GB2153365A

GB2153365A - Process for the preparation of N-acyl-derivatives of aspartame

Info

Publication number: GB2153365A
Application number: GB08432449A
Authority: GB
Inventors: Sergio Auricchio; Francesco Minisci
Original assignee: Pierrel SpA
Current assignee: Pierrel SpA
Priority date: 1984-01-26
Filing date: 1984-12-21
Publication date: 1985-08-21
Also published as: IT8419324A0; AU3720584A; FR2558838A1; KR850005409A; ZA85229B; GB8432449D0

Abstract

A process for obtaining the alpha -isomer of N-acyl-derivatives of aspartame (as an example, N-formyl- or N-benzyloxycarbonyl-aspartame) via condensation of the N-protected aspartic acid anhydride with the methyl ester of L-phenyl-alanine, in the presence of non-polar or slightly polar solvents and slightly basic aprotic solvents.

Description

SPECIFICATION Process for the preparation of n-acyl-derivatives of aspartame Aspartame, namely the methyl ester of a-L-aspartyl-L-phenyl-alanine is an artificial sweetener of great interest. Several studies have been carried out aimed at attaining increasingly simple and economic production methods of aspartame.

One of the most investigated methods is based on the aminolysis of the aspartic acid anhydride with the methyl ester of L-phenylalanine, in which the amino group of the aspartic acid is protected as a salt (see, for instance, Bull. Chem. Soc. Japan 46, 2611, 1973; U.S.

Patent 3,901,871 and Chem. Abs., 78, 160119, 1973), or by means of the usual protecting groups employed in peptide synthesis such as, for instance, benzyloxycarbonyl, tert-butoxycarbonyl, formyl 2-nitrophenyl-sulfonyl and analogs thereof (see, for instance, Swiss Patent 508,590, British Patent 1,339,101, Dutch Patent Application 7007176 and Chem. Abs. 80, 48405, 1974).

Such aminolysis leads indeed to a mixture of a-and fi-amides:

wherein P is a protecting group of the amino function.

The regioselectivity is of paramount importance for the industrial success of the process, as the isomer, contrary to the a-isomer, has a bitter taste and, consequently, must be necessarily removed.

Therefore, the formation of the isomer represents a drawback for two reasons: it decreases the overall yields of useful product and creates a lot of problems in the purification step which becomes more expensive the higher the percentage of the isomer.

Various methods for obtaining the isomer in a predominant proportion over the a-isomer have been reported in the literature such as, for instance, the use of low temperatures, the employment of an excess of L-phenyl-alanine methyl ester, or the introduction of various additives into the reaction mixture, (see, for example, Russian Chemical Review, 50, 316,1981). Illustrative Jditives described in the literature are, for instance, acetic acid (Bull, Chem. Soc. Jap. 46, 1893, 1973), carbon dioxide (U.S. Patent 3,901,871), ortho-phosphoric acid (German Publication 2,233,535), the mixtures of weak acids and lower aliphatic alkanols (Bull. Chem. Soc. Jap. 46, 2611, 1973) and the mixtures of alcohols and strong acids (British Patents 1,481,186 and Chem. Abs., 85, 6055, 1976).

The use of these additives, or of an excess of L-phenylalanine methyl ester, complicates both the practical exploitation of the process and the separation of the reaction products or the possible recovery of the excess of phenylalanine, which, incidentally, is by far the most expensive reactant among those employed.

We have now surprisingly found that, if the N-protected aspartic acid anhydride of formula I

wherein R is hydrogen and R1 is a group selected from formyl, benzyloxycarbonyl, benzyloxycarbonyl substituted on the phenyl ring by one or two groups selected from (C, 4)alkyl, (C14)alkoxy, halo, nitro and trifluoromethyl, and tert-butoxycarbonyl, or R and R, taken together with the adjacent nitrogen atom represent a phthalimido moiety, is reacted with L-phenyl-alanine methyl ester of formula Il

it is possible to obtain N-protected a-aspartame of formula Ill

wherein R and R, are as above defined, with a high regioselectivity degree, provided that the condensation reaction is carried out in the presence of a suitable organic solvent selected from non-polar or slightly polar solvents and slightly basic aprotic solvents, under particular concentration conditions which depend on the nature of the employed solvent.

