WO2009027998A2

WO2009027998A2 - A process for the preparation of alanine-nca (alanine-n-carboxyanhydride) from alanine with tri phosgene

Info

Publication number: WO2009027998A2
Application number: PCT/IN2008/000540
Authority: WO
Inventors: Satyanarayana Kota; Venkateswarlu Tallapaneni; Bhujanga Rao Adibhatla Kali Satya; Nannapaneni Venkaiah Chowdary
Original assignee: Natco Pharma Ltd
Current assignee: Natco Pharma Ltd
Priority date: 2007-08-27
Filing date: 2008-08-26
Publication date: 2009-03-05
Anticipated expiration: 2010-02-27
Also published as: WO2009027998A3

Abstract

Present invention describes a commercial process for preparation of tyrosine-NCA from alanine (IV) and triphosgene (V) that is economically viable, robust, and scalable linearly without using toxic phosgene gas as shown in the scheme 2.Advantages of Present Invention: 1. In the present process hazardous phosgene is replaced with triphosgene and liberated gases are fed to a scrubber, hence it is safe and environmental friendly. 2. The process is suitable for a commercial preparation. 3. The process produces alanine-NCA of purity of > 99%, chlorides < 0.01% and free alanine <0.01%.

Description

AN IMPROVED PROCESS FOR PREPARATION OF

ALANINE-NCA

Field ofihe Invention

Alanine-N-Carboxy anhydride (Alanine-NCA, I) is one of the four amino acid building blocks used in the preparation of glatiramer acetate. The present invention describes a process for preparation of alanine-NCA by industrially convenient, environmentally friendly and safe process involving the reaction of triphosgene and alanine. Alanine-NCA obtained by this process is > 99.0% pure with <0.01% chloride content and substantially free from alanine impurity.

Background of the Invention

Glatiramer acetate is a synthetic peptide polymer used in the treatment of relapsing multiple sclerosis. (M. M. Mouradain, Pharmacology & Therapeutics, 98, 245-255, 2003). It is a random polymer of L-alanine, L-tyrosine, L-Glutamic acid, L-lysine and is prepared by polymerization N-Carboxy anhydrides (NCAs) of L-Tyrosine, L-alanine, L- δ-benzyl L-glutamic acid and L-ε-trifmoroacetyl lysine. The above polymerization reaction is highly sensitive, and the final polymer quality depends on the quality of input amino acid NCAs. We found that, whenever above amino acid NCAs do not meet the stringent quality specifications, glatiramer acetate obtained failed in average molecular weight and amino acid composition specifications. In order to obtain glatiramer acetate meeting to specifications, we use NCAs, which are to be >99% pure and chloride content 0.01% and free amino acid content <0.01 %. Amino acid N-carboxyanhydrides (amino acid -NCAs) are chemically 3-substituted oxazolin-2, 5-diones. Leuchs first prepared NCAs in 1906 via intermolecular cyclization of N-alkyl or N-aryloxycarbonyl amino acid halides. Hence these are also known as Leuchs anhydrides (Leuchs H., Chem. Ber., 41, 1721, 1906). This method involves preparation of alkoxy aryl carbamates, conversion to acid halides, and thermally induced cyclization. This method is obsolete and of historical interest only. Amino acid -NCAs are highly unstable at room temperature and needs to be stored around -20⁰C.

Since the first preparation of NCAs, many different procedures have been reported. Fuchs-Farthing is the most popular method for preparation of amino acid-NCAs (III) both at academic and industrial level (Farthing, A., J. Chem. Soc, 3213-3217, 1950; Fuchs F. Chem. Ber., 55, 2943, 1922). This preparation, involves reaction of amino acid (II) with phosgene in variety of solvents (Scheme 1). Many variants of phosgenation such as phosgenation of amino acid copper complexes, N-trimethylsilyloxcarbony amino acid trimethylsilyl ester and N-carboxy amino acid sodium salts (T. J. Blacklock, R. Hirschmann, and D. F. Verber, Peptides, Volume 9, pp39, academic press, 1987) are reported. Use of phosgene is major disadvantage in these methods. Phosgene is highly hazardous gas, and requires specialized handling and personal protection equipment. Hence it not preferred method in the industry.

