CA1318460C - Process for the production of ó-aspartylphenylalanine derivatives - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

a-spartylphenylalanine derivatives are produced by converting a .beta.-aspar-tylphenylalanine derivative of the formula:

wherein R2 and R3 are each hydrogen or Cl 4 alkyl in a non-alcoholic solvent into an imide intermediate of the formula:

where R1 is hydrogen or Cl 4 alkyl; and thereafter converting said imide into an a-aspartylphenylalanine derivative of the following formula in the presence of a base:

Description

~ 3 ~ 0 CKGROt~ND OE~ THE INVENTION
Field of the ~nvention:
.
The present invention relates to imide compounds, to a process for the production of the imide compounds from ~-aspartylphenylalanine derivatives (~-AP
derivatives), and to a process for the production of -aspartylphenylalanine derivatives (-AP; derivatives) from the ~-AP derivatives.

Description of the Background-Many processes are known for producing ~-L-aspartyl-L-phenylalanine methyl ester (-APM) which is useful as a sweetening agent. Many of the conventional processes comprise protecting the amino group of L-asparti~ acid (L-Asp) by some means, for example, with a carboben20xy group, a formyl group, a hydrogen halide, or the like, and then dehydrating and condensing the protected a~partic acid with L-phenylalanine methyl ester to obtain an N-protected-L-aspartyl-L-phenylalanine methyl ester. Removal of the protective group then yields a-APM. However, in all of ..

~ 3 ~

these proce~ses, the formation oE ~-L aspartyl-L-phenylalanine methyl ester (~-APM) as a by-product is inevitable.
Another rnethod which i5 known involves the conversion oE a ~-aspartylphenylalanine derivative (formula (2)) into an -aspartylpherlylalanine derivative (formula (3)) and an -aspartylphenylalanine anhydride derivative (DKP derivative) as shown in the following equation. This process is described in Japanese Patent Application Laid-open No. 277696/1986.

C:H2Ph NH2fEICo2~2 NH2CHCONHCHC02R5 CH2CONHCHCO2R3 -> CH2CO2R4 ) CH2Ph (3) Although the -AP derivative may be converted into the desired a-APM easily and in high yield, when the DKP derivative is employed as a reactant, it gives only a low yield of ~-APM and large amount:s of by-products are formed in the reaction. Further, the by~products which form are changed into substances which can never be converted into -~PM. Therefore, in order to maximize conversion to ~-APM, the conversion oE
material into the DKP derivative is to ke avoided.

~ 3 ~ 6 0 SV~lMARY OF THE INVENTION
Accordingly, one ob~ect of the present invention is to provide a method of synthesi~ing -aspartylphenyla:Lanine from B-APM or ~-AP while avoidiny the Eormation of a- aspartylphenylalanine anhydride.
Br.iefly, this object and other objects o~ the present invention as hereinaEter will become more readily apparent can be attained in a method for synthesizing a-aspartylphenylalanine by converting a ~-aspartylphenylalanine derivative of the formula:

NH2 ICHCO2R2, CH2coNHfHco2R3 CH2Ph (2) where R2 and R3 are each hydrogen or Cl_4 alkyl in a non-alcoholic solvent in~o an imide intermediate of the ormula:

., ~ NCHC02Rl C~2CO
C~2Ph (1) where R1 is hydrogen or Cl_4 alkyl; and thereafter converting said imide into an a~
aspartylphenylalanine derivative of the following formula in the presence of a base ~ 3 ~

fH2Ph - -CH2Co2Ra~
(3 wherein R4 and R5 are each hydrogen or cl_~ alkyl.
~ nother aspect of the invention i.s the provision of a cyclic imide oE formula (1) prepared from ~-aspartylphenylalanine as well as a method of synthesizing the imide compound.

BRIEF DESCRIPTION OF THE DRAWING
A more complete appreciation o~ the invention and many of the attendant advantages thereo~ will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing, wherein:
the FIGURE is an NMR spectrum (60 MHz) of the imidomethyl ester intermediate of the pxesent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the process of the invention a ~-AP derivative is conver-ted into an ~-AP derivative by a first reaction in which a ~-AP derivative is placed in a non-alcoholic solvent in the presence or absence of an acid catalyst with stirring or without stirring. A reaction ~ 3 ~

