Preparation and application of a methylene blue intermediate

Abstract

Methylene blue is a drug with wide-ranging medical applications. Developing a synthetic process for preparing high-purity pharmaceutical-grade methylene blue is of great significance. This invention uses 3,7-dibromo-10H-phenthiazide as a starting material, and acylation with benzoyl chloride to obtain intermediate compound 2. Compound 2 is then subjected to a copper-catalyzed coupling reaction to obtain intermediate compound 3. Compound 3 is then subjected to a non-metal-catalyzed oxidation reaction to finally obtain the high-purity target product, methylene blue. This synthetic process has the advantages of mild reaction conditions, simple post-processing, and high product quality.

Description

Technical Field

[0001] This invention belongs to the field of drug synthesis process development, and specifically relates to a method for preparing a methylene blue intermediate. Background Technology

[0002] Methylene blue (structural formula shown below) is a chemical with a wide range of medical applications, primarily in surgical procedures, treatments, and diagnostics. Originally used in medicine as an effective antidote to treat methemoglobinemia, nitrite poisoning, and cyanide poisoning, methylene blue has been applied in recent years as a fluorescent staining agent in sentinel lymph node biopsies for breast cancer, helping to accurately identify lesions.

[0003] The preparation and purification of methylene blue have been disclosed in several patents and documents. For example, CN 101084204B discloses a chemical synthesis and purification method for diaminophenthiazine onion compounds, including methylene blue. Using N,N-dimethylaniline as a raw material, the method involves steps such as nitrosation, reduction, thiosulfate oxidation, oxidative coupling, cyclization, and chlorination to obtain methylene blue that meets pharmaceutical quality requirements. However, this method is cumbersome, time-consuming, and has an overall yield of less than 10%. Furthermore, the use of potassium dichromate as an oxidant exacerbates the post-processing, pollutes the environment, and is unsuitable for industrial production.

[0004] The literature Tetrahedron, 1997, 53, 10083-10092 discloses a method for preparing methylene blue and its analogues from phenothiazine. The method involves converting phenothiazine to 3,7-dibromophenothiazine 5-onium bromide via bromination, followed by reaction of the 3,7-dibromophenothiazine 5-onium bromide with a disubstituted amine in ethanol or chloroform to obtain bromide salts of methylene blue analogues in good yields. However, this method uses chloroform as the reaction solvent, and alcoholic solvents readily generate competing reactions that produce difficult-to-remove impurities, making it unsuitable for industrial production.

[0005] CN 110382472 B discloses a method for preparing 3,7-bis(dimethylamino)phenothiazine-5-onium iodide. The method involves a one-pot reaction to convert phenothiazine to the corresponding 3,7-diiodophenothiazine-5-onium iodide, followed by conversion of this intermediate to methylene blue iodonium salt in the presence of the nucleophile dimethylamine. This one-pot reaction is carried out in the same reaction vessel and solvent system, reducing the need for intermediate separation and purification, and lowering the risk of degradation of unstable intermediates during separation / purification steps. However, the reaction requires an excessive amount of elemental iodine, resulting in high production costs and increased costs associated with iodide ion separation and purification.

[0006] Therefore, there is an urgent need for a method for preparing methylene blue that is simple to operate, environmentally friendly, has a high yield, and produces high-quality products, which is conducive to industrial production. Summary of the Invention

[0007] The above-mentioned methods for preparing methylene blue have technical problems such as harsh reaction conditions, cumbersome operation, high economic and time costs, and low yield and purity. This invention provides a method for preparing pharmaceutical methylene blue, which has the characteristics of simple operation, low cost, high yield and high product purity.

[0008] The technical solution of this invention comprises a three-step reaction: Step 1, acylation reaction: The chemical reaction equation is as follows: The preparation method provided by the present invention uses 3,7-dibromo-10H-phenthiazide as the starting material and obtains compound 1 through an acylation reaction.

[0009] Using 3,7-dibromo-10H-phenthiazide as a starting material, a certain amount of solvent was added to a reaction flask and stirred to dissolve it. Then, sodium hydroxide and benzoyl chloride were added to the reaction flask in a certain proportion, and the reaction was stirred at room temperature until the starting material was completely converted. Compound 1 was obtained by extraction, drying, and concentration.

[0010] Preferably, in the reaction, benzoyl chloride is in the amount of 1.0-1.2 equivalents and sodium hydroxide is in the amount of 5.0 equivalents.

[0011] Preferably, the solvent in the reaction is a mixture of dichloromethane and water in a ratio of 1:1, the reaction temperature is room temperature, and the reaction time is 24 hours.

[0012] The second step, the coupling reaction: The chemical reaction equation is as follows: Compound 1 was added to a reaction flask at a certain temperature, and solvent was added and stirred until homogeneous. A certain proportion of dimethylaminetetrahydrofuran solution was then added. L -Proline ligand and copper catalyst were reacted under stirring at a certain temperature until the starting material was completely converted. The reaction solution was extracted, dried, and concentrated to obtain compound 2.

