A method for synthesizing folic acid

Abstract

The present invention discloses a method for synthesizing folic acid. Using epichlorohydrin as the starting material, folic acid is synthesized through steps such as ring opening, amino protection, oxidation, cyclization, condensation, and deprotection. The present invention has the characteristics of low-cost and readily available raw materials, simple process, convenient operation, mild conditions, and less wastewater discharge, and thus has great implementation value and social and economic benefits.

Description

(1) Technical Field

[0001] The present invention relates to the field of synthesis of pharmaceutical and chemical intermediates. In particular, the method uses epichlorohydrin as the starting material to prepare a new method for the anti-anemia drug folic acid. (2) Technical Background

[0002] Folic acid is a water-soluble vitamin, named after its rich content in green leaves, also known as pteroylglutamic acid. Folic acid can be used as an anti-anemia drug, with a content of 50 - 70 μg / 100 g in tea leaves, which promotes the formation of normal red blood cells. In the material metabolism of tea plants, folic acid participates in the transmission of methyl groups, and also participates in the synthesis of amino acids and nucleic acids in the body, and promotes the generation of red blood cells together with vitamin B 12 Folic acid is used for various megaloblastic anemias, especially suitable for pregnant women and infants with megaloblastic anemia. Therefore, studying the synthesis method of folic acid not only has theoretical research value, but also has certain economic benefits.

[0003] The structural formula of folic acid (Ι) is as follows:

[0004]

[0005] Before the present invention, the main synthesis methods of folic acid (Ι) are as follows:

[0006] 1) In 1948, the initial industrial synthesis route for synthesizing folic acid using dibromopropionaldehyde as the raw material was reported in Patent US2562223. The crude folic acid product was prepared by reacting N-(4-aminobenzoyl)-L-glutamic acid, 2,4,5-triamino-6-hydroxypyrimidine sulfate with 2,3-dibromopropionaldehyde in a buffer solution of acetic acid / sodium acetate at room temperature for 5 - 9 hours in a mixed solvent of water and ethanol. During this period, the 2,3-dibromopropionaldehyde solution dissolved in ethanol was slowly added to the reaction solution for heat preservation reaction. Finally, after acidification, washing, and purification, the pure folic acid product was obtained, and the total yield was only 20%. The synthesis route is as follows:

[0007]

[0008] In this route, 2,4,5-triamino-6-hydroxypyrimidine and N-(4-aminobenzoyl)-L-glutamic acid need to be dissolved in a large amount of water, and the substrate 2,3-dibromopropionaldehyde is difficult to prepare and expensive. This process has been phased out.

[0009] 2) In 2014, the method for synthesizing folic acid using trihalopropene as the raw material was reported in Patent CN103896945. 1,2,3-trihalopropene (1,2,3-trichloropropene or 1,2,3-tribromopropene), 2,4,5-triamino-6-hydroxypyrimidine sulfate, and N-(4-aminobenzoyl)-L-glutamic acid were first reacted in isopropanol at 40 °C in the presence of a phase transfer catalyst tetrabutylammonium bromide to obtain a crude product, and then the crude product was oxidized by hydrogen peroxide to obtain pure folic acid. The synthesis route is as follows:

[0010]

[0011] During the reaction process of this route, the use of the oxidant has a destructive effect on folic acid, and the substrate 1,2,3-trihalopropene is unstable and has a high price, so it is not applicable to industrial production.

[0012] 3) Currently in China, folic acid is mainly prepared using 1,1,3-trichloroacetone as the raw material. As reported in the route of Patent CN103896945 in 2014, 1,1,3-trichloroacetone first reacts with N-(4-aminobenzoyl)-L-glutamic acid, and then 3,4,5-triamino-6-hydroxypyrimidine is added. During this process, 10% Na2CO3 solution is used to neutralize the acid generated in the reaction and maintain the pH at 3.5. After the reaction is completed, the crude folic acid is obtained by suction filtration. The crude product is made into pure folic acid after alkali dissolution and acid extraction, and the total yield can reach 55%.

[0013]

[0014] This process mainly has the following disadvantages: 1. The purity of the raw material trichloroacetone is not high (about 67%), there are many by-products, and a large amount of wastewater is generated. 2. The preparation of 1,1,3-trichloroacetone requires the use of hydrogen chloride gas, which will generate a large amount of waste gas and does not meet the requirements of green chemistry. These have restricted the industrial development of folic acid. (III) Summary of the Invention

[0015] Aiming at the above problems existing in the prior art, the present invention aims to provide a new method for synthesizing folic acid that is simple to operate, low in cost, and has mild reaction conditions suitable for industrial production.

