HK1175166A1 - Methods for the production of l-carnitine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently producing L-carnitine having a very high purity.SOLUTION: The method includes: (a) preparing a solution comprising at least 5% (w/w) carnitine in a first solvent and in which the carnitine is a mixture of D-carnitine and L-carnitine; (b) administering a L-carnitine crystal into the solution; (c) adding a second solvent in which L-carnitine is insoluble or does not have a low solubility; and (d) isolating crystals comprising L-carnitine. The first solvent is selected from ethanol, methanol, water, acetonitrile or a mixture thereof, and the second solvent is selected from acetone, isopropanol, iso-butanol, 2-propanol, 1-pentanol, 2-butanone, methyl acetate, ethyl acetate, butyl acetate, tetrahydrofuran, toluene or a mixture thereof.

Description

Process for the manufacture of L-carnitine

Technical Field

The present invention relates to a process for the manufacture of L-carnitine.

Background

Carnitine (vitamin Bt; 3-hydroxy-4-trimethylammonio-butyrate) is a quaternary ammonium compound biosynthesized from amino acids (lysine and methionine). In living cells, it is required to transport fatty acids from the cytosol to the mitochondria during breakdown of lipids to produce metabolic energy. It is used as a nutritional supplement.

Carnitine exists in two stereoisomeric forms. The biologically active form is L-carnitine, whereas the enantiomer, D-carnitine, is not biologically active. When L-carnitine is produced in an industrial process, it is desirable to produce the biologically active L-form in high purity. High purity L-carnitine can be obtained by microbiological methods. EP 0195944 discloses a microbiological process in which L-carnitine is produced from crotonobetaine (crotonobetain) and betaine butyrate in a bioreactor with the aid of specific microorganisms. A mixture of optically pure L-carnitine and butyrate betaine is obtained. To remove the butyrate betaine, the final product was recrystallized from methanol and isobutanol.

Another microbiological method for obtaining substantially pure L-carnitine is disclosed in DD 296702. L-carnitine in the culture solution is depleted by electrodialysis and recrystallization of L-carnitine. In this microbiological process, essentially no D-carnitine is produced and therefore no enantiomeric separation step is required. The L-carnitine is recrystallized from a solvent to remove other substances.

Obtaining high purity L-carnitine using non-microbial or non-enzymatic methods is more complicated. Using organic synthesis, a mixture of D-and L-carnitine is generally obtained. To obtain pure L-carnitine, DE 68901889T 2 teaches the use of ruthenium phosphine complexes in stereoselective asymmetric hydrogenation reactions. The complexes are relatively complex and expensive and are therefore unsuitable for the large-scale preparation of L-carnitine in an industrial process.

Thus, processes have been developed to isolate L-carnitine from a mixture of L-and D-carnitine. Generally these methods are based on the conversion of carnitine into a salt with an optically active acid and the separation of L-from D-carnitine with the different physical properties (e.g. solubility) that L-and D-carnitine have.

In this respect DD 93347 discloses a process for the separation of D-and L-carnitine from alcohol solutions in the presence of camphoric acid, dibenzoyltartaric acid or a combination thereof. Due to the different solubility of the salts, D-carnitine can be separated from L-carnitine.

DE 3536093 discloses a process for preparing L-carnitine from a racemic mixture, in which D-and L-carnitine are converted into optically active salts using dibenzoyl-L-tartaric acid, followed by fractional crystallization.

However, the processes for converting carnitine into optically active salts or acids are relatively complicated, since they include the steps of adding a separating agent and removing it after the separation process. This results in a relatively time and labor efficient overall process.

Problems to be solved by the invention

The problem to be solved by the present invention is to provide a method for producing L-carnitine that can solve the above problems.

The process should be suitable for the manufacture of high purity L-carnitine from a mixture of enantiomers comprising L-and D-carnitine. The optical purity can thereby be increased significantly and the yield should also be high.

The process should be carried out in a simple manner. In particular, the use of other compounds which have to be removed afterwards, for example optically active auxiliaries, should be avoided. Furthermore, the number of process steps should be relatively small and the processes should not require complex equipment. In general, the method should be cost-effective and labor-efficient.

