WO1999003853A1

WO1999003853A1 - Method for producing ascorbate

Info

Publication number: WO1999003853A1
Application number: PCT/CH1998/000271
Authority: WO
Inventors: Milan Oklobdzija; Marjian Hohnjec
Original assignee: Enco Engineering Chur Ag
Priority date: 1997-07-16
Filing date: 1998-06-23
Publication date: 1999-01-28
Also published as: EP1000050A1

Abstract

According to the inventive method, 2-keto-L-gulonic acid is converted to 2-keto-L-gulonic acid ester in a two-stage esterification process, before being further processed to ascorbate. The two-stage esterification enables efficient removal of the water introduced by the starting materials and arising in the reaction from the reaction solution. Said water removal from the reaction balance brings about an increase in esterification yields and a reduction in side products during the lactonization step as a result of the anhydrous ester, thereby economically enabling greater yields and higher purity of the synthesized ascorbate.

Description

Process for the production of ascorbate

TECHNICAL AREA

The present invention relates to a process for preparing ascorbate, which is a precursor for ascorbic acid, starting from 2-keto-L-gulonic acid, in which 2-keto-L-gulonic acid ester is formed with an alanol and in the presence of an at least catalytic amount of acid .

STATE OF THE ART

Although vitamin C (ascorbic acid) is present in many natural substances, synthetic methods are used for industrial production. The most widely used methods are based on the more or less modified so-called Reichstein synthesis (Reichstein and Grüssner, Helv. Chim. Acta 17, 311 (1934)), which starts from glucose. Diacetone-2-keto-L-gulonic acid occurs as an intermediate.

More recently, biotechnological processes have also been proposed for ascorbic acid synthesis, in which 2-keto-L-gulonic acid is obtained by fermentation processes (for example EP 0 278 447; Jap.Pat. 40154/1976; EP 0 292 303). 2-Keto-L-gulonic acid is isolated from the fermentation solution as a monohydrate and is prepared in two steps using methods similar to those of Reichstein synthesis Ascorbate implemented, which can then be processed into vitamin C.

In a first stage, 2-keto-L-gulonic acid is usually esterified with a lower alkanol, preferably methanol or ethanol, in the presence of an acid catalyst, e.g. Sulfuric acid, hydrochloric acid, sulfonic acids or strongly acidic ion exchange resins. In a second step, the ester formed is lactonized to sodium ascorbate in the presence of bases such as sodium bicarbonate or sodium hydroxide. This ascorbate can then be converted to ascorbic acid using an ion exchange process.

A major problem with the esterification / lactonization reaction described above is the presence of water in the reaction mixture. One equivalent of water is brought into the reaction mixture as water of crystallization with the starting product 2-keto-L-gulonic acid monohydrate, another is formed during the esterification and a third during the lactonization. During esterification, water affects the location of the

Reaction equilibrium to the disadvantage of the ester, which results in a lower yield. In the lactonization step, water also favors side reactions in the presence of bases, which lead to the yellowing of the product ascorbate. With a large proportion of water in the reaction mixture, it becomes correspondingly difficult to produce vitamin C in sufficient purity and whiteness. In addition, in the presence of water, part of the ester is hydrolyzed and causes losses in the separation of the sodium ascorbate, which is separated from the reaction mixture as a solid after lactonization. The higher the water content in the mother liquor, the greater the solubility of the sodium ascorbate and thus the loss with the mother liquor.

The yields in esterification / lactonization of the above type are typically 90 percent, the purity of the ascorbate is 89 percent. Various methods have been proposed to reduce the water content during esterification. Water can be removed during the reaction by continuously distilling off the water / alcohol mixture, treating the distillate with molecular sieves and recycling the alcohol thus dried (Pol. Pat. 57,042; Pol. Pat. 57,573). Another possibility mentioned was the continuous distillation of the water / alcohol mixture and replacement with fresh dry alcohol during the reaction [EP 0 535 927]. In both cases, a large amount of alcohol must be distilled off with a correspondingly large amount of energy. In addition, reaction times of up to 10 hours are necessary, with the risk of decomposition and side reactions.