[0016] To achieve the above object, the present invention adopts the following technical solutions:

[0017] The specific route is as follows:

[0018]

[0019] The present invention provides a method for synthesizing folic acid, comprising the following steps:

[0020] a) Epichlorohydrin, a catalyst, organic solvent A, and ethyl p-aminobenzoate are stirred and reacted at 25°C to 90°C (preferably 60 - 80°C) for 10 - 24 hours (particularly preferably stirred and reacted at 70 - 75°C for 20 h). The resulting reaction solution A is subjected to post-treatment A to obtain compound (IV);

[0021]

[0022] The catalyst described above is one or a mixture of two or more of stannous chloride dihydrate, lithium bromide, and aluminum trifluoromethanesulfonate (preferably lithium bromide). The molar ratio of epichlorohydrin, ethyl p-aminobenzoate to the catalyst is 1:1.0 - 1.5:0.02 - 0.15 (preferably 1:1.1:0.05 - 0.10, particularly preferably 1:1.1:0.05);

[0023] b) Dissolve the compound (IV) obtained in step a) in organic solvent B, and dropwise add di-tert-butyl dicarbonate (commonly known as Boc anhydride). After the addition is complete, stir and react at 25°C to 70°C for 10 - 24 hours (preferably stir and react at 40 - 45°C for 24 h). The resulting reaction solution B is subjected to post-treatment B to obtain compound (V);

[0024]

[0025] The molar ratio of the compound (IV) to di-tert-butyl dicarbonate is 1:0.8 - 2 (preferably 1:1.2 - 1.5, particularly preferably 1:1.5);

[0026] c) Dissolve the compound (V) obtained in step b), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), and potassium bromide in organic solvent C. At -5°C to 25°C (preferably 0 - 5°C), dropwise add a saturated aqueous solution of sodium hypochlorite in sodium bicarbonate (to form a 2,2,6,6-tetramethylpiperidine 1-oxyl (commonly known as TEMPO) / sodium hypochlorite / potassium bromide oxidation system). After the addition is complete, stir and react at -5°C to 25°C for 0.5 - 1.5 hours (preferably stir and react at 0 - 5°C for 0.5 hour). The resulting reaction solution C is subjected to post-treatment C to obtain compound (VI);

[0027]

[0028] The molar ratio of the compound (V), 2,2,6,6-tetramethylpiperidine-N-oxide, potassium bromide to sodium hypochlorite in the saturated aqueous sodium bicarbonate solution is 1.0:0.05 - 0.25:0.02 - 0.15:1.0 - 2.0 (preferably 1.0:0.10 - 0.15:0.05 - 0.1:1.1 - 1.5, particularly preferably 1:0.1:0.1:1.5); the volume ratio of the organic solvent C to the saturated aqueous sodium bicarbonate solution of sodium hypochlorite is 0.8 - 1.5:1 (preferably 1.0 - 1.2:1);

[0029] d) Dissolve the compound (VII) and the basic substance in the organic solvent D, stir and react for the first time at room temperature for 0.5 - 2 hours (preferably 1 h), add the compound (VI) described in step c), and stir and react for the second time at 50°C - 150°C for 5 - 10 hours (preferably at 90 - 120°C, most preferably at 120°C for the second stirring reaction for 7 h), and then stir and react for the third time in the air at 20 - 50°C for 10 - 30 hours (preferably stir and react for the third time at 25 - 35°C for 12 - 20 hours, particularly preferably stir and react for the third time at 25°C for 12 hours). The obtained reaction solution D is subjected to post-treatment D to obtain the compound (VIII);

[0030]

[0031] The molar ratio of the compound (VI), the basic substance to the compound (VII) is 1:2.5 - 6.0:0.9 - 1.5 (preferably 1:3 - 5:1.0 - 1.2, most preferably 1:4:1);

[0032] e) Dissolve the compound (VIII) described in step d) in ethanol, add an aqueous sodium hydroxide solution of 1.5 - 5 M (preferably 2 - 4 M, particularly preferably 2 M) (to construct a hydrolysis system), carry out a hydrolysis reaction at 40 - 80 °C for 3 - 7 hours (preferably a hydrolysis reaction at 50 °C for 5 hours), add acetic acid to adjust the pH of the reaction solution to 3 - 4, and distill under reduced pressure (to recover the solvent) to obtain a crude product containing the intermediate (IX); dissolve the crude product containing the intermediate (IX) in N,N - dimethylformamide, add a condensing agent, and carry out a first stirring reaction at 30 °C to 50 °C for 2 - 5 hours (preferably a first stirring at 50 °C for 2 hours), add the compound (X), and carry out a second stirring reaction at 50 °C to 80 °C for 4 - 10 hours (preferably a second stirring reaction at 70 °C for 6 hours), and subject the obtained reaction solution E to post - treatment E to obtain the compound (XI); the condensing agent is one or a mixture of two or more of 1 - hydroxybenzotriazole (HOBT), 2 - chloro - 4,6 - dimethoxy - 1,3,5 - triazine (CDMT), N,N'-carbonyldiimidazole (CDI), N,N'-dicyclohexylcarbodiimide (DCC) (preferably N,N'-carbonyldiimidazole); the molar ratio of the compound (VIII), sodium hydroxide contained in the aqueous sodium hydroxide solution, the condensing agent to the compound (X) is 1:4 - 15:1.0 - 1.8:0.9 - 1.5 (preferably 1:5 - 10:1.1 - 1.5:1.0 - 1.2, most preferably 1:5.5:1.2:1.2);

