Method for preparing and purifying an N-alkylated aspartame derivative

A method is disclosed for preparing and purifying N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl ester from aspartame and 3,3-dimethylbutyraldehyde by hydrogenation in an organic solvent solution followed by the formation of an aqueous/organic solvent solution having an organic solvent content of about 17% to about 30% by weight of the aqueous/organic solvent solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved method of preparing and purifying an 
N-alkylated aspartame derivative which is particularly useful as a 
sweetening agent. 
2. Related Background Art 
It is known that various N-substituted derivatives of aspartame, such as 
disclosed in U.S. Pat. No. 5,480,668, are useful as sweetening agents. In 
particular, the N-alkylated aspartame derivative, 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester, is known as an extremely potent sweetening agent since its 
sweetening potency, on a weight basis, has been reported to be at least 50 
times that of aspartame and about 10,000 times that of sucrose. 
Since sweetening agents are mainly employed in foods for human consumption, 
it is extremely important that such sweetening agents be produced using 
methods which provide highly purified product. Such methods must also be 
commercially practicable, i.e., capable of use on an industrial scale and 
economically efficient. 
U.S. Pat. No. 5,510,508 describes a method for preparing 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester of the formula 
##STR1## 
comprising treating an aqueous acetic acid/alcoholic solution of aspartame 
and 3,3-dimethylbutyraldehyde, at room temperature, with hydrogen at a 
pressure less than or equal to 1 bar (0.1 MPa) in the presence of a 
catalyst based on platinum or palladium. The product is purified by 
precipitation and filtration after the alcohol is removed from the solvent 
under vacuum. 
There is, however, a need to prepare 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester of even greater purity than known in the art, particularly for use 
as a sweetening agent for human consumption. 
SUMMARY OF THE INVENTION 
This invention relates to a method for providing a highly purified 
N-alkylated aspartame derivative that may be used as a sweetening agent. 
In particular, the invention relates to a method for preparing 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester of the formula 
##STR2## 
comprising the steps of (i) treating a mixture of aspartame and 
3,3-dimethylbutyraldehyde in an organic solvent with hydrogen in the 
presence of a hydrogenation catalyst at a temperature and pressure 
effective to form an organic solvent solution of 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester; (ii) filtering the organic solvent solution to remove the 
hydrogenation catalyst; and (iii) forming an aqueous/organic solvent 
solution from the organic solvent solution to precipitate the 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester from the aqueous/organic solvent solution. Significantly, the 
aqueous/organic solvent solution has an amount of organic solvent of about 
17% to about 30% by weight of the aqueous/organic solvent solution. A 
particularly preferred organic solvent for use in this method is methanol. 
The precipitate is recovered using standard filtration techniques to 
provide highly purified N-N-3,3-dimethylbutyl)- 
L-.alpha.-aspartyl!-L-phenylalanine 1-methyl ester. 
Another embodiment of this invention is directed to a method for purifying 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester comprising the steps of preparing an organic solvent solution of 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester and thereafter forming an aqueous/organic solvent solution from the 
organic solvent solution to precipitate the 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester from the aqueous/organic solvent solution. Again, the 
aqueous/organic solvent solution has an amount of organic solvent of about 
17% to about 30% by weight of the aqueous/organic solvent solution. The 
purification method of this invention allows for the recovery of highly 
purified N-N-3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 
1-methyl ester by filtration of the precipitate while the impurities 
remain in the aqueous/organic solvent solution. 
DETAILED DESCRIPTION OF THE INVENTION 
This invention is directed to an improved method of preparing and purifying 
an N-alkylated aspartame derivative, namely 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester. This N-alkylated aspartame derivative is a highly potent sweetening 
agent. 
