Preparation of amino acid derivatives

Abstract: A hydroxy-carboxylate ester is converted to an amino-acid-amide by replacement of the -OH group with -N₃, via, e.g., CH₃SO₂Cl and NaN₃; reacting with amine to convert the ester to amide; and hydrogenating -N₃ to -NH₂. The present process alleviates the conventional use of blocking-deblocking procedures.

Description: The present invention relates to a process for forming azido-amides. These are useful in the synthesis of optically active amino-amides which are useful in the preparation of sweetners and for other purposes. W. R. Huber, J.A.C.S. 77 , 112-116 (1955) discloses a method (p. 112) for making racemic mixtures of aminoacyl glycerides by treating an alpha-halogen ester with sodium azide to form an alpha-azido ester, which was then saponified to give the alpha-azido acid. This acid was then converted to the acid chloride with thionyl chloride, which was, in turn, reacted with glycerol, or a monoglyceride, or a diglyceride, to form a tri-, di-, or mono-alpha-azidoacyl glyceride. The formation of the glyceride was accomplished via a ketene intermediate and yielded a racemic mixture. The formal glyceride was hydrogenated to convert the azido group to an amino group. On p. 113, Col. 1, the paper states that an alpha-aminoacyl glyceride was treated with an alpha-azidoacyl chloride "to yield an alpha-azido-acylaminoacyl glyceride which, on hydrogenation, yielded the 'dipeptide-glyceride'." The relevant reactions are considered to be: In the Huber paper R is identified as a long chain fatty residue, such as laurin, palmitin, stearin, etc. Note that the ester moiety (glyceryl) is retained intact and is not converted to amide. My ester group, on the other hand, reacts with and is replaced by amine to form an amide group. In the Huber synthesis, the ester moiety (glyceryl) is retained in tact and is not converted to an amide, as presently achieved. Further, Huber's synthesis provides a product which is a racemic mixture and is not capable of achieving an optically active product nor one which is substantially free of one enantiomer as is possible by the present invention. The conversion of alkylsulphonyl esters to azido esters has been disclosed in DE-A-3,320,140 and US-A-3,869,407 in relation to arabinose and citric acid derivatives. As synthesis for forming optically active amino-amides in four sequential steps has now been devised. The present synthesis utilizes readily available hydroxy carboxylic acid esters and provides substantially one enantiomer of the resultant amino-amide product. The present invention provides a process of forming an azido-amide comprising (I) reacting the -OH group of a hydroxy-carboxylic ester of the formula: R-CH(OH)-(R') n -COOR'' wherein R is H or alkyl of 1-18 carbons; R' is -CH₂-; n is 0-8; and R'' is alkyl of 1-18 carbons, with an azido-coupler, thereby to replace the hydroxyl group with an azide-reactable group or to replace the hydrogen atom of the hydroxyl group and thereby convert the hydroxyl group into an azide reactable group; (II) reacting the product (I) with an azide salt, thereby to form an azido-carboxylic ester of the formula: R-CH(N₃)-(R') n -COOR'' wherein R, R', R'' and n are as hereinbefore defined; and (III) reacting the azido-carboxylic ester thus formed, or the corresponding carboxylic acid obtained by hydrolysis, with an amine which is a linear or branched alkylamine of the formula H x N(C n H 2n+1 ) 3-x wherein n = 1 to 18 and x = 1 or 2; a cycloalkylamine of the formula: wherein n is independently 1 to 8, y is 0 or 1 and x is independently 0 to 4, or the amine has the formula H₂NR° wherein R° is a branched member selected from the group consisting of fenchyl, diisopropyl carbinyl, d-methyl-t-butylcarbinyl, d-ethyl-5-butylcarbinyl, di-t-buylcarbinyl, 2-methylthio-2,4-dimethylpentane-3-yl, where at least one of R₁, R₂, R₃, R₄ is alkyl having from 1 to 4 carbon atoms and the remainder are hydrogen or alkyl having from 1 to 4 carbon atoms; X is O, S, SO, SO₂, C=O or CHOH; m is zero, 1, 2, 3 or 4; n and p are each zero, 1, 2 or 3, and the sum of n + p is not greater than 3; the sum of the carbon atoms in R₁, R₂, R₃ and R₄ is not greater than 6; and when both or R₁ and R₂ or R₃ and R₄ are alkyl they are methyl or ethyl. A hydroxy-carboxylate ester is converted to an amino-amide in four sequential steps: In Step I the hydroxyl group is replaced by a group capable of reacting with an azide salt. This group is called herein an azido-coupler, and it will be defined and described in detail later in this specification. The reaction in this first Step I is (using methane sulfonyl chloride as the azido-coupler exemplar with triethylamine as the base to neutralize by-product HCl): (I) R-CH(OH)-(R') n -C(:O)OR'' + CH₃SO₂Cl + Et₃N → R-CH(OSO₂CH₃)-(R') n -COOR'' + Et₃N.HCl. The thus-modified