Preparation method of high-purity isosulfur blue

Abstract

The invention provides a preparation method of high-purity isosulfur blue, which comprises the following steps: by taking 2-chlorobenzaldehyde as a raw material, after sulfonation reaction of fuming sulfuric acid, adding calcium carbonate to convert excessive sulfuric acid into calcium sulfate precipitate, and filtering to remove the calcium sulfate precipitate, thereby reducing inorganic salt residues in an intermediate I; carrying out substitution reaction on the intermediate I under the action of sodium sulfite and sodium hydrogen sulfite, adding a calcium chloride solution after the reaction to enable sulfite to form calcium sulfite precipitate, and carrying out filtration, vacuum concentration and methanol pulping to obtain a high-purity intermediate II; the intermediate II reacts with N, N-diethylaniline to generate an intermediate III, then the intermediate III is oxidized by silver oxide to generate an isosulfur blue silver salt, sodium carbonate is added to directly convert the silver salt into a sodium salt form, and a high-purity isosulfur blue finished product is obtained through ethyl acetate crystallization. According to the method, inorganic salt residues are remarkably reduced, the product purity and yield are improved, meanwhile, the technological process is simplified, the solvent usage amount is reduced, operation is safe and environmentally friendly, and industrial production is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of pharmaceutical chemistry, and particularly to a method for preparing high-purity isosulfan blue. Background Art

[0002] Isosulfan Blue is the first tumor tracer approved by the US FDA for lymphatic imaging and is also the most widely used tracer for sentinel lymph node biopsy in breast cancer currently popular in Europe and America. It is widely used in the diagnosis and treatment of breast cancer and has important clinical significance. The preparation of isosulfan blue involves multiple chemical reactions, including sulfonation, alkylation, and oxidation. The complexity of these processes and the requirements for high-purity products have made its preparation technology a research hotspot in the field of pharmaceutical chemistry.

[0003] The existing isosulfan blue preparation technology mainly uses 2-chlorobenzaldehyde as the starting material and prepares isosulfan blue through steps such as sulfonation, alkylation, and oxidation. However, the following main defects exist in the existing methods:

[0004] For example, Patent US2006 / 224003A1 discloses a method for preparing isosulfan blue intermediates. This route uses 2-chlorobenzaldehyde as the raw material and sulfonates it with 23% and 65% fuming sulfuric acid at 80 - 85°C. After HPLC monitors that the reaction is complete, the reaction solution is dropped into cold water (3v / w), sodium chloride (2.4w / w) is added, and the mixture is stirred for crystallization. After filtration and drying, 2-chloro-5-sulfobenzaldehyde is obtained; then 2-chloro-5-sulfobenzaldehyde is heated to 100 - 105°C in an aqueous solution of 40% sodium hydroxide and an aqueous solution of sodium sulfite for reaction. After the reaction is complete, the solvent is removed by reduced pressure concentration to obtain sodium benzaldehyde-2,5-disulfonate; sodium benzaldehyde-2,5-disulfonate reacts with N,N-diethylaniline in a hydrochloric acid solution to obtain 4-[bis[4-(diethylamino)phenyl]methyl]-2,5-benzenedisulfonic acid, and the disodium salt is obtained by treatment with sodium hydroxide in 95% ethanol; then it is oxidized with ammonium dichromate and 40% sulfuric acid in a methanol solution to obtain isosulfan blue acid, and the isosulfan blue finished product is obtained by salifying with sodium carbonate in a methanol solution. No desalting operation is carried out during the preparation of the intermediates 2-chloro-5-sulfobenzaldehyde and sodium benzaldehyde-2,5-disulfonate in this method, resulting in a large amount of residue of sulfates, sulfites, and sodium chloride, seriously affecting the purity of the intermediates and the finished product.

[0005] For another example, in the sulfonation process of Patent US2008 / 0293963A1, the reaction involves reacting 2-chlorobenzaldehyde with 2.0 equivalents of 20% oleum. After the reaction is complete, the reaction mixture is poured into ice water, and then solid sodium chloride is added and stirred to produce a cream-colored precipitate. After filtration, washing with ether, and drying, 2-chloro-5-sulfobenzaldehyde is obtained; 2-chlorobenzaldehyde-5-sulfonic acid, sodium sulfite, sodium bisulfite, and water are injected into a pressure vessel, and the reaction mixture in the vessel is stirred and heated under the following conditions, and the reaction is carried out at a pressure of 140 - 150 psi. After the reaction is complete and the reaction mixture is cooled, it is poured into methanol with stirring (such that the water content accounts for 20% of the total volume), precipitated, filtered, and the mother liquor is concentrated under reduced pressure to remove the solvent. The obtained solid is slurried with methanol, filtered, and a light yellow solid is obtained. The solid is dissolved in N,N-dimethylformamide and stirred to dissolve at room temperature, then filtered. The filtrate is precipitated with dichloromethane to obtain a light yellow solid; then it reacts with N,N-diethylaniline in glacial acetic acid to obtain 4-[bis[4-(diethylamino)phenyl]methyl]-2,5-benzenedisulfonic acid; it is oxidized with silver oxide in a methanol solution to obtain the silver salt of isothiocyanine blue, and the isothiocyanine blue acid is obtained through acidification in an acidification bed; then it forms a salt with sodium bicarbonate in an isothiocyanine blue acid aqueous solution, and the isothiocyanine blue finished product is obtained through crystallization with acetone.

