JPS5935920B2 - Method for producing 2-ketogulonate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for the preparation of 2-ketocuronic acid, its alkyl esters and salts by selective reduction of 2,5-diketogluconic acid; its alkyl esters or salts thereof.

2-Ketocuronic acid is an intermediate for the production of ascorbic acid.

Complete reduction of 2,5-diketogluconic acid with excess sodium borohydride has been reported as part of the structural determination of this acid.

Ag, Biol. Chem28, 819 (1964)
, J. Biol., Chem. 204, 34 (1953)
and Antonie Van Leeuwenhoec 37, 185 (1971).

Catalytic reduction of 2,5-diketogluconic acid using Raney-nickel catalyst and hydrogen produces 2-ketogluconic acid and 2
- A mixture of ketocuronic acids is obtained in low yields. In this case 2-
Ketogluconic acid is the main product. (Agr, Bi
ol, Chem, 28, 819 (1964). No. 4,159,990, 2,5-diketogluconate is reduced with one equivalent of alkali metal borohydride to form a mixture of 2-ketocuronic acid and 2-ketogluconic acid. did. 52,5-diketogluconate can be reduced with large regioselectivity and stereoselectivity to produce high yields of the desired 2-ketocuronic acid, followed by amine-borane reducing agent at pH ca. 2-7. It can be converted to ascorbic acid using

Thus, for example, the process provides yields of 2-ketocuronic acid and 2-ketogluconic acid of greater than 94%, with the product mixture being approximately 96% the desired 2-ketocronic acid. Such high yields can be obtained without carrying out the reduction reaction in the presence of a boron-complexing agent, as is required for optimal yields of reduction with alkali metal borohydrides.

In addition, 2-ketocuronic acid produced by this method is
It has been found that in the presence of excess amine-borane reducing agent, no further reduction (ie, reduction of the 2-keto group) occurs at a balanced and reasonable rate. However, the 2-keto group of 2-ketocuronic acid is readily reduced by the alkali metal borohydride, making the optimal 2,5 with low yield and using excess alkali metal borohydride.
-diketogluconate The reduction rate or conversion of the starting material cannot be increased. Furthermore, this 2,5-diketogluconate is most stable under the acidic conditions used in the amine-borane reduction of the present invention. The present invention relates to a method for producing 2-ketogulonate, which includes 2,
5-diketogluconate with the general formula RtR2HN-BH,
amine-borane is reduced with pyridine-borane in a solution with a pH of about 2 to 7 at a temperature of about -20 to 70°C.

In addition, R and R2 in the above general formula are each selected from alkyl having 1 to 4 carbon atoms, and the above 2,5-diketogluconate is selected from 2,5-diketogluconic acid and its alkali metal salts. selected from those having selected counterions. A preferred amine-borane reducing agent of the general formula R, R2HN-BH, is dimethylamine-borane.

Pyridine-borane is also a preferred amine-borane. The reaction is preferably carried out at a temperature of 0 to 25°C, preferably at a pH in the range of 4 to 6.

Preferred 2,5-diketogluconate starting materials include 2
, 5-diketogluconic acid, and sodium 2,5-diketogluconate. The method of the present invention involves the site-selective reduction of 2,5-diketogluconate (R
stereoselective reduction (egiOselectivereductiOn) and stereoselective reduction
ctiOn).

The reaction product is primarily 2-ketogulonate with only a small amount (i.e., about 2-12%) of 2-ketogluconate formed. This reaction product is therefore suitable for the production of ascorbic acid by conventional methods, for example by base-catalyzed lactonization of lower alkyl esters of 2-ketocuronic acid. If necessary, the small amount of 2-ketocronic acid present in the reaction product may be converted to erythorbitic acid in a similar manner. This conversion is 2-
This can be done separately or simultaneously with converting ketogulonate to ascorbic acid. The 2,5-digatogluconate used as a starting material in the present invention is 2,5-
Either diketogluconic acid or a salt of this acid can be used.

2,5- as used in this specification and claims.
Diketogluconates, 2-ketogulonates and 2-ketogluconates include the free acids and appropriate alkyl esters and salts thereof as previously described. 2
, 5-diketogluconic acid and its salts are made by any means known in the art.

