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1. WO1985002609 - PREPARATION DE PHENYL ALANINES PAR HYDROGENATION DE PHENYL SERINES

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PREPARATION OF PHENYL ALANINES BY HYDROGENATION OF PHENYL SERINES
Technical Field

The present invention relates to a large scale process for the preparation of 3-Phenyl alanine
derivatives of the general formula



In the general formulae as referred to hereinafter the symbols are defined as follows:
R stands for hydrogen or C1-4 alkyl
R2 stands for hydrogen or CO-R4 - wherein
stands for C1-4 alkyl, aralkyl or aryl
stands for hydrogen or -CO-R4 wherein R4 is as given above.

Background Art

Phenyl alanine is a known essential amino acid. Recently its importance has increased since it is the active ingredient of the sweetening agent known as
"aspartam" /methyl-L-aspartyl-L-phenyl-alaninate/.
L-Phenyl alanine is prepared by fermentation in the industry. This process is rather expensive as at the end of the fermentation the product has to be isolated from a big volume of water containing a great amount of inorganic ions, and the purification of the water is
rather complicated /EU Pat. Appln. 85959/.
Several classical aaino acid synthesis methods are known for the preparation of phenyl alanine : such as the Strecker synthesis using the expensive phenyl acetaldehyde [Bull. Chem. Soc. Jap. 46. /1978/ 1865-66] or the Erlenmayer azlactone syntheis in several steps from N-acyl glycine with an average total yield etc.
[J. Org. Chem. 37. No. 18 /1972/ 2616].

Disclosure of the Invention

The present invention relates to the preparation of phenyl alanine derivatives of the general formula I by reacting a phenyl serine derivative of the general formula



or a salt thereof formed with inorganic or organic acid with hydrogen in the presence of a catalyst and optionally N-acylating said product.
When using gasious hydrogen metal palladium may be used as a catalyst, preferably in the form of 10 % palladium on charcoal in an amount of 10 to 50 % by weight related to the weight of the starting material.
The reaction is performed in a protic solvent in the presence of C1-4 alcohol, water or a C1-4 straight carboxylic acid. A mixture of said solvents may be used as well. As a preferred embodiment glacial acetic acid may be used as a solvent.
One of the possible embodiments of our process can be conducted by introducing hydrogen gas in the
presence of 1 to 5 moles of a mineral acid calculated to the phenyl serine derivative.
Hydrogenation may be conducted at a pressure of 1 to 10 atm., preferably at 3-5 atm. The preferred reaction temperature range is from 20 to 120 °C, preferably 40-70 °C.
As mineral acids sulfuric acid or hydrochloric acid can preferably be used.
As another preferred embodiment of the process according to the invention the required hydrogen is
formed in situ from a substance which releases hydrogen under the conditions of the reaction on the surface of the catalyst in the reaction mixture. As such substances hydrazine, formic acid, cyclohexene or cyclohexadiene are preferred.
As protic solvents particularly carboxylic acids may be used. If in the end product the carboxylic or amino group contains a substituent different from hydrogen then as starting material the correspending N-substituted compound of the general formula II can be employed.
Particularly if esters are prepared phenyl serine is used as starting material which upon hydrogenation results in phenyl alanine ester.
If N-acyl derivatives are to be prepared the acylation can be performed both before and after hydrogenation. Acylation may be carried out by methods known per se, e.g. with carboxylic acid anhydride. [J . Biol. Chem. 98 /1932/ page 295].
If the formyl derivatives are prepared then the reaction can preferably be prepared by using formic acid hydrogenating agent and at the same time as acylating agent.
In order to prepare N-formyl phenylalanine of the formula



falling within the scope of the compounds of the general formula I formic acid is particularly preferred as hydrogen releasing agent. Phenyl serine of the formula


is then used as starting material and by reacting same with formic acid, hydrogenation and the formylation of the amino group can be carried out in one step. For this reaction 1-40 %, preferably 10-20 % by weight of 10 % palladium on charcoal catalyst can be used calculated on the weight of the starting material. The reaction can be performed with 2 to 20 moles formic acid calculated on phenyl serine, preferably in the presence of glacial acetic acid. As a solvent formic acid can be used in the presence of water or without it.
Phenyl alanine and derivatives thereof can be prepared under large scale conditions economically and in good quality by using the process of the invention.
Industrial synthesis can be conducted from economically available glycine prepared by large scale synthesis.
Phenyl serine derivatives of the general formula II which serve as starting materials for the hydrogenation according to the invention are known and can be prepared from glycine.
According to a preferred embodiment of the invention glycine and benzaldehyde are reacted under alkaline conditions in a two-layer solvent system. One of the two layers consists of a water inmiscible organic solvent.
The other layer comprises a water miscible organic solvent and/or water. The reaction takes place in the presence of a phase transfer catalyst.
As phase transer catalysts 0.01-1.0 mole equivalent quaternary ammonium salt calculated on glycine can be used. Reaction is carried out at 15-100 °C, preferably at 25-60 °C. The phenyl serine alkali salt thus prepared can be further used in another reaction /e.g. dehydroxylation/, without isolation. As water inmiscible solvents aromatic hydrocarbons, such as benzene or toluene may be used. As water miscible organic solvents C1-4 alcohol or acetone may be used.
Phenyl alanine prepared according to the invention may appear in optically active form or racemic form. As starting material erithro or threo D-, L- or DL-phenyl serine may be employed. If the DL-form is used racemic phenyl alanine is formed according to the process of the invention. The product is then resolved by method known per se [J. Prakt. Chem. 9-/1959/ 104.].
Further details of the invention can be found in the following Examples which serve merely for illustration and not for limitation.