As a matter of fact, it is known that diprotic polar solvents, e.g. dimethylsulfoxide, dimethylformamide and the like, always afford the isomer in great predominant proportions ( > 80%) and, accordingly, they must be avoided. The nucleophilic protic solvents, like water, the alcohols and analogues. are to be avoided as well, as they may react with the anhydride of the aspartic acid, thus giving rise to side-reactions which considerably decrease the yields of aspartame. Also the strong basic solvents such as, for instance, pyridine, pycolines, amines and so on, lead essentiaily to the isomer and, consequently, they cannot advantageously be employed.

On the contrary, the solvents useful for the purpose of the present invention are selected from non polar-, slightly polar- and slightly basic aproptic solvents, non-polar and slightly polar solvents being preferred.

Examples of non-polar or slightly polar solvents which can advantageously be employed in the process of the present invention are the aliphatic and aromatic hydrocarbons, optionally substituted by halogen atoms, such as benzene, toluene, xylene, n-hexane, n-pentane, 1,2dichloroethane, chloroform, carbon tetrachloride, methylene chloride and analogs threof, whereas examples of slightly basic aprotic solvents are the ethers, tetrahydrofuran, dioxane and the like.

Among the non-polar and the slightly polar solvents, the aromatic hydrocarbons, like toluene, and the halogenated aliphatic hydrocarbons, like methylene chloride or 1,2-dichloroethane, are preferably employed; these solvents possess the advantage of dissolving the reactants to a sufficient extent and contemporaneously displaying a negligible degree of solubility towards the end reaction products, which can easily be recovered from the medium in the form of crystalline precipitates; the mother liquors can be recycled as such.

For obtaining a high a/ss ratio, however, the sole use of these types of solvents is not sufficient in itself; also the steady concentration of the reactants in the reaction mixture is an important feature as well.

In particular, if non-polar or slightly polar solvents like hydrocarbons or halogenated hydrocarbons are adopted, it is necessary to maintain the steady concentration of L-phenylalanine methyl ester during the reaction at a relatively low level. In fact, if stoichiometric ratios of N-protected aspartic acid anhydride/L-phenyl-alanine methyl ester are employed in these solvents, the ratio of a/ss-N-protected aspartame considerably varies with the concentration of the reactants, in the sense that this ratio increases with the dilution (see, for instance, Tables 1 < 7).

A particularly effective practical solution is that of slowly adding L-phenyl-alanine methyl ester to the N-protected aspartic acid anhydride solution: in fact, remarkably different results are obtained if the two reactants either are suddenly mixed together (lower a/ss ratio), of if the methyl ester of L-phenyl-alanine is added to the N-protected aspartic acid an hydride solution in a predetermined period of time (higher a/p ratio, Table 5). Considering that the reaction rate is sufficiently high already at room temperature, it is possible to keep a low steady concentration of L-phenyl-alanine methyl ester by adding this reactant in an interval of time comprises between about 30 minutes and about 3 hours, thus obtaining the a-isomer with a high regioselectivity degree.

Considering that the reaction rate increases with the temperature, the addition times of Lphenyl-alanine methyl ester can be reduced by increasing the temperature and, contemporaneously, always keeping a low steady concentration of L-phenyl-alanine methyl ester. Accordingly, if the addition of the methyl ester of L-phenyl-alanine is carried out at 50"C, the same selectivity (a/fl = 80/20) obtained at 20"C in 60 minutes is indeed achieved in 6 minutes (Table 5).

On the contrary, leaving unchanged the other conditions, the lowest selectivity degrees in aisomer are obtained by adding slowly the N-protected aspartic acid anhydride to the solution of the methyl ester of L-phenyl-alanine.

A different, but equally selective solution is that of pouring both the reactants (L-phenylalanine methyl ester and N-protected aspartic acid anhydride) into the reaction solvent over a sufficiently long period of time (preferably 10-120 minutes, depending on the temperature), in order to keep the concentration of the reactants always low.

From the results obtained with a series of experiments, we have found that, when non-polar or slightly polar solvents are employed, it is possible to obtain a great selectivity (averagely higher than 70%) in a-aspartame if the reaction is performed under such conditions that the steady concentration of L-phenyl-alanine methyl ester is maintained below 0.2 M.

The slightly basic solvents are preferably selected from the ethers. The ways these solvents are employed in order to obtain a high selectivity in a-isomer are, for certain aspects, opposite to those described for the non-polar or slightly-polar ones. In these instances, a necessary condition for obtaining the a-isomer with a high selectivity degree, is that the concentration of the N protected aspartic acid anhydride is always relatively high, irrespective of the concentration of the methyl ester of L-phenyl-alanine.

