brings significant reduction of producing costs. 
     One of the benefits of this invention is also that carbon dioxide used as a solvent is inexpensive, incombustible and non-toxic. This brings savings in explosion protection of the equipment and buildings and in controlling the hazards of solvent effluents. Thus the improved occupational safety is also an advantage of this method. The plant using carbon dioxide as a solvent does not cause harmful solvent effluents to the environment. 
     It is characteristic of the method according to this invention that the racemic derivative of epoxypropionic acid is dissolved in carbon dioxide under elevated pressure. The obtained solution is charged to the chromatographic column at 0°-120° C. temperature, most preferably at 30°-60° C. and under elevated pressure, most preferably 150-300 bar. The continuous eluant flow passes through the chromatographic column. The eluant is carbon dioxide, where a small amount of a modificator, most preferably 0.1-1 weight-%, most preferably small molecular alcohol or water, has been added to, if needed. The chromatographic column has been packed with solid packing material. The packing material has been prepared by coating suitable particles, most preferably silica gel particles, with suitable chiral materials, most preferably with cellulose esters or cellulose carbamates. 
     The composition of the eluant flow coming out from the chromatographic column is observed with a suitable detector and the flow is divided in successive parts so that one fraction contains the product, in other words the desired optical isomer of the derivative of epoxypropionic acid of the desired purity. The eluant flow coming out from the chromatographic column can further be divided, for example, into two fractions. The other fraction can still contain a considerable amount of the desired optical isomer and it can be returned to the racemic mixture to be charged to the column. The last fraction contains almost exclusively the non-desired optical isomer from the racemic mixture and it is removed. 
     Batches of the racemic derivatives of epoxypropionic acid dissolved in carbon dioxide are charged into the chromatographic column consecutively so that the more quickly eluting isomer of the batch will not get mixed with the slowest eluting isomer of the previous batch. 
     The pressure of the fractions of the eluant flowing out from the chromatographic column is reduced and/or the temperature is elevated so that the derivatives of epoxypropionic acid dissolved in them precipitate. Most preferably the eluant flow is allowed to expand adiabatically to the pressure of 50-60 bar. The precipitated derivatives of epoxypropionic acid are transferred with the eluant flow to pressure vessels where the carbon dioxide eluant is evaporated and the product is led into atmospheric pressure. 
     The carbon dioxide eluant evaporated in pressure vessels is led, after purifying and liquifying, if needed, to the inlet of the chromatographic column. 
    
    
     The following examples clarify the invention. 
     EXAMPLE 1 
     The racemic methyl ester of (p-methoxyphenyl)epoxypropionic acid was dissolved in carbon dioxide in pressure vessel at 40° C. under the pressure of 260 bar. The concentration of the dissolved ester in the carbon dioxide phase was 8 weight-%. 
     The carbon dioxide solution containing the methyl ester of (p-methoxyphenyl)epoxypropionic acid obtained from the dissolving vessel was conducted for ten seconds to the carbon dioxide flow, the temperature of which was 40° C. and the pressure 250 bar. The obtained mixture was led to the chromatographic column filled with silica gel particles coated with cellulose-tris-(3,5-dimethylphenylcarbamate). 
     Part of the eluant flow coming out from the chromatographic column was conducted continuously to the detector (FID). According to the signal given by flame ionization detector, the batch of the methyl ester of racemic (p-methoxyphenyl)epoxypropionic acid charged to the chromatographic column was divided in the column almost completely into two pure enantiomers, which came out from the column after 24 minutes and at intervals of 4 minutes. 
     EXAMPLE 2 
     The packing material of the chromatographic column and the conditions in the arrangements described in example 1 were varied. The methyl ester of (p-methoxyphenyl)epoxypropionic acid was still used as a separable racemic mixture. 
     
         ______________________________________                                    
                                       Interval                           
Experi-                         Retention                                 
                                       between                            
ment  Temp-   Pres-             time of                                   
                                       the optical                        
num-  erature sure   Packing material                                     
                                first peak                                
                                       isomer                             
ber   °C.                                                          
              bar    of the column                                        
                                min    peaks min                          
______________________________________                                    
1     120     100    crown ether                                          
                                 9     0                                  
2     120     100    brancod    18     0                                  
                     polysiloxane                                         
3     60      150    microcrystalline                                     
                                45     1                                  
                     cellulosetriacetate                                  
4     40      250    microcrystalline                                     
                                10     4                                  
                     cellulosetriacetate                                  
5     40      200    cellulose-tris-(3,5-                                 
                                40     6                                  
                     dimethyl-phenyl)                                     
                     carbamate                                            
6     35      300    cellulose-tris-(p-                                   
                                22     7                                  
                     methylphenyl)                                        
                     carbamate                                            
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     EXAMPLE 3 
     The arrangements described in example 1 were varied regarding to the eluant flow entering to the chromatographic column. Cellulose-tris-(3,5-dimethylphenylcarbamate) was used as packing material of the chromatographic column. 
     
         ______________________________________                                    
                        Retention time                                    
Experi-         Temp-   of the first                                      
                                  Interval between                        
ment   Pressure erature eluating  the enantiomer                          
number bar      °C.                                                
                        enantiomer min                                    
                                  peaks min                               
______________________________________                                    
 7     180      40      48        8                                       
 8     200      40      40        6                                       
 9     250      40      23        4                                       
10     300      40      22        3                                       
11     200      30      22        6                                       
12     250      30      22        4                                       
13     200      20      17        0                                       
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     EXAMPLE 4 
     The arrangements described in example 1 were varied regarding to the amount of the racemic mixture charged to the chromatographic column. 
     
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                   Retention                                              
      Amount of the                                                       
                   time of   Interval                                     
Experi-                                                                   
      charged rasemic                                                     
                   the first between                                      
ment  mixture g/kg of                                                     
                   eluating  the     Measured                             
num-  the column&#39;s enantiomer                                             
                             enantiomer                                   
                                     resolution                           
ber   packing material                                                    
                   min       peaks min                                    
                                     R.sub.M                              
______________________________________                                    
14    0.003        23        4       1.44                                 
15    0.01         39        5       0.92                                 
16    0.02         21        3       0.94                                 
17    0.04         24        3       0.68                                 
18    0.08         21        3       0.56                                 
19    0.16         23        3       0.58                                 
20    0.32         22        2       0.38                                 
21    1            22        1.9     0.31                                 
22    5            21        1.5     0.22                                 
23    10           21        1.6     0.18                                 
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     Resolution has been used as measure in evaluating the experimental results presented in example 4. The measured resolution Rs is the difference between the retention times of the eluted peaks divided with the bottom width of the latter eluting peak. 
     There is a certain dependance between the obtained resolution results and the amount of the charged racemic mixture in the experiments described in example 4. When the resolution is very low, the eluted peaks of the optically pure isomers almost overlap and the amount of the pure isomer obtained as product from the charged amount of racemic mixture is small. The economic manufacturing process requires that the resolution is at least in the range of 0.1-0.2.