occurs in which the imide c~mpound o~ formula (1) is formed as an intermediate. A second reaction is then initiated by addiny a basic catalyst to the reaction medium during the flrst reaction or after the ~irst reaction has been completed.
The ~~AP derivative which is Eorllled by the process of the present invention may be easily converted into ~-APM by any known technique. For example 7 ~-APM may be produced in the Eorm o~ the hydrochloride (-APM ~HCl) by placing the ester in an aqueous solution containing HCl and methanol (Japanese Patent Application Laid-open No. 129258/1984).
The starting material of the present process is the ~-AP derivative of formula (2). Preferred specific embodiments'oE the ~-AP derivative include B-aspartylphenylalanine dimethyl ester (~-aspartylphenylalanine methyl ester in which the -carboxyl group o~ the aspartic acid residue has been methyl esterified) (~-APMz) and R-aspartylphenylalanine -methyl ester (~-aspartylphenylalanine in which the ~-carboxyl group of the aspartic acid residue has ~een methyl esterified) (~-A(M)P). The starting material can to-lerate the presencé of ~-AP derivatives.
The ~-AP derivative of formula (2) may be produced by any known method. For example, the ~-AP derivative $ ~ ~ ~

of formula (2), where ~2 is hydrogenr may be prepared by protecting the a-carboxyl group o~ the aspartic acid by, e.g. r benzyl esterification, and then protecting the N-terminal of the amino acid with a conventionally used protecting group. The resulting compound is then reacted with a phenylalanine alkyl ester in the presence o~ a condensing ayent such as dicyclohexylcarbodiimide, thereby yielding an N-protected-aspartylphenylalanine alkyl ester and thereafter removing the N-protecting ~roup and the benzyl ester group in a conventional manner.
In the case of the ~-AP derivative where R3 is hydrogen/ the compound may be prepared by alkylating the -carboxyl group of aspartic acid, protecting the N-t~rminal of the acid with a conventinal protecting group, reacting the protected compound with phenylalanine whose ~-carboxy.l has been benzyl esterified in the presence o~ a condensing agent as in the above-described technique, and thereater removing the N-protecting group and the benzyl ester group in a conventional manner.
Still further, in the case where R2 and R3.of the ~-AP derivative are both alkyl groups, the compound may be prepared by condensing an N-protected -~- carboxyl-alkylated-aspartic acid with an ~carboxyl -alkylated-phenylalanine by the process -7- ~ 3 ~

described above, and thereafter removing the N-protectin~ group in a conventional manner.
Furthermore, where R2 and R3 oE the ~-~P derivative are both hydrogen, the compound may be prepared by the alkali saponiEication of any o~ the above three alkyl esters.
The ~-AP derivative of formula (2) may be employed as such, as an N-protected -~- aspartylphenylalanine alkyl ester (or dialkyl ester) in the reaction without removing the N-protecting group.
Further, the ~-AP derivative oE formula (2) may be in the form o~ a salt such as the hydrochloridel a sulfate or the like.
The synthesis conditions for the conversion of the compound of ~ormula (2) to the compound of formula (1) and compound of the formula (3) are clescribed below.
The solvent which is ernployed in the reaction medium is not particularly limited and may be selected from a wide group of solvents as long as alcoholic solvents ar~ excluded Erom the first-stage reaction in which the ~-AP derivative is converted to the imide oE
~ormula (1). Suitable solvents include aromatic hydrocarbons such as benzene, toluene, ~r the like;
halogenated hydrocarbons such as dichloroethane, or the like; h~drocarbons such as pentanel hexane, or the like; ketones such as acetone, methyl ethyl ketone, 1 3 ~

diethyl ketone, or the like; fatty acid esters such as ethyl acetate, butyl acetate, or the like; fatty acids such a~ ~cetic acid, propionic acid, or the like;
alcohols such as methanol, ethanol, isopropanol, butanol, or the like; and water. O~ course, mixtures of various solvents may also be employed. Especially preferred for the ~irst-stage reactioll are aromatic hydrocarbon, halogenated hydrocarbon, ketone and fatty acid ester solvents. For the second-stage reaction, in addition to the above preferred solvents of the first stage, alcoholic solvents and mixed solvents of water and alcohol may be used. When water is added, it is used in an amount of 1-10 mole per mole of the compound of formula (1) The amount of the solvent employled is not particularly limited, but, in general, 0.5-200 times by weight of the solvent is employed based on the amount of ~-AP derivative employed.
The catalyst which is used in the above-described first-stage reaction may be omitted, but if an acid is addedl the rate of the intermediate derivative formation is increased and thus the use of an acid catalyst is preferred. While the acid catalyst added is not particularly limited, preferred acids include mineral acids such as sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and the like; Eatty acids 9 ~L~i~6~