Summary of The Invention

Unexpectedly, the problem to be solved by the present invention is solved by the process described in the claims. Further embodiments of the invention are disclosed throughout the specification.

The subject of the present invention is a process for the manufacture of L-carnitine comprising the following steps:

(a) providing a solution comprising at least 5% (w/w) carnitine in a first solvent, wherein the carnitine is a mixture of D-and L-carnitine,

(b) optionally seeding the solution with L-carnitine crystals,

(c) adding a second solvent in which L-carnitine is insoluble or has low solubility,

(d) isolating crystals comprising L-carnitine.

Carnitine is a zwitterion that contains a carboxylic acid group and a quaternary ammonium group. The carnitine used in step (a) is preferably the zwitterionic carnitine. However, salts, such as chloride, sulfate or nitrate salts, may also be used. The carnitine used in step (a) is preferably not a salt of carnitine with an optically active anion.

By the process of the invention, L-carnitine is obtained in an increased purity. The process of the invention is therefore also a process for purifying L-carnitine, or a process for obtaining L-carnitine in high purity, or a process for increasing the enantiomeric excess of L-carnitine.

According to the invention, crystalline L-carnitine is obtained from a solution comprising D-and L-carnitine. In a preferred embodiment, the solution in step (a) consists essentially of the solvent and carnitine. In the process of the invention, the enantiomeric excess (e.e.) of L-carnitine is increased. Preferably, the enantiomeric excess is increased by more than 1% or more than 2%. The increase in enantiomeric excess depends on the enantiomeric excess of the initial solution in step (a). Enantiomeric excess (e.e.) is defined as the absolute difference between the mole fractions of each enantiomer expressed as a percentage. For example, the enantiomeric excess of a sample containing 90% L-isomer and 10% D-isomer is 80% L-isomer.

In a preferred embodiment of the invention, the carnitine in step (a) comprises more than 50%, 80%, 90%, 95% or 98% (e.e.) L-carnitine. Preferably, the L-carnitine isolated in step (d) comprises more than 90%, 98%, 99% or 99.5% (e.e.) L-carnitine. Preferably, when the solution used in step (a) comprises more than 95% (e.e.) L-carnitine, in step (d) crystals comprising more than 99% (e.e.) L-carnitine are obtained.

In a preferred embodiment of the present invention, the first solvent is selected from the group consisting of ethanol, methanol, water, acetonitrile and a mixture thereof. The solvent is a good solvent for carnitine. This means that the solubility of carnitine at room temperature is at least 5%, preferably at least 10% or at least 20% (w/w). Preferably, the first solvent is ethanol. In particular embodiments, the first solvent may comprise no more than 0.5%, no more than 2%, or no more than 5% (w/w) water. Thus, the solvent may be technical grade.

In a preferred embodiment, the solution in step (a) is a saturated or supersaturated solution. It is also preferred that the solution is close to saturation, meaning that the concentration of carnitine is above 80% or above 90% of the saturation concentration. In a preferred embodiment of the invention, the concentration of total carnitine in the first solvent is between 5 and 75%. In ethanol, the concentration is preferably 10 to 50% or 15 to 40% (w/w). In methanol, the concentration is preferably 20 to 70% (w/w).

Preferably, the carnitine dissolved in the solution of step (a) is pure carnitine or highly pure carnitine. In this embodiment, the solution of step (a) contains only small amounts of other substances. The amount of other substances may be less than 0.5, less than 1, or less than 2% (w/w), based on the amount of total carnitine (D-and L-) in the solution. In another embodiment, the carnitine dissolved in the solution of step (a) comprises an amount of by-products or other compounds, for example starting compounds from a manufacturing process. In this embodiment, the solution of step (a) may comprise no more than 5%, no more than 10% or no more than 15% (w/w) of other compounds based on the total amount of carnitine. For example, the synthetic carnitine may comprise less than 1% (w/w) hydroxycrotonic acid.

In a preferred embodiment, the solution of step (a) is substantially free of water. This means that the carnitine and solvent should be substantially free of water or contain as little water as possible. The process of the present invention has been found to be more effective in the presence of only small amounts of water. Preferably, the total water content of the solution of step (a) is less than 2%, less than 1% or less than 0.5% (w/w).