The N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester is prepared by first treating a mixture of aspartame and 
3,3-dimethylbutyraldehyde in an organic solvent with hydrogen in presence 
of a hydrogenation catalyst. Aspartame and 3,3-dimethylbutyraldehyde are 
readily available starting materials, which are typically combined in a 
substantially equivalent molar ratio, e.g. about 1:1. Higher molar amounts 
of the aldehyde are more likely to lead to the generation of impurities, 
while excess molar amounts of aspartame are not preferred due to waste and 
cost. 
The hydrogenation reaction is conducted in an organic solvent, preferably 
an alcohol and most preferably methanol. If desired, water may be present 
in the reaction mixture so long as the organic solvent is present in an 
amount greater than about 30% by weight of the total weight of water and 
organic solvent. Essentially the concentration of organic solvent must be 
great enough to solubilize the 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester so as to avoid aggregation with the catalyst which is removed via 
filtration. Other exemplary organic solvents include tetrahydrofuran, 
ethyl acetate, and the like. 
The hydrogenation catalyst may be selected from catalysts based on 
palladium or platinum such as for example, platinum on activated carbon, 
palladium on activated carbon, platinum black or palladium black. Other 
hydrogenation catalysts include, without limitation, nickel on silica, 
nickel on silica and alumina, Raney nickel, ruthenium black, ruthenium on 
carbon, palladium hydroxide on carbon, palladium oxide, rhodium black, 
rhodium on carbon and rhodium on alumina. The hydrogenation catalysts 
based on palladium or platinum are most preferred. The catalyst is present 
in an amount effective to produce the N-alkylated aspartame derivative in 
an acceptable yield. Generally, the weight ratio of catalyst to aspartame 
is about 0.01:1 to about 0.25:1, most preferably about 0.02:1. 
The pH of the reaction mixture is typically between about 4.0 to about 6.5, 
most preferably about 5.0 to about 5.5. If desired, the pH of the reaction 
mixture can be adjusted by the addition of common acids or bases. 
The components of the reaction mixture are hydrogenated in the presence of 
the hydrogenation catalyst under a hydrogen atmosphere. Typically the 
pressure of the hydrogen is held at about 5 psi to about 100 psi, most 
preferably about 30 psi to about 35 psi. 
The hydrogenation reaction is conducted by first mixing the aspartame, and 
3,3-dimethylbutyraldehyde, organic solvent, and catalyst to form a slurry, 
which is then hydrogenated at a temperature of about 20.degree. C. to 
about 30.degree. C., preferably about 22.degree. C. to about 26.degree. C. 
for approximately 2 to 48 hours, and most preferably for 12 to 16 hours. 
The resulting organic solvent solution containing 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester is then filtered to remove the hydrogenation catalyst. Standard 
filtration techniques may be employed. Preferably a filtering aid, such as 
celite, is added and the solution is filtered with a sparkle filter. 
After filtration, an aqueous/organic solvent solution having organic 
solvent in an amount of about 17% to 30% by weight of the aqueous/organic 
solvent solution is formed. The aqueous/organic solvent solution can be 
formed in any way that results in a solution having the above-described 
amount of organic solvent. For example, the organic solvent from the 
organic solvent solution containing 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester can be first reduced by, for example, rotary evaporation or 
distillation, and then replaced by water to achieve the desired organic 
solvent content. Or more preferably, after the water is added the organic 
solvent is further reduced by distillation to achieve the desired organic 
solvent content. It is also possible to add water to the organic solvent 
solution which is then reduced to achieve an organic solvent content of 
about 17% to about 30%, more preferably about 17% to about 25% by weight 
of the aqueous/organic solvent solution. Yet another possibility, if 
desired, is to add water and remove the organic solvent simultaneously. If 
the organic solvent is reduced to less than 17%, it is also possible to 
add back organic solvent to form an organic solvent solution having an 
organic solvent content between about 17 to about 30%. 
After the aqueous/organic solvent solution having the specified organic 
solvent content is formed, the solution is held for about 2 to about 24 
hours, most preferably at least about 12 hours and at a temperature of 
about 5.degree.-25.degree. C., most preferably about 10.degree.-15.degree. 