Best mode of carrying out the Invention

Example 1
To a mixture of 2000 ml. of glacial acetic acid, 135 ml. of 96 % sulfuric acid and 20 ml. water, 50 g. of 10 % palladium on charcoal catalyst are added and the mixture is vigoruouly stirred at atmospheric pressure under hydrogen. To the thus prehydrated mixture 100 g /0.552 mole/ of DL-threo-3-phenyl-serine monohydrate are added and the reaction mixture is further stirred at 20 °C. When the theoretical amount of hydrogen /13.0 1/ is consumed the catalyst is removed by filtration, washed with 2 x 100 ml. of hot glacial acetic acid, whereafter 308.3 g. of anhydrous sodium acetate are added to the combined filtrates, the precipitated salt is filtrated, washed with 2 x 100 ml. of hot glacial acetic acid and the glacial acetic acid layer is
evaporated in vacuo at 30-40 °C. The obtained crystalline residue is dissolved in 800 ml. methanol and the pH is adjusted to pH = 4.5 by adding concentrated aqueous ammonia solution, the mixture is cooled and the precipitated product is filtered. Yield: 68.6 g. of DL-3-phenyl-alanine. /75.2 %/

Example 2
In a mixture of 200 ml. of glacial acetic acid, 80 ml. of 96 % sulfuric acid and 200 ml. of water 10 g. of 10% palladium on charcoal are added and the mixture is vigorously stirred under hydrogen atmosphere at
5-60 °C for 20 minutes. 100 g. of DL-phenyl serine are then added and the mixture is hydrogenated under similar conditions for 12 hours. The catalyst is filtered, washed twice with 50 ml. of hot galcial acetic acid and the mixture is then boiled. 180.0 g of anhydrous calcium chloride are then added to the hot solution in small portions. When the addition is completed the solution is hot filtered and worked up according to Example 1.
75.5 g. of DL-phenyl alanine are obtained. Yield: 82.7 %.

Example 3
2.20 g. of DL-phenyl serine monohydrate are dissolved in 20 ml. of glacial acetic acid whereafter 1.0 g. of palladium on charcoal is added. 12.0 ml. of formic acid are added dropwise, the purity of which is

98 % . The mixture is maintained at 95 °C for 4 hours, filtered and the filtrate is evaporated and the residue is recrystallized from acetone.
Yield: 2.07 g. of DL-N-formyl-alanine. Yield: 86.7 %.

Purity: 98 % M.p. 168-170 °C.

Example 4
2.1 g. of L-phenyl serine ethyl ester are dissolved in 15 ml. of glacial acetic acid, 0.80 g of palladium on charcoal is added and 20 ml. of hydrazine are added dropwise. The mixture is kept at 100 °C for 6 hours, filtered and evaporated in vacuo. The product is purified by column chromatography. 1.3 g. of L-phenylalanine-ethyl ester is obtained. Yield: 65.4 %, purity:

95 %. Similarly cyclchexene cr cyclohexediene may be used instead cf hydrazine hydrate.

Example 5
5.0 g. of N-acetyl-phenyl serine are dissolved in 60 ml. of glacial acetic acid and 2.0 g of palladium on charcoal and 30 ml formic acid of 98 % purity are added and the reaction mixture is kept at 80 °C for 3 hours. The mixture is filtered, the filtrate is evaporated and the residue is recrystallized from a mixture of ethanol and water. 4.44 g. of DL-N-acetyl phenyl alanine are obtained. Yield: 92.0 % . Purity: 96.0 %, m.p.:
142-143 °C.