[0033]

[0034] f): Dissolve the compound (XI) and trifluoroacetic acid described in step e) in dichloromethane, and carry out a stirring reaction at 20 - 40 °C for 6 - 12 hours (preferably keep the temperature at 40 °C for 6 hours for this step, and the purpose of this step is to remove the tert - butoxycarbonyl protecting group), and subject the obtained reaction solution F to post - treatment F to obtain a crude product containing the intermediate (XII);

[0035] Dissolve the crude product containing the intermediate (XII) in ethanol, add an aqueous sodium hydroxide solution of 1.5 - 5 M (preferably 2 - 4 M, particularly preferably 2 M), carry out a hydrolysis reaction at 40 - 80 °C for 6 - 14 hours (preferably stir at 50 °C for 12 h), distill off ethanol under reduced pressure, adjust the pH to 3 - 4 with acetic acid, carry out suction filtration (to remove the solvent), and vacuum - dry the obtained filter cake to obtain folic acid, i.e., the compound (I);

[0036]

[0037] The volume of the trifluoroacetic acid is 2 - 10 mL / g based on the mass of the compound (XI), preferably 3 - 6 mL / g, and most preferably 3.4 mL / g;

[0038] The molar ratio of the compound (XI) to sodium hydroxide contained in the aqueous sodium hydroxide solution is 1:3 - 15 (preferably 1:4 - 10, most preferably 1:6).

[0039] Furthermore, in step a), the organic solvent A is one or a mixture of two or more of chlorobenzene, toluene, xylene, dichloromethane, 1,2 - dichloroethane, and chloroform, and is preferably toluene.

[0040] Still further, the volume of the organic solvent A is 10 - 20 mL / g based on the mass of the epichlorohydrin, and is preferably 12 - 16 mL / g.

[0041] In one embodiment of the present invention, the post - treatment A in step a) is as follows: The obtained reaction solution A is cooled to room temperature, washed with saturated brine, the aqueous layer is extracted with toluene, the organic layers are combined, concentrated hydrochloric acid is added until a solid precipitates, suction filtration is carried out, the obtained filtrate is distilled under reduced pressure, dried, and the compound (IV) is obtained.

[0042] In the above post - treatment A, the unreacted compound (III) hydrochloride cake can be recovered by suction filtration, and the solvent can be recovered by distillation under reduced pressure.

[0043] Furthermore, in step b), the organic solvent B is selected from one or a mixture of two or more of the following: ethanol, methanol, isopropanol, dichloromethane, 1,2 - dichloroethane, and chloroform; the organic solvent B is preferably ethanol.

[0044] Still further, in step b), the volume of the organic solvent B is 2 - 8 mL / g based on the mass of the compound (IV) (preferably 3 - 6 mL / g).

[0045] In one embodiment of the present invention, in step b), the post - treatment B is as follows: The obtained reaction solution is distilled under reduced pressure (to remove and recover the solvent), petroleum ether is added for pulping, suction filtration is carried out (to separate and recover the unreacted ethyl 4 - ((3 - chloro - 2 - hydroxypropyl)amino)benzoate), and the obtained filtrate is distilled under reduced pressure to obtain the compound (V).

[0046] Furthermore, in step c), the organic solvent C is one or a mixture of two or more of dichloromethane, ethyl acetate, and acetone, and is preferably dichloromethane.

[0047] Still further, the volume of the organic solvent C is 2 - 8 mL / g based on the mass of the compound (V), and is preferably 3 - 5 mL / g.

[0048] In one embodiment of the present invention, the post - treatment C is as follows: Sodium thiosulfate is added to the reaction solution C to quench the reaction, allowed to stand for layering, the aqueous layer is extracted with dichloromethane, the organic layers are combined, distilled under reduced pressure (to recover the solvent), dried, and the compound (VI) is obtained.

[0049] Further, in step d), the basic substance is selected from one or more mixtures of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and sodium acetate; preferably sodium bicarbonate.

[0050] Further, in step d), the organic solvent D is selected from one or more of the following mixed solvents: 1,2-dichloroethane, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, preferably dimethyl sulfoxide; the volume of the organic solvent D is 2-10 mL / g based on the mass of compound (VI), preferably 3-5 mL / g.

[0051] In one embodiment of the present invention, the post-treatment D in step d) is: subjecting the reaction solution to reduced pressure distillation (removing dimethyl sulfoxide), washing the concentrated reaction solution obtained with saturated brine, extracting with ethyl acetate, combining the organic layers, subjecting to reduced pressure distillation (recovering the solvent), recrystallizing the obtained crude product with dichloromethane and petroleum ether, filtering by suction, and drying the obtained filter cake to obtain the compound (VIII).

[0052] Further, in step e), the volume of the sodium hydroxide aqueous solution is 3-10 mL / g based on the mass of compound (VIII), preferably 5-7 mL / g.

