Preparation of N-Z-protected N-methylated amino acids

Details Preparation of N-Z-protected N-methylated amino acids Preparation of N-Z-protected N-methylated amino acids

1996-11-19

2001-04-17

Gerstl, Robert (Department: 1626)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

Reexamination Certificate

active

06218572

ABSTRACT:

The present invention relates to a novel process for preparing N-protected N-alkylated amino acids. N-alkylated amino acids are important constituents of highly biologically active peptides. Examples of such peptides are cyclosporins (Wenger; Agnew. Chem. 1985, 97, 88) and dolastatins (G. Pettit; J. Nat. Prod. 1981, 44, 482).
The preparation of N-monoalkylated amino acids without using N-protective groups has achieved no practical significance.
The processes described in the literature require excesses of reagents which are costly and difficult to handle (silver oxide, sodium hydride) (DE 2855786; Benoiton; Can. J. Chen. 1977, 55, 906). In addition, the esterification of the acid functionality (Olsen, J. Org. Chem. 1970, 35, 1912) which often occurs at the same time is unwanted and troublesome. N-protected N-alkylated amino acid derivatives are particularly advantageous for use in peptide chemistry because the alkylated amino group cannot react and the acid functionality does not have to be unblocked.
The most practicable process presented to date is that of Runge (WO 90/06914), wherein t-butyloxycarbonyl-protected amino acids are mixed with alkyl iodide, and the methylation takes place after addition of potassium tert-butanolate.
However, the protective group chosen for the process described in WO 90/06914 is not optimal for preparing larger amounts of N-protected N-alkylated amino acids. When the benzyloxycarbonyl protective group (abbreviated to Z hereinafter) is used in place of the t-butyloxycarbonyl group under the conditions of WO 90/06914, only a very moderate yield of the required N-alkylated amino acid is obtained, in addition to many byproducts.
The use of the Z protective group under the conditions described by Runge has not hitherto been described, presumably because it is evident from the relevant literature (Fieser & Fieser; Reagents for Organic Synthesis Vol. 1, 110 Wiley 1967; Houben-Weyl Vol. XV/1, 64) that the Z protective group is readily hydrolyzed under alkaline conditions.
We have found, surprisingly, that the required product can be obtained in very good yield and purity when the reaction is carried out suitably.
The present invention relates to a process for preparing N-proctected N-alkylated amino acids of the formula I:
where
R
S
is a conventional protective group in peptide synthetis,
R
1
is the side chain of a proteinogenous amino acid or derivative thereof,
R
2
is hydrogen, alkyl, alkenyl, alkynyl, aryl or hetaryl and
R
3
is C
1-2
-alkyl
which comprises mixing a compound of the formula II:
where R
1
, R
2
and R
s
have the abovementioned meanings, with a solution of potassium tert-butanolate in a non-protic organic solvent and subsequently adding a C
1-2
-alkyl halide.
The substituents in formula I preferably have the following meanings:
R
1
the residue of a proteinogenous amino acid, in particular H, CH
3
, CH
2
—CH
2
—CH
3
, CH(CH
3
)—CH
2
—CH
3
, CH
2
—CH—(CH
3
)
2
, CH
2
—C
6
H
5
, CH
2
—C
6
H
4
O—C(CH
3
)
3
, CH
2
—C
6
H
4
OCH
2
—C
6
H
5
, CH
2
—O—CH
3
, CH
2
—O—C(CH
3
)
3
, CH
2
—O—Si(CH
3
)
3
, CH
2
—O—CH
2
—C
6
H
5
, CH
2
—C
6
H
4
O—Si(CH
3
)
3
, CH(CH
3
)—O—CH
3
, CH(CH
3
)—O—C(CH
3
)
3
, CH(CH
3
)—O—Si(CH
3
)
3
, CH(CH
3
)—O—CH
2
—C
6
H
5
, CH
2
—S—C(C
6
H
5
)
3
, CH
2
—S—CH(C
6
H
5
)
2
, CH
2
—S—CH
2
—C
6
H
5
, CH
2
CH
2
CH
2
CH
2
N—(CO)
2
C
6
H
4
, particularly CH—(CH
3
)
2
,
R
2
H, C
1-4
-alkyl, in particular CH
3
, CH
2
—CH
3
, CH
2
—CH
2
—CH
3
, CH(CH
3
)—CH
2
—CH
3
, CH
2
—CH
2
—CH
2
—CH
3
, CH
2
—CH—(CH
3
)
2
, C(CH
3
)
3
, CH
2
—CH═CH
2
, CH
2
—C≡CH, C
6
H
5
, C
6
H
4
CH
3
,
R
3
methyl,
R
S
COOCH
2
—C