[0013] Preferably, the coupling agent used in the reaction is a dimethylaminetetrahydrofuran solution.

[0014] Preferably, the molar ratio of compound 2 to dimethylamine in the reaction is 1:2.4-1:10.

[0015] Preferably, the anti-copper catalyst is cuprous iodide, cuprous chloride, copper acetate, or copper sulfate, and the addition amount is 20 mol.

[0016] Preferably, the ligand used in the reaction is L -Proline, added at 20 mol%. Preferably, the solvent in the reaction is at least one of toluene, tetrahydrofuran, dimethyl sulfoxide, acetonitrile, and N,N-dimethylformamide.

[0017] Preferably, the reaction temperature for synthesizing compound 2 is 25-130 degrees Celsius, and the reaction time is 12-24 hours.

[0018] The third step, the oxidation reaction: The chemical reaction equation is as follows: Compound 2 was added to a reaction flask at a certain temperature, and a certain amount of reaction solvent was added and stirred to dissolve it. Then, a certain amount of oxidant p-benzoquinone and concentrated hydrochloric acid were added to the reaction flask, and the reaction was stirred at room temperature until the starting materials were completely converted. Compound 3 was obtained by filtration.

[0019] Preferably, the amount of p-benzoquinone in the reaction is 1.0-1.2 equivalents, and the amount of concentrated hydrochloric acid is 20.0 equivalents.

[0020] Preferably, the solvent used in the reaction is at least one of methanol, ethanol, isopropanol, and distilled water.

[0021] Preferably, the reaction is carried out at a temperature of 25-80 degrees Celsius for 12-24 hours. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments.

[0023] The following embodiments are intended to better enable those skilled in the art to understand the present invention. However, the descriptions in the embodiments are for illustrative purposes only and should not, and will not, limit the scope of the invention in any way.

[0024] Example 1 A method for preparing pharmaceutical-grade methylene blue includes the following three reaction steps: Step 1, acylation reaction: 3,7-Dibromo-10H-phenthiazide (28 mmol, 10.0 g) and sodium hydroxide (0.14 mol, 5.6 g) were added to a 500 mL reaction flask, followed by the addition of 200 mL of DCM / H₂O mixture and stirring to dissolve. Benzoyl chloride (29.4 mmol, 4.1 g) was added dropwise to the reaction flask, and the mixture was stirred at room temperature for 18 hours. TLC analysis confirmed complete conversion of the starting material. The reaction mixture was separated, and the aqueous phase was extracted with dichloromethane. The combined organic phases were dried over anhydrous sodium sulfate and concentrated under reduced pressure to remove the solvent, yielding 20.0 g of compound 1 (63% yield).

[0025] The second step, the coupling reaction: Compound 1 (43 mmol, 20.0 g) was added to a 1000 mL reaction flask and dissolved in 400 mL of acetonitrile. Cuprous iodide (8.6 mmol, 1.6 g) was then added to the reaction flask. L 1-Proline (8.6 mmol, 1.0 g) and dimethylaminetetrahydrofuran solution (0.43 mol, 200 mL) were reacted at 80 °C with stirring for 24 hours. TLC analysis showed complete conversion of the starting material. The reaction solution was cooled to room temperature and slowly poured into 500 mL of deionized water. Extraction was performed with dichloromethane as an organic solvent. The organic phase was collected, dried, and concentrated under reduced pressure to remove the solvent. The product was crystallized from dichloromethane to give a dark green solid, with a yield of 67%.

[0026] The third step, the oxidation reaction: Compound 2 (26 mmol, 10.1 g) was added to a 500 mL reaction flask, followed by 200 mL of ethanol. Concentrated hydrochloric acid (0.52 mol, 50 mL) was slowly added under ice-water bath with stirring to dissolve the compound. p-benzoquinone (26 mmol, 2.8 g) was then added to the reaction mixture, and the mixture was reacted at room temperature for 12 hours. TLC analysis showed complete conversion of the starting material. The mixture was filtered to obtain a blue-black solid, which was dried under vacuum to yield 7.2 g of target compound 3, with a yield of 87% and a purity > 99.8% as determined by HPLC.

[0027] Example 2 Step 1, acylation reaction: 3,7-Dibromo-10H-phenthiazide (28 mmol, 10.0 g) was added to a 500 mL reaction flask, and sodium hydroxide (0.14 mol, 5.6 g) was added to the flask and dissolved in DCM / H2O. The mixture was then cooled in an ice-water bath. Benzoyl chloride (33.6 mmol, 4.7 g) was added dropwise to the flask, and the mixture was stirred at room temperature for 18 hours. TLC analysis showed complete conversion of the starting material. The mixture was separated into liquid and aqueous phases, extracted with dichloromethane, and the organic phases were combined. The organic phase was dried over anhydrous sodium sulfate, concentrated, and dried under vacuum at room temperature to give 26.0 g of compound 1, with a yield of 81%.

