PROCESS FOR PRODUCING CHIRAL COMPOUNDS AS PURE OPTICAL ISOMERS

Abstract: A process for producing chiral compounds of high optical purity wherein enzymes catalyse a reaction between chiral and non-chiral starting materials in supercritical carbon dioxide so that principally only one of the optically active isomers of the chiral starting material reacts, and produces a new chiral compound which has higher optical purity than the starting material. By using supercritical carbon dioxide as the reaction medium the problems and risks of handling large amounts of organic solvents can be avoided.

Description: PROCESS FOR PRODUCING CHIRAL COMPOUNDS AS PURE OPTICAL ISOMERS The object of the present invention is a production method for optically active 5 isomers of chiral compounds wherein enzymes catalyze a reaction between a chiral and a non-chiral starting material in supercritical carbon dioxide so that mainly only one of the optically active isomers of the chiral starting material reacts and produces a new chiral product which has a higher enantiomeric purity than the optical purity of the starting material. 10 A considerable amount (40 %) of the produced pharmaceuticals today are molecules which contain one or more chiral centers. Also, a large part of the agricultural pesricides and herbicides are chiral compounds. It is known that generally only one of the optically active isomers of a chiral compound, 15 enantiomer, has the desired pharmacological or biological effect. However, only about 12 % of pharmaceuticals are currently produced as pure optical isomers. The major part is produced and used as racemic mixtures. The non-desired enantiomers of a racemic mixture are often without any 20 pharmaceutical effect. Some may cause side effects and in the worst case they may be poisonous. Apparently the major part of chiral pharmaceuticals and agrochemicals should be produced and used as pure optical isomers. New legislation and development of new, more economical production techniques bring demands and possibilities particularly to produce new chiral 25 pharmaceuticals as pure optical isomers. In prior art a method is known wherein optically active compounds are synthesized, without the use of enzymes, by using intermediates produced through asymmetric epoxidation (For example: K.B. Sharpless, Chemtech Nov. 30 1985, page 682). The production of these chiral reagents is complicated and they are very expensive. The whole method is expensive and this is a partial reason for not producing pharmaceuticals and agrochemicals more widely as pure optical isomers. Equally known is to produce enantiomerically pure chiral compounds by separating the enantiomers of a racemic mixture by chromatographic methods. ( G.Gϋbitz . Chromatographia Vol. 30, No 9/10, 1990, 555 - 564). Liquid chromatography where a mixture of organic solvents are used as the eluent is 5 available for this purpose. Several chiral stationary phases may be used. However, a general purpose chiral stationary phase has not been developed. Therefore a specific chiral separation method has to be developed for each separation case. In a liquid chromatographic production method one has to use large amounts of organic solvents which are flammable, they pose an occupational health risk and they require extensive precautions to avoid environmental problems. They also leave small solvent residues in the products. The desired optically pure product has to be separated from the eluent by evaporating the solvents. The solvents have to be purified prior to reuse by distillation which consumes large amounts of energy. It is further known to make use of the stereospecificity of enzymes to catalyze the synthesis of a certain enatiomer of a chiral compound in organic solvents. (M.Schneider, Performance Chemicals, April 1989, 28 - 31). The solvents which are used in this method are flammable, toxic and the products have to be recovered from the solvents by distillation much in the same way as in the liquid chromatographic separation methods of a racemic mixture. Large amounts of very dilute organic solutions have to be processed in this method. However, it has recently been discovered that many enzymes catalyze non-enantiospecific reactions not only in organic solvents but also in supercritical carbon dioxide. (For example US Pat. 4 925 790 and A. Marty et.al., Biotechnology Letters, Vol 12, No 1, 1990, 11 - 16). Supercritical carbon dioxide is defined as carbon dioxide in the state where its temperature is higher than the critical temperature (32 C) and pressure higher than the critical pressure (72 bar). In this supercritical state carbon dioxide can be used as solvent for volatile and hydrophobic compounds. Supercritical carbon dioxide is a non-toxic, non-flammable and relatively cheap solvent. By using supercritical carbon dioxide as reaction medium one can avoid the occupational protection and effluent problems and the risks of fire associated with the use of large amounts of organic solvents. A further advantage is that the recovery of carbon dioxide for reuse consumes only a fraction of the energy which is required to recover organic solvents by distillation. We have