Thus, in this case, if stoichiometric ratios of N-protected aspartic acid anhydride/L-phenyl alanine methyl ester in dioxane are adopted, the ratio of a/fl-N-protected aspartame consider ably varies with the concentration: more precisely, this ratio decreases with the dilution i.e., the contrary of the situation which was observed with non-basic solvents. If the N-protected aspartic acid anhydride is slowly added to the solution of L-phenyl-alanine methyl ester, the selectivity is in any case low, whereas, if the methyl ester of L-phenyl-alanine is slowly added to the solution of N-protected aspartic acid anhydride, the selectivity in a-isomer remarkably increases with the concentration of the anhydride (see Table 8 z 2).

In this case, the experimental results show that, on average, when the concentration of the Nprotected aspartic acid anhydride is higher than 0.1 M, the reaction preferably affords the a isomer.

With both solvent classes, the reactions are generally carried out at temperatures between 0 and 100"C, preferably 0 and 80"C, in a period of time preferably between 5 and 120 minutes.

The ratio of N-protected aspartic acid anhydride/L-phenyl-alanine methyl ester is preferably between 1 and 1.5.

In all instances, the a-isomer of N-protected aspartame, containing small amounts of the ss- isomer, precipitates from the reaction solvent and can thus be recovered by filtration, decantation or centrifugation, while the mother liquors can be recycled as such for subsequent operations.

In any case, the phenomenon of the selectivity is more significant and, accordingly, a higher selectivity is achieved, when the amino group of the aspartic acid an hydride is protected as the formyl or benzyloxycarbonyl derivative: this is particularly useful for the purpose of the present invention, if it is borne in mind that the de-protection of the amino group occurs in the easiest way in the case of these two derivatives.

Thus, in accordance with a preferred embodiment of the present invention, the condensation is carried out by employing the N-formyl- or N-benzyloxycarbonyl-aspartic acid anhydride and Lphenyl-alanine methyl ester at a concentration lower than 0.2 M. According to a most preferred embodiment, the reacts(' is performed in toluene, methylene chloride or 1,2-dichloroethane and the concentration of L-phenylalanine methyl ester in the reaction mixture is kept below 0.05 M.

The following Examples are provided for the purpose of better illustrating the invention, but in no way they must be construed as a limitation of the scope of the invention itself.

EXAMPLES 1-3 (Table 1) 6 Milliliters of toluene, which were previously mixed with the amount of L-phenyl-alanine methyl ester indicated in Table 1, were immediately poured into 6 ml of toluene mixed with the amount of N-formyl-aspartic acid anhydride, again indicated in Table 1, at 20"C under stirring.

The mixture was stirred for a further 30 minutes at the above same temperature, then 30 ml of acetonitrile were added, and the solution was HPLC-analyzed (Merck RP 18 7,um column), by eluting with an aqueous solution of phosphate buffer (pH 2.5): acetonitrile = 70:30 (v/v), with a 1 ml/min flow, and using a-aspartame as the internal standard. The obtained results are reported in Table 1.

EXAMPLES 4-6 (Table 2) 10 Milliliters of toluene, which were previously mixed with the amount of L-phenylalanine methyl ester indicated in Table 2, were poured into 10 ml of toluene admixed with the amount of N-benzyioxycarbonylaspartic acid anhydride again indicated in Table 2, at 20"C under stirring. Stirring was carried on at the same temperature for an additional 30 minutes, then the reaction mixture was made homogeneous by the addition of acetonitrile and, subsequently, it was HPLC-analyzed (Merck RP 18 7 gm column) by eluting with an aqueous solution of phosphate buffer (pH 4): acetonitrile = 70:30, with a 1.2 ml/min flow. The obtained yields and selectivities are reported in Table 2.

EXAMPLES 7-9 (Table 3) By operating as described in Examples 4 6, but employing the 3-phthalimide-succinic anhydride (I,

instead of the N-benzyloxycarbonyl-aspartic acid anhydride, the results reported in Table 3 were obtained.

EXAMPLES 10-12 (Table 4) 6 Milliliters of 1 ,2-dichloroethane, which were previously mixed with the amount of L-phenylalanine methyl ester indicated in Table 4, were immediately poured into 6 ml of 1,2dichloroethane mixed with the amount of N-formyl aspartic acid anhydride. again indicated in Table 4, at 20"C under stirring. After stirring at the same temperature for 30 minutes and adding 30 ml of acetonitrile, the solution was HPLC-analyzed as described in Example 1-3. The obtained yields and selectivities are reported in Table 4.