[0053] In the above technical solution, in step e), the volume of the N,N-dimethylformamide is 2-10 mL / g based on the mass of the compound (VIII), preferably 5-7 mL / g.

[0054] In one embodiment of the present invention, the post-treatment E in step e) is: adding saturated brine to the reaction solution E, extracting the obtained aqueous layer with dichloromethane, combining the organic layers, drying with anhydrous sodium sulfate, subjecting to reduced pressure distillation (removing the solvent), recrystallizing with ethanol and water, filtering by suction, and drying the obtained filter cake to obtain the compound (XI).

[0055] Further, in step f), the volume of the dichloromethane is 2-10 mL / g based on the mass of the compound (XI), preferably 3-6 mL / g.

[0056] Further, in step f), the volume of the ethanol is 2-10 mL / g based on the mass of the compound (XI), preferably 4-7 mL / g.

[0057] Further, in step f), the volume of the sodium hydroxide aqueous solution (2M) is 3-10 mL / g based on the mass of the compound (XI), preferably 5-7 mL / g.

[0058] In one embodiment of the present invention, the post-treatment F is as follows: saturated brine is added to the reaction solution F, the obtained aqueous layer is extracted with dichloromethane, the organic layers are combined and distilled under reduced pressure (to remove the solvent) to obtain the crude product containing the intermediate (XII).

[0059] The capital letters after each of the above substances are only for distinguishing the reagents in different stages for convenient description and have no other special meanings.

[0060] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in the following aspects: low-cost and easily available raw materials, mild conditions, simple operation, high product quality and yield, and low production cost. One of the innovative points of the present invention is that epichlorohydrin is used as the starting material, and folic acid is synthesized through steps such as ring opening, amino protection, oxidation, cyclization, condensation, and deprotection, providing a brand-new route for the synthesis of folic acid and having significant social and economic benefits. (IV) Specific Embodiments

[0061] The present invention will be further described below through specific examples, but the protection scope of the present invention is not limited thereto.

[0062] The operation process of recrystallization in the present invention is a conventional operation in the art. Specifically, under reflux conditions, the crude product is first dissolved in Solvent 1, Solvent 2 is added dropwise and stirring is continued for crystallization. After crystallization is completed, it is cooled to room temperature and filtered by suction. The obtained filter cake is the target product.

[0063] Example 1: Preparation of Ethyl 4-((3-chloro-2-hydroxypropyl)amino)benzoate (IV)

[0064] 10.90 g (66 mmol) of ethyl p-aminobenzoate, 5.54 g (60 mmol) of epichlorohydrin, and 794 mg (3 mmol) of lithium bromide were added to 80 ml of toluene in a two-necked flask. The reaction solution was magnetically stirred at 70 - 75 °C for 20 h. After the reaction was completed, the reaction was stopped. After cooling to room temperature, it was washed with saturated brine, allowed to stand for layer separation, the aqueous layer was extracted with 50 ml of toluene, the organic layers were combined, and concentrated hydrochloric acid (concentration 36% - 38%) was added until a solid precipitated. It was filtered by suction, the solid was separated and the filtrate was collected, and the filter cake of the unreacted compound (III) hydrochloride was recovered. The solvent in the filtrate was recovered by distillation under reduced pressure, and after drying, 12.83 g of a white solid (IV) was obtained, with a yield of 83% and a purity of 97%.

[0065] Spectral characterization of compound (IV):

[0066] White solid, mp 67 - 68 °C; 11H NMR (400 MHz, CDCl3) δ 7.86 (d, J = 8.7 Hz, 2H), 6.59 (d, J = 8.8 Hz, 2H), 4.31 (q, J = 7.1 Hz, 2H), 4.13–4.06 (m, 1H), 3.66 (m, 2H), 3.44 (dd, J = 13.3, 4.3 Hz, 1H), 3.28 (dd, J = 13.4, 7.2 Hz, 1H), 2.97 (s, 1H), 1.36 (t, J = 7.1 Hz, 3H). 13 13C NMR (100 MHz, CDCl3) δ 165.8, 145.4, 131.0, 129.2, 119.1, 68.3, 61.3, 47.9, 47.7, 14.3.

[0067] Example 2: Preparation of Ethyl 4-(tert-butoxycarbonyl)(3-chloro-2-hydroxypropyl)amino)benzoate (V)

[0068] 5.15 g (20 mmol) of ethyl 4-((3-chloro-2-hydroxypropyl)amino)benzoate and 20 ml of ethanol were added to a single-necked flask, and then 6.54 g (30 mmol) of di-tert-butyl dicarbonate was added dropwise. The reaction solution was stirred at 40 - 45 °C for 24 h. After the reaction was completed, the solvent was removed and recovered by distillation under reduced pressure, and then 25 ml of petroleum ether was added for pulping. The unreacted ethyl 4-((3-chloro-2-hydroxypropyl)amino)benzoate was recovered by filtration and separation, and the obtained filtrate was distilled under reduced pressure to obtain 4.79 g of ethyl 4-(tert-butoxycarbonyl)(3-chloro-2-hydroxypropyl)amino)benzoate as a yellow oil, with a yield of 67% and a purity of 95.5%.