such as formic acid, acetic acid, propionic acid, and the like; and Lewis acids such a~ aluminunl chloride, and the like. The amount of the acid ~ddecl is not particularly critical, but it is ~dvantag~o~s from an economical point of view to use 0.001-20 mole per mole of the ~-AP derivative, and in general, 0.01-2 mole per mole is used.
A basic catalyst is employed in the second-stage reaction. Suitable basic catalysts include the likes of inorganic bases such as sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbona~e, sodium bicarbonate and the like and or~anic bases such as triethylamine, and the like. The amount of the base employed preferably ranges from 0.01 ~ mole per mole of the imide derivative used in this reaction from an economic point of view. Further, the addition of an inorganic or organic compound of a metal such as zinc, copper, nickel or the like can be made. Suitable compounds include ZnC12, ZnSO4, CuC12, NiC12, and the like. The compound is used in an amount of 0.01-2 mole, preerably 0.1-1.0 mole per mole of the imide of formula (1). The compound within these amounts increase the yield of product.
The reaction temperature for the first stage is normally in th~ range of -10 to 200C. However, at low temperatures, the reaction rate is diminished. While it ~31~
is not necessary to employ excesslvely hicJh temperatures, the temperature employed i6 usually in the ran~e oE 0-150C.
The reactiorl temperature for the second stage is normally in the range of -30 to 100C. If the temperature is too high, side reactions such as polymerization will occur. Accordingly, the temperature is usually in the range of -20 to 80C.
The reaction time for the first stage varies depending on the amount of the catalyst used and the reaction temperature. Normally, it is within the range of from 10 minutes to 30 hours. The reaction time for the second stage varies depending on the amount of the catalyst used and the reaction temperature, and is normally within the range of from 5 minutes to 10 hours.
The present process has the advantage that the ~-AP derivative may be synthesized from the ~-AP derivative without the formation of the DKP
derivative or with the formation of the DKP derivative in only very small amounts. ~urther, the by-products in the liquid reaction mixture are mainly the ~-AP
derivative of formula (2). Thus, the reaction medium can be recycled as a starting material to conduct further processing.

--ll--~ 3 ~ 3 ~

The metllod Eor converting the reaction mixtur~ of the present invention containiny the ~-AP into --APM
and separating the a-~PM may be effected by ally known method. For example, although it is possible to separate the -AP derivative from the reaction medium, it is generally a~vantageous to remove the solvent by distillation, and then convert the ~-AP into a-APM HCl by the method described above (Japanese Patent Application Laid-open No. 129258/1984) and then obtain ~-APM .
Having generally described this invention, a further understanding can be obtained by reEerence to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unle~s otherwise specified.

EXAMPLES
Example 1 To 1.2 1 o a toluene solution containing 61.6 g of ~-L-aspartyl-h-phenylalanine dimethyl ester (~-L-aspartyl-L-phenylalanine methyl ester in which the -carboxyl group of the L-aspartic acid has been methyl esterified) (~-APM2(L/L)) was added 1.2 ml of 98 wt% sulfuric acid. The reaction medium was heated with stirrin~ at 103C for 11 hours. The medium was then cooled to 30C, and toluene was added until the reaction mixture reached 1.5 1 in total volume.

~ 3 ~
A 1.0 ml sample was taken from this reaction mixture and quantitatively analy~ed for the intermediate deriva~ive; N~ carbomethoxy-~-phenyl~
ethyl)-2-aminosucci~lmide (imidomethyl ester) by high performance liquid chromatography (HPLC). The yield oE
the imidomethyl ester was ~8.7~, and the yie:Ld of the ~-APM2 (L/L) starting material was 1.0%.
The physical property values of the formed imidomethyl ester are as follows:
Elemental Analysis for C14H16N2O4:
Calculated: C: 60.86, H: 5.84, N: 10.14 Found: C: 60.78, H, 5.80, N. 10.21.
The NMR spectrum of the imidomethyl ester i5 shown in FIGURE 1.
Furthe~, 900 ml of the above-described reaction mixture was distilled to remove the toluene under reduced pressurer then 600 ml of methanol was added thereto and cooled to 20C. Thereafter, 120.0 ml of a methanol solution of 1 mole/l of potassium hydroxide was added thereto and reacted for an hour. A 1.0 ml sample of this reaction mixture was taken and quantitatively-analyzed for -aspartylphenylalanine dimethyl este~r (-aspartylphenylalanine-methyl ester in which the ~-carboxyl group of the aspartic acid has been methyl esterified) ~-APM~. The yield of a-APM2 obtained, based on the imidomethyl ester, was 55.4%.