Another problem arises when using 2-keto-L-gulonic acid produced by fermentation, which can sometimes contain strong impurities such as proteins, starting materials and acids formed during the side reaction in the fermentation. Such impurities are difficult to separate and can adversely affect the course of the reaction and the purification of the end product ascorbic acid.

PRESENTATION OF THE INVENTION

It is an object of the present invention to improve the process of the type mentioned in terms of yield, purity and economy. These and other objects are achieved according to the present invention by a method as specified in claim 1.

In the process described, 2-keto-L-gulonic acid is accordingly converted to 2-keto-L-gulonic acid ester in a two-stage esterification process, the solution formed after the first esterification step being at least partially evaporated and the residue formed being subjected to a second esterification process. The two stages of Esterification allows the water introduced by the starting materials and the water formed during the reaction to be effectively removed from the reaction solution. This removal of water from the reaction equilibrium has the following advantages:

- The esterification proceeds with increased yield. The more complete implementation of 2-keto-L-gulonic acid makes it possible to obtain an ascorbate with a significantly higher purity from the subsequent lactonization. A subsequent reaction in the ion exchanger and above all the crystallization of ascorbic acid can then be carried out correspondingly more efficiently.

- As a result of the anhydrous or practically completely anhydrous ester, less by-products leading to yellowing occur in the lactonization step. The cleaning of the possible end product ascorbic acid is also much more economical. For a sodium ascorbate produced by the process according to the invention, for example, an absorption of 0.315 could be measured for a 16.7% aqueous solution at 400 nm, while a corresponding solution of sodium ascorbate produced in the esterification without removal of the water had an absorption of 0.351 .

- Less ester is lost through hydrolysis during lactonization.

- As a result of less solubility, less product is lost with the mother liquor when the sodium ascorbate formed during the lactonization is separated off (e.g. 0.78% compared to 1.76% in the case of single-stage esterification).

- Overall, both a greater yield and a higher purity of the synthesized ascorbate are achieved in a more economical manner. Compared to the water removal methods mentioned in the introduction, the method according to the invention has the following advantages:

- Compared to distilling off the alcohol / water mixture and drying the distillate with molecular sieves, shorter reaction times are achieved and less energy is used for alcohol evaporation.

- Compared to the method by distilling off the alcohol / water mixture and replacing it with dry alcohol, shorter reaction times, less energy for alcohol evaporation and less energy for the regeneration of the alcohol are also used.

Advantageous refinements of the method according to the invention are specified in the dependent claims.

In particular, in the first esterification step 2-keto-L-gulonic acid is preferably in methanol or ethanol and in the presence of an acid such as sulfuric acid, hydrochloric acid or sulfonic acids, at reaction temperatures preferably between room temperature and 85 ° C., with preferred reaction times between 30 and 180 minutes, converted to the corresponding methyl or ethyl ester. A yield of 95% is typically achieved, with about 5% remaining unreacted 2-keto-L-gulonic acid. The reaction mixture is then preferably evaporated to dryness, ie all water is removed. Since the corresponding esters are very stable, there are no losses due to decomposition. In a second esterification step, the addition of preferably fresh, dry methanol or ethanol and in the presence of an acid under the same conditions as in the first step, the remaining 2-keto-L-gulonic acid is esterified. The subsequent lactonization is carried out by adding a base, preferably sodium bicarbonate, at reaction temperatures of preferably room temperature to 80 ° C. and preferred reaction times between 30 and 240 minutes. That so Sodium ascorbate obtained can then be dissolved in water and converted to ascorbic acid using ion exchangers. Pure ascorbic acid can finally be obtained by crystallization, separation and subsequent drying.

A partial reduction in the water content in the esterification can be achieved by drying the 2-keto-L-gulonic acid monohydrate to the corresponding anhydride before adding it to the esterification. The total yield in esterification / lactonization can be increased to typically 93% and the purity of sodium ascorbate to 92%. However, the yellowing during lactonization cannot be completely prevented.