ester is then reacted with an azide salt; this completes the replacement of -OH with -N₃. Thus: (II) R-CH(OSO₂CH₃)-(R') n -COOR'' + NaN₃ → R-CH(N₃)-(R') n -COOR''+ NaSO₃CH₃. The azido-ester of reaction (II) is treated with an amine to yield the corresponding amide according to the following reaction: (III) R-CH(N₃)-(R') n -COOR''+ H₂N-R''' → R-CH(N₃)-(R') n -CONHR''' + R''OH. The azido-amide of reaction (III) may be hydrogenated to form the amino-amide according to reaction (IV) below: (IV) R-CH(N₃)-(R') n -CONHR''' + H₂ → R-CH(NH₂)-(R') n -CONHR'''. In each of the above equations: R is H or alkyl of 1-18 carbons; R' is -CH₂-; n is 0-8; R'' is alkyl of 1-18 carbons; and R''' is a monovalent organic radical. The present invention provides a synthesis for converting a hydroxy-carboxylate to an amino-amide in a series of simple steps, using generally inexpensive reactants and reagents. In the present process the hydroxyl group of the hydroxyl carboxylic ester is converted to a primary amino group, and the carboxylate group is converted to an amide group, all without the use of a blocking agent. The present invention is particularly useful in the preparation of the D-alanine-amides. In the past the only practical way to prepare the D-alanine-amides required starting with D-alanine, blocking the amino group, esterifying, reacting with an amine to convert the ester group to an amide group, and finally deblocking to form the desired D-alanine-amide. This blocking-deblocking process is described in U.S. Patent 4,411,925, Example 16, where N-t-butoxy-carbonyl-D-alanine is reacted with 3-amino-2,2,4,4-tetra-methylthietane to form the corresponding t-Boc-D-alanine-amide, which was then hydrolyzed to form the free alpha-amino-amide in 43% yield. D-alanine is relatively expensive. The present process has unexpectedly been found to achieve the same end product (D-alanine-amide) starting from lactic acid ester, which is relatively cheap. The D-enantiomer is formed in reaction II and no other transformation occurs during subsequent reactions which change the integrity of the optical center. The Hydroxy Carboxylic Esters The process is applicable to the esters of hydroxy carboxylic acids generally. These acids include: Alpha-hydroxy acids , e.g., glycolic, lactic, alpha-hydroxy butyric, alpha-hydroxy-n- and -iso-valeric acids; etc.; Beta-hydroxy acids , e.g., beta-hydroxy propionic acid, beta-hydroxybutyric, beta-hydroxy valeric acid; etc.; Gamma-hydroxy acids , e.g., gamma-hydroxy butyric acid, gamma-hydroxy valeric acid, etc.; Other hydroxy carboxylic acids which are useful include delta-hydroxy valeric acid, epsilon-hydroxy caproic acid, and the like. Azido-Coupler In Step I of the 4-step invention process, the hydroxyl group is replaced by a group capable of reacting with an azide salt (i.e., an azide-reactable group). This can be accomplished with what is herein called an "azido-coupler." There are at least two basic types of azido-coupler. One type (preferred) reacts with and replaces the H atom of the -OH group. Another type replaces the whole -OH group. The first type includes, by way of example, methane sulfonylchloride, toluene sulfonylchloride, benzyl sulfonylchloride, and the like. The class reaction (with methane sulfonylchloride as exemplar) is given above. The resulting methane sulfonyl derivative reacts readily with the azide salt in Step II, as indicated above. The second type of azido-coupler includes, by way of example, PX₃ or PX₅ where X represents a halogen atom, preferably chlorine or bromine; and SOCl₂. The class reaction (with thionyl chloride as exemplar) is -CH(OH)- + SOCl₂ → -CH(Cl)- + HCl +SO₂. The chloro-compound reacts readily with the azide salt in Step II above. The Azide Salt The cheapest and easiest to use is sodium azide, NaN₃ although KN₃, TlN₃, and other azide salts may be used. As is well known, the azides are heat-sensitive, and care should be taken in their use. The Amine Substantially any primary or secondary amine is operable in the present invention process. The following are useful: Alkyl amines (linear or branched) : H x N(C n H 2n+1 ) 3-x where n = 1 to 18 and x = 1 or 2. Cycloalkyl amines : where n is independently 1-8; y is O or 1; and x is independently 0-4. Aromatic amines (aralkyl, alkaryl, and the like): where n = 1-4 and x = independently 0-5. Carbo- and heterocyclic amines : This group includes H₂NR°, where R°, e.g., is a branched member selected from the group consisting of fenchyl, diisopropyl carbinyl, d-methyl-t-butylcarbinyl, d-ethyl-t-butylcarbinyl, di-t-butylcarbinyl, 2-methylthio-2,4-dimethylpentan-3-yl, where at least one of R₁, R₂, R₃, R₄ is alkyl having from 1 to 4 carbon atoms and the remainder are hydrogen or alkyl having from 1 to 4 carbon atoms; X is O, S, SO, SO₂, C=O or CHOH; m is zero, 1, 2, 3, or 4; n and p are each zero, 1, 