6
H
5
(=Z), COOC(CH
3
)
2
—C
6
H
5
(=DMZ), COOCH
2
—C
6
H
4
(4-Br) (=BZ), COOCH
2
—C
6
H
4
(4-Cl)(=CZ), COOCH
2
—C
6
H
4
(3-Cl)(=3CZ), COOCH
2
—C
6
H
4
(2-Cl) (=2CZ), COOCH
2
—C
6
H
4
(4-OCH
3
) (=MOZ), COOCH
2
—C
6
H
4
(4-NO
2
)(=NZ), COOCH
2
—C
6
H
4
(2-NO
2
) (=2NZ), COOCH
2
—C
6
H
4
(4-OCOCH
3
)(=AcOZ).
The particularly preferred protective group is Z.
The reaction according to the invention is expediently carried out under an inert protective gas such as helium or argon. It is particularly advantageous to use nitrogen.
Suitable solvents for the reaction are non-protic solvents, in particular tetrahydrofuran, 1,2-dimethoxyethane, diethoxymethane, dioxane, dichloromethane, trichloromethane, tetrachloromethane and N,N-dimethylethyleneurea, N,N-dimethylpropyleneurea and N-methylpyrrolidone.
Tetrahydrofuran is preferably used.
The bases used are sodium t-butanolate or potassium t-butanolate. Potassium t-butanolate is preferably employed. From 2.2 to 10, preferably 3.5 to 6, equivalents of the base are used, based on II.
Suitable alkylating agents are methyl and ethyl bromides and, in particular, methyl and ethyl iodides. From 1.2 to 6, in particular 2.5, equivalents of the alkylating agent are used, based on II.
The reaction takes place at from −40° C. to +1000° C. It is advantageously carried out at from −10° C. to +20° C.
The reaction can be worked up by distillation, extraction, crystallization, chromatography or a combination thereof. It is preferred to carry out an extraction after acidification and subsequently to crystallize the compound I.
The acidification can be carried out with acids such as hydrochloric acid, sulfuric acid, phosphoric acid, hydrobromic acid, acetic acid, methanesulfonic acid. Sulfuric acid is preferably used. Extractants which can be used are water-immiscible solvents such as pentane, hexane, heptane, octane, petroleum ether, ethyl acetate, diethyl ether, diisopropyl ether, methyl t-butyl ether, dichloromethane, xylene, chloroform, tetrachloromethane, benzene, toluene, particularly preferably toluene. The crystallization can take place from organic solvents such as pentane, hexane, heptane, octane, petroleum ether, ethyl acetate, dichloromethane, chloroform, tetrachloromethane, benzene, toluene, xylene, acetone, 2-butanone, methanol, ethanol, n-propanol, isopropanol, diethyl ether, diisopropyl ether, methyl t-butyl ether and mixtures thereof, preferably toluene and toluene/heptane mixtures. If the amino acids contain other reactive groups, for example cysteine, serine, tyrosine, lysine, threonine, these must be protected during the reaction.
Particular advantages of the process are that
1) low-cost starting materials with advantageous handling properties are used (Z-amino acids),
2) no simultaneous esterification takes place,
3) the yield is much greater than in processes disclosed in the literature,
4) the isolated product has high purity and can be used without further purification for subsequent reactions, and
5) no racemization takes place.


REFERENCES:
patent: 4250085 (1981-02-01), Atherton et al.
patent: 28 55 786 (1978-12-01), None
patent: 90/06914 (1990-06-01), None
Greene, Protective Groups in Organic Synthesis pp. 407, 412, 441, 442, 1991.*
Mcomie, Protective Groups in Organic Chemistry pp. 56-59, 1973.*
Green, Protective Groups in Organic Synthesis pp. 315-316, 327-330, 335-338, 441-444, 1991.*
Green Protective Groups in Organic Synthesis 2nd Ed pp. 335-340, 1991.*
Synthetic Comm., 24(17), 2429, 1994.*
McOmie, Protective Groups in Organic Chemistry pp. 56-57, 1973.*
Wenger,Angew. Chem., vol. 97, p. 88-96.
Pettit et al.,J. Nat. Prod., 1981, vol. 44, pp. 482-485, Jul./Aug. 1981.
Cheung et al.,Can. J. Chem., vol. 55, 1977. pp. 906-910.
Olsen,J. Org. Chem., vol. 35, No. 6, 1970, pp. 1912-1915.
Fieser & Fieser, Reagents for Organic Synthesis vol. 1, 110 Wiley 1967.
Houben-Weyl vol. XV/a, 64.

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