[0028] The second step, the coupling reaction: Compound 1 (43 mmol, 20.0 g) was added to a 1000 mL reaction flask and dissolved in 400 mL of toluene. CuCl (8.6 mmol, 0.85 g) was then added to the reaction flask. L 1-Proline (8.6 mmol, 1.0 g) and dimethylaminetetrahydrofuran solution (0.43 mol, 200 mL) were reacted at 110 °C with stirring for 24 hours. TLC analysis showed complete conversion of the starting material. The reaction solution was cooled to room temperature and slowly poured into 500 mL of deionized water. Extraction was performed with dichloromethane as an organic solvent. The organic phase was collected, dried, and concentrated under reduced pressure to remove the solvent. The product was crystallized from dichloromethane to give 12.3 g of a dark green solid, with a yield of 74%.

[0029] The third step, the oxidation reaction: Compound 2 (32 mmol, 12.3 g) was added to a 500 mL reaction flask, followed by 200 mL of distilled water. Concentrated hydrochloric acid (0.64 mol, 60 mL) was slowly added under an ice-water bath, and the mixture was stirred until dissolved. p-benzoquinone (33.6 mmol, 3.6 g) was added to the reaction solution, and the mixture was stirred for approximately 15 min. The reaction was allowed to proceed at room temperature for 24 hours. TLC analysis confirmed complete conversion of the starting material. The mixture was filtered to obtain a blue-black solid, which was then vacuum dried to yield 9.2 g of the target compound 3, with a yield of 90% and a purity > 99.6% as determined by HPLC. Characterization data of compound 1: 1 H NMR (400 MHz, CDCl3) δ 7.55 (d, J = 1.9 Hz, 2H), 7.39 – 7.31 (m,3H), 7.25 (hept, J = 2.9 Hz, 6H). Characterization data of compound 2: 1H NMR (400 MHz, CDCl3) δ 7.37 (dt, J = 7.0, 1.5 Hz, 2H), 7.27 (td, J= 6.8, 6.3, 2.8 Hz, 2H), 7.21 (dd, J = 8.1, 6.4 Hz, 3H), 6.73 (d, J = 2.8 Hz, 2H), 6.50 – 6.40 (m, 2H), 2.91 (s, 12H). Characterization data of compound 3: 1 H NMR (400 MHz, D2O) δ 6.90 (s, 2H), 6.83 – 6.59 (m, 2H), 6.47 (s,2H), 2.86 (s, 12H).

Claims

1. A method for synthesizing a key intermediate (compound 2) of pharmaceutical methylene blue, characterized in that, Prepared via the following reaction: 。 2. The synthesis method according to claim 1, characterized in that, The copper catalysts used in the reaction to synthesize compound 2 were cuprous iodide, cuprous chloride, copper acetate, and copper sulfate, with an addition amount of 20 mol.

3. The synthesis method according to claim 1, characterized in that, The ligands used in the reaction to synthesize compound 2 are L -Proline, added at a rate of 20 mol%.

4. The synthesis method according to claim 1, characterized in that, The solvent used in the reaction to synthesize compound 2 is at least one of toluene, tetrahydrofuran, dimethyl sulfoxide, acetonitrile, and N,N-dimethylformamide.

5. The synthesis method according to claim 1, characterized in that, The reaction temperature for synthesizing compound 2 is 25-130 degrees Celsius.

6. According to the synthesis method of claim 1, compound 1 is prepared by the following reaction: 。 7. The synthesis method according to claim 6, characterized in that, The amount of benzoyl chloride used in the reaction is 1.0-1.2 equivalents.

8. The synthesis method according to claim 6, characterized in that, The amount of sodium hydroxide used in the reaction is 5.0 equivalents.

9. The synthesis method according to claim 6, characterized in that, The solvent used in the reaction is a mixture of dichloromethane and water in a volume ratio of 1:

1.

10. The synthesis method according to claim 6, characterized in that, The reaction temperature is room temperature, and the reaction time is 12-24 hours.

11. A method for synthesizing pharmaceutical methylene blue (compound 3) according to claim 1, characterized in that, Prepared via the following reaction: 。 12. The synthesis method according to claim 11, characterized in that, The oxidant used in the reaction is benzoquinone, and the amount used is 1.05 equivalents.

13. The synthesis method according to claim 11, characterized in that, The solvent used in the reaction is at least one of methanol, ethanol, isopropanol, and distilled water.

14. The synthesis method according to claim 11, characterized in that, The reaction temperature is 25-80 degrees Celsius.

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