2, or 3, and the sum of n + p is not greater than 3; the sum of the carbon atoms in R₁, R₂, R₃, and R₄ is not greater than 6; and when both or R₁ and R₂ or R₃ and R₄ are alkyl they are methyl or ethyl. A preferred amine of this group is 3-amino-2,2,4,4-tetramethylthietane. Amines of the above carbo- and hetero-cyclic groups are known. See U.S. Patent 4,517,379, especially at Col. 12, lines 14 et seq. The contents of U.S. Patent 4,517,379 in its entirety are herein incorporated by reference. In summary, the amine can be an amino group attached to substantially any organic radical. Hydrogenation The hydrogenation reaction of step (IV) can be carried out using conventional hydrogenation conditions and catalysts. For example, the reaction can be carried out using hydrogen (low to moderate pressure) in the presence of a supported catalyst of Pd, Pt, Rh and the like or a Raney catalyst of Co, Ni, Fe and the like. It has been observed that the hydrogenation can be accomplished under mild conditions and even with the use of a Raney catalyst without the need of additional hydrogen being present, as illustrated by Example 3 hereinbelow. This was possible because Raney catalysts conventionally carry a substantial amount of adsorbed hydrogen, resulting from the activation process and because the hydrogen requirement in the Example was comparatively modest. The following Examples are given for illustrative purposes only and are not meant to be a limitation on the present invention as defined by the claims appended hereto. All parts are by weight unless otherwise stated. Example 1 An equal molar solution of (S)-ethyl lactate and triethylamine in dry methylene chloride was treated with an equal molar amount of methane sulfonylchloride at 5-10°C. After the addition was complete, the mixture was filtered to remove triethylamine hydrochloride. The filtrate was then washed with water, dried over MgSO₄, and concentrated to give a high yield of ethyl-(S)-2[(methylsulfonyl)oxy]propionate. Example 2 One mole of the compound made in Example 1 was added with vigorous stirring to a solution of 1.1 mole of sodium azide in 200 mL of water and 100 mL of methanol. The resulting mixture was stirred for 24 hours, extracted with methylene chloride, dried over MgSO₄ and concentrated in vacuo at 25°C to give (R)-ethyl-2-azidopropionate in nearly quantitative yield. Example 3 A 14.3 g sample of (R)-ethyl-2-azidopropionate was taken up in 50 mL of isopropylamine and allowed to stand for 24 hours. After stripping excess amine, 14.8 g of pure N-isopropyl-2-azidopropionamide was obtained. The amide thus obtained, 3.12 g, was stirred with 5.0 g of Raney nickel catalyst in 40 mL of isopropanol until nitrogen evolution ceased. The solution was filtered and concentrated in vacuo at 40°C to give 2.5 g of crude N-isopropyl-D-alanine amide. Example 4 CH₃CH(N₃)COOC₂H₅→ CH₃CH(N₃)COOH Into a 2L beaker was placed 200 mL of methanol, 800 mL of water, and 425 g (2.97 moles) of the azido-ester of Example 2. The resulting biphasic system was then saponified with 6N sodium hydroxide at 0-7°C. pH = 12.2. After complete saponification the pH was lowered to 8.0 and the solution was extracted with 2 X 500 mL of CH₂Cl₂. The methanol was then removed in vacuo and the aqueous phase acidified to pH = 1.0 with concentrated HCl. The resulting solution was then extracted with 3 X 300 mL CH₂Cl₂, washed with saturated brine, dried over MgSO₄, filtered, and concentrated in vacuo at 25°C to give 323.25 g ( 95% yield) of (R)-2-azido propionic acid as a pale yellow oil. Example 5 Into a sealed flask was placed 1.15 g of (R)-2-azido-propionic acid, 25 mL of dry tetrahydrofurane and 1.1 g of triethylamine. The resulting solution was chilled to -10° to -7°C and treated with 1.36 g of isobutylchloroformate. After 5 minutes a solution of 1.45 g of 3-amino-2,2,4,4-tetramethylthietane in 10 mL of tetrahydrofurane was added. The solution was allowed to warm to room temperature and was stirred an additional 6 hours. The solution was then stripped of solvent, taken up in dichloromethane, and washed with 3 X 25 mL of 1N hydrochloric acid and 1 X 25 mL of saturated sodium bicarbonate solution. The organic phase was then dried over MgSO₄, filtered, and concentrated in vacuo at 40°C to give 1.94 g of azido-amide. Example 6 The azido-amide, 1.0 g, was dissolved in 25 mL of isopropyl alcohol and cooled to 0 - 5°C. To this was added 2.5 g of Raney nickel catalyst. After the vigorous evolution of nitrogen ceased (4 to 10 minutes) the reaction mass was filtered from the catalyst and concentrated in vacuo at 40°C to give a green oil. This material was taken up in ether, washed with a pH = 9.0 solution of ethylene diamine tetraacetic acid, dried over Na₂SO₄ and concentrated to give 795 mg of a colorless oil homogeneous on silica gel TLC with consistent spectra properties.

