METHOD FOR INCREASING THE ALPHA TO BETA RATIO IN AN ASPARTAME COUPLING REACTION

Abstract: An improved coupling step of an alpha -APM manufacturing process is disclosed. The solvents in the coupling step are adjusted in one of two ways to improve the alpha - to beta - isomer ratio of the product of the coupling step, alpha , beta protected L-aspartyl-L-phenylalanine or lower alkyl ester thereof. In one manner, the solvent may comprise an increased concentration of a carboxylic acid such as acetic acid alone. In the other manner, one or more additional solvents along with a carboxylic acid such as acetic acid in conventionally known concentrations are used.

Description: METHOD FOR INCREASING THE ALPHA*TO BETA RATIO IN AN ASPARTAME COUPLING REACTION This application is a continuation-in-part application of U.S. Serial No. 07/219,613, filed on July 14, 1988, which is incorporated by reference herein. BACKGROUND OF THE INVENTION This invention relates to methods of manufacturing L-aspartyl-L- phenylalanine methyl ester hydrochloride (APM(HCl)), which is used to prepare alpha-L-aspartyl-L-phenylalanine methyl ester (α-APM), a sweetening agent which is approximately 200 times sweeter than sucrose. More particularly, this invention relates to methods for increasing the ratio of α to β isomers in a process for manufacturing L-aspartyl-L-phenylalanine methyl ester hydrochloride. In a conventional method for manufacturing α-APM, L-phenylalanine and N-formyl aspartic anhydride are coupled in the presence of acetic acid to yield an isomeric mixture of α, β-N-formyl-L- aspartyl-L-phenylalanine. This mixture is subsequently deformylated and esterified to produce α-APM(HCl), which is used to prepare α-APM. In conventional manufacturing methods, the coupling of L-phenylalanine and N-formyl aspartic anhydride produces an isomeric mixture having an α to β ratio of about 3.0-3.8 to 1. This coupling typically takes place in the presence of 11-16 moles of acetic acid per mole of L-phenylalanine as a solvent. Because the β isomer is not sweet, it must be removed from the mixture. Thus, it is desirable to have a minimal amount of the β isomer. However, attempts to minimize the amount of β isomer must not make the process cost inefficient. Conventionally, the coupling must be carried*out at elevated temperatures so that the reaction slurry does not solidify. However, the elevated temperature decreases the selectivity of the reaction, which reduces the α β ratio. Thus, an improved yield of the α-isomer in the coupling step of an APM manufacturing process is desired. The present invention teaches such a process which produces such an improved yield. SUMMARY OF THE INVENTION The present invention teaches an improved N-formyl aspartic anhydride and L-phenylalanine or its lower alkyl ester coupling step which results in an improved α to β ratio. The coupling step may take place in the presence of an increased amount of acetic acid, or in the presence of one or more solvents and a straight chain or branched C 2 -C 10 carboxylic acid, preferably acetic acid. Both the addition of one or more solvents and an increased amount of the carboxylic acid improve the ratio of α to β isomer in the product, N-formyl-L-aspartyl-L-phenylalanine or lower alkyl esters thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In a preferred embodiment, the coupling step of an α-APM manufacturing process is conducted in the presence of an amount of acetic acid greater than 50 moles per mole of L-phenylalanine or in the presence of a C 2 -C 10 carboxylic acid, preferably acetic acid, in any amount along with one or more additional solvents. In one APM manufacturing process, a protected L-aspartic anhydride such as N-formyl-L-aspartic anhydride is first produced, typically by formylating L-aspartic acid in a reaction mixture of formic acid and acetic anhydride. The N-formyl-L- aspartic anhydride is then coupled with L-phenylalanine or a lower alkyl ester thereof ("L-Phe"). The reaction step is typically carried out in the presence of acetic acid, which acts as a solvent. The acetic acid has conventionally been added in amounts of 11-16 moles per mole of L-phenylalanine or lower alkyl ester thereof. The coupling step produces an isomeric mixture of α, β-N-formyl-L-aspartyl-L- phenylalanine or lower alkyl ester thereof, with an α- isomeric yield of 65-75% of the mixture considered successful. The isomeric mixture is then subjected to additional steps to produce