Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-08-21
2001-05-01
McKane, Joseph K. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reexamination Certificate
active
06225493
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the purification of N-[N-(3,3-dimethylbutyl)-L-&agr;-aspartyl]-L-phenylalanine 1-methyl ester (&agr;-neotame) by resolution of the isomers of neotame.
2. Related Background Art
N-[N-(3,3-dimethylbutyl)-L-&agr;-aspartyl]-L-phenylalanine 1-methyl ester (&agr;-neotame) is a high potency dipeptide sweetener (about 8000× sweeter than sucrose) that has the formula
Its &bgr;-isomer, N-[N-(3,3-dimethylbutyl)-L-&bgr;-aspartyl]-L-phenylalanine 1-methyl ester (&bgr;-neotame), has the structure
The chemical synthesis of &agr;-neotame is disclosed in U.S. Pat. No. 5,480,668, U.S. Pat. No. 5,510,508 and U.S. Pat. No. 5,728,862, the disclosure of each of which is incorporated by reference herein. U.S. Pat. Nos. 5,510,508 and 5,728,862 describe the conventional synthesis of &agr;-neotame by hydrogenation of a mixture of aspartame and 3,3-dimethylbutyraldehyde with a catalyst such as Pd on carbon.
A number of other synthetic routes to neotame have been explored. Certain of these routes, for example, the coupling of phenylalanine methyl ester to neo-aspartic anhydride as disclosed in U.S. Pat. No. 6,077,962, and methods which employ Z-aspartame (&agr;- and &bgr;-mixture) as a starting material, as disclosed in U.S. Pat. No. 5,302,743 and JP 60-075497, do not yield pure &agr;-neotame. In fact, both the desired &agr;-isomer and the undesirable &bgr;-isomer of neotame are obtained. Since &agr;-neotame is mainly employed in foods for human consumption, it is extremely important that &agr;-neotame exist in a highly purified state. Thus, it is clear that there is a need for a purification strategy which selectively produces the pure &agr;-isomer of neotame.
SUMMARY OF THE INVENTION
This invention is directed to a process of purifying N-[N-(3,3-dimethylbutyl)-L-&agr;-aspartyl]-L-phenylalanine 1-methyl ester (&agr;-neotame) comprising the steps of providing a mixture of (i) the &agr;- and &bgr;-isomers of neotame and (ii) an organic solvent; contacting the mixture with aqueous metal ion such that the &bgr;-isomer chelates the metal ion and forms a metal complex; and partitioning the metal complex from the &agr;-isomer of neotame.
DETAILED DESCRIPTION
The present invention is directed to a purification scheme for N-[N-(3,3-dimethylbutyl)-L-&agr;-aspartyl]-L-phenylalanine 1-methyl ester (&agr;-neotame). The purification scheme of the present invention is particularly suitable for use when neotame is produced by any method which results in the coexistence of both the &agr;- and &bgr;-isomers of neotame.
According to the present invention, a mixture of the &agr;- and &bgr;-isomers of neotame in an organic solvent is contacted with aqueous metal ion such that the &bgr;-isomer chelates the metal ion and forms a metal complex; the metal complex is then partitioned from the &agr;-isomer of neotame. In effect, the separation is between a complex of one isomer and the zwitterionic form of the other isomer.
According to the first step of the present invention, a mixture of (i) the &agr;- and &bgr;-isomers of neotame and (ii) an organic solvent is provided.
The mixture can take the form of a reaction mixture in which neo-aspartic anhydride was coupled with phenylalanine methyl ester or a reaction mixture in which Z-aspartame (mixture of &agr;- and &bgr;-aspartame) was employed to form neotame. Alternatively, the mixture can be formed by adding an organic solvent to such a reaction mixture or to a dry mixture of the &agr;- and &bgr;-isomers of neotame.