[0069] Spectral characterization of compound (V):

[0070] Yellow liquid; 1 1H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 8.6 Hz, 2H), 7.31 (d, J = 8.6 Hz, 2H), 4.38 (q, J = 7.1 Hz, 2H), 4.06 (m, 1H), 3.93 (dd, J = 14.7, 7.4 Hz, 1H), 3.82 (dd, J = 14.6, 3.9 Hz, 1H), 3.61–3.50 (m, 2H), 1.43 (s, 9H), 1.40 (t, J = 7.1 Hz, 3H). 13 13C NMR (100 MHz, CDCl3) δ 166.1, 154.0, 142.7, 132.0, 123.4, 123.3, 81.9, 69.5, 61.2, 52.2, 45.0, 28.1, 14.4.

[0071] Example 3: Preparation of Ethyl 4-((tert-butoxycarbonyl)(3-chloro-2-oxopropyl)amino)benzoate (VI)

[0072] 5.33 g (15 mmol) of ethyl 4-(tert-butoxycarbonyl)(3-chloro-2-hydroxypropyl)amino)benzoate, 234 mg (0.15 mmol) of TEMPO, and 180 mg (0.15 mmol) of potassium bromide were added to a single-necked flask and dissolved in 25 ml of dichloromethane. A saturated sodium bicarbonate solution containing sodium hypochlorite (22.5 mol) (25 ml) was slowly added dropwise at 0 - 5 °C. After the addition was complete, the reaction was stirred at 0 - 5 °C for 0.5 h. After the substrate was completely reacted, the reaction was quenched with 4.27 g (27 mmol) of sodium thiosulfate. The layers were separated, the aqueous layer was extracted with dichloromethane, and the organic layers were combined. The solvent was recovered by distillation under reduced pressure, and after drying, 5.02 g of white solid ethyl 4-((tert-butoxycarbonyl)(3-chloro-2-oxopropyl)amino)benzoate was obtained, with a yield of 94% and a purity of 99%.

[0073] Spectral characterization of compound (VI):

[0074] White solid, mp 130 - 132 °C; 1 H NMR (400 MHz, CDCl3) δ 8.01–7.95 (m, 2H), 7.43 (d, J = 8.3 Hz, 2H), 6.73 (s, 0H), 4.35 (q, J = 7.1 Hz, 2H), 1.52 (s, 4H), 1.38 (t, J = 7.1 Hz, 2H). 13 C NMR (100 MHz, CDCl3) δ 198.9, 166.0, 154.3, 142.3, 131.9, 123.5, 123.2, 81.9, 61.2, 52.1, 49.6, 28.1, 14.4.

[0075] Example 4: Preparation of Ethyl 4-(((2-amino-4-hydroxypteridin-6-yl)methyl)(tert-butoxycarbonyl)amino)benzoate (VIII)

[0076] In a round-bottom flask, 3.36 g (40 mmol) of sodium bicarbonate and 2.39 g (10 mmol) of 2,4,5-triamino-6-hydroxypyrimidine sulfate were weighed and dissolved in 10 ml of dimethyl sulfoxide. The reaction solution was stirred at room temperature for 1 hour. 3.56 g (10 mmol) of ethyl 4-((tert-butoxycarbonyl)(3-chloro-2-oxopropyl)amino)benzoate was added to the reaction solution, and the reaction solution was heated to 120 °C and stirred for 7 h. After the substrate reaction was complete, the reaction solution was cooled to room temperature and stirred in air for 12 h. The reaction was stopped, and dimethyl sulfoxide was removed by distillation under reduced pressure. The concentrated reaction solution was washed with saturated brine and extracted with ethyl acetate (50 ml × 2). The organic layers were combined, the solvent was recovered by distillation under reduced pressure, and the obtained crude product was recrystallized with 60 ml of dichloromethane and 15 ml of petroleum ether. After filtration, the solid was dried in a vacuum drying oven at 50 °C for 10 hours. 3.30 g of yellow solid ethyl 4-(((2-amino-4-hydroxypterin-6-yl)methyl)(tert-butoxycarbonyl)amino)benzoate was obtained, with a yield of 75% and a purity of 94.4%.

[0077] Spectral characterization of compound (VIII):

[0078] Yellow solid, mp; 240 - 242 °C; 1 1H NMR (400 MHz, CDCl3) δ 7.90 (d, J = 7.6 Hz, 2H), 7.47 (d, J = 7.6 Hz, 2H), 4.89 (s, 2H), 4.28 (q, J = 5.9 Hz, 2H), 1.38 (s, 9H), 1.29 (t, J = 5.9 Hz, 3H). 13C NMR (100 MHz, DMSO-d6) δ 173.6, 166.0, 159.5, 157.1, 154.1, 147.5, 146.2, 142.3, 136.5, 131.9, 123.5, 123.2, 81.9, 61.2, 46.0, 28.1, 14.4.