C. This hold period allows for substantial precipitation of the 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester which is then removed using standard filtration techniques. 
The resulting filtrate, which may contain 10-15% of the N-alkylated 
aspartame derivative can be, if desired, recycled into the process. The 
recovered solid, which is preferably washed with water and then dried, is 
highly purified 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester. 
As noted previously, another embodiment of this invention is directed to a 
method for purifying 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester, no matter how prepared. In this purification method the 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester may be formed in an organic solvent solution or added to organic 
solvent. As noted previously, the organic solvent solution may contain 
water as long as the 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester is solubilized in the organic solvent solution. The formation of the 
aqueous/organic solvent solution for this purification method is the same 
as previously described for the method of preparing 
N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1-methyl 
ester.

The Examples which follow are intended as an illustration of certain 
preferred embodiments of the invention, and no limitation of the invention 
is implied. 
EXAMPLE 1 
A slurry of .alpha.-aspartame a (29.43 g, 0.1 mol) and 
3,3-dimethylbutyraldehyde (10 g, 0.1 mol) was formed in methanol (500 ml). 
To the slurry was added a palladium catalyst, Pd/C (4%, 50% wet, 1.2 g). 
The mixture was hydrogenated at 30 psi at room temperature for about 12 to 
16 hours. The hydrogenated mixture was then filtered through a Celite bed 
and the bed was washed with methanol (50 ml). The methanol was reduced to 
about half the volume (250 ml) on a rotary evaporator under reduced 
pressure and then water (250 ml) was added to it. The remaining methanol 
was distilled to a level of about 17-25% methanol in the resulting 
aqueous/methanol solution. The aqueous/methanol solution was stirred at 
10.degree. to 15.degree. C. for 2 to 12 hours. The precipitated solid was 
filtered, washed with water (50 ml) and dried in a vacuum oven at 
40.degree. C. (house vacuum) for 16 hours to yield 19.65-24.57 g of 
N-N-(3,3 -dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine 1 -methyl 
ester (52-65%) as a white solid (&gt;97% pure by HPLC). 
COMATIVE EXAMPLE 1 
Aspartame (50-60 g/L), 3,3-dimethylbutyraldehyde (20-30 g/L), methanol (30 
ml) and a 0.1M aqueous solution of acetic acid (60 ml) were mixed with a 
palladium catalyst, Pd/C (10%; 1 g). The mixture was hydrogenated at 14.5 
psi (0.1 MPa) at room temperature for about 2 hours. The methanol was then 
removed by evaporation and a white solid precipitate was recovered. The 
reaction was only 60-70% complete. After the product was filtered off, 
dried and washed with hexane, the resulting 
N-N-(3,3-dimethylbutyl)-L-.alpha. aspartyl!-L-phenylalanine 1-methyl 
ester was found by HPLC to contain about 20 to 30% (0% of methanol) 
aspartame, 1 to 2% N,N-dineohexyl aspartame and 1-2% of a late eluter 
believed to be dineohexyl lactone. When the methanol level was distilled 
to a level of about 5-10% methanol in the resulting aqueous/methanol 
solution, the percentage of aspartame in the final isolated material went 
down to 3-5%. 
EXAMPLE 2 
N-N-(3,3-dimethylbutyl)-L-.alpha. aspartyl!-L-phenylalanine 1-methylester 
was prepared in a manner similar to Comparative Example 1 with the 
exception that the methanol was not completely distilled off, but instead 
left at 20-24% by weight of the hydrogenated aqueous/methanol solution. 
The precipate was recovered by filtration, washed with water and dried. 
The resulting N-N-(3,3-dimethylbutyl-L-aspartyl!-L-phenylalanine 1-methyl 
ester had a purity of &gt;99% by HPLC. 
Other variations and modifications of this invention will be obvious to 
those skilled in this art. This invention is not to be limited except as 
set forth in the following claims.