Claims: Process of forming an azido-amide comprising (I) reacting the -OH group of a hydroxy-carboxylic ester of the formula: R-CH(OH)-(R') n -COOR'' wherein R is H or alkyl of 1-18 carbons; R' is -CH₂-; n is 0-8; and R'' is alkyl of 1-18 carbons, with an azido-coupler, thereby to replace the hydroxyl group with an azide-reactable group or to replace the hydrogen atom of the hydroxyl group and thereby convert the hydroxyl group into an azide reactable group; (II) reacting the product (I) with an azide salt, thereby to form an azido-carboxylic ester of the formula: R-CH(N₃)-(R') n -COOR'' wherein R, R', R'' and n are as hereinbefore defined; and (III) reacting the azido-carboxylic ester thus formed, or the corresponding carboxylic acid obtained by hydrolysis, with an amine which is a linear or branched alkylamine of the formula H x N(C n H 2n+1 ) 3-x wherein n = 1 to 18 and x = 1 or 2; a cycloalkylamine of the formula: wherein n is independently 1 to 8, y is 0 or 1 and x is independently 0 to 4, or the amine has the formula H₂NR° wherein R° is a branched member selected from the group consisting of fenchyl, diisopropyl carbinyl, d-methyl-t-butylcarbinyl, d-ethyl-5-butylcarbinyl, di-t-buylcarbinyl, 2-methylthio-2,4-dimethylpentane-3-yl, where at least one of R₁, R₂, R₃, R₄ is alkyl having from 1 to 4 carbon atoms and the remainder are hydrogen or alkyl having from 1 to 4 carbon atoms; X is O, S, SO, SO₂, C=O or CHOH; m is zero, 1, 2, 3 or 4; n and p are each zero, 1, 2 or 3, and the sum of n + p is not greater than 3; the sum of the carbon atoms in R₁, R₂, R₃ and R₄ is not greater than 6; and when both or R₁ and R₂ or R₃ and R₄ are alkyl they are methyl or ethyl. A process according to claim 1, wherein the azide-coupler is methane sulphonylchloride, toluene sulphonylchloride or benzyl sulphonylchloride. A process according to claim 1 or 2, wherein the hydroxy-carboxylic ester is an ester of an α-, β- or γ-hydroxy carboxylic acid. A process according to claim 1, 2 or 3 wherein the azide salt is sodium azide. Process according to claim 3 wherein the hydroxy-carboxylic ester is (S)-ethyl lactate; the azido-coupler is methane sulfonylchloride; and the azide salt is sodium azide. Process according to any one of the preceding claims wherein the amine is isopropylamine or 3-amino-2,2,4,4-tetramethylthietane. Process for the preparation of an amino-amide which comprises the conversion of a hydroxy-carboxylic ester of the formula R-CH(OH)-(R') n -COOR'' wherein the various symbols are as defined in claim 1, into an azido-amide by a process according to any one of the preceding claims, followed by the catalytic hydrogenation of the azido-amide to convert it to an amino-amide.

Inventors: KUPPER ROBERT JOE, KUPPER, ROBERT JOE
Applicants: HAMPSHIRE CHEMICAL CORP, HAMPSHIRE CHEMICAL CORPORATION
Application year: 1989
Main classification: C07C247
IPC codes: C07C67, C07D337, C07C241, C07C247, C07D335, C07C227, C07D331, C07C309, C07D333