WO1999058493A2

WO1999058493A2 - Process for the preparation of optically pure phenylalanine

Info

Publication number: WO1999058493A2
Application number: PCT/EP1999/002862
Authority: WO
Inventors: Pietro Allegrini; Giuseppe Barreca; Alessandro Rosi; Giorgio Soriato
Original assignee: Zambon Group SpA; Zambon SpA
Current assignee: Zambon Group SpA; Zambon SpA
Priority date: 1998-05-11
Filing date: 1999-04-28
Publication date: 1999-11-18
Anticipated expiration: 2000-11-11
Also published as: WO1999058493A3; ITMI981014A1; IT1299210B1

Abstract

A process for the preparation of optically pure D- or L-phenylalanine consisting of the crystallisation from acetic acid of a mixture enantiomerically enriched in one of the two enantiomers is described.

Description

Process for the preparation of optically pure phenylalanine

The present invention relates to a process for the preparation of optically pure phenylalanine and, more particularly, it relates to a process for the preparation of D- or L-phenylalanine by crystallisation of enantiomerically enriched mixtures.

D-phenylalanine and L-phenylalanine are widely known compounds (The Merck Index - XII ed., no. 7425, page 1253).

There are several enzymatic or chemical methods for the preparation of D-phenylalanine and of L-phenylalanine. As an example, we can cite the methods described in the patent US

4,262,092 (Ethyl Corporation), in the International patent application WO 85/03932

(Alkaloida Vegyeszeti Gyar), in the Belgian patent no. 883322 (A.E.C. Societe de Chimie

Organique et Biologique), in the International patent application WO 80/01571 (Agence

Nationale de Valorisation de la Recherche), in the German patent application no. 27 49 203 (Riedel-de Haen AG), in the European patent application no. 0 132 373 (Ajinomoto Co.,

Inc.), in the European patent application no. 0 133 053 (Nippon Kayaku KK) and in the

European patent application no. 0 406 124 (Rhone-Poulenc Chimie).

Most of the known methods shows the problem of the contamination of the desired phenylalanine with the other enantiomer. This drawback is particularly evident in the methods for the preparation of D- or L- phenylalanine by resolution where it is often necessary to use repeated crystallisations, generally from water, to obtain an optically pure compound.

We have now found that, crystallising from acetic acid an enantiomerically enriched mixture of D- or L-phenylalanine, the desired optically pure enantiomer is obtained with good yields though carrying out a single crystallisation.

Therefore, object of the present invention is a process for the preparation of optically pure phenylalanine consisting of the crystallisation in acetic acid of a mixture enantiomerically enriched in one of the two enantiomers.

The process of the invention allows to obtain one or the other of the two phenylalanine enantiomers in optically pure form with a single crystallisation even starting from mixtures with a 70-75% enantiomeric excess. In the present context, optically pure phenylalanine means a phenylalanine with an enantiomeric excess (ee) higher than 99%.

The used acetic acid is glacial acetic acid. The amount of acetic acid can vary from 2 to 10 times by weight with respect to the mixture to be purified, preferably from 3 to 8 times by weight.

The crystallisation procedure foresees first the dissolution into acetic acid of the mixture to be purified by heating to a temperature generally from 60°C to 90°C.

After cooling to a temperature from 40°C to 70°C, the crystallisation can be optionally seeded by adding a little amount of the desired phenylalanine in optically pure form.

The filtration of the crystallised product is generally carried out after keeping the mixture at room temperature for a period variable from some minutes to some hours.

The process of the present invention is particularly useful for the purification of enantiomerically enriched mixtures deriving from resolutions of D,L-phenylalanine by fractional crystallisation of diastereoisomeric salts but it can be efficiently used in every case an increase of the optical purity of a mixture enantiomerically enriched of one of the two phenylalanine enantiomers is desired.

In fact, as already underlined, the process object of the present invention allows to obtain D- phenylalanine or L-phenylalanine in optically pure form (ee>99%) with a single crystallisation though starting from a low optical purity, for example from mixtures with a

70-75% enantiomeri excess.

In order to illustrate the present invention the following examples are now given.

Example 1

Purification of a mixture of L- and D-phenylalanine (relative ratio 95:5) In a 100 ml flask, equipped with magnetic stirrer and reflux condenser, L-phenylalanine (19 g) and D-phenylalanine (1 g) were dissolved at 90°C in glacial acetic acid (63 g).

The resultant solution was cooled at 65°C before adding a little amount of L-phenylalanine as crystallisation seed.

The resultant suspension was cooled at 20°C in about 1 hour and kept under these conditions for 18 hours. The solid was filtered and washed with glacial acetic acid (10.5 g). After drying under vacuum at 70°C up to constant weight a product (16.3 g) with the following characteristics was obtained:

D-phenylalanine (HPLC determination on chiral column after derivatization to N-BOC phenylalanine): <0.1%

HPLC titre: 100%

[α]_D ²⁰ (c=2, H₂O): -33.7°

Example 2

Purification of a mixture of D- and L-phenylalanine (relative ratio 95:5) In a 100 ml flask, equipped with magnetic stirrer and reflux condenser, D-phenylalanine (9.5 g) and L-phenylalanine (0.5 g) were dissolved at 90°C in glacial acetic acid (73.5 g).