Generally, 2,5-diketogluconate is produced as a calcium salt in aqueous solution by fermentation using methods well known in the fermentation industry and can be used directly in this form as the starting material of the present invention. This 2,5-diketogluconate can also be produced by fermentation in the presence of other ions such as sodium, and the sodium 2,5-diketogluconate thus obtained is also of the invention. can be used directly in the method of The reduction of 2,5-diketogluconate is performed by converting a solution of this 2,5-diketogluconate into a compound of the general formula RlR2HN-BH, where R and R2 are the same as defined above. This is done in contact with an effective amount of amine-borane or pyridine-borane.

Preferably, the reaction is carried out in an aqueous solution optionally containing an organic co-solvent, such as alkanols having 1 to 4 carbon atoms, alkanediols having 2 to 4 carbon atoms, and the like. Methanol is preferred as the co-solvent. 2,
The concentration of 5-diketogluconate is not limited, but is about 5-
A range of 20 weight percent is preferred.

The concentration of 2,5-diketogluconate produced by fermentation is generally within this range, thereby providing a suitable aqueous solution of the starting material used in the process of the invention. When utilizing an alkyl ester as starting material, the reaction is carried out in an anhydrous solution, such as an alkanol, especially methanol. In all cases, it is not necessary that all of the 2,5-diketogluconate be dissolved in the solvent; it is sufficient if a substantial portion of the starting material is in solution. Amine-boranes useful as reducing agents in the process of the invention are well known in the art and generally available commercially.

For example, C.F. Lane, Aldrichimica 6;51 (
(1973). If necessary, these can be prepared using known methods, for example by reacting diborane with a suitable amine of the general formula RlR2NH to form an amine-borane RlR2HN-BH3.
It can be created by creating. This reaction is generally conducted at a temperature of about 0°C or less. The amount of amine-borane used in the reduction reaction is determined by the amount of amine-borane present in this reaction mixture that can be converted into the desired reaction product.
- The amount of diketogluconate starting material will be determined.

Preferably, enough amine-borane is used to convert all of the 2,5-diketogluconate starting material present in the reaction mixture. This is because it gives the optimum yield of a given 2-ketogluconate suitable for subsequent conversion to ascorbic acid. However, if necessary, a smaller amount of amine-borane reducing agent may be used to achieve a low degree of conversion, i.e. to reduce only a portion of the 2,5-digatogluconate present in the reaction mixture. . Unreacted 2,5-diketogluconate can then be recycled and used for further reduction reactions. It will be understood that this specification and claims encompass other ways of implementing the reduction that will be readily apparent to those skilled in the art, as well as the above-described ways of implementing the invention. Further, this reduction can be carried out in a batch manner or in a continuous manner, but is not limited thereto. It will be appreciated that one mole of amine-borane contains three equivalents of hydride ion.

Thus, per mole of 2,5-diketogluconate starting material present in the reaction mixture, from about 0.30 to about 0.
High yields of the desired 2-ketogulonates are made using 40 moles, preferably about 0.33 moles of amine-borane. However, the 2-keto group of the product 2-ketogulonate is reduced very slowly by excess amine-borane reducing agent. Particularly when R1 and R, are both other than hydrogen, the rate of reduction of the 5-keto group can be controlled, if necessary, by using a relatively large amount of reducing agent, e.g. About 2-3 moles of amine-borane can be used quickly. This excess ensures complete conversion of the 2,5-diketogluconate starting material in the reaction mixture. The amine-borane reducing agent can be added to the solution of 2,5-diketogluconate in the patch at the beginning of the reaction or while the reaction is progressing. It can also be added as a solid or as a solution. During the reduction of 2,5-diketogluconate with amine-borane, the pH of the solution
should be kept between about 2 and 7, preferably between about 4 and 6.

To maintain the pH in the above range, mineral acids such as hydrochloric acid, sulfuric acid, phosphoric acid, etc. or organic acids such as lower alkyl carboxylic acids, such as alkyl carboxylic acids having 1 to 6 carbon atoms, may be added to the reaction mixture. can. The pH of aqueous solutions of sodium or calcium 2,5-diketogluconate produced by fermentation is usually below 5, and therefore such solutions are suitable for use in the reduction method of the present invention. The time required to complete the reduction depends on the temperature and amounts of reagents used.