[0006] Similarly, the intermediate 2-chloro-5-sulfobenzaldehyde is not subjected to desalting treatment. Desalting treatment is added in the post-treatment process of preparing disodium 2,5-disulfobenzoate, but the process is too cumbersome and the yield is low, which is not conducive to industrial production; using silver oxide as an oxidant, a large amount of methanol is required for washing the filter cake in the post-treatment, and it needs to be treated through an acidification bed, with a cumbersome process and high cost, which is not conducive to commercial production.

[0007] In addition, ammonium dichromate and concentrated sulfuric acid are often used as oxidants in traditional methods. These reagents are highly toxic and have a potential explosion risk, and impurities are easily introduced during the operation, which is not conducive to industrial production.

[0008] Therefore, it is necessary to provide an improved method for preparing high-purity isothiocyanine blue to solve the above problems. Summary of the Invention

[0009] According to the above-mentioned technical problems existing in the existing method for preparing isothiocyanine blue, such as salt residue, improper selection of oxidant, and process complexity, a method for preparing high-purity isothiocyanine blue is provided.

[0010] After the sulfonation reaction of the present invention, it is treated by adding calcium carbonate, so that the excessive sulfuric acid in the reaction is converted into calcium sulfate precipitate and removed by filtration, effectively reducing the residues of sulfates and other inorganic salts in intermediate I and improving the purity of the intermediate; in the preparation of intermediate II, by adding calcium chloride solution, the excessive sulfite is precipitated as calcium sulfite, further removing the inorganic salt impurities and ensuring the smooth progress of the subsequent reaction; in the preparation process of isothiocyanine blue finished product, after the oxidation is completed, sodium carbonate is directly added to treat the reaction solution to displace isothiocyanine blue in the form of silver salt into sodium salt, avoiding the complex operations of acidification - neutralization in the traditional process and reducing the generation of impurities at the same time; through the crystallization and methanol pulping process, the residual impurities are effectively removed, the post - treatment process is simplified, and the yield and purity of the product are significantly improved. The operation of the present invention is simple, the solvent usage is small, the product quality is stable, and it is conducive to industrial production.

[0011] The technical means adopted by the present invention are as follows:

[0012] A method for preparing high - purity isothiocyanine blue, comprising the following steps:

[0013] S1. Preparation of intermediate I: 2 - chlorobenzaldehyde is subjected to a sulfonation reaction with fuming sulfuric acid. After the reaction is completed, it is quenched with ice water, the temperature is raised, and calcium carbonate is added to adjust the pH to 6 - 7. After filtration, sodium carbonate is added to the filtrate to adjust the pH to 8 - 9. After filtration, the filtrate is concentrated under reduced pressure, cooled for crystallization, filtered, and dried to obtain intermediate I;

[0014] S2. Preparation of intermediate II: Intermediate I reacts with sodium sulfite in water under the action of sodium bisulfite by refluxing. Calcium chloride solution is added to precipitate calcium sulfite. After filtration, the filtrate is concentrated under reduced pressure, cooled for crystallization, methanol pulping, filtered, and dried to obtain intermediate II;

[0015] S3. Preparation of intermediate III: Intermediate II and N,N - diethylaniline are heated and refluxed in glacial acetic acid. After cooling, methanol is added and stirred, and intermediate III is obtained by filtration;

[0016] S4. Preparation of isothiocyanine blue finished product: Intermediate III and silver oxide are subjected to an oxidation reaction in methanol, and anhydrous sodium carbonate is added to displace it into sodium salt. After filtration, ethyl acetate is added to the filtrate for crystallization, filtered, and dried to obtain high - purity isothiocyanine blue finished product.

[0017] Further, in step S1, the concentration of fuming sulfuric acid is 20% - 60%, the sulfonation reaction temperature is controlled at 10 - 15°C, and after the sulfonation is completed, the temperature is raised to 63 - 68°C and stirred for reaction for 25 ± 1 hour.

