Chemistry: natural resins or derivatives; peptides or proteins; – Peptides of 3 to 100 amino acid residues – Synthesis of peptides
Reexamination Certificate
1999-10-25
2001-04-24
Low, Christopher S. F. (Department: 1653)
Chemistry: natural resins or derivatives; peptides or proteins;
Peptides of 3 to 100 amino acid residues
Synthesis of peptides
C530S338000, C530S345000, C560S001000
Reexamination Certificate
active
06222013
ABSTRACT:
The invention relates to a process for the esterification of an amino acid or peptide in which the amino acid or peptide is converted into the corresponding ester in the presence of a monoalkyl hydrosulphate having the general formula ROSO
3
H, where R represents an alkyl group.
A similar process is known from Khim. Ind (Sofiya), 58(10), 445-7, 1986, which describes the esterification of aliphatic amino acids with the aid of methylhydrosulphate.
A drawback of the known process is that in theory at least 2 equivalents of methylhydrosulphate are required for the esterification and in experiments even 3.75 equivalents to achieve a yield of 93-94% while the reaction time required for this is relatively long.
The invention provides a process for the esterification of amino acids and peptides that does not have the aforementioned drawbacks.
This is achieved according to the invention in that the hydrosulphate is prepared, in the presence of the amino acid or peptide, from chlorosulphonic acid and an alcohol having the general formula ROH, where R has the same meaning as above.
Suprisingly, it has been found that a conversion higher than 99% can be achieved, in a reaction time of less than an hour, using a substantially equimolar amount of chlorosulphonic acid calculated with respect to the amount of aminoacid or peptide. Contrary to expectations, no reaction was found to take place between the amino function of the amino acid or peptide and chlorosulphonic acid to form the corresponding N-substituted amidosulphonic acid. Another important advantage of the process according to the invention is that the reaction can be carried out at high amino acid or peptide concentrations in alcohol and that high ester concentrations proved possible.
While it is true that PL-B-159729 discloses an esterification in a methanol solution which contains methylhydrosulphate (CH
3
OSO
3
H), with the methylhydrosulphate being formed in situ from methanol and chlorosulphonic acid, the said patent publication describes the esterification of 3-amino-pyrazine-2-carboxylic acid. Besides the fact that the said patent publication deals with a fundamentally different compound, it claims that the esterification requires in theory at least 2 equivalents of chlorosulphonic acid (ClSO
3
H), 4 equivalents of chlorosulphonic acid being needed in experiments, and the reaction time amounting to many hours.
In principle, all amino acids and peptides, which may be optically active, for example dipeptides, can be esterified by the process according to the invention, for example &agr;-amino acids, both aromatic and aliphatic, in particular phenylglycine, which may or may not be substituted, for example p-hydroxyphenylglycine, phenylalanine, tyrosine, proline and valine. The hydroxy function of for example-hydroxyphenylglycine, too, proved not to react with chlorosulphonic acid. Known dipeptides are for example L-alanyl-L-proline (AlaPro) or L-aspartyl-L-phenylalanine methyl ester (APM). It has been found that essentially no racemisation occurred in the esterification of optionally active compounds.
An alcohol use may in principle be made of any alcohol which may normally be used for acid-catalysed esterification, for example alcohols having the formula ROH, where R represents an alkyl group, in particular an alkyl group having 1-20 C atoms. Preferably, use is make of alcohols having the formula ROH, where R represents an alkyl group, in particular a lower alkyl group having 1-5 C atoms, for example methanol, ethanol, (i- or n-)propanol, (i- or n-)-butanol. Particularly good results are achieved with primary alcohols.
The temperature at which the esterification reaction takes place is not particularly critical and in practice usually lies between 30° C. and the reflux temperature, preferably between 50° C. and 120° C. The optimum temperature may vary depending on the desired ester, that is, the applied alcohol and amino acid or peptide. It has been found that the formation of the methyl ester usually proceeds optimally at the reflux temperature, whereas a temperature somewhat lower than the reflux temperature often gives better results for the higher alkyl esters. The optimum temperature can readily be determined by those skilled in the art.
Any inert, organic solvent can be used as solvent (or suspending agent) in the process according to the invention. It is preferred for the alcohol employed in the esterification to be used as a solvent.
The chlorosulphonic acid to amino acid or peptide molar ration preferably is between 0.8 and 2.0, in particular, a (slightly) more than equimolar ratio of chlorosulphonic acid to amino acid or peptide is chosen, for example between 1.0 and 1.3.
In a suitable embodiment of the process according to the invention the amino acid or peptide to be esterified is suspended or dissolved in the alcohol. Subsequently, chlorosulphonic acid is added, preferably metered drop-wise for a period of for example 2-20 minutes, preferably 4-10 minutes, to this solution, optionally with cooling it the temperature is not to rise directly to the reflux temperature or the envisages esterification temperature. Subsequently, the temperature is raised to the desired level, for example between 50 and 120° C. and the reaction mixture is kept at this temperature for a period of for example between 10 minutes and 10 hours depending on the temperature and the desired ester. When the conversion is (virtually) complete the ester is recovered, for example by evaporating the alcohol and extraction at increased pH using an (in essence commonly known) organic solvent that is immiscible or poorly miscible with water, for example toluene, dichloromethane or methylisobutylketone (MIBK). In some cases, for example in the case of the esters of p-hydroxyphenylglycine, it will also be possible to recover the ester directly via crystallization from water at increased pH (pH 8-10), whether or not after completely or partially evaporating the alcohol.
The invention is now illustrated by the examples without being limited thereto.
REFERENCES:
patent: 5424476 (1995-06-01), Takemoto
patent: 0 544 205 (1993-06-01), None
Chemical Abstracts, vol. 107, No. 13, Sep. 28, 1987, Columbus, Ohio, US; abstract No. 115953h. I. Rusev et al: “Esterification of alpha-amino carboxylic acids with monomethyl sulfate” p. 665; XP002050388 cited in the application see abstract & Khim. Ind., vol. 58, No. 10, 1986, pp. 445-447.
Chemical Abstracts, vol. 122, No. 7, Feb. 13, 1995, Columbus Ohio, US; abstract No. 51410w. A Wolksi et al.: “Method for preparation of the mothyl ester of 3-aminopyrazine-2-carboxylic acid” p. 1097; XP002050389 cited in the application see abstract & PL 159 729 A (Polfa) Jan. 29, 1993.
Chemical Abstracts, vol. 119, No. 11, Sep. 13, 1993, Columbus, Ohio, US; abstract No. 117812h. R. A. El-Sayed et al: “Some new reactions of cinnamoylmorpholine derivatives with amino acids” p. 999; XP002050390 see abstract & Pak. J. Sci. Ind. Res., vol. 35, No. 11, pp. 434-437.
Boesten Wilhelmus H. J.
Quaedflieg Peter J. L. M.
DSM NV
Low Christopher S. F.
Lukton David
Pillsbury Madison & Sutro LLP
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