discovered that certain enzymes can selectively catalyze the reactions of only one enantiomer of a chiral compound in supercritical carbon dioxide. This discovery provides a new production method for the pure optically active isomers of chiral compounds. The purpose of the present invention is to provide a new, more economical method for the production of optically active isomers of chiral compounds. The object of the invention is a method wherein, differing from previous methods, supercritical carbon dioxide is used as a reaction medium in enzymatically catalyzed enantiomerically specific reactions. The advantages of the present invention include that only one, non-flammable, non-toxic and cheap solvent, carbon dioxide, is used as solvent for the starting materials, as a reaction medium in enzymatically catalyzed enantiomerically specific reactions and possibly also in the fractionation of the reaction mixture and in the purification of the products. Another advantage of the present invention is that carbon dioxide provides an atmosphere which protects the starting materials and products from oxidation. A further advantage of the present invention is that the solvent power of supercritical carbon dioxide can be tuned. By this we mean that the solvent power of supercritical carbon dioxide is strongly dependent on its density. This property makes it possible to precipitate the reaction mixture from supercritical carbon dioxide simply by lowering the pressure of the supercritical solution. In this way one can separate the carbon dioxide from the reaction mixture and the carbon dioxide can then be circulated for reuse with very small energy consumption. A still further advantage of the present invention is that supercritical carbon dioxide has very good mass-transfer properties compared to liquid solvents. This property of carbon dioxide accelerates the dissolution of starting materials and promotes the reactions which are controlled by mass transfer. Another advantage of the present invention is that the production process is virtually closed. There are no solvent effluents, the production of waste water is very small, the process, is well protected from external microbe 5 contamination and the parts of the process may be easily sterilized when necessary. Carbon dioxide does not leave any harmful solvent residues. One advantageous embodiment of the invention is the resolution of a racemic mixture of a chiral compound. In this embodiment the racemic mixture and a non-chiral reagent are brought to contact with enzymes so that principally only one of the enatiomers of the racemic mixture reacts. The obtained chiral reaction product is enantiomerically pure and is physico-chemical ly so different from the racemic mixture that it can be separated from the reaction mixture by conventional methods. For example by extraction, crystallization or evaporation. Other optically pure compounds can then be produced from the obtained enantiomerically pure product by conventional chemical methods. A still further advantage of the present invention, when compared to e.g. resolution made with chiral reagents, is that cheap, simple compounds like carboxylic acids or alcohols can be used instead of the expensive chiral reagents. The following examples describe some of the preferred embodiments of the present invention. Example 1. 4.9 g of a lipase (EC 3.1.1.3) produced by Mucormiehθi yeast, immobilized on a ionexchange resin, was weighed into a 250 ml pressure vessel, which was closed and rinsed with atmospheric pressure carbon dioxide to remove air. The pressure vessel was warmed to 50 C and it was pressurized with carbon dioxide to 150 bar. 0.3 cc water was fed to the pressure vessel with carbon dioxide along with 45 mmol racemic ibuprofen and 105 mmol propanol, butanol or amylalcohol. The contents of the pressure vessel was agitated with a magnetic stirrer. Small samples of the reaction mixture were taken during the reaction and they were analyzed using gas chromatography and liquid chromatograhy using chiral packed columns. After the experiment the contents of the pressure vessel was directed through a pressure reduction valve into another vessel which was kept in atmospheric pressure. During pressure reduction the components of the reaction mixture precipitated from carbon dioxide. They formed droplets which settled to the bottom of the vessel. The carbon dioxide which was the solvent for the reaction mixture evaporated and was removed from the top of the vessel. According to analysis results the esterification rate of the S-enantiomer of ibuprofen with these alcohols was approximately nine times the rate of esterification of the R-enantiomer of ibuprofen. Table 1. The enantiomeric purity of the S-ibuprofen propylester formed in the lipase catalyzed reaction between racemic ibuprofen and alcohols after 25 % of the racemic ibuprofen had reacted. Example 2 The procedure of example 1 was repeated varying the amount of water which was fed to the reactor along with carbon dioxide. This was done to find the optimum moisture for the reaction rate and selectivity. Starting materials for the reaction were racemic ibuprofen and propanol. The