However, generally the reaction time is relatively short, about 15
The reaction is substantially complete within minutes to about 3 hours. When this selective reduction is complete, the remaining unreacted 2,5-diketogluconate can be recycled for further reaction or heated with an acid or base and then filtered to effectively remove the 2,5-diketogluconate. can be removed. The 2-ketogulonate prepared by the above method can be prepared by filtering the reaction mixture, adjusting the filtrate to a pH of between about 1.5 and 2 by adding an acid such as concentrated sulfuric acid, and filtering off the precipitate that forms. can be separated along with smaller amounts of 2-ketogluconate. The water or water-organic co-solvent can be removed, for example by freeze drying or heating under reduced pressure, to collect the desired product. The ratio of 2-ketocuronic acid to 2-ketogluconic acid in the mixture is 1
50cm (5 feet) of 0V-210 [Ohio Valley Specialty Co., Ltd. (0hi0Va11eySpe
cia1tyC0. )] It was determined by gas-liquid chromatography of bertrimethylsilylated methyl ester at 135°C using a column. However, other analyzes can also be used, for example liquid chromatography or thin layer chromatography. The 2-ketocuronic acid formed by the reduction method of the present invention can be easily converted to ascorbic acid by methods known in the art. The small amount of 2-ketogluconate present in the reaction mixture can be separated off, for example by chromatography, and the 2-ketogulonate can be converted to ascorbic acid. However, a mixture of 2-ketogulonates containing a small amount of 2-ketogluconate can be used directly in the next reaction, where the 2-ketogluconate is converted to erythorpic acid. This ersorpic acid can be separated from the ascorpic acid formed. For this reason,
For example, mixtures of 2-keto acids can be converted to methyl esters by refluxing in methanol for about 3 to 24 hours in the presence of an acidic catalyst (eg, hydrochloric acid) or a sulfonic acid ion exchange resin. Other esters can be made in this manner using the appropriate alcohol. When the starting material is the alkyl ester selective reduction of 2,5-diketogluconic acid, the ester is formed directly.

The mixture of methyl esters can be separated and then refluxed in methanol in the presence of a base such as sodium bicarbonate in an inert atmosphere. Upon cooling, sodium ascorbate and sodium erythorbate precipitate out.
Collect the coarse salt by filtration, mix it with water, and add
DOwex) 50 [Tau Chemical Company (Dowchemi)
calCO. )] Deionize with a cationic exchange resin. After removing the water, ascorbic acid and elsorpic acid are recrystallized from methanol-water. Ascorbic acid containing a small amount of elsorpic acid is obtained. If necessary, ascorbic acid may be obtained by recrystallization, for example from a 4:1 methanol-water solution. Other suitable solvents or cosolvents can be used if desired. If desired, the methyl esters of 2-ketocuronic acid and 2-ketogluconic acid can be separated and converted to ascorbic acid and erythorpic acid, respectively, using the same conditions as described above for the mixture of esters. Ascorbic acid can also be prepared by combining a small amount of 2-ketogluconate obtained by the method of the invention with a suitable organic solvent (e.g. benzene, toluene,
xylene, etc.) from about 50°C to 130°C (preferably 6°C).
0°C to 90°C) in the presence of an acid selected from hydrochloric acid, hydrobromic acid, sulfuric acid and sulfonic acid ion exchange resins (although other similar acids may also be used). You can also make one.

After about 3 to 12 hours of heating, depending on the reaction used, the lactonization of 2-katogulonate to ascorbic acid is substantially complete. In this process, erythorpic acid is not generated from the small amount of 2-ketogluconate present, making this process a simple method for selectively forming ascorbic acid from the reaction product of the amine-borane reduction of the present invention. will be given. Next, the present invention will be explained with reference to Examples.

However, it is to be understood that the invention is not limited to the specific details of these embodiments. Example 1 15% (w/v) aqueous solution of 2,5-diketogluconic acid (
Temperature 6-8℃, PH3. O) 100-, 4.6ν(
1.07 mol) of borane-dimethylamine complex was added.