In a preferred embodiment of the invention, step (a) comprises heating the solution, preferably until all carnitine is dissolved, for example to a temperature above 40 ℃ or above 50 ℃. Optionally, residual solid carnitine and/or residual solids may be subsequently removed, for example by filtration. In step (a), the temperature may be adjusted to 40 to 80 ℃ or 50 to 75 ℃. The choice of temperature depends on the solvent. When ethanol is used, temperatures of 50 to 75 ℃, for example about 65 ℃, are preferred.

In step (b), the solution from step (a) may be seeded with L-carnitine crystals. Preferably, the solution in which the seeding is carried out is a saturated or supersaturated solution. Supersaturated solutions can be obtained as follows: in step (a) a saturated solution is prepared at an elevated temperature and the saturated solution is slowly cooled so that no precipitation or crystallization of carnitine occurs.

By seeding and incubating the solution with L-carnitine crystals, the growth of the crystals can be initiated. Thus, when the process of the present invention comprises a seeding step of step (b), the average size of the finally obtained crystals will be higher. However, carnitine crystals having high purity can be obtained in high yield without adding seed crystals in the seed-crystal step of step (b). The carnitine crystals consist essentially of L-carnitine or are enriched in L-carnitine. Only a small amount of seed crystals is needed for seeding. The seed crystals should have a high purity and be very fine. Preferably, the seed crystals should be added while the solution is still clear (i.e., while no or substantially no crystals or precipitates are spontaneously formed). This can be achieved by seeding at elevated temperature. In a preferred embodiment of the invention, in the seeding step (b), the temperature of the solution is about 25 to 50 ℃, preferably 30 to 45 ℃. However, during and after seeding and before addition of the second solvent, the solubility of carnitine is still rather high and essentially no crystals or only a limited rapid formation of crystals is observed.

In a preferred embodiment, the temperature of the solution is lowered after seeding with crystals. Preferably, the temperature is reduced to 10 to 30 ℃, for example to about 20 ℃.

In step (c), a second solvent is added in which the L-carnitine is insoluble or has low solubility. During and after the addition of the second solvent, crystallization of L-carnitine is observed. During the crystallization of L-carnitine, the L-carnitine in the solution is depleted and it becomes a suspension. Thus, after crystallization, the composition may be considered as a solution or suspension. When referring to "solution" with respect to step (b) and subsequent steps, this is meant.

In a preferred embodiment of the present invention, the second solvent is selected from the group consisting of acetone, isopropanol, isobutanol, 2-propanol, 1-pentanol, 2-butanone, methyl acetate, ethyl acetate, butyl acetate, tetrahydrofuran, toluene, and mixtures thereof. Preferably, the second solvent is acetone.

The second solvent is a solvent in which L-carnitine is insoluble or low in solubility. In a preferred embodiment of the invention, the solubility of L-carnitine in the second solvent is less than 3%, less than 2% or less than 1% (w/w) at 25 ℃. Upon addition of the second solvent, the overall solubility of carnitine in the solution decreases and carnitine crystallizes. Thus, upon addition of the second solvent, solid carnitine crystals are formed in solution.

In a very preferred embodiment of the invention, the first solvent is ethanol and the second solvent is acetone.

In a preferred embodiment of the present invention, in step (c), the ratio of the first solvent to the second solvent is 1:1 to 1:10(w/w), more preferably between 1:1.5 to 1:6 or 1:2 to 1:4 (w/w). The ratio should be adjusted so that the solubility of carnitine in the solvent mixture is significantly reduced, thereby crystallizing a large portion of the total carnitine.

In a preferred embodiment of the present invention, the temperature of the solution is adjusted to 10 ℃ to 30 ℃ after the optional seeding step (b), or before or during step (C), or after the addition of the second solvent in step (C). However, the second solvent may be added before or after the temperature of the solution is reduced.

Preferably, in step (c), the second solvent is added slowly (e.g., dropwise). In a preferred embodiment of the invention, in step (c), the second solvent is added to the solution over a period of from 20 minutes to 8 hours or from 1 hour to 6 hours. During the addition of acetone and incubation, L-carnitine crystallizes. The amount of crystallized L-carnitine can generally be increased by incubation at low temperatures.