In order to avoid phosgene, many substitutes are developed; diphosgene and triphosgene are generally used substitutes.

Scheme 1

Diphosgene (trichlomethylchloroformate, TCF) is high boiling (127-128° C for TCF, versus 8.2⁰C for phosgene) hence easy to handle and is less toxic than phosgene. Some amino acid NCAs were prepared using diphosgene at a small scale by reacting amino acid with diphosgene (Katakai R., Iizuka Y., J. Org. Chem., 50, 715-716, 1985). However, diphosgene is not available in quantities required at industrial scale. Triphosgene (bistrichloromethyl carbonate) is crystalline compound and is less hazardous than phosgene and diphosgene. In the recent times it has become a popular substitute of phosgene and is available abundantly at a cheap price. Daly and Poche for the first time prepared amino acid NCAs using triphosgene, their paper describes synthesis of stearyl glutamate, DL-2-aminostearic acid, δ-benzyl L-glutamate, O-benzyl-L-tyrosine, L-phenyl alanine, L-leucine, L-alanine in THF in 58.5-89.5% yield (Daly, W.H. & Poche, D., Tetrahedron Lettres, 29, 5859-5862, 1988), later Wilder and Mobashery (Wilder, R., and Mobashery, S, Journals of Organic Chemistry, 57, 2755-56, 1992) reported the synthesis of L-valine, O-benzyl-L-tyrosine, L-phenylalanine, δ-benzyl L-glutamatic acid, glycine, Boc-L-lysine in 65-83% yield in anhydrous ethyl acetate and triethylamine system using triphosgene.

Recent, US patents 6,479,665 (2002) and 6, 603,016 (2002) gives a very general method of preparation of α,β,γ amino acid NCAs using phosgene, diphosgene, triphosgene in presence of unsaturated organic compound, such as α-pinene in order to trap liberated hydrochloric acid and less than 1000 mbar to give oc,β,γ amino acid NCAs free of chlorides. US patents 6,479,665, has the disadvantage of creating negative pressure, which creates process-engineering difficulties. Similarly, US patent 6, 603,016 procedures involves an additional process chemical such as α-pinene to trap hydrogen chloride liberated in the reaction. Which increases the cost of production. In addition, both these patents do not mention anything regarding the purity of NCAs, and amount of free amino acid in the NCAs obtained by the above processes. These quality specifications are highly important to produce glatiramer acetate meeting quality specifications.

Alanine -NCA was prepared using phosgene (Astbury, W.T., Dalgliesh, C.E., Darmon, S.E., Sutherland, G.B.B.M., Nature, 162, 596,1948; Hirschmann et al., J. Am. Chem. Soc. 93, 2746-2754, 1971;Bailey.J.L., J. Chem. Soc. 3461-3466; Lindberg. Et al., J. Med. Chem. Vol. 21, 448-456, 1978; Thomas J. Blacklock et al., J. Org. Chem. 53, 836-844, 1988.). However, none of the above methods describe, alanine-NCA prepared using triphosgene having >99.0% assay, <0.01% chloride content and free from alanine. Present invention describes a commercial process for preparation of tyrosine-NCA from alanine (IV) and triphosgene (V) that is economically viable, robust, and scalable linearly without using toxic phosgene gas as shown in the scheme 2.

Scheme 2

Detailed Description of the Present Invention

Moisture causes polymerization of amino acid NCAs. Therefore, the moisture present in alanine can also cause polymerization of alanine NCA formed during the reaction media. To prevent polymerization, vacuum dried alanine with moisture content <0.01% is used in the reaction.

One of other difficulties in the process is choice of solvent and the purity of it. The critical parameter is content of moisture in the solvent. Solvents such as ethyl acetate, dioxane, tetrahydrofuran, and acetonitrile are used with moisture content less than 0.01%. In this invention, solvents are dried using desiccants such as calcium chloride, molecular sieves for overnight, and then double distilled, checked for moisture content. Solvents with moisture content less than 0.01% are only used in the process. The particle size of alanine plays critical role in the preparation of alanine- NCA. When the commercial alanine with an average particle size of about 50 microns is used, the yield of alanine-NCA was about 50%. However, when the particle size of alanine is reduced to less than 200 microns, the rate of reaction is faster and the yield increased to > 80%. The product purity is more 95% also.