2-Keto-L-gulonic acid monohydrate obtained from fermentation processes is often heavily contaminated with by-products and starting materials which are difficult to separate. In such cases, the following variant of the method according to the invention is advantageously used, in which such impurities are efficiently separated off in the esterification stage. For this purpose, after the first esterification step, the pure 2-keto-L-gulonic acid ester is separated off after cooling and crystallization, the mother liquor is concentrated and, after cooling and crystallizing again, further esters are separated off. After evaporation of the remaining mother liquor, the second esterification step is then carried out. After cooling and crystallization, further 2-keto-L-gulonic acid esters are obtained in pure form from the reaction mixture. Impurities remain essentially in the residual mother liquor. The further processing of the pure ester makes it possible to obtain a sodium ascorbate from the lactonization, which can be further processed to pharmaceutical quality. Ion exchange and crystallization can also be carried out much more efficiently due to the greatly reduced impurities. If the isolated ester is additionally subjected to a further esterification step before the lactonization, the proportion of 2-keto-L-gulonic acid can be reduced even further. Ascorbate produced in this way is of pharmaceutical quality in accordance with the purity conditions required by Pharmacopoeia.

In addition to using the process according to the invention for 2-keto-L-gulonic acid monohydrate obtained from fermentation processes, it can also be used very efficiently for diacetone-2-keto-L-gulonic acid monohydrate (2,3: 4,6-diisopropylidene obtained by the Reichstein method) -2-oxo-L-gulonic acid monohydrate) can be used. For this purpose, diacetone-2-keto-L-gulonic acid monohydrate is advantageously first hydrolyzed to 2-keto-L-gulonic acid, which is then converted further to the ester and sodium ascorbate in a two-stage esterification process according to the invention. A separation of hydrolysis and esterification is advantageous because the difficult separation of acetone / alcohol mixtures can be avoided.

The method according to the invention can be explained in more detail by the following examples, but is in no way restricted to it:

Rpis i l i

Esterification and lactonization:

50 g of 2-keto-L-gulonic acid monohydrate, content 99.38% and 200 ml of methanol are placed in a 500 ml glass vessel. 0.5 ml of concentrated sulfuric acid is added dropwise with stirring. The esterification (step I) is then carried out with stirring and heating to reflux temperature for 60 minutes. The reaction mixture is then evaporated to dryness under vacuum at 50 mbar and a bath temperature of 85 ° C. 200 ml of methanol and 0.5 ml Given sulfuric acid. Then esterification is continued for 60 minutes under the same conditions as for esterification I (step II). The reaction mixture is then cooled to 30 ° C. and 21.4 g of sodium bicarbonate are added with stirring. After heating, the lactonization is carried out at 63 to 65 ° C for 3 hours. After cooling to 25 ° C., the product is separated off by filtration, washed twice with 10 ml of methanol each time and then dried under vacuum. 46.52 g of sodium ascorbate, 94.8% content, are obtained (esterification / lactonization yield 95.03% of theory).

0.36 g of sodium ascorbate is lost with the mother liquor. To determine the whiteness, a sample of the sodium ascorbate obtained is dissolved in water. For a 16.7 percent solution, an absorption of 0.315 is measured at 400 nm. An analogous reaction, but without evaporation after the first esterification, gives the following comparative values: 46.92 g of sodium ascorbate, 89% content, are obtained (esterification / lactonization yield 90% of theory). With the mother liquor, 0.82 g of sodium ascorbate is lost. The measurement of the degree of whiteness under the same conditions as above gives an absorption of 0.351.

Ion exchange and crystallization:

46.5 g sodium ascorbate are dissolved in water. A system of cation and anion exchange columns is used for the conversion to ascorbic acid. 230 ml of Amberlit 200 C are used for the cation column and 30 ml of Amberlit IRA 900 for the anion column. The ascorbic acid solution obtained is treated with activated carbon. After concentration, 37.125 g of pure ascorbic acid, 100% content, are obtained by crystallization and drying. The overall yield of 2-keto-L-gulonic acid monohydrate to ascorbic acid is 90.03% of theory The ascorbic acid obtained meets all requirements of British Pharmacopoeia or US Pharmacopoeia. The thermal stability is measured as follows: 3 g during 45 Heated to 120 ° C for minutes and then dissolved in 15 ml of water. The absorption at 420 nm, 3 cm cuvette, gives 0.012.

