Method for preparing β-alaninamides

Abstract

The invention relates to β-alanine amides of general formula (I), wherein: R1 represents hydrogen or C1-6 alkyl that is optionally substituted by hydroxy, amino, carboxy, carbamoyl, methylmercapto, guanidino, optionally substituted aryl or heteroaryl, and; R2 represents hydrogen or R1 and R2, together, form a group of formula —(CH2)n—, wherein n is 3 or 4. Said β-alanine amides are produced without using an amino protective group by reacting the corresponding amine with a cyanoacetic ester in order to form an acetamide and by effecting a subsequent catalytic hydrogenation. The method is suited, in particular, for producing carcinine (β-alanyl-histamine, R?1 =imidazol-4-ylmethyl, R2=H), a naturally occurring pseudo dipeptide, which is used as an active ingredient having an antioxidative effect in medicaments and cosmetics.

Description

The present invention relates to a method for preparing amides of β-alanine, in particular pseudodipeptides such as, for example, carcinine. It furthermore relates to novel cyanoacetamides as intermediates in the method of the invention.

Carcinine (β-alanylhistamine) is a naturally occurring pseudodipeptide of the structure below, which can be isolated from animal tissue. Carcinine has an antioxidative action and has been proposed as drug for certain complications of diabetes (U.S. Pat. No. 5,561,110), in particular cataract (U.S. Pat. No. 5,792,784), and as component of cosmetic preparations (U.S. Pat. No. 6,280,715).

The known syntheses of carcinine from the components histamine and β-alanine require the use of protective groups and/or activated derivatives and are therefore not very suitable for the inexpensive preparation of large amounts.

It was therefore an object of the present invention to provide a method suitable for the technical synthesis of carcinine and other pseudodipeptides of β-alanine, which does not require the use of protective groups or expensive derivatives and which uses only readily accessible starting materials.

According to the invention, this object is achieved by the method of the invention.

The β-alaninamides which can be produced according to the invention have the general formula

Here, R¹ is hydrogen or C_1-6-alkyl which is unsubstituted or substituted with hydroxy, amino, carboxy, carbamoyl, methylmercapto, guanidino, unsubstituted or substituted aryl or heteroaryl and R² is hydrogen, or R¹ and R² together form a group of the formula —(CH₂)_n— where n is 3 or 4.

These compounds may be present in a neutral form or, after protonation of the primary amino group, as salts with acids. Some of the compounds, in particular those containing imidazolyl radicals, may also be present in a plurality of tautomeric forms or as a mixture of such forms.

Here and below, C_1-6-alkyl means all linear or branched primary, secondary or tertiary alkyl groups having from 1 to 6 carbon atoms, i.e. for example methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, iso-butyl, pentyl, isopentyl, neopentyl, 2-methylbutyl, hexyl, 2-methylpentyl, 3-methylpentyl etc. This applies accordingly to C_1-10-alkyl, in which case, for example, groups such as octyl or 2-ethylhexyl are also included in addition to the groups already mentioned.

Aryl means mono- or polycyclic carbocyclic aromatic groups such as, in particular, phenyl or naphthyl and, accordingly, heteroaryl means mono- or polycyclic heterocyclic aromatic groups having one or more heteroatoms, in particular imidazolyl or indolyl. Where appropriate, aryl groups may also have one or more of the abovementioned substituents, in particular hydroxyl groups as in 4-hydroxyphenyl, for example.

It has been found that amines of the general formula

in which R¹ and R² are as defined above, can be reacted with a cyanoacetic ester of the general formula

in which R³ is C_1-10-alkyl, to give a cyanoacetamide of the general formula

in which R¹ and R² are as defined above, or a corresponding salt, and the cyanoacetamide (IV) can be converted to the target compound (I) or a corresponding salt by catalytic hydrogenation.

R¹ is preferably hydrogen or unsubstituted or substituted C_1-4-alkyl, in particular methyl, isopropyl, isobutyl, sec-butyl, indol-3-ylmethyl, benzyl, p-hydroxybenzyl, 2-(methylsulfanyl)ethyl, hydroxymethyl, 1-hydroxyethyl, carbamoylmethyl, 2-carbamoyl-ethyl, carboxymethyl, 2-carboxyethyl, 4-aminobutyl or 3-guanidinopropyl.

R² is preferably hydrogen.

Particularly preferably, R¹ is imidazol-4-ylmethyl or 3-methylimidazol-4-ylmethyl and R² is hydrogen.

R³ is preferably methyl or ethyl.

If the amine (II) is present as a salt in which the primary amino group is protonated, the latter must first be deprotonated by adding a base. Bases which may be used here are in principle all bases which are more basic than the primary amino group. Preference is given to using a medium-strength to strong base. Examples of compounds suitable for this are alkali metal hydroxides such as sodium or potassium hydroxide, tertiary amines such as triethylamine, 4-dimethylaminopyridine, 1,4-diaza[2.2.2]bicyclooctane, bicyclic amidines (“DBN”, “DBU”) and, in nonaqueous solvents, alkali metal alkoxides such as sodium methoxide or sodium ethoxide and, in aprotic solvents, also alkali metal hydrides and amides, such as, for example, sodium hydride or sodium amide. The base is preferably used in stoichiometric or nearly stoichiometric amounts.