α-ΔPM hydrochloride. The mixture is deblocked by adding an effective amount of hydrochloric acid. Residual acetic acid and an acid such as formic acid are then removed from the mixture. The deformylated isomers are then esterified by addition of an effective amount of methanol, water, and hydrochloric acid to yield α- and β- APM hydrochloride. The α-APM hydrochloride is then precipitated out of the mixture. The β- isomer, which is present in relatively high amounts, can then be recovered. It has been discovered that the α- to β- isomer ratio, and thus, the resulting yield, can be increased during the step in which N-formyl-L-aspartic anhydride and L-phenylalanine are coupled. The yield can be improved by adjusting the solvents in one of two manners. In one method, acetic acid is added in an amount of between at least 50 to about 250 moles per mole of L-phenylalanine. In the second method, at least two different solvents, one of which is a C 2 -C 10 carboxylic acid, preferably acetic acid, are used. An amount of two or more solvents including a carboxylic acid similar to or greater than that conventionally used (11-16 moles per mole of Phe or lower alkyl ester thereof) is preferred. It has surprisingly been found that a minimal amount of an additional solvent is effective to significantly increase the α to β ratio of the coupling product. It has also surprisingly been found that the α to β ratio continues t-o increase as the amount of solvents used increases. The process works equally well in the coupling of benzyloxycarbonyl-aspartic anhydride and L-phenylalanine or lower alkyl esters thereof. This process is especially preferred in a coupling process with L-phenylalanine methyl ester. The benzyloxycarbonyl group is removed by hydrogenation and the α-isomer is similarly precipitated. Any organic solvent may be used as an additional solvent in the process. The solvents may be selected from the group including, but not limited to hydrocarbons, aromatic hydrocarbons, halocarboπs, secondary or tertiary alcohols, ketones, esters, ethers, lower alkyl phosphates, and nitriles. One of the additional solvents is preferably an ether. The additional solvent or solvents used with the carboxylic acid are preferably selected from the group including, but not limited to methyl tert-butyl ether, ethyl tert-butyl ether, toluene, xylene, cymene, ether, anisole, furfurylacetate, methyl ethyl ketone, 2-propanol, and triethyl phosphate. Methyl tert-butyl ether is an especially preferred solvent. The solvents may be added in any amount relative to each other to produce an increased molar ratio. A preferred range of carboxylic acid to second solvent molar ratios is 1:10 to 25:1. An especially preferred range of carboxylic acid to second solvent molar ratio is 1:2 to 2:1. The minimum amount of total solvent added is preferably 10 moles per mole of L-phenylalanine or lower alkyl ester thereof, with no real maximum limitation. In fact, increased amounts of the solvents show increased yield of the α-isomer. However, it may become cost-ineffective or process facilities may not be large enough to accommodate large quantities of solvent. A preferred total solvent amount is 144 to 723 moles of total solvent per mole of L-phenylalanine or lower alkyl ester thereof. Especially preferred is a combination of 76.8 moles of MTBE and 76.8 moles of acetic acid per mole of L-phenylalanine. The coupling reaction may be carried out at temperatures as low as -15°C. This low temperature apparently reduces the rate of reaction which results in an additional increase in the α to β ratio of the coupling product. A preferred temperature range is -5°C to 50°C. The following examples detail the effects of dilute coupling, and the use of additional solvents on the α- to β- isomer ratio of the N-formyl-L-aspartyl L- phenylalanine and the yield of the α- product. EXAMPLE 1—DILUTED COUPLING OF N-FORMYL-L-APSARTIC ANHYDRIDE AND L-PHENYLALANINE (ACETIC ACID ONLY SOLVENT) In each of the following couplings, acetic acid and L-phenylalanine were added in the amounts below. The coupling reaction was carried out at the temperatures shown below for 5 hours. Run # AcOH : LPhe Temperature α to β Ratio (Mole Ratio) e C 1 11.1:1 20-25 75.0/25.0 2 127.3:1 20-25 81.3/18.7 3 233.3:1 20-25 83.5/16.5 4 233.3:1 50 81.2/18.8 5 1166.7:1 20-25 84.0/16.0 The α-N-formyl-L-aspartyl-L-phenylalanine isolated yield calculated from Run #4 was 80.3 molar percent from L-Phe with over 