At πpi pl 7.

100 g of 2-keto-L-gulonic acid anhydride, content 99.4%, and 200 ml of methanol are placed in a 500 ml glass vessel. 0.5 ml of concentrated sulfuric acid is added dropwise with stirring. The esterification is then carried out with stirring and heating to reflux temperature for 90 minutes (step

I) carried out. The reaction mixture is then evaporated to dryness under vacuum at 50 mbar and a bath temperature of 85 ° C. 200 ml of methanol and 0.5 ml of sulfuric acid are added to the residue. Then esterification is continued for 90 minutes under the same conditions as for esterification I (step

II). The reaction mixture is then cooled to 30 ° C. and 46.5 g of sodium bicarbonate are added. After heating, the lactonization is carried out at 63 to 65 ° C for 3.5 hours. After cooling to 25 ° C., the product is separated off by filtration, washed twice with 20 ml of methanol each time and then dried under vacuum. 102.56 g of sodium ascorbate, 94.09% content, are obtained (esterification / lactonization yield 95.13% of theory).

111.06 g of 2-keto-L-gulonic acid monohydrate, content 89.5%, and 260 ml of methanol are placed in a 500 ml glass vessel. With stirring, 1.0 ml of concentrated sulfuric acid is added dropwise. The esterification (step I) is then carried out with stirring and heating to reflux temperature for 90 minutes. 35 g of methanol are then evaporated from the reaction mixture under vacuum. After cooling to 10 ° C., the resulting 2-keto-L-gulonic acid methyl ester crystals are filtered off, washed twice with 10 ml of methanol each time and dried. This gives 66.01 g of product. The mother liquor is vacuumed until crystallization begins evaporated, then cooled to 10 ° C. The crystals formed are filtered off, washed twice with 5 ml of methanol each time and dried. This gives a further 21.20 g of product. The mother liquor is evaporated to a constant weight under vacuum at 60 mbar and 50 ° C. 40 ml of methanol and 0.15 ml of sulfuric acid are added to the residue. Then esterification is continued for 90 minutes under the same conditions as for esterification I (step II). The reaction mixture is then evaporated in vacuo until crystals appear, then cooled to 4 ° C. The crystals formed are filtered off, washed twice with 4 ml of methanol each time and dried. A further 6.85 g of product are obtained. A total of 94.06 g of 2-keto-L-gulonic acid methyl ester are obtained (96.44% of theory).

The 94.06 g of methyl 2-keto-L-gulonic acid obtained are suspended in 170 ml of methanol and 38.7 g of sodium bicarbonate are added. Lactonization is carried out at 63 to 65 ° C for 3 hours. After cooling to 25 ° C., the product is separated off by filtration, washed twice with 20 ml of methanol each time and dried under vacuum. 88.05 g of sodium ascorbate, content 97.8%, are obtained (esterification / lactonization yield 89.22% of theory).

R ia -ipl 4.

50 g of diacetone-2-keto-L-gulonic acid monohydrate, 4 ml of water and 0.5 ml of sulfuric acid are placed in a 250 ml glass vessel. The reaction mixture is heated with stirring. The resulting solution is refluxed at a bath temperature of 90 ° C for 60 minutes. Then the split off acetone, 20.5 ml, is distilled off. 100 ml of methanol and then 0.5 ml of sulfuric acid are added dropwise to the residue. The esterification (step I) is then carried out with stirring and heating to reflux temperature for 60 minutes. The reaction mixture is then under vacuum at 50 mb and a bath temperature of 80 ° C to dryness evaporated. 50 ml of methanol and 0.2 ml of sulfuric acid are added to the residue. Then esterification is continued for 60 minutes under the same conditions as for esterification I (step II). The reaction mixture is then cooled to 30 ° C. and 15.5 g of sodium bicarbonate are added. After heating, the lactonization is carried out at 63 to 65 ° C for 3 hours. After cooling to 25 ° C., the product is separated off by filtration, washed twice with 10 ml of methanol each time and then dried under vacuum. 32.13 g of sodium ascorbate, content 95.27%, are obtained (yield hydrolysis / esterification / lactonization 90.3% of theory).