Example 6
6.0 g. of L-N-formyl-phenyl serine are dissolved in 80 ml. of glacial acetic acid, 1.5 g. of palladium on charcoal are added as well as 35 ml. of formic acid of purity of 98% . The mixture is kept at 100 °C for 4.5 hours and worked up according to Example 5. It is
crystallized from formic acid. 5.59 g. of L-N-formyl-phenyl alanine are obtained. Yield: 96 % . Purity: 95 % . M.p.: 169-170 °C. [α]D20= + 73 ° /c=1, ethanol/

Example 7
30 g. of 10 % palladium on charcoal catalyst are admixed with 100 ml. of water whereafter 200.0 g of phenyl serine monohydrate are added in 1400 ml. of 85% formic acid in nitrogen atmosphere. The mixture is boiled for 4 hours, filtered while hot and the catalyst is washed with hot formic acid and the filtrate is evaporated. To the obtained white crystalline substance 300 ml. of 20 % hydrochloric acid are added and the mixture is heated under reflux for 1 hour, cooled, stirred at 0-5 °C for 2 hours and the precipitated crystalline substance is filtered, washed with some 20 % hydrochloric acid and dried. Yield: 92.9 %.
187.4 g. of DL-3-pnenyl alanine hydrochloride are obtained. M.p.: 239-240 °C.

Example 8
30 g. of 10 % palladium on charcoal catalyst are heated in a mixture of 540 ml. of glacial acetic acid and 90 ml. of 85 % formic acid for 20 minutes whereafter a solution of 200 g. phenyl serine monohydrate in 540 ml. of 85 % formic acid and 360 ml. of glacial acetic acid are added dropwise at 106-107 °C within 2 hours. The mixture is then boiled for further 2.5 hours and the mixture is worked up as given above. Yield: 95.4 %. 192.3 g. of DL-3-phenyl-alanine hydrochloride.

Example 9
40 ml. of glacial acetic acid and 6 ml. of formic acid are admixed and in the mixture 2.3 g. of 10 % palladium on charcoal catalyst are suspended and the mixture is heated under stirring to reflux temperature /105-110 °C/. At this temperature a solution of 22.6 g. of ammonium salt of N-formyl-phenyl serine in 100 ml. of glacial acetic acid and 16 ml. of formic acid is added dropwise within 1 hour and it is heated to boiling for 1 hour. The catalyst is filtered while hot, the mixture is washed with some hot glacial acetic acid, the filtrate is evaporated from water bath to 30 to 32 g. in vacuo. To the obtained syrup 30 ml. of 20 % hydrochloric acid are added, the solution is boiled for 1 hour and evaporated from water bath in vacuo. The residue /28-32 g. / is taken up in 200 ml. 90 % methanol and the pH is adjusted to 6-7 with c.c. aqueous ammonia. The solution is allowed to stand overnight in a refrigerator, the precipitated product is filtered, washed with methanol and dried to constant weight. Yield: 15.5 g. /84.6 %/, of DL-3-phenyl alanine. Purity: 90 % .
The filtered catalyst can be used in the next reaction without any further treatment according to all Examples.

Example 10
1.5 g. of 10 % palladium on charcoal catalyst are suspended in 5 ml. water and a solution of 9.80 g. of L-phenyl-serine methyl ester and 3.5 g. of ammonium formiate in 75 ml. of 85 % formic acid is added. The solution is boiled for 3 hours and filtered, the filtrate is evaporated in vacuo. 17.4 g. of syrup like residue are allowed to stand at room temperature for 48 hours in 200 ml. methanol in hydrochloric acid. The mixture is evaporated and the resudue is triturated with ether, the obtained crystalline substance is filtered and dried. 8.62 g. /80.1 %/ of L-phenyl alanine methyl ester hydrochloride axe obtained. M.p.: 158-160 °C.
[α]D25º= + 32.2° /c=2, ethanol/

Example 11
In 1500 ml. of water 80.0 g. /2.0 mole/ of sodium hydroxide and 75.0 g./1.0 mole/ of glycine are dissolved at 15 °C. 212.0 g. /2.0 mole/ of benzaldehyde and 10.0 g. of triethyl benzyl ammonium hydroxide are added in 200 ml. of benzene, the mixture is stirred for 2 hours at 50 °C, cooled, whereafter 150 ml. of glacial acetic acid are added and the two layers are separated. From the benzene layer 105.0 g. of benzaldehyde are recovered. The aqueous layer is evaporated in vacuo to 590.0 g. To the residue

1000.0 ml. of 100 % formic acid are added and the obtained homogeneous solution is added to 20.0 g. 10 % palladium on charcoal suspended in 1000 ml. glacial acetic acid.
The mixture is boiled for 3 hours, the catalyst is filtered off and the filtrate is evaporated in vacuo, to the residue 300 ml. of concentrated hydrochloric acid are added, the mixture is boiled for 1 hour, then cooled and stirred for 2 hours at 0-5 °C. The precipitated DL-3-phenyl alanine hydrochloride is filtered, washed with some
1:1 hydrochloric acid, dried. Yield: 244.8 g. containing 160.6 g. of hydrochloride of phenyl alanine /79.7 %/ and 84.2 g. sodium chloride. The obtained product is stirred in 1000 ml. absolute methanol at room temperature /25 °C/, and the undissolved sodium chloride is removed by filtration, the filtrate is adjusted to pH = 6.5 with concentrated aqueous ammonia and the mixture is allowed to stand overnight at 0-5 °C. The precipitated crystals are filtered, washed with some 80 % methanol, dried.
Yield: 122.1 g. /74 %/ of DL-3-phenyl alanine.