Any organic solvent which has a different degree of solvation power for the metal complex of the &bgr;-isomer and the &agr;-isomer of neotame is suitable for use in the present invention. Suitable organic solvents include, without limitation, glycerol, toluene, chloroform, dichloromethane, butyl acetate, ethyl acetate and mixtures thereof. One of ordinary skill in the art will readily appreciate that the greater the difference between the solubilities of the metal complex of the &bgr;-isomer and the &agr;-isomer of neotame in a given solvent, the more useful the solvent will be for purposes of the present invention.
According to the second step of the present invention, the mixture is contacted with aqueous metal ion such that the &bgr;-isomer of neotame chelates the metal ion to form a metal complex. Suitable metal ions include, without limitation, Cu
2+
, Zn
2+
, Fe
2+
, Fe
3+
, Mn
2+
, Ca
2+
and Mg
2+
. Preferably the metal ion is Cu
2+
. One of ordinary skill in the art will readily appreciate that any metal ion capable of forming a metal complex with the &bgr;-isomer of neotame is suitable for use in the present invention. Typically a 1-2 equivalent amount, and preferably an equimolar amount, of metal ion is used as compared to the amount of &bgr;-isomer present.
Typically, contacting the mixture with aqueous metal ion is accomplished by adding to the mixture of the &agr;- and &bgr;-isomers an aqueous solution containing a metal salt such as a sulfate, chloride or nitrate dissolved therein.
After addition of the metal ion to the solution, the pH is optionally but preferably manipulated to be in a range from about 2 to about 9, more preferably from about 3 to about 8, and most preferably from about 4 to about 7. The formation of a metal complex of the &bgr;-isomer of neotame is maximized at these pH conditions. Typically, the pH is manipulated by the addition of an acid, a base or both.
Simultaneously with the formation of the metal complex of the &bgr;-isomer, a biphasic system comprising an aqueous layer and an organic layer is created. The organic layer primarily contains the metal complex of the &bgr;-isomer of neotame, while the aqueous layer primarily contains the &agr;-isomer of neotame.
According to the third step of the present invention, the metal complex is partitioned from the &agr;-isomer of neotame.
Partitioning may be achieved by any known means, including, without limitation, liquid/liquid extraction, solid/liquid extraction and chromatography. One of ordinary skill in this art will readily appreciate that the metal complex may exist as a solid or in solution, depending upon the organic solvent used, the purification conditions, etc. Likewise, one of ordinary skill in this art will readily appreciate that the &agr;-isomer exists in solution at this point. If liquid/liquid extraction is to be carried out, i.e., both the metal complex and the &agr;-isomer are in solution, a simple separatory funnel, continuous extractor or counter-current device may be employed. If chromatography is to be used, any stationary phase/solvent combination which enables resolution of the &agr;-isomer is suitable.
After partitioning the metal complex from the &agr;-isomer, the aqueous solution containing the &agr;-isomer may optionally be acidified. Preferably, a pH of about 2-3 is achieved in such a step. This acidification step is typically accomplished by adding an acid to the aqueous solution containing &agr;-neotame and aids in the purification of the &agr;-neotame.
After partitioning the metal complex from the &agr;-isomer, the &agr;-isomer solution can be treated in order to form neotame crystals. Optionally, the solution may be concentrated or further processed and then treated such that &agr;-neotame crystals form. Such treatments include, without limitation, the addition of water or the addition of another solvent which causes &agr;-neotame crystals to form. Furthermore, other additives which facilitate the formation of &agr;-neotame crystals may be added. An example of such an additive is &agr;-neotame seed crystals. Additionally, the treatments may include the selection and maintenance of an appropriate temperature or temperature range at which to conduct the treatments. Optionally the treatments may also include the selection and maintenance of an appropriate pH or pH range at which to conduct the treatments.
REFERENCES:
patent: 5432262 (1995-07-01), Takemoto et al.
patent: 550171
Haar, Jr. Joseph P.
Prakash Indra
Zhao Robert Y.
Fitzpatrick ,Cella, Harper & Scinto
McKane Joseph K.
Murray Joseph
The NutraSweet Company
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