94.06 g of methyl 2-keto-L-gulonic acid, prepared analogously to the method described in Example 3, is suspended in 190 ml of methanol and 0.5 ml of concentrated sulfuric acid are added dropwise with stirring. The mixture is then heated to reflux temperature for 60 minutes with stirring in order to esterify remaining residues of 2-keto-L-gulonic acid. After cooling to 40 ° C, 40.3 g of sodium bicarbonate are added. Lactonization is carried out at 63 to 65 ° C for 3 hours. After cooling to 25 ° C., the product is separated off by filtration, washed twice with 20 ml of methanol each time and dried under vacuum. This gives 89.53 g of sodium ascorbate, content 98.75 (yield: esterification / lactonization 95.24% of theory).

25 g of the sodium ascorbate obtained are dissolved in 19.5 ml of demineralized water. 1.1 g of ascorbic acid are added to neutralize traces of sodium bicarbonate that are still present. The solution is heated to 75 ° C and 75 ml of hot methanol is added dropwise. The suspension is cooled for a further 30 minutes and then slowly cooled to 20 ° C. The product is filtered off and washed with a methanol / water mixture and dried under vacuum. 23.04 g of dry, pure sodium ascorbate with a content of 99.4% are obtained, which corresponds to the purity conditions required by Pharmacopoeia.

Claims

PATENT CLAIMS

1. Process for the preparation of ascorbate starting from 2-keto-L-gulonic acid, in which 2-keto-L-gulonic acid ester is formed with an alkanol and in the presence of an at least catalytic amount of acid, characterized in that the esterification in two stages is carried out, the solution formed after the first esterification step being at least partially evaporated and the residue formed being subjected to a second esterification process.

2. The method according to claim 1, characterized in that 2-keto-L-gulonic acid monohydrate is used as the starting material.

3. The method according to claim 1, characterized in that the starting material 2-keto-L-gulonic acid is at least partially converted by drying to 2-keto-L-gulonic acid anhydride and then subjected to the esterification steps.

4. The method according to claim 1, characterized in that the starting material 2-keto-L-gulonic acid by hydrolysis of diacetone-2-keto-L-gulonic acid (2, 3; 4, 6-diisopropylidene-2-oxo-L-gulonic acid ) is made with a strong acid.

5. The method according to claim 4, characterized in that diacetone-2-keto-L-gulonic acid monohydrate is used as the starting material.

6. The method according to any one of claims 4 or 5, characterized in that after the hydrolysis the split off acetone is separated off before the esterification steps.

7. The method according to claim 1, characterized in that the 2-keto-L-gulonic ester is lactonized by adding a base to an ascorbate.

8. The method according to claim 1, characterized in that the ester formed in the first esterification step is at least partially separated from the solution before it is at least partially evaporated.

9. The method according to claim 1, characterized in that the ester formed in the second esterification step is at least partially separated from the solution.

10. The method according to claim 1, characterized in that the esters formed in the two esterification steps are each at least partially separated from the solution.

11. The method according to any one of claims 8 to 10, characterized in that the ester is separated from the mother liquor by means of crystallization and subsequent filtration or centrifugation.

12. The method according to any one of claims 8 to 10, characterized in that only the isolated ester is further processed in the lactonization to ascorbate.

13. The method according to claim 1, characterized in that the reaction solution from the first and / or the second esterification step is / are evaporated to dryness.

14. The method according to claim 1, characterized in that the first stage of the esterification is carried out at temperatures between room temperature and 85 ° C in the presence of a strong acid for 30-120 minutes, the reaction mixture is then evaporated to dryness and then a second stage of Esterification is carried out under the same conditions.

15. The method according to any one of claims 8 to 12, characterized in that the isolated ester before lactonization is treated in alcoholic solution with an at least catalytic amount of a strong acid.