. -13- ~3~

The methyl ester oE ~-aspartylpilenylalanine anhydride was not detected, and ~-APM2 was detected in an amount of 35O1~.

Example 2 150 ml oE the reaction mixture of the imidomethyl ester obtained in Example 1 (the reaction mixture diluted with toluene to 1.5 1 in total volume) was distilled under reduced pressure to remove the toluene. After adding 100 ml of methanol, it was cooled to 0C, and 0.8 g of sodium hydroxide was added, and the medium was stirred for 1.5 hoursO
When quantitatively analyzed by HPLCr ~-APM2 had been produced in an amount of 52.8% based on the imidomethyl ester.

Exam~le 3 75 ml of the reaction mixkure of the lmidometh~l ester obtained in Example 1 (the reaction mixture diluted with toluene to l.S 1 in total volume) was distilled under reduced pressure to remove the toluene.
AEter adding 100 ml of acetone, it was cooled to 5C, then 1~ ml of a 1 M/l aqueous potassium hydroxide solution was added and reacted for 1.5 hours.
When quantitatively analyzed by HPLC, ~ 3 ~ 0 aspartylphenylalanine (~-AP) had been produced in a yield oE 3~.5~ based on the imidomethyl ester.

Exa~ple 4 The reaction was accomplished in a manner similar to that in Example 3 except that ater removiny toluene by distillation, water was added instead oE acetone.
When -AP was quantitatively analyzed by HPLC, it had been produced in a yield 28.8~ based on the imidomethyl ester.

Example 5 _ 75 ml of the reaction mixture of the irnidomethyl ester obtained in Example 1 (the reaction mixture diluted with toluene to 1.5 1 in total volume) was maintained at 40C/ 10.0 ml of a methanol solution oE 1 M/l pota~sium hydroxide was added, and the resulting solution was stirred for 30 minutes.
A~ter the reaction, the toluene and ~he methanol was removed by distillation under reduced pressure, 20.0 ml of a 1 M/l aqueous sodium hydroxide solution was added, and reacted at 40C for 3 hours to effect saponification, followed by quantitative analysis for ~-AP by HPLC. It was found that ~-AP had been produced in an amount of 29.0% based on the imidomethyl ester.

.. . .. . .. . . .

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Example 6 To 100 ml of methanol were added 28~0 9 of ~-L- aspartylphenylalanine and 6.0 ml of 98 wt~
sulfuric acid, followed by heating at reflux for 4.5 hours. Then, the methanol was removed by distillation under reduced pressure and 200 ml of toluene was added. The resulting mixture was reacted at 103C with stirring for 15 hours.
This reaction mixture was quantitatively analyzed or the imidomethyl ester by HPLC and it was found that the imidomethyl ester had been produced ln a yield of 99.5%.

Example 7 28.0 g of ~-A(M)P was suspended in 500 ml of toluene. A 0.6 ml amount of 98 wt% sulfuric acid was added and the mixture was stirred at 103C for 20 hours. After cooling to 20C, 250 ml of a 1 M/l aqueous sodium hydroxide solution was added and stirred for an hour. ~-AP was quantitatively analyzed by HPLC
and it was found that AP had been produced in an amount of 36.5~ based on ~-AtM)P.

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3.57 of B-L- aspartyl-L-phenylalanine d.imethyl ester h~drochloride (~-APM2~L/L)) was suspended in 100 ml o~ toluene, 0.21 g of triethylamine was added, and stirred at 100C for 30 hours. Then, 20 ml o~ a methanol solution o~ 1 M/l potassium hydroxide was added,.and reacted at 20C for 30 minutes. Thereafter, 20 ml of a 1 M/l aqueou~ potassium hydroxide solution was added, and the resulting mixture was sti.rred for 3 hours. Quantitative analysis by HPLC showed that ~-AP had been produced in an amount of 48.3%.

Exarnple_9 To 32.2 g of N-formyl -~ L-phenylalanine methyl ester were added 100 ml of methanol and 6.1 ml of 9`8 wt% sulfuric acid Eollowed by heating at reflux for 6 hours. Thereafter, the methanol was removed by distillation under reduced pressure, 200 ml of toluene was added, and reacted at 103C with stirring for 15 hours. This reaction mixture was ~uantitatively analyzed Eor the imidomethyl ester by HPLC, and it was found that it had been produced in an amount of 90.5~.
` Further, in this example, when 30.8 g oE N-formyl -~ L-aspartyl-L-phenylalanine was employed as the starting material, the imidomethyl ester was produced in a yield oE 91.3%.