[0079] Example 5: Preparation of diethyl 4-(((2-amino-4-hydroxypterin-6-yl)methyl)(tert-butoxycarbonyl)amino)benzoyl-L-glutamate (XI)

[0080] 1.32 g (3 mmol) of ethyl 4-(((2-amino-4-hydroxypterin-6-yl)methyl)(tert-butoxycarbonyl)amino)benzoate was added to a round-bottom flask. After adding 8 ml of ethanol and stirring to dissolve the substrate, 8 ml of 2 M sodium hydroxide solution was added, and the reaction solution was reacted at 50 °C for 5 hours. After the reaction was completed, the pH of the reaction solution was adjusted to 3 - 4 with acetic acid, and the solvent was removed by distillation under reduced pressure to obtain intermediate (IX). The obtained intermediate was directly subjected to the next reaction without separation and purification.

[0081] The obtained intermediate (IX) was dissolved in 7 ml of DMF, 0.58 g (3.6 mmol) of N,N-carbonyldiimidazole was added, and the mixture was stirred at 50 °C for 2 hours. The temperature was raised to 70 °C, then 0.41 g of diethyl L-glutamate 0.41 g (3.6 mmol) was added, and the reaction solution was kept at the reaction temperature for 6 hours. After the reaction was completed, it was washed with saturated brine and extracted with dichloromethane. The organic layer was separated, dried over anhydrous sodium sulfate. The solvent was removed by distillation under reduced pressure, and the obtained crude product was recrystallized from 40 ml of ethanol and 20 ml of water. After filtration, the solid was placed in a vacuum drying oven at 85 °C and dried for 10 hours. 1.45 g of diethyl 4-(((2-amino-4-hydroxypterin-6-yl)methyl)(tert-butoxycarbonyl)amino)benzoyl-L-glutamate was obtained, with a yield of 85% and a purity of 92.1%.

[0082] Example 6: Preparation of folic acid (I)

[0083] 2.99 g (5 mmol) of diethyl 4-(((2-amino-4-hydroxypterin-6-yl)methyl)(tert-butoxycarbonyl)amino)benzoyl-L-glutamate, 10 ml of trifluoroacetic acid and 10 ml of dichloromethane were added to a single-necked flask, and the reaction solution was kept at 40 °C for 6 hours. After the reaction was completed, it was washed with saturated brine and extracted with dichloromethane. The organic layer was collected and the solvent was removed by distillation under reduced pressure to obtain a yellow solid. The obtained yellow solid was dissolved in 15 ml of ethanol, and then 15 ml of sodium hydroxide solution (2 M) was added. The reaction solution was stirred at 50 °C for 12 h. After the reaction was completed, ethanol was removed and recovered by distillation under reduced pressure. The pH of the reaction solution was adjusted to 3-4 with acetic acid, and a solid precipitated out. The reaction solution was placed in a low-temperature refrigerator at 0-5 °C overnight. Filtration was carried out, and the solid precipitated by cooling was collected. The solid was placed in a vacuum drying oven at 85 °C and dried for 12 hours to obtain 1.50 g of yellow solid folic acid, with a yield of 68% and a purity of 95.7%.

[0084] Spectral characterization of compound (I):

[0085] Yellow solid, mp 248-250 °C; 1 1H NMR (400 MHz, DMSO-d6) δ 12.31 (s, 2H), 11.55 (s, 1H), 8.67 (s, 1H), 8.16 (d, J = 7.7 Hz, 1H), 7.67 (d, J = 8.8 Hz, 2H), 6.96 (t, J = 6.1 Hz, 2H), 6.66 (d, J = 8.8 Hz, 2H), 4.51 (d, J = 5.7 Hz, 2H), 4.36 (m, 1H), 2.34 (t, J = 7.5 Hz, 2H), 2.12–1.98 (m, 1H), 2.01–1.87 (m, 1H).

[0086] Examples 7 to 20:

[0087] Preparation of ethyl 4-((3-chloro-2-hydroxypropyl)amino)benzoate (IV). The preparation method of each example was repeated as in Example 1, with the difference that certain reaction conditions were changed (such as the type of reaction solvent, reaction temperature, type and dosage of catalyst, etc.). The specific changed reaction conditions and corresponding reaction effects in each example are shown in Table 1-2 below.

[0088] Table 1

[0089]

[0090] Table 2

[0091]

[0092]

[0093] Examples 21 to 30:

[0094] Preparation of ethyl 4-((tert-butoxycarbonyl)(3-chloro-2-hydroxypropyl)amino)benzoate (V). The preparation method of each example was repeated as in Example 2, with the difference that certain reaction conditions were changed (such as the type of reaction solvent B, reaction temperature, and the molar ratio of ethyl 4-((3-chloro-2-hydroxypropyl)amino)benzoate (IV) to Boc anhydride, etc.). The specific changed reaction conditions and corresponding reaction effects in each example are shown in Tables 3-4 below.