Preferably, the composition is incubated at a reduced temperature after the addition of the second solvent. For example, the temperature may be reduced to 5 to 20 ℃ or below 15 ℃. The composition may be incubated at this temperature for 10 minutes to 2 days or 30 minutes to 24 hours. In a preferred embodiment of the invention, after the addition of the second solvent in step (C), the composition is incubated at a temperature below 20 ℃ for 10 minutes to 2 days.

In step (d), the solid crystals are isolated by known methods, such as filtration or precipitation. Optionally, the crystals are washed, preferably with a second solvent. The solvent is removed by drying (optionally under reduced pressure). For example, acetone may be removed at 55 ℃ at a pressure of 100bar or less.

According to the invention, the total solid carnitine isolated in step (d) is called "crystals". These solids were found to have a crystalline structure. However, particularly upon rapid addition of the second solvent, the solid carnitine "crystals" may also comprise (at least partially) a carnitine precipitate.

The total yield of L-carnitine is preferably above 80% or above 85%, based on the total L-carnitine in the starting solution.

In a preferred embodiment of the invention, the method comprises the steps of:

(a) providing an ethanol solution comprising at least 5% (w/w) carnitine, wherein the carnitine comprises at least 50% (e.e.) L-carnitine, wherein the solution is heated until all carnitine is dissolved,

(a1) the temperature of the solution is adjusted to 25 to 50C,

(b) the solution is seeded with L-carnitine crystals,

(b 1) Optionally adjusting the temperature to 10 to 30 ℃,

(c) adding acetone in such an amount that the ratio of ethanol/acetone is 1:1 to 1:10(w/w),

(c1) optionally cooling the composition to a temperature below 20 ℃,

(d) the crystals comprising L-carnitine are preferably isolated by filtration.

In the above process, steps (a) to (d) are carried out continuously.

In a particular embodiment of the invention, the entire process of the invention is repeated using the crystals obtained in step (d). In this embodiment, a solution of crystals in a first solvent is prepared. When the entire process is repeated two or more times, highly pure L-carnitine can be obtained even from carnitine of low enantiomeric purity.

The method of the present invention solves the above problems. By the process of the invention, highly pure L-carnitine can be obtained from a mixture of D-and L-carnitine. In the separation of mixtures of D-and L-carnitine, it is not necessary to add optically active compounds as disclosed in DE 3536093 or DD 93347. In the process of the invention, carnitine is not converted into an optically active salt in an intermediate step. Surprisingly, the process of the present invention is highly efficient, since it is generally assumed in the art that it is difficult to effectively separate zwitterions without optically active additives (see DD 93347, column 2, lines 8-11). It is therefore difficult to envisage that a significantly increased L-enantiomeric form can be obtained by the relatively simple process of the invention.

The method of the present invention can be carried out in a small number of process steps and in a simple manner. The method does not require precipitation of an optically active salt of L-carnitine, and isolation and decomposition are performed by an additional crystallization step. The process provides L-carnitine in high enantiomeric purity and in high yield. The process of the present invention is less costly and labor intensive than processes known in the art. The process of the invention can be used for purifying L-carnitine from a mixture of enantiomers, for example obtained by industrial synthesis.

Examples

Example 1

100g of carnitine and 300g of ethanol were introduced into a laboratory reactor. The reactor was heated to 65 ℃ and stirred until all carnitine had dissolved. Thereafter, the temperature of the reactor was set to 37 ℃. At 37 ℃, pure L-carnitine seed crystals are added. The temperature of the reactor was cooled to 20 ℃ at a rate of-0.2K/min. 900g of acetone were added over 2 hours at 20 ℃. Thereafter, the suspension was cooled to 10 ℃. The solid was isolated at 10 ℃ and washed with acetone and dried at 55 ℃ and <100 mbar.

As a result, 86.1g of crystalline white dry solid was obtained. The solid contained 99.036% (w/w) total carnitine. Its enantiomeric purity was 99.60% (e.e.). The residual solvent content was 349mg/kg ethanol and 386mg/kg acetone. The total yield of L-carnitine was 88.6%.