Example 12
1.5 g. 10 % palladium on charcoal catalyst is covered with 5 ml. water and a solution of 10.4 g N-acetyl-threo-L-3-phenyl-serine and 3.5 g. of ammonium formiate in 70 ml. of formic acid is added. The mixture is boiled for 3 hours, the catalyst is filtered and washed with 5 ml. c.c. hydrochloric acid and cooled.
The precipitated N-acetyl-3-phenyl-L-alanine is filtered and washed with water. Yield: 8.1 g 78 %, m.p.: 169-171 °C [α]D = +44.8 /c=2, ethanol/ [α ]D = -60.8° /c=2, ethanol/, 3.5 g. ammonium formiate, 35 ml. of glacial acetic acid and 32 ml. 85 % formic acid solution are added to a solution of 3 g. 10 % polladium on charcoal in 10 ml. glacial acetic acid and the mixture is boiled for 5 hours. The catalyst is filtered off, the mixture is evaporated, dissolved in a mixture of 200 ml. methanol and 800 ml. of water while hot, a mixture of 10 ml. methahol and 10 ml. of cone, hydrochloric acid is added and the precipitated N-benzoyl-L-phenyl alanine is filtered. Yield: 5-5 g. 41 %, m.p.: 134-136 °C.
αD = -15.8° /c=2, ethanol/
αD = 17.5° /c=4, 0.25 m potassium hydroxide solution/

Preparation of the starting materials
Example 1
Preparation of DL-threo-3-phenyl serine
12.0 g. /0.30 mole/ of sodium hydroxide are dissolved in 50 ml. water and 10.0 g. /0.13 mole/ of glycine are added. To the reaction mixture 25 ml. of benzene and 1.2 g. /0.005 mole/ of triethyl benzyl ammonium chloride are added and the mixture is heated to 50 °C. 28.35 g. /0.26 mole/ of benzaldehyde are added and the mixture is kept at 50 °C for 2 hours, whereafter "it is cooled, acidified, filtered and washed with aqueous ethanol. After drying in vacuo at 100 °C 21.60 g. of DL-threo-3-phenyl serine are obtained . After drying the product on air at 80 °C 23 .8 g . of DL-threo-3-phenyl serine monohydrate are obtained.

Example 2
Acetylation of DL-threo-3-phenyl serine
10.8 g. of phenyl serine are dissolved in 11.2 g. hydroxide and 80 ml. water. At 0 °C 16.8 ml. /0.18 mole/ of acetic acid anhydrid are added. After the addition the mixture is stirred for 1 hour at 0 °C then for 1 hour at room temperature whereafter 22.8 ml. of 36 % by weight hydrochloric acid are added at 0 °C. The precipitated product is filtered and washed with some water. 11.2 g. of DL-threo-N-acetyl-3-phenyl serine are obtained.
M.p.: 150-152 °C.

Example 3
Formylation of EL-threo-3-phenyl serine
9.0 g. /0.05 mole/ of DL-threo-3-phenyl serine and 8 ml. of formamide are stirred for 2 hours at 100 °C. The reaction mixture is cooled and the product is precipitated with 22 ml. of acetone, filtered and washed with acetone. 9.5 g. /83.7 %/ of the ammonium salt of DL-threo-2-formylamindo-3-phenyl serine are obtained.
M.p.: 169-170 °C.

Analytical method for analysing pehnyl serine, phenyl alanine and derivatives

Equipment: high pressure liquid chromatograph of type

Hawlett-Packard 10843
Column: Hawlett-Packard RP-8 /5u, 200 x 4.6 mm/
Eluant: A: 0.004M H3PO4: 0.016 M KH2PO4, 20 % CH3OH
B: CH3OH
Flow rate: 1 ml/min
Injected volume: 10.0 μI
Detection: UV detector, 215 mm
Quantitative determination: 1 mg/ml /methanol-water/
external standard
retention time: min.
1. Eluant: % B 0.0
phenyl serine: 3.70
phenyl alanine: 5.0
2. Eluant: % B 20.0
N-formyl phenyl serine 4.0
N-formyl phenyl alanine 6.0
N-acetyl phenyl serine 4.40
N-acetyl phenyl alanine 7.60
phenyl serine methyl ester 3.70
phenyl alanine methyl ester 4.80