[0018] Further, in step S1, after the sulfonation reaction, ice water is added for quenching, the temperature is raised to 50 - 60°C, the mass ratio of calcium carbonate to fuming sulfuric acid is greater than 1:1, and the pH is adjusted to 6 - 7; the weight ratio of sodium carbonate to 2-chlorobenzaldehyde is greater than 0.6:1, and the pH is adjusted to 8 - 9.

[0019] Further, in step S2, Intermediate I: sodium sulfite: sodium bisulfite (molar ratio) = 1:(1.5 - 2.5):(0.15 - 0.25 ) ， The heating reflux reaction time is 3 - 4 hours.

[0020] Further, in step S2, the addition amount of the calcium chloride solution is the same as the molar ratio of the remaining sodium sulfite after reflux until the precipitation is complete.

[0021] Further, in step S3, N,N-diethylaniline: Intermediate II (molar ratio) = (2.0 - 2.5):1 (i.e., the molar ratio of N,N-diethylaniline to Intermediate II is 2.0 - 2.5:1), until the reaction is complete, the heating reflux reaction temperature is 115 - 120°C, and the time is 3 - 4 hours.

[0022] Further, in step S4, silver oxide: Intermediate III (molar ratio) = (1.8 - 2.2):1, the oxidation reaction temperature is 25 - 30°C, and the time is 16 - 18 hours.

[0023] Further, in step S4, anhydrous sodium carbonate: Intermediate III (molar ratio) (1.8 - 2.2):1, the reaction temperature is 50 - 60°C, and the reaction time is 6 - 8 hours.

[0024] Further, in the crystallization process of step S4, ethyl acetate is used as the solvent, the crystallization temperature is 25 - 30°C, and the crystallization time is 1 - 2 hours.

[0025] Further, the purity of the prepared isothiazole blue finished product is not less than 99.8%, and the residual inorganic salt content is not higher than 0.1%.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. By improving the post-treatment steps of the sulfonation reaction, the present invention adopts the calcium carbonate precipitation method to replace the traditional salting-out method, converts the excessive sulfuric acid in the reaction into calcium sulfate precipitation, effectively reduces the residual inorganic salts (such as sodium sulfate, sodium sulfite, sodium chloride) in Intermediate I, avoids the eutectic of inorganic salts and the product, improves the purity of Intermediate I (up to 99.62%), simplifies the operation steps at the same time, and reduces the difficulty of subsequent desalting and refining. In addition, by controlling the crystallization temperature and concentration conditions, the yield of Intermediate I is significantly improved (up to 87.02%).

[0028] 2. During the conversion of intermediate I to intermediate II in the present invention, by introducing calcium chloride solution, the excess sulfite in the reaction forms insoluble calcium sulfite precipitate, which is effectively removed by filtration, reducing the influence of inorganic salt impurities on subsequent steps, avoiding the complexity of multiple precipitation and filtration operations in the traditional method, and at the same time improving the product purity (up to 99.75%) and yield (up to 91.99%). Methanol pulping further removes trace residual impurities, ensuring the high quality of intermediate II.

[0029] 3. In the preparation of isothiocyanine blue finished product, the present invention uses silver oxide as an oxidant, which significantly reduces toxicity and safety risks compared with the traditional use of concentrated sulfuric acid and ammonium dichromate. After the oxidation is completed, sodium carbonate is directly added to replace the silver salt with sodium salt, avoiding the introduction of the complex process of acidification - neutralization in the traditional process, and at the same time reducing the possibility of impurity generation. By optimizing the crystallization conditions (such as using ethyl acetate as the crystallization solvent), the purity (up to 99.80%) and yield (up to 80%) of the finished product are improved, and the operation process is significantly simplified, making the process more suitable for industrial production.

[0030] 4. The present invention effectively removes inorganic salt impurities by adopting the processes of directional precipitation and pulping refining in each intermediate preparation step, ensuring the purity and stability of the product; the improved process avoids the accumulation and diffusion of impurities in the traditional process, and the finally prepared isothiocyanine blue finished product has higher chemical purity and thermal stability, which can meet the high-quality requirements for clinical use.

[0031] In summary, the present invention optimizes the key steps in the isothiocyanine blue preparation process, solving the problems of excessive inorganic salt residues, unsafe oxidation reaction, and complex process in the traditional method. Through innovative processes such as calcium carbonate precipitation, silver oxide oxidation, and efficient crystallization, the purity of intermediates and finished products is significantly improved, the product yield is increased, and at the same time the process complexity and safety risks are reduced. The method of the present invention is simple to operate, highly controllable, and environmentally friendly, suitable for large-scale industrial production, providing a reliable technical guarantee for the efficient preparation of isothiocyanine blue. For the above reasons, the present invention can be widely promoted in the field of pharmaceutical chemistry technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 It is a purity detection diagram of intermediate I in Example 1 of the present invention.