results are collected in table 2. According to the results an advantageous water content for the reaction rate is 0.35 - 1.3 mL water in a litre of carbon dioxide. The water content had no effect on the enantiomeric purity of the produced ibuprofen ester. Table 2. The effect of water content of supercritical carbon dioxide on the conversion rate of racemic ibuprofen and on the enantiomeric purity of the resulting chiral ibuprofen ester in lipase catalyzed esterification reactions between ibuprofen and propanol. Example 3. In the procedure of example 1 , the concentration of non-chiral propanol, which reacted with the racemic ibuprofen, was varied in order to find out the effect of propanol excess on the ibuprofen reaction rate and on the enantiomeric purity of the produced ester. In these experiments the concentration of racemic ibuprofen was 12 mmol per litre. The results are collected in table 3. In this set of experiments the advantageous propanol excess was 3 - 16 mol propanol / mol ibuprofen. The propanol excess had no decisive effect on the enantiomeric purity of the obtained S-ibuprofen ester. Table 3. The effect of propanol concentration on the conversion rate of racemic ibuprofen and on the enantiomeric purity of the produced S-ibuprofen ester in the lipase catalyzed esterification reaction. Example 4. In the procedure of example 1 the reaction temperature and pressure were varied. The effect on the rate of reaction between ibuprofen and propanol and on the enantiomeric purity of the obtained S-ibuprofen ester is shown in table 4. Table 4. Temperature and pressure effects on the rate of reaction between racemic ibuprofen and propanol and on the enantiomeric purity of the product in lipase catalyzed esterification reaction. Example 5. In the procedure of example 1 the lipase-group (EC 3.1.1.3) enzyme was varied. 5 The starting material for the reaction was racemic ibuprofen and amyl alcohol. The results are shown in table 5. Table 5. The effect of enzyme on the rate of reaction between racemic ibuprofen and amylacohol and on the enantiomeric purity of the produced S-ibuprofen amyl ester. Example 6. The procedure of example 1 was repeated except that 45 mmol 2-octanol was used as the chiral, racemic starting material and 105 mmol hexanoic acid as the non-chiral reagent. The production rate of the produced ester was 12.5 mmol ester per kg lipase and hour. The enantiomeric purity of the obtained R-2-octylester was 84 % when 50 % of the racemic 2-octanol had reacted. Example 7. The procedure of example 1 was repeated except that 45 mmol ibuprofen propylester was used as the chiral, racemic starting material and 105 mmol amylalcohol as the non-chiral reagent. The rate of the alcoholysis reaction was 17.5 mmol ibuprofen amylester per kg lipase and hour. The enantiomeric purity of the obtained S-ibuprofen amylester was 81 % when 25 % of the racemic starting material had reacted.

Claims: Claims: 1. A method for producing optically active isomers of chiral compounds, said method comprising the steps of: a) one or more compounds, each having at least one chiral center and one or more non-chiral compounds, which react with these chiral compounds, are dissolved in supercritical carbon dioxide, which can contain other non-reacting components; and b) the obtained solution of chiral and non-chiral compounds in supercritical carbon dioxide is brought to contact with enzymes so that principally only one of the stereoisomeric forms of the chiral compounds reacts with the non-chiral compounds. 2. A method as claimed in claim 1 wherein said chiral compound, which is dissolved in supercritical carbon dioxide, forms a racemic mixture. 3. A method as claimed in claim 1 wherein the chiral compounds which are dissolved in supercritical carbon dioxide are carboxylic acids, alcohols or esters. 4. A method as claimed in claim 1 wherein the chiral compound which is dissolved in supercritical carbon dioxide is racemic ibuprofen. 5. A method as claimed in claim 1 wherein the non-chiral compounds which are dissolved in supercritical carbon dioxide is carboxylic acid, alcohol or ester. 6. A method as claimed in claim 1 wherein supercritical carbon dioxide contains in addition 0.4 - 1.5 mL water per litre of carbon dioxide. 7. A method as claimed in claim 1 wherein the dissolution of chiral and non-chiral compounds, and the contact of the so obtained solution in supercritical carbon dioxide with enzymes happens in the temperature of 35 - 70 C and under 80 - 250 bar pressure. 8. A method as claimed in claim 1 wherein the enzymes, which the supercritical carbon dioxide solution is brought to contact with, belong to the lipase class (EC 3.1.1.3). 9. A method as claimed in claim 1 wherein carbon dioxide is separated from the starting materials and reaction products dissolved in it by bringing the carbon dioxide solution to pressure which is lower than 80 bar and to temperature which is lower than 35 C.

Inventors: AALTONEN OLLI, RANTAKYLAE MARKKU, AALTONEN, OLLI, RANTAKYLAE, MARKKU
Applicants: VALTION TEKNILLINEN, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1992
Main classification: C07B57
IPC codes: B01D11, C12P7, C12P41