[0095] Table 3

[0096]

[0097] Table 4

[0098]

[0099] Examples 31 to 36:

[0100] Preparation of ethyl 4-((tert-butoxycarbonyl)(3-chloro-2-oxopropyl)amino)benzoate (VI). The preparation method of each example was repeated as in Example 3, with the difference that certain reaction conditions were changed (such as the type of reaction solvent C, the molar ratio of ethyl 4-((tert-butoxycarbonyl)(3-chloro-2-hydroxypropyl)amino)benzoate (V), TEMPO, sodium hypochlorite, and potassium bromide, etc.). The specific changed reaction conditions and corresponding reaction effects in each example are shown in Table 5 below.

[0101] Table 5

[0102]

[0103] Examples 37 to 47:

[0104] Preparation of ethyl 4 - ((((2 - amino - 4 - hydroxypterin - 6 - yl)methyl)(tert - butoxycarbonyl)amino)benzoate (VIII). The preparation method of each example was repeated as in Example 4, except that certain reaction conditions were changed (such as the type of reaction solvent D, reaction temperature, type and amount of base, etc.). The specific reaction conditions changed in each example and the corresponding reaction effects are shown in Tables 6 - 7 below.

[0105] Table 6

[0106]

[0107] Table 7

[0108]

[0109] Examples 48 to 53:

[0110] Preparation of ethyl 4 - ((((2 - amino - 4 - hydroxypterin - 6 - yl)methyl)(tert - butoxycarbonyl)amino)benzoate (VIII). The preparation method of each example was repeated as in Example 5, except that certain reaction conditions were changed (such as the type of condensing agent, the molar ratio of ethyl 4 - ((((2 - amino - 4 - hydroxypterin - 6 - yl)methyl)(tert - butoxycarbonyl)amino)benzoate (VIII), diethyl L - glutamate (X) and the condensing agent). The specific reaction conditions changed in each example and the corresponding reaction effects are shown in Table 8 below.

[0111] Table 8

[0112]

[0113] The content described in this specification is only a list of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms stated in the examples.

Claims

1. A method for synthesizing folic acid, characterized in that The method comprises the following steps: a) Epichlorohydrin, a catalyst, organic solvent A and ethyl p-aminobenzoate are stirred and reacted at 25°C to 90°C for 10 to 24 hours. The obtained reaction solution A is subjected to post-treatment A, cooled to room temperature, washed with saturated brine, the aqueous layer is extracted with toluene, the organic layers are combined, concentrated hydrochloric acid is added until a solid precipitates, filtered by suction, the obtained filtrate is distilled under reduced pressure and dried to obtain compound (IV); The catalyst is one or a mixture of two or more of stannous chloride dihydrate, lithium bromide, and aluminum trifluoromethanesulfonate. The molar ratio of epichlorohydrin, ethyl p-aminobenzoate to the catalyst is 1:1.0 - 1.5:0.02 to 0.15; b) Compound (IV) obtained in step a) is dissolved in organic solvent B, di-tert-butyl dicarbonate is added dropwise. After completion of the addition, it is stirred and reacted at 25°C to 70°C for 10 to 24 hours. The obtained reaction solution B is distilled under reduced pressure, slurried with petroleum ether, filtered by suction, and the obtained filtrate is distilled under reduced pressure to obtain compound (V); The molar ratio of compound (IV) to di-tert-butyl dicarbonate is 1:0.8 to 2; c) Compound (V) obtained in step b), 2,2,6,6-tetramethylpiperidine-N-oxide, and potassium bromide are dissolved in organic solvent C. An aqueous solution of sodium hypochlorite saturated with sodium bicarbonate is added dropwise at -5°C to 25°C. After completion of the addition, it is stirred and reacted at -5°C to 25°C for 0.5 to 1.5 hours. The obtained reaction solution C is quenched with sodium thiosulfate, allowed to stand and separate into layers, the aqueous layer is extracted with dichloromethane, the organic layers are combined, distilled under reduced pressure and dried to obtain compound (VI); The molar ratio of compound (V), 2,2,6,6-tetramethylpiperidine-N-oxide, potassium bromide to sodium hypochlorite in the aqueous solution of sodium hypochlorite saturated with sodium bicarbonate is 1.0:0.05 to 0.25:0.02 to 0.15:1.0 to 2.0; The volume ratio of organic solvent C to the aqueous solution of sodium hypochlorite saturated with sodium bicarbonate is 0.8 - 1.5:1; d) Compound (VII) and a basic substance are dissolved in organic solvent D, and stirred and reacted for the first time at room temperature for 0.5 - 2 hours. Compound (VI) obtained in step c) is added, and stirred and reacted for the second time at 50°C to 150°C for 5 to 10 hours, and then stirred and reacted for the third time at 20 - 50°C in air for 10 - 30 hours. The obtained reaction solution D is distilled under reduced pressure. The concentrated reaction solution is washed with saturated brine, extracted with ethyl acetate, the organic layers are combined, distilled under reduced pressure, the obtained crude product is recrystallized with dichloromethane and petroleum ether, filtered by suction, and the obtained filter cake is dried to obtain compound (VIII); The molar ratio of compound (VI), the basic substance to compound (VII) is 1:2.5 - 6.0:0.9 - 1.5; The basic substance is selected from one or a mixture of two or more of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and sodium acetate; e) Dissolve the compound (VIII) described in step d) in ethanol, add 1.5 - 5M aqueous sodium hydroxide solution, carry out a hydrolysis reaction at 40 - 80 °C for 3 - 7 hours, add acetic acid to adjust the pH of the reaction solution to 3 - 4, and perform vacuum distillation to obtain a crude product containing intermediate (IX); dissolve the crude product containing intermediate (IX) in N,N - dimethylformamide, add a condensing agent, stir and react for the first time at 30 °C to 50 °C for 2 - 5 hours, add compound (X), stir and react for the second time at 50 °C to 80 °C for 4 - 10 hours, add saturated brine to the obtained reaction solution E, extract the aqueous layer with dichloromethane, combine the organic layers, dry with anhydrous sodium sulfate, perform vacuum distillation, recrystallize with ethanol and water, filter by suction, and dry the obtained filter cake to obtain compound (XI); the condensing agent is one or a mixture of two or more of 1 - hydroxybenzotriazole, 2 - chloro - 4,6 - dimethoxy - 1,3,5 - triazine, N,N'-carbonyldiimidazole, N,N'-dicyclohexylcarbodiimide; the molar ratio of the compound (VIII), sodium hydroxide contained in the aqueous sodium hydroxide solution, the condensing agent to compound (X) is 1:4 - 15:1.0 - 1.8:0.9 - 1.5; f): Dissolve the compound (XI) and trifluoroacetic acid described in step e) in dichloromethane, stir and react at 20 - 40 °C for 6 - 12 hours, add saturated brine to the obtained reaction solution F, extract the aqueous layer with dichloromethane, combine the organic layers and perform vacuum distillation to obtain the crude product containing intermediate (XII), and obtain the crude product containing intermediate (XII); Dissolve the crude product containing intermediate (XII) in ethanol, add 1.5 - 5M aqueous sodium hydroxide solution, carry out a hydrolysis reaction at 40 - 80 °C for 6 - 14 hours, distill off ethanol under reduced pressure, adjust the pH to 3 - 4 with acetic acid, filter by suction, and vacuum - dry the obtained filter cake to obtain folic acid, namely compound (I); The volume of the trifluoroacetic acid is 2 - 10 mL / g based on the mass of the compound (XI); The molar ratio of the compound (XI) to sodium hydroxide contained in the aqueous sodium hydroxide solution is 1:3 - 15.