EXAMPLES 13-19 (Table 5) A mixture of N-formyl-aspartic acid anhydride, and 6 ml of 1.2-dichloroethane (0.4 M) was mixed under stirring with 6 ml of a solution (0.4 M) of L-phenylalanine methyl ester at the temperature and in the period of time reported in Table 5. The resulting mixture was subsequently stirred for 30 minutes, mixed with 30 ml of acetonitrile and the obtained solution was HPLC-analyzed as illustrated in Examples 1-3. The obtained results are given in Table 5.

EXAMPLES 20-22 (Table 6) The procedures of Examples 4-6 were repeated, but using 1,2-dichloroethane instead of toluene. Yields and selectivities are reported in Table 6.

EXAMPLES 23-25 (Table 7) The procedures of Examples 7-9 were repeated, but using 1 2-dichloroethane instead of toluene. The obtained results are reported in Table 7.

EXAMPLES 26-28 (Table 8) The procedures of Examples 4-6 were repeated, but using tetrahydrofuran instead of toluene.

The obtained results are reported in Table 8.

EXAMPLES 29-31 (Table 9) The procedures of Examples 7-9 were repeated, but using tetrahydrofuran instead of toluene.

The obtained results are reported in Table 9.

EXAMPLES 32-33 (Table 10) 6 Milliliters of dioxane, which were previously mixed with the amount of L-phenyl-alanine methyl ester indicated in Table 10 were immediately mixed with 6 ml of dioxane mixed with the amount of N-formyl-aspartic acid anhydride again indicated in Table 10, at 20"C under stirring.

The reaction mixture was stirred at the same temperature for a further 30 minutes, then 30 ml of acetonitrile were added and the obtained solution was HPLC-analyzed as described in Examples 1-3. The obtained yields are reported in Table 10.

EXAMPLES 34-35 (Table 10) A mixture of N-formyl-aspartic acid anhydride, in the amount indicated in Table 10, and 6 ml of dioxane, was mixed under stirring at 20at, in 60 minutes, with 6 ml of a solution of Lphenyl-alanine methyl ester in the amount again indicated in Table 10. Once the addition was terminated, the mixture was stirred at 20"C for an additional 30 minutes, mixed with 30 ml of acetonitrile and the obtained solution was HPLC-analyzed as described in Examples 1-3. The obtained results are reported in Table 10.

EXAMPLES 36-38 (Table 11) The procedures of Examples 4-6 were repeated, but using dioxane instead of toluene. The obtained results are reported in Table 11.

EXAMPLES 39-41 (Table 12) The procedures of Examples 7-9 were repeated, but using dioxane instead of toluene. The results given in Table 1 2 were obtained.

EXAMPLES 42-44 (Table 13) The procedures of Examples 10-12 were repeated, with the sole difference that dimethylsulfoxide was employed in place of 1 ,2-dichloroethane. The obtained results are reported in Table 13.

EXAMPLE 45 (Table 13) By operating substantially as described in Examples 1 7 and 34, with the only difference being that dimethylsulfoxide was employed instead of 1 2-dichloroethane and dioxane, the results reported in Table 1 3 were obtained.

EXAMPLES 46-48 (Table 14) The procedures of Examples 4-6 were repeated but using dimethylsulfoxide instead of toluene. The obtained results are reported in Table 1 4.

EXAMPLES 49-51 (Table 15) The procedures of Examples 7-9 were repeated but employing dimethylsulfoxide instead of toluene. The obtained results are given in Table 1 5.

TABLE 1 Effect of the concentration of the reactants on the ratio of α/ss-formylaspartame in toluene at 20 C.

Example L-Phenylalanine N-Formyl-aspartic α-Formylaspartame ss-Formylaspartame Over N methyl ester acid anhydride % % all (molarity) (Molarity) yield % 1 0.8 0.8 52.5 47.5 94 2 0.2 0.2 63.7 36.3 93 3 0.02 0.02 78.3 21.7 96 TABLE 2 - Reactions @, TOLUENE

Over Example L-Phenylalanine N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- all N methyl ester aspartic acid anhy- α;-aspartame ss-aspartame yield (Molarity) dride (Molarity) % % % 4 0.8 0.8 61 39 96 5 0.2 0.2 74 26 98 6 0.02 0.02 85 15 96 TABLE 3 - REACTIONS in TOLUENE

N-3-Phthalimido- N-2-Phthalimido- Over L-Phenylalanine 3-Phthalimido-suc succinoyl-phenyl- succinoyl-phenyl- all Example methyl ester cinic acid anhydri- alanine methyl ester alanine methyl ester yeild N (Molarity) de (Molarity) (α;-isomer) (ss-isomer) % 7 0.6 0.8 46 52 96 8 0.2 0.2 53 47 98 9 0.02 0.02 57 43 98 TABLE 4 - Effect of the concentration of the reactants on the ratio α-ss-formyl aspartame in 1,2-dichloroethane at 20 C.