96% purity of the α- isomer in the filter cake. EXAMPLE 2—DILUTED COUPLING OF N-FORMYL-L-ASPARTIC ANHYDRIDE AND L-PHENYLALANINE (ACETIC ACID & SECOND SOLVENT) In each of the following couplings, 36.6 ml. of acetic acid was added per gram of L-phenylalanine and 80.6 ml. of the second solvent was added per gram of L-phenylalanine. The coupling reaction wήs carried out for 5 hours at 5°C and 17 hours at 20-22°C. Second Solvent a/β Ratio α-FAP yield Hexane 3.52 77.90 Cyclohexane 4.36 81.35 Benzene 4.04 80.16 Toluene 6.26 86.23 Xylene 6.02 85.80 Mesitylene 6.34 86.37 Tetramethyl benzene 6.24 86.18 Ethyl benzene 6.36 86.42 Diethyl benzene 6.05 85.82 Cumene 6.05 85.82 Cymene 5.67 85.01 Dichloroethylene 4.12 80.45 Chloroform 4.72 82.53 Carbon tetrachloride 5.89 85.49 2-Propanol 4.18 80.72 Methylethylketone 4.16 80.65 Methyl acetate 5.21 83.90 Ethyl acetate 3.63 78.40 Ether 6.98 87.47 Methyl t-butylether 8.15 89.07 Dimethoxyethylenglycol 6.14 86.00 Diethoxyethylenglycol 6.41 86.51 t-Butoxy-2-methoxyethane 5.90 85.50 t-Butoxy-2-ethoxyethane 6.92 87.40 2-Methoxyethylether 4.37 81.38 1.4 Dioxan 7.17 87.80 Tetrahydrofuran 3.89 79.57 Furfuryl acetate 5.06 83.50 Acetonitrile 3.57 78.13 Triethylphosphate 4.47 81.73 EXAMPLE 3- DILUTED COUPLING OF N-FORMYL-L-ASPARTIC ANHYDRIDE AND L-PHENYLALANINE WITH SECOND AND THIRD SOLVENTS In each of the following couplings, 36.6 ml. of acetic acid was added per gram of L-phenylalanine, and 40.3 ml. of the second and third solvents was added per gram of L-phenylalanine. The reaction temperature was 5°C for 5 hours and 20-22°C for 17 hours. Second solvent Third Solvent α β Ratio α-FAP Yield 1.4 Dioxane Toluene 8.22 89.16 1.4 Dioxane Xylene 8.28 89.23 Dimethoxyethylenglycol Toluene 6.70 87.02 Dimethoxyethylenglycol Xylene 5.96 85.64 Dimethoxyethylenglycol Cymene 6.34 86.37 Diethoxyethylenglycol Toluene 7.03 87.60 Diethoxyethylenglycol Xylene 6.85 87.20 Methyl t-butylether Toluene 7.46 88.18 Methyl t-butylether Xylene 7.98 88.90 EXAMPLE 4—DILUTED COUPLING OF N-FORMYL-L-ASPARTIC ANHYDRIDE AND L-PHENYLALANINE VITH METHYL TERT-BUTYL ETHER Acetic acid and methyl tert-butyl ether were added in a variety of concentrations to a coupling reaction of L-phenylalanine and N-formyl-L-aspartic anhydride at 20°C. The mixture was stirred at this temperature for 30 minutes, then agitation was stopped and the mixture was left at 20°C for 3 hours. The crystals of the isomeric mixture of the reaction product precipitated out and the slurry was agitated for two more hours and a sample of the slurry was taken. Molar Ratio Molar Ratio α-N-formyl-aspartyl- α β AcOH/L- •Phe MTBE/L-Phe phenylalanine Ratio 11.1/1 1.79/1 79.5 3.87 11.1/1 2.73/1 79.9 3.97 11.1/1 5.47/1 84.5 5.42 11.1/1 10.93/1 86.3 6.57 EXAMPLE 5—COUPLING OF N-FORMYL-L- ASPARTIC ANHYDRIDE AND PHENYLALANINE METHYL ESTER (ACETIC ACID & METHYL TERT-BUTYL ETHER) L-phenylalanine methyl ester (3 g. , 0.017 moles) was dissolved in methyl tert-butyl ether (164 ml., 1.410 moles) and 82 ml. (1.435 moles) of acetic acid. This solution was cooled and 2.66 g. (0.0186 moles) of N-formyl aspartic anhydride was added. The mixture was stirred at 5°C for 5 hours. The resulting N-formyl-L-aspartyl-L-phenylalanine methyl ester mixture contained 88.3% of the α- isomer and 11.7% of the β- isomer. EXAMPLE 6—COUPLING OF N-BENZYLOXYCARBONYL L-ASPARTIC ANHYDRIDE AND PHENYLALANINE METHYL ESTER (ACETIC ACID & METHYL TERT-BUTYL ETHER) N-Benzyloxycarbonyl aspartic anhydride (4.33 g. , 0.0174 moles) was added to a mixture of methyl tert-butyl ether (150 ml., 1.292 moles) and acetic acid (82 ml., 1.435 moles). This mixture was cooled to 5°C. A solution of 3 g. (0.0167 moles) of L-phenylalanine methyl ester and 14 ml. (0.12 moles) of methyl tert-butyl ether was added. The resulting mixture was stirred for 3 hours at 5°C. The resulting N-Benzyloxycarbonyl-L-aspartic L-phenylalanine methyl ester mixture contained 83.9% of the α- isomer and 16.1% of the β- isomer. EXAMPLE 7—COUPLING OF N-FORMYL-L-ASPARTIC ANHYDRIDE AND L-PHENYLALANINE (PROPIONIC ACID AND METHYL TERT-BUTYL ETHER) L-phenylalanine (2.73 g. , 0.0165 moles) was added to a mixture of propionic acid (100 ml., 0.9 moles) and methyl tert-butyl ether (200 ml., 1.72 moles). The resulting mixture was cooled to 5°C and 2.65 g. (0.019 moles) of N-formyl aspartic anhydride was added. The mixture was stirred at 5°C for 5 hours. The resulting N-formyl-L-aspartyl-L-phenylalanine yielded 86.8% of the α- isomer and 13.2% of the β- isomer.