During the reaction of triphosgene with alanine at the refluxing temperature, if the vent is kept open, the atmospheric air is sucked, and moisture entered into the reaction vessel induces polymerization of alanine-NCA formed in the reaction. To control moisture- induced polymerization during the reaction, the reaction is conducted in presence of inert atmosphere. The inert atmosphere is created by nitrogen or argon, preferable by nitrogen gas.

The reaction of triphosgene with alanine liberates hydrogen chloride, this react with free alanine to give alanine hydrochloride. The formation of hydrochloride salt hinders progress of reaction and invariable results in alanine- NCA having chloride impurities. The scrubber neutralizes liberated toxic gases. The reservoir of the scrubber is filled with alkalis such as ammonia, alkali metal hydroxides.

Mode of addition of triphosgene is critical to achieve highly pure alanine-NCA. When triphosgene is added in single lot, the reaction is incomplete. In order to force the increase reaction to completion, triphosgene is added in the lots. However, handling triphosgene in the lots is not convenient. To solve this problem, a solution of triphosgene is in the solvent of reaction is made and added in lots.

There are number of methods described in the literature to determine the purity of NCAs, however, we found procedure of Berger et al., involving titration of alanine NCA with sodium methoxide in presence of thymol blue indicator is accurate, simple and precise (Berger, A., SeIa, M., Katachlski, E., Analytical Chemistry, 25, 1554-1555, 1953). Chlorides content is determined by argentometry. Free alanine in alanine is determined by thin layer chromatography using ninhyrin detection system. Accordingly, the main objective of the present invention is to provide an improved process for the preparation of alanine-NCA of formula I possessing very high purity.

Another objective of the present invention is to provide an improved process for the preparation of alanine-NCA possessing very high purity, with >99.0% assay, chlorides content <0.01% and free alanine content 0.01%.

Still another objective of the present invention is to provide an improved process for the preparation of alanine-NCA possessing very high purity, with >99.0% assay, chlorides content <0.01% and free alanine content 0.01% suitable for preparation of glatiramer acetate.

Still another objective of the present invention is to provide an improved process for the preparation alanine-NCA possessing very high purity, with >99.0% assay, chlorides content <0.01% and free alanine content 0.01% suitable for preparation of glatiramer acetate meeting quality specifications for average molecular weight and amino acid analysis.

Summary of the Invention.

The present invention describes a process for preparation of highly pure alanine-NCA suitable for preparation of glatiramer acetate. The invention involves reaction of purified alanine with triphosgene, wherein triphosgene is added in lots as solution in purified solvent. Alanine-NCA obtained by this process is >99.0% pure and contains <0.01% chlorides and free alanine.

Accordingly the present invention has been developed based on our finding that the qualities of input amino acid -NCAs influence the average molecular weight and amino acid analysis of glatiramer acetate. The details of the invention are described in examples given below which are provided to illustrate the invention only and therefore should not be construed to limit the scope of the present invention.

Example 1: Preparation of alanine-NCA

Into a 100 L glass reactor, ethyl acetate (37L, MC<0.01%), powdered and vacuum dried alanine (500g,particle size < 200 micron, MC < 0.01%) are added. The out let of reactor is connected to a scrubber. The reservoir of the scrubber is filled with 10% sodium hydroxide solution and inert atmosphere is created by applying nitrogen. Triphosgene (500g) is dissolved in ethyl acetate (5L) and charged into the addition tank. Reaction mixture is heated to 70-80⁰C, and then triphosgene solution is added in portions during 4- 6h. After the addition of about 4.2L triphosgene solution, the reaction is clear solution. Then, the remaining triphosgene solution is added and reaction maintained 70-80⁰C for further Ih. The reaction mixture is cooled to 60⁰C, the filtered through a hyflow bed under nitrogen atmosphere. The filtered reaction mixture is charged to the reactor and solvent removed to 1/3 volume, then hexane (30L, MCO.01%,) is added and stirred for Ih at 0-5⁰C. Separated crystalline product is filtered and dried under nitrogen atmosphere and stored at -20⁰C. Yield: 50Og (87.7%), assay = 99.7% (non-aqueous titration), Chlorides = 0.01% (argentometry), free alanine < 0.01 % (TLC).