After 1 hour, analysis by high pressure liquid chromatography [formate form Aminex A-21]
Resin, ammonium formate. Solution pH5. O] indicated that complete reduction had occurred.

At this point, 30% acetone was added and the solution was slowly poured into a slurry of 150% DOwex 50 ion exchange resin (hydrogen form). After the hydrogen was released, the resin was removed by filtration, the solvent was removed using a rotary evaporator, and the residue was poured into 200 ml of anhydrous methanol. Amerlyst
) 15 ion exchange resin catalyst (20-, hydrogen form) was added and the azeotrope of methanol and trimethylborate was distilled off at normal pressure. This distillation was accompanied by esterification of 2-ketogluconic acid. When the methanol solution is filtered and reduced to 30°C, crystallization progresses and 6.9t of methyl 2-ketochlorinate (melting point 152-154°C, literature says 15°C) is separated.
3-154°C) was obtained. Evaporate the mother liquor in vacuo to obtain 2-
The ratio of methyl ketochronate to methyl 2-ketogluconate is 0V-2 with 7J MeF23 (150 CTrL (5 feet)).
(by gas-liquid partition chromatography (Glpc) analysis of Berucylated methyl ester at 135°C using a 10 column) (2-
Ketocuronic acid (corresponding to 2-ketogluconic acid) was made into a solid. Example 2 The method of Example 1 was repeated using 6NHCl while keeping the pH at 3.5.

High pressure liquid chromatography (Hlp) after 3 hours at O°C.
c) Analysis [calcium form Dowex (DOw)]
ex) 50 ion exchange fat, 0.01 M CaC12 buffer - PH8] showed that the ratio of 2-ketogulonate to 2-ketogluconate was 94:6. The reactants were treated as described in Example 1, with a 96:4 ratio after esterification.
A mixture (according to 7/Pc analysis) of methyl 2-ketogulonate and methyl 2-ketogluconate was obtained. Example 3
-23 The reduction of sodium 2,5-diketogluconate was accomplished using a number of amine-boranes at different temperatures and pH conditions.

i.e. 10.5% (w/
To the stirred solution of sodium 2,5-diketogluconate in v) was added 1 part of solid amine-borane. To ensure the completeness of the reaction, Hp/c analysis [formate form of aminex (A
minex)-25 resin, ammonium formate buffer at pH 5.3] and the yield was determined by HPLC analysis with an internal standard based on the hydride equivalents used. The ratios of 2-ketocronic acid and 2-ketogluconic acid of Examples 3 to 20 were converted to the corresponding methyl esters as described in Example 1 and subjected to Glpc analysis of these corresponding vertrimethylsilylated methyl esters ( 150
CTrL (5 feet) 0V-210 column, 135°C
). The ratios of 2-ketocuronic acid and 2-ketogluconic acid in Examples 21-23 were converted to alcolpic acid and erythorpic acid, and their corresponding vertrimethylsilylated methyl esters were determined by the Glpc analysis described above. The results obtained were as follows:

Claims (1)

[Scope of Claims] 1. A 2,5-diketogluconate selected from the group consisting of 2,5-diketogluconic acid and an alkali metal salt of the acid in a solution having a pH of about 2 to 7 and a temperature of about -20°C to 70°C.
The formula R_1R_2HN・BH_3 (R_1 in the formula
and R_2 are each selected from the group consisting of alkyl having 1 to 4 carbon atoms) and pyridine-borane.
- A method for producing ketogulonate. 2 The above amine-borane has the formula R_1R_2HN・BH_
3. The method of claim 1, wherein the amine-borane of 3 is an amine-borane. 3. The method of claim 2, wherein R_1 and R_2 are each methyl. 4. The method according to claim 1, wherein the amine-borane is pyridine-borane. 5. A process according to claim 1, wherein the reduction is carried out in an aqueous solution at a pH of 4 to 6 and at a temperature of 0°C to 25°C. 6. The method of claim 5, wherein the 2,5-diketogluconate is sodium 2,5-diketogluconate and is used in an amount of 5 to 20% by weight of the solution. 7. Process according to claim 6, in which from 0.30 to 0.4 moles of amine-borane are used.