-17- ~3~

E x a mE~l e 1 O
To O.Y l o a toluene solution containiny 30.8 g o~ ~-APM2 was added 0.55 ml of 98 wt~ sulfuric acid.
The resulting mixture was heated with stirring, and reacted at 100C Eor 6 hours. The imidomethyl ester ,was produced in a yield of 98.0%. ThereaEter, the toluene was removed by distillation under reduced pressure. 0.4 ml of methanol was added to the resulting material, and the mix-tu,re was divided equally into 5 portions. To the respective portions, the 'ingredients shown below were added follow,ed by the addition to each portion of 21 ml of a methanol solution of 2 M/l potassium hydroxide at 20C with stirring. Ten minutes later, ~-APM2 was quantitatively analyzed for by HPLC. The results obtained for each portion are shown in the table~

1~--Expe r i - Y i e ld oE
ment No. Additive -APM~

Based on the Based on t:he imidomethyl imidomethyl ester ester (mole tim~s) (~) 1 ZnSO 7H O 0 5 65.7 2 ZnC12 0.5 65.7 H2O 3.5 3 CuC12 0.5 58.2 H2O 3.5 4 ZnC12 0.5 57.3 Ion exchange resin* 50 ml * "Diaion~CR10" produced by Mitsubishi Chemical Induskries, CO., Ltd.

Comparat ve Example To the whole volume of the reaction mixture which had been treated with potassium hydroxide in Example 1 texcluding 1.0 ml taken as the sample) were added 50 ml of a 35% a~ueous hydrochloric acid solution and 50 ml of water, followed ~y concentration of the reaction medium under reduced pressure to a liquid volume o about 100 ml. To this concentrated li~uor was added 6.0 ml of methanol, maintained at 20C with stirring for 7 days, and then maintained at 5C for 2 days. ~he separated APM HCl crystals were filtered, 300 ml of water was added, and the mixture was neutraliæed with a 10~ aqueous Na2CO3 solution to adjust the pH to 4.5.

,R~D~

-19- 131~

The mixture was heated to 60C ~o dissolve the ~PM and was left to stand at 5C for 24 hours. ~he separated crystals were filtered and dried by heating under reduced pressure, thereby yielding 12.1 g oE -APM crystals ~34.3~ based on ~-APM2).
Havlng generally descri.bed this invention, a further understanding can be obtained by reference to certain specific examples which,are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise spec,ified.
This application is a division of Canadian Application No. 570,705 filed June 29, 19,88.

Claims

The embodiments of the invention in Which an exclusive property or privilege is claimed are defined as follows:
1. A process for the production of an a-aspar-tyl-phenylalanine derivative, which comprises:
converting a .beta.-aspartylphenylalanine derivative of the formula:

(2) wherein R2 and R3 are each hydrogen or Cl 4 alkyl in a non-alcoholic solvent into an imide intermediate of the formula:

(1) where R1 is hydrogen or Cl 4 alkyl; and thereafter converting said imide into an a-aspartylphenylalanine derivative of the following formula in the presence of a base:

(3) wherein R4 and R5 are each hydrogen or Cl-4 alkyl.
2. The process according to claim 1, wherein each of R1, R2, R3, R4 and R5 is hydrogen or methyl.

3. The process according to claim 1, wherein the solvent for the reaction is water or an organic solvent.
4. The process according to claim 1, wherein the .beta.-aspartylphenylalanine derivative and/or the imide inter-mediate is reacted in a suspension or solution.
5. The process according to claim 4, wherein said suspension or solution is stirred.
6. A process for the production of an .alpha.-aspartyl-phenylalanine derivative, which comprises:
converting an imide of the formula:

(1) wherein R1 is hydrogen or C1-4 alkyl, into an .alpha.-aspartyl-phenylalanine derivative of the formula:

(3) wherein R4 and R5 each represent hydrogen or a C1-4 alkyl.
7. The process according to claim 6, wherein each of R1, R4 and R5 is hydrogen or methyl.
8. The process according to claim 6, wherein the solvent for the reaction is water or an organic solvent.
9. The process according to claim 6, wherein said imide is reacted in a suspension or solution.

10. The process according to claim 9, wherein said suspension or solution is stirred.