Claims: WHAT IS CLAIMED IS: 1. A method of producing an isomeric mixture of α, β protected L-aspartyl-L-phenylalanine or lower alkyl ester thereof comprising: coupling a protected aspartic anhydride and L-phenylalanine or said lower alkyl ester in the presence of a C 2 to C 10 carboxylic acid in an amount greater than about 50 moles per mole of L-phenylalanine or lower alkyl ester. 2. The method of Claim 1 wherein said protected aspartic anhydride is N-formyl aspartic anhydride. 3. The method of Claim 1 wherein said protected aspartic anhydride is benzyloxycarbonyl aspartic anhydride. 4. The method of Claim 1 wherein said carboxylic acid is acetic acid. 5. The method of Claim 1 wherein said protected L-aspartyl-L- phenylalanine or lower alkyl ester thereof comprises at least 80% of the α isomer. 6. The method of Claim 1 wherein said lower alkyl ester of L-phenylalanine is L-phenylalanine methyl ester. 7. A method of producing an isomeric mixture of α, β protected L-aspartyl-L-phenylalanine or lower alkyl ester thereof comprising: coupling a protected aspartic anhydride and L-phenylalanine or a lower alkyl ester thereof in the presence of a first solvent comprising a C 2 -C 10 carboxylic acid and at least one additional solvent selected from the group consisting of hydrocarbons, aromatic hydrocarbons, halocarbons, secondary alcohols, tertiary alcohols, ketones, esters, ethers, lower alkyl phosphates and nitriles, said additional solvent added in an amount to produce a total solvent molar amount effective to produce a ratio of α-isomer to β-isomer greater than the ratio of α-isomer to β-isomer produced by the use of an equivalent molar amount of said carboxylic acid alone. 8. The method of Claim 7 comprising the addition of at least two additional solvents. 9. The method of Claim 7 wherein said carboxylic acid is acetic acid. 10. The method of Claim 7 wherein said carboxylic acid is propionic acid. 11. The method of Claim 7 wherein said additional solvent is selected from the group consisting of methyl tert-butyl ether, ethyl tert-butyl ether, hexane, cyclohexane, benzene, toluene, xylene, mesitylene, tetramethyl benzene, ethyl benzene, diethyl benzene, cumene, cymene, dichloroethylene, chloroform, carbon tetrachloride, 2-propanol, methylethyl ketone, methyl acetate, ethyl acetate, ether, dimethoxyethylenglycol, diethoxyethylenglycol, t-butoxy-2-methoxyethane, t-butoxy-2- ethoxyethane, 2-methoxyethylether, 1.4 dioxane, tetrahydrofuran, furfuryl acetate, acetonitrile, and triethylphosphate. 12. The method of Claim 7 wherein said additional solvent comprises methyl tert-butyl ether. 13. The method of Claim 7 wherein said protected L-aspartyl-L- phenylalanine or lower alkyl ester thereof comprises at least 77% of the α-isomer. 14. The method of Claim 7 wherein said lower alkyl ester of L-phenylalanine comprises L-phenylalanine methyl ester. 15. The method of Claim 7 wherein said coupling is carried out in the presence of a total amount of acetic acid and additional solvent which is at least 6 moles per mole of L-phenylalanine. 16. The method of Claim 15 wherein the ratio of acetic acid to additional solvent is in the range between 1:10 and 25:1. 17. The method of Claim 16 wherein the ratio of acetic acid to additional solvent is in the range between 1:2 and 2:1. 18. The method of Claim 15 wherein the total solvent amount comprises about 144 to 723 moles of solvent per mole of L-phenylalanine or lower alkyl ester thereof. 19. The method of Claim 18 wherein the total solvent amount comprises about 154 to 308 moles of solvent per mole of L-phenylalanine or lower alkyl ester thereof. 20. The method of Claim 7 wherein said coupling reaction is carried out at a temperature in the range of -15°C to 50°C. 21. The method of Claim 20 wherein said coupling reaction is carried out at a temperature in the range of -5°C to 25°C.

Inventors: GELMAN YEFIM, GELMAN, YEFIM
Applicants: NUTRASWEET CO, THE NUTRASWEET COMPANY
Application year: 1991
Main classification: C07C229
IPC codes: C07C9, C07C229, C07K5