Example 2

A laboratory reactor was charged with 60.2g of carnitine and 60g of methanol. The reactor was heated to 50 ℃ and stirred until all carnitine had dissolved. Thereafter, the temperature of the reactor was set to 25 ℃. At 25 ℃, 0.74g of pure L-carnitine seed crystals are added. The temperature of the reactor was cooled to 20 ℃ at a rate of-0.2K/min. 180g of acetone were added over 1 hour at 20 ℃. Thereafter, the suspension was cooled to 10 ℃ within 50 minutes. The suspension was stirred at this temperature for a further 30 minutes. Thereafter, the solid was filtered through a Nutsch filter (Nutsch filter), washed twice with approximately 60g of acetone and subsequently dried at 55 ℃ and a pressure of 250mbar for 8 hours.

As a result, 45.78g of crystalline white dried solid was obtained. The solid contained 98.94% (w/w) total carnitine. Its enantiomeric purity was 99.78% (e.e.).

Example 3

A laboratory reactor was charged with 30.1g carnitine and 90g ethanol. The reactor was heated to 65 ℃ and stirred until all carnitine had dissolved. Thereafter, the temperature of the reactor was set to 37 ℃. At 37 ℃, 0.91g of pure L-carnitine seed crystals are added. The temperature of the reactor was cooled to 20 ℃ at a rate of-0.2K/min. 270g of ethyl acetate are added at 20 ℃ over 1 hour. Thereafter, the suspension was cooled to 10 ℃ over 50 minutes. The suspension was stirred at this temperature for a further 30 minutes. Thereafter, the solid was filtered through a Nutsch filter, washed twice with approximately 30g of ethyl acetate and subsequently dried at 55 ℃ and a pressure of 250mbar for 8 hours.

As a result, 28.66g of crystalline white dry solid was obtained. The solid contained 98.51% (w/w) total carnitine. Its enantiomeric purity was 99.46% (e.e.).

Claims (13)

1. A process for the manufacture of L-carnitine comprising the steps of:

(a) providing a solution comprising at least 5% (w/w) carnitine in a first solvent, wherein the carnitine is a mixture of D-and L-carnitine,

(b) optionally seeding the solution with L-carnitine crystals,

(c) adding a second solvent in which L-carnitine is insoluble or has low solubility,

(d) isolating the crystals comprising L-carnitine,

wherein the first solvent is selected from the group consisting of ethanol, methanol, water, acetonitrile and mixtures thereof, and

wherein the second solvent is selected from the group consisting of acetone, isopropanol, isobutanol, 2-propanol, 1-pentanol, 2-butanone, methyl acetate, ethyl acetate, butyl acetate, tetrahydrofuran, toluene, and mixtures thereof.

2. The method of claim 1, wherein in step (a), the concentration of total carnitine in the first solvent is from 5 to 75% (w/w).

3. The process of claim 1, wherein in step (a), the carnitine comprises greater than 80% (e.e.) L-carnitine, and in step (d), the crystals comprise greater than 95% (e.e.) L-carnitine.

4. The method of claim 1, wherein step (a) comprises heating the solution to a temperature above 40 ℃.

5. The method of claim 1, wherein the solubility of L-carnitine in the second solvent is less than 2% (w/w) at 25 ℃.

6. The process of claim 1, wherein in the seeding step (b), the temperature of the solution is from 25 to 50 ℃.

7. The method of claim 1, wherein in step (c), the ratio of the first solvent to the second solvent is from 1:1 to 1:10 (w/w).

8. The method of claim 1, wherein the temperature of the solution is adjusted to 10 ℃ to 30 ℃ after seeding step (b), before or during step (c), or after the addition of the second solvent in step (c).

9. The method of claim 1, wherein in step (c), the second solvent is added to the solution over a time span of 20 minutes to 24 hours.

10. The method of claim 1, wherein after the addition of the second solvent in step (c), the composition is incubated at a temperature of less than 20 ℃ for 10 minutes to 2 days.

11. The method of claim 1, wherein in step (d), the crystals comprise greater than 99% (e.e.) L-carnitine.

12. The process of claim 1, wherein in step (d), the crystals are isolated by filtration or precipitation.

13. The method of any one of the preceding claims, wherein the first solvent is ethanol and the second solvent is acetone.