Over Example L-Phenylalanine N-Formyl-aspartic α-Formylaspartame ss-Formylaspartame all N methyl ester acid anhydride % % yield (Molarity) (Molarity) % 10 0.8 0.8 64.2 35.8 94 11 0.2 0.2 76.5 23.5 96 12 0.02 0.02 84.7 15.3 98 TABLE 5 - Effect of the addition rate of L-phenyl-alanine methyl ester (0.4 M in 1,2-dichloroethane) to the N-formyl-aspartic acid anhydride (0.4 M in 1,2-dichloroethane) on the ratio of α/ss formylaspartame.

Example Temperature Addition time α-Formylaspartame ss-Formylaspartame Overallyield N ( C) (minutes) % % % 13 20 - 69.3 30.6 94 14 20 6 71.5 28.5 96 15 20 12 77.2 22.8 95 16 20 24 78.0 22.0 98 17 20 60 79.6 20.4 98 18 20 180 82.6 17.4 97 19 50 6 79.9 20.1 96 TABLE 6 - Reaction in @,2-dichloroethane

Example L-Phenylalanine N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- N-Benzyloxycarbo- Over N methyl ester aspartic acid anhy- α-aspartame nyl-ss-aspartame all (Molarity) dride % % yield (Molarity) % 20 0.8 0.8 65 35 96 21 0.2 0.2 77 23 98 22 0.02 0.02 81 19 92 TABLE 7 - Reactions in 1,2-dichloroethane

Example L-pHenylalanine 3-Phthalimido-succi- N-3-Phthalimido-suc- N-2-Phthalimido-suc- Over methyl ester nic acid anhydride cinoyl-phenylalanine cinoyl-phenylalanine all N (Molarity) (Molarity) methyl ester methyl ester yield (α;-isomer)% (ss-isomer)% % 23 0.8 0.8 53 47 99 24 0.2 0.2 57 43 99 25 0.02 0.02 60 40 98 TABLE 8 - Reactions in TETRAHYDROFURAN

Example L-Phenylalanine N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- Over methyl ester aspartic acid anhy- α;-aspartame ss-aspartame all N (Molarity) dride % % yield (Molarity) % 26 0.8 0.8 63 37 95 27 0.2 0.2 58 42 97 28 0.02 0.02 41 59 97 TABLE 9 - Reactions in TETRAHYDROFURAN

Example L-Phenylalanine 3-Phthalimido-suc- N-3-Phthalimido-suc- N-2-Phthalimido-suc- Over methyl ester cinic acid anhydride cincyl-phenylalanine cinoyl-phenylalanine all N (Molarity) (Molarity) methyl ester methyl ester Yield (α ;-isomer)% (ss-isomer)% % 29 0.8 0.8 46 54 98 30 0.2 0.2 44 56 99 31 0.02 0.02 16 84 99 TABLE 10 - Effects of the concentration of the reactants, and of the addition rate of L-phenylalanine methyl ester, on the ratio of α/ss-formylaspartame in Dioxane at 20 C

Example L-Phenylalanine N-Formyl-aspartic Addition time α;-Formyl- ss-Formyl- Overall N methyl ester acid anhydride of the ester aspartame aspartame yield (Molarity) (Molarity) (Minutes) % % % 32 0.2 0.2 - 77.1 22.9 97 33 0.02 0.02 - 58.0 42.0 94 34 0.4 0.4 60 80.5 49.5 96 35 0.02 0.02 60 51.3 48.7 93 TABLE 11 - Reactions in DIOXANE

Example L-Phenylalanine N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- Over methyl ester aspartic acid anhy- α;-aspartame ss-aspartame all N (Molarity) dride % % yield (Molarity) % 36 0.8 0.8 77 23 93 37 0.2 0.2 58 42 96 38 0.02 0.02 48 52 95 TABLE 12 - Reactions in DIOXANE