2. The method for synthesizing folic acid according to claim 1, characterized in that: In step a), the organic solvent A is one or a mixed solvent of two or more of chlorobenzene, toluene, xylene, dichloromethane, 1,2 - dichloroethane, chloroform; The volume of the organic solvent A is 10 - 20 mL / g based on the mass of the epichlorohydrin.

3. The method for synthesizing folic acid according to claim 1, characterized in that: The catalyst described in step a) is lithium bromide.

4. The method for synthesizing folic acid according to claim 1, wherein: In step b), the organic solvent B is selected from one or a mixed solvent of two or more of the following: ethanol, methanol, isopropanol, dichloromethane, 1,2 - dichloroethane, chloroform; In step b), the volume of the organic solvent B is 2 - 8 mL / g based on the mass of the compound (IV).

5. The method for synthesizing folic acid according to claim 1, wherein: In step c), the organic solvent C is one or a mixture of two or more of dichloromethane, ethyl acetate, acetone; The volume of the organic solvent C is 2 - 8 mL / g based on the mass of compound (V).

6. The method for synthesizing folic acid according to claim 1, characterized in that: In step d), the organic solvent D is selected from one or more of the following mixed solvents: 1,2-dichloroethane, ethanol, N,N-dimethylformamide, dimethyl sulfoxide; the volume of the organic solvent D is 2 to 10 mL / g based on the mass of the compound (VI).

7. The method for synthesizing folic acid according to claim 1, characterized in that: In step e), the volume of the sodium hydroxide aqueous solution is 3 to 10 mL / g based on the mass of the compound (VIII); In step e), the volume of the N,N-dimethylformamide is 2 to 10 mL / g based on the mass of the compound (VIII).

8. The method for synthesizing folic acid according to claim 1, wherein: The condensing agent in step e) is N,N'-carbonyldiimidazole.

9. The method for synthesizing folic acid according to claim 1, characterized in that: In step f), the volume of the dichloromethane is 2 - 10 mL / g based on the mass of the compound (XI); In step f), the volume of the ethanol is 2 - 10 mL / g based on the mass of the compound (XI); In step f), the volume of the sodium hydroxide aqueous solution is 3 to 10 mL / g based on the mass of the compound (XI).

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