Preferred solvents for the first stage are polar protic or aprotic solvents such as water, C_1-4-alkanols such as, for example, methanol or ethanol and amides such as, for example, N,N-dimethylformamide, N,N-dimethyl-acetamide, 1-methyl-2-pyrrolidone or 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU).

Preferred catalysts used for the hydrogenation are metal catalysts based on nickel, cobalt, copper, rhodium, palladium, ruthenium or platinum, which are, where appropriate, applied to a support. These include, for example, nickel and cobalt catalysts of the Raney type, finely divided platinum (obtained, for example, by reduction of PtO₂), rhodium, palladium or platinum on activated carbon or aluminum oxide or cobalt on silicon dioxide (silica).

Particular preference is given to Raney nickel and Raney cobalt and to rhodium on activated carbon or aluminum oxide.

Solvents which may be used in the hydrogenation are the usual solvents for hydrogenation of nitrites to amines, such as, for example, water, concentrated aqueous ammonia solution, methanol, ethanol, N,N-dimethyl-formamide or mixtures of said solvents.

The cyanoacetamides of the general formula

in which R¹ and R² are as defined above, and the salts thereof are novel and are likewise subject matter of the invention.

Preference is given to those cyanoacetamides (IV) in which R¹ is unsubstituted or substituted imidazol-4-ylmethyl and R² is hydrogen. Examples of suitable substituents here are C_1-6-alkyl groups, in particular methyl.

Particularly preferred cyanoacetamides (IV) are N-(cyanoacetyl)histamine of the formula

and the salts and tautomers thereof and also N-(cyanoacetyl)-3-methylhistamine of the formula

and the salts thereof.

The following examples illustrate the carrying-out of the method of the invention and the preparation of the compounds of the invention and are not to be regarded as being limiting.

EXAMPLE 1

2-Cyano-N-[2-(1(3)H-imidazol-4-yl)ethyl]acetamide (N-cyanoacetylhistamine)

7.60 g (67.2 mmol) of ethyl cyanoacetate were added dropwise at 60° C. to a solution of 5.00 g (45 mmol) of histamine in 50 g of ethanol. After 2 h at 80° C., the reaction mixture was concentrated in a rotary evaporator and the residue was purified by means of flash column chromatography on silica gel (eluent: ethyl acetate→ethyl acetate/methanol 1:1 gradient).

Yield: 6.61 g (82%) ¹H-NMR (DMSO-d₆, 400 MHz): δ=11.8 (br., 1H); 8.28 (t, 1H), 7.52 (s, 1H); 6.80 (s, 1H); 3.60 (s, 2H); 3.30 (q, 2H); 2.65 (t, 2H).

¹³C NMR (DMSO-d₆, 100 MHz): δ=161.92; 134.63; 134.21; 116.64; 116.13; 39.25; 26.63; 25.23.

LC-MS: m/z=179 ([M+H]⁺), 161, 149, 138, 112, 95, 83.

EXAMPLE 2

3-Amino-N-[2-(1H-imidazol-4-yl-ethyl]propionamide (β-alanylhistamine, carcinine)

92 mg of Rh/Al₂O₃ (5% Rh) were added to a solution of 1.00 g (5.61 mmol) of 2-cyano-N-[2-(1(3)H-imidazol-4-yl)ethyl]acetamide (prepared according to example 1) in 11.4 g of ethanol and 7.6 g of concentrated aqueous ammonia solution. The mixture was hydrogenated at 90° C. and a hydrogen pressure of 50 bar for 2 h. The catalyst was filtered off via Celite® and the filtrate was concentrated in a rotary evaporator. After drying at 50° C./20 mbar for a relatively long time, 0.89 g of crude carcinine with a content (HPLC) of 73% was obtained.

¹H-NMR (DMSO-d₆, 400 MHz): δ=12 (br., 1H); 7.95 (t, 1H), 7.50 (s, 1H); 6.77 (s, 1H); 3.6 (br., 1H); 3.25 (q, 2H); 2.74 (t, 2H); 2.63 (t, 2H); 2.15 (t, 2H). LC-MS: m/z=183 ([M+H]⁺), 166, 148, 124, 95.

Claims

1. A method for preparing β-alaninamides of the formula:

2. A method for preparing β-alaninamides of the formula:

3. The method as clamed in claim 2 wherein the catalyst used is the second stage is selected from the group consisting of rhodium on activated carbon, rhodium on aluminum oxide, Raney nickel and Raney cobalt.

4. N-(Cyanoacetyl) histamine of the formula:

5. N-(Cyanoacetyl)-3-methylhistamine of the formula:

6. The method as claimed in claim 1, wherein the catalyst used in the second stage is selected from the group consisting of rhodium on activated carbon, rhodium on aluminum oxide, Raney nickel and Raney cobalt.

7. The method as claimed in claim 2, wherein the catalyst used in the second stage is selected from the group consisting of rhodium on activated carbon, rhodium on aluminum oxide, Raney nickel and Raney cobalt.

8. A cyanoacetamide of the formula:

9. The method as claimed in claim 2, wherein R1 is unsubstituted imadazol-4-ylmethyl.

10. The cyanoacetoamide (IV) as claimed in claim 8, wherein R1 is unsubstituted imadazol-4-ylmethyl and R2 is hydrogen.