The resultant solution was cooled at 55°C before adding a little amount of D-phenylalanine as crystallisation seed.

The resultant suspension was cooled at 20°C in about 1 hour and kept under these conditions for 18 hours. The solid was filtered and washed with glacial acetic acid (2x5.5 g).

After drying under vacuum at 70°C up to constant weight a product (7.4 g) with the following characteristics was obtained:

L-phenylalanine (HPLC determination on chiral column after derivatization to N-BOC phenylalanine): <0.1% HPLC titre: 100%

[α]_D ²⁰ (c=2, H₂O): +34.1°

Example 3

Purification of a mixture of D- and L-phenylalanine (relative ratio 90:10)

In a 100 ml flask, equipped with magnetic stirrer and reflux condenser, D-phenylalanine (9 g) and L-phenylalanine (1 g) were dissolved at 90°C in glacial acetic acid (70 g).

The resultant solution was cooled at 56-58°C before adding a little amount of D- phenylalanine as crystallisation seed.

The resultant suspension was cooled at 20°C in about 2 hours and kept under these conditions for 1 hour. The solid was filtered and washed with glacial acetic acid (10 g). After drying under vacuum at 70°C up to constant weight a product (7 g) with the following characteristics was obtained:

L-phenylalanine (HPLC determination on chiral column after derivatization to N-BOC phenylalanine): =0.2% HPLC titre: 100%

[α]_D ²⁰ (c=2, H₂O): +34.2°

Example 4

Purification of crude D-phenylalanine

Crude D-phenylalanine (29.5 g; enantiomeric ratio 94.5:5.5 - HPLC titre 94.5%), obtained from (S)-2-bromo-3-phenyl-propionic acid and ammonia according to the method described in Berichte, 39, 4002 (1906), and glacial acetic acid (100.0 g) were charged into a 250 ml reactor, equipped with a double jacket, mechanic stirrer, thermometer and reflux condenser.

The mixture was heated at 90°C under stirring; the resultant solution was added with Norit charcoal (0.8 g) and the mixture was kept under stirring at 90°C for 30 minutes before filtering on paper.

The reactor and the filter were washed with hot glacial acetic acid (12 g at 80°C) and the washings were collected with the filtrate into a 250 ml flask with mechanic stirrer, thermometer and reflux condenser. The product was brought into solution again at 90°C, let spontaneously cool up to 50°C under slow stirring and the solution was seeded with optically pure D-phenylalanine (0.05 g).

At about 45°C a copious precipitation of a white solid was observed.

After 1 hour from the reaching of 20°C, the solid was separated from the mother liquors by filtration on a porous filter and washed with glacial acetic acid (2x7.5 g) and subsequently with toluene (2x5 g). The resultant wet product (31.0 g) was dried in oven at 80°C under vacuum for 2 days to give D-phenylalanine (18.0 g) with the following characteristics:

HPLC titre: 100%

L-phenylalanine: 0.2%

[α]_D ²⁰ (c=2, H₂0): +33.6° Example 5

Purification of crude D-phenylalanine

Crude D-phenylalanine (14.4 g; enantiomeric ratio 88:12 - HPLC titre 95.9%), obtained from (S)-2-bromo-3-phenyl-propionic acid and ammonia according to the method described in Berichte, 39, 4002 (1906), and glacial acetic acid (80 g) were charged into a 250 ml flask, equipped with mechanic stirrer, thermometer and reflux condenser.

The mixture was heated at 70°C; the resultant solution was added with L4S charcoal (0.5 g) and the mixture was kept under stirring at 70°C for 30 minutes before filtering. The filter was washed with hot glacial acetic acid (10 g at 70°C) and the washings were collected with the filtrate into a 250 ml flask with mechanic stirrer, thermometer and reflux condenser. The solution was cooled up to 40°C and seeded with optically pure D- phenylalanine.

After 30 minutes at 40°C, the mixture was cooled at 20°C in about 1 hour and filtered. The solid was washed with glacial acetic acid (5 g) and subsequently with 95% ethanol (3x4 g)-

The resultant wet product was dried in oven at 60°C under vacuum for 24 hours to give D- phenylalanine (8.1 g) with the following characteristics:

HPLC titre: 100% L-phenylalanine: 0.1%

Claims

1) A process for the preparation of optically pure phenylalanine consisting of the crystallisation from acetic acid of a mixture enantiomerically enriched in one of the two enantiomers.

2) A process according to claim 1 for the preparation of D-phenylalanine.

3) A process according to claim 1 for the preparation of L-phenylalanine.

4) A process according to claim 1 wherein the amount of acetic acid is from 2 to 10 times by weight with respect to the mixture to be purified. 5) A process according to claim 4 wherein the amount of acetic acid is from 3 to 8 times by weight.