Example L-Phenylalanine 3-Phthalimido-succi N-3-Phthalimido- N-2-Phthalimido-suc- Over methyl ester nic acid anhydride succinoyl-phenylala- cinoyl-phenylalanine all N (Molarity) (Molarity) nine methyl ester methyl ester Yield (α;-isomer)% (ss-isomer)% % 49 0.8 0.8 44 46 94 40 0.2 0.2 49 51 97 41 0.02 0.02 41 59 98 TABLE 13 - Ratio between α- and ss-aspartame in dimethylsulfoxide at 20 C

Example L-Phenylalanine N-Formyl-aspartic Addition time α;-Formyl- ss-Formyl- Overall N methyl ester acid anhydride of the ester aspartame aspartame yield (Molarity) (Molarity) (Minutes) % % % 42 0.8 0.8 - 20.6 79.4 94 43 0.2 0.2 - 19.1 80.9 93 44 0.02 0.02 - 18.6 81.4 95 45 0.4 0.4 60 17.0 83.0 94 TABLE 14 - Reaction in DIMETHYLSULFOXIDE

Example L-Phenylalanine N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- N-Benzyloxycarbonyl- Over methyl ester aspartic acid anhy- α;-aspartame ss-aspartame all N (Molarity) dride % % yield (Molarity) % 46 0.8 0.8 19 81 93 47 0.2 0.2 4 96 95 48 0.02 0.02 9 91 98 TABLE 15 - Reactions in DIMETHYLSULFOXIDE

Example L-Phenylalanine 3-Phthalimido-suc- N-3-Phthalimido-suc- N-2-Phthalimido-suc- Over methyl ester cinic acid anhydride cincyl-phenylalanine cinoyl-phenylalanine all N (Molarity) (Molarity) methyl ester methyl ester Yield (α ;-isomer)% (ss-isomer)% % 59 0.8 0.8 8 92 99 50 0.2 0.2 3 97 99 51 0.02 0.02 9 91 99

Claims

1. A process for preparing N-protected-a-aspartame of formula Ill

wherein R is hydrogen and R1 is a group selected from formyl, benzyloxycarbonyl, benzyloxycarbonyl substituted on the phenyl ring by 1 or 2 groups selected from (C,-C4)alkyl, (Ct 4)alkoxy, halo, nitro and trifluoromethyl, and tert-butoxycarbonyl, or R and R1, taken together with the adjacent nitrogen atom, represent a phthalimido group, which comprises reacting N-protectedaspartic acid anhydride of formula I

wherein R and R1 are as above defined, with L-phenyl-alanine methyl ester of formula II

in the absence of additives, said process being carried out:: a) in the presence of a non-polar or slightly polar solvent, keeping the steady concentration of L-phenyl-alanine methyl ester below 0.2 M, or b) in the presence of an aprotic slightly basic solvent, keeping the concentration of the Nprotected-aspartic acid anhydride above 0.1 M.

2. A process as defined in claim 1, wherein the non-polar or slightly polar solvent is a solvent selected from aliphatic or aromatic hydrocarbons, optionally substituted by halogen atoms.

3. A process as defined in claim 2, wherein the solvent is selected from benzene, toluene, xylene, methylene chloride and 1 2-dichloro-ethane.

4. A process as defined in claim 2 or 3, wherein the concentration of L-phenyl-alanine methyl ester is kept below 0.05 M.

5. A process as defined in claim 1, wherein the slightly basic aprotic solvent is selected from the class of the ethers.

6. A process as defined in claims 2, 3 or 4, wherein the steady concentration of L-phenylalanine methyl ester is kept at the desired low level either by operating in diluted solutions or by gradually adding the methyl ester of L-phenyl-alanine to the mixture of the solvent with the Nprotected-aspartic acid an hydride.

7. A process as defined in claim 5, wherein the concentration of the N-protected-aspartic acid anhydride is kept above 0.2 M.

8. A process as defined in any one of the preceding claims, wherein a ratio of n-protected aspartic acid anhydride/L-phenyl-alanine methyl ester of between 1 and 1.5 is employed.

9. A process as defined in any one of the preceding claims, carried out at temperatures between 0 and 100"C.

1 0. A process as defined in any one of the preceding claims, wherein R is hydrogen and R is formyl or benzyloxcarbonyl.

11. A process as defined in claim 10, wherein R1 is formyl.

1 2. N-3-Phthalimido-succinoyl-phenyl-alanine methyl ester.

1 3. A process for preparing N-protected-a-aspartame substantially as described herein and exemplified.

1 4. N-protected-a-aspartame when prepared by the process of any one of claims 1 to 11 or 13.