Example 2: Preparation of alanine-NCA in tetrahydrofuran

Powdered and vacuum dried alanine (1Og, particle size < 200 micron, MC < 0.01%) is added to tetrahydrofuran (50OmL, MC <0.1) and heated to reflux temperature. A solution of triphosgene (1Og) in tetrahydrofuran is added during course of 6h. Reaction mixture is heated till clear solution is obtained, filtered and solvent distilled to obtain a concentrate.

To the concentrate, hexane (400 mL, MCO.1%) is added and cooled to 0-5° C. The separated crystalline material is filtered under nitrogen, and packed under nitrogen atmosphere and stored at -20° C.

Yield: 3.7 (32.5%), assay = 99.1%, Chlorides <0.01%, free alanine < 0.01% (TLC). Example 3: Preparation of alanine-NC A in acetonitrile

Powdered and vacuum dried alanine (1Og, particle size < 200 micron, MC < 0.01%) is added to acetonitrile (50OmL, MC <0.1) and heated to reflux temperature. A solution of triphosgene (1Og) in acetonitrile is added during course of 4h. Reaction mixture is heated a till clear solution is obtained, filtered and solvent distilled to obtain a concentrate. To the concentrate, hexane (500 mL, MCO.1%) is added and cooled to 0-5° C. The separated crystalline material is filtered under nitrogen, and packed under nitrogen atmosphere and stored at -20° C. Yield: 5.1g (44.8%), assay = 99.4%, Chlorides = 0.01%, free alanine < 0.01%.

Advantages of Present Invention:

1. In the present process hazardous phosgene is replaced with triphosgene and liberated gases are fed to a scrubber, hence it is safe and environmental friendly.

2. The process is suitable for a commercial preparation. 3. The process produces alanine-NCA of purity of > 99%, chlorides < 0.01% and free alanine <0.01%.

Claims

We Claim:

1. An improved process for the preparation of alanine-NCA of formula (I)

Which comprises reacting purified alanine with triphosgene as a solution in suitable purified solvents, and which is added in portions at a temperature hi the range 0-120⁰C to obtain the compound of the formula (I), alanine-NCA with high purity and which is suitable for preparation of glatiramer acetate.

2. A process as claimed in claim 1 wherein the purified solvent employed is selected from organic solvents, acetonitrile, dioxane, ethyl acetate, tetrahydrofuran, and preferably ethyl acetate and moisture content of purified solvent is less than <0.01%.

3. A process as claimed in claim 1 wherein the moisture content of purified alanine employed is less than <0.01%.

4. A process as claimed in claim 1 wherein triphosgene is added as a solution in purified solvent in lots.

5. A process as claimed in claims 1 wherein the preferred temperature of the reaction is 50 to 80 °C.

6. A process as claimed in claims 1 wherein the used particle size of alanine is < 200 micron.

7. A process as claimed in claim 1, wherein liberated toxic gases from the reaction between alanine and triphosgene are neutralized by a scrubber, and the reservoir of the scrubber is filled with ammonia, alkali metal hydroxides such as sodium hydroxide etc. in an inert atmosphere created by gases such as argon or nitrogen.

8. A process as claimed in claim 1 wherein alanine-NCA of purity >99.0% is obtained.

9. A process claimed as in claim 1, wherein free chloride content < 0.01% in the alanine-NCA

10. A process claimed as in claim 1, wherein the content of free alanine is < 0.01% in alanine-NCA.

11. An improved process for the preparation of alanine-NCA of the formula (I) substantially as herein described with reference to the Examples 1 to 3.