[0034] Figure 2 It is the purity detection chart of Intermediate II in Example 1 of the present invention.

[0035] Figure 3 It is the purity detection chart of Intermediate III in Example 1 of the present invention.

[0036] Figure 4 It is the purity detection chart of isothiocyanine blue prepared in Example 1 of the present invention.

[0037] Figure 5 It is the purity detection chart of Intermediate I in Example 2 of the present invention.

[0038] Figure 6 It is the purity detection chart of Intermediate II in Example 3 of the present invention.

[0039] Figure 7 It is the purity detection chart of isothiocyanine blue prepared in Example 4 of the present invention.

[0040] Figure 8 It is the purity detection chart of Intermediate I in Comparative Example 1 of the present invention.

[0041] Figure 9 It is the purity detection chart of Intermediate II in Comparative Example 2 of the present invention.

[0042] Figure 10 It is the purity detection chart of Intermediate III in Comparative Example 3 of the present invention.

[0043] Figure 11 It is the purity detection chart of isothiocyanine blue prepared in Comparative Example 4 of the present invention. Detailed implementation manners

[0044] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0045] The present invention provides a method for preparing high-purity isothiocyanine blue, including the following steps:

[0046] S1. Preparation of Intermediate I: Using 2-chlorobenzaldehyde as a raw material, sulfonating it with 20% - 60% fuming sulfuric acid. After the reaction ends, the reaction solution is quenched in ice water. The mixture is heated to 50 - 60°C, and calcium carbonate solid is added to adjust the pH to 6 - 7. Then, filtration is carried out (the filter cake is calcium sulfate). Sodium carbonate solid is added to the filtrate to adjust the pH to 8 - 9, and filtration is carried out again (the filter cake is calcium carbonate). The filtrate is concentrated under reduced pressure until a large amount of solid precipitates, and crystallization is carried out at 5 - 10°C. After filtration and drying, Intermediate I is obtained.

[0047] S2. Preparation of Intermediate II: Intermediate I undergoes a substitution reaction with sodium sulfite in water under the action of sodium bisulfite. After the reaction is complete, an aqueous calcium chloride solution is added, and calcium sulfite solid precipitates out. The mixture is filtered, and the filtrate is concentrated under reduced pressure until a large amount of solid precipitates. Crystallization is carried out at 5 - 10 °C, and then filtered. The solid is slurried with methanol, filtered, and dried to obtain Intermediate II.

[0048] S3. Preparation of Intermediate III: Intermediate II and N,N - diethylaniline are heated under reflux in glacial acetic acid. After cooling, methanol is added and stirred, and then Intermediate III is obtained by filtration.

[0049] S4. Preparation of Isothiocyanine Blue Product: Intermediate III is oxidized by silver oxide in a methanol solution, and the reaction is carried out at 25 - 30 °C for 16 h. After the reaction is completed, sodium carbonate is added to the reaction solution, and the reaction continues at 40 - 60 °C for 6 - 8 h. After the reaction is completed, the temperature is cooled to 25 - 30 °C and filtered. Ethyl acetate is added to the mother liquor for crystallization, and the high - purity Isothiocyanine Blue product is obtained by filtration.

[0050] Example 1

[0051] The present invention provides a method for preparing high - purity Isothiocyanine Blue, which specifically includes the following steps:

[0052] S1. Preparation of Intermediate I

[0053] 100 g of 20% fuming sulfuric acid is added to a 250 - ml three - necked flask, and the mixture is placed in a cold well and stirred while cooling to 5 - 10 °C. 25 g of 2 - chlorobenzaldehyde is added dropwise, and the temperature is controlled ≤15 °C. After the addition is complete, the mixture is stirred at 10 - 15 °C for 2 h. Then the temperature is raised to 63 - 68 °C and stirred for reaction for 25 ± 1 h. After the reaction is completed, the temperature is cooled to 20 - 30 °C, and the reaction is quenched by adding it to ice while stirring. The aqueous solution is heated to 50 - 55 °C, and calcium carbonate is slowly added while stirring. After the addition is complete (pH ~ 6), the mixture is stirred and dispersed for 1.5 - 2 h, and then filtered. Sodium carbonate is added to the mother liquor to adjust the pH to 8, and then filtered. The mother liquors are combined and concentrated under reduced pressure until a large amount of solid precipitates. Crystallization is carried out at 5 - 10 °C for 1.5 - 2 h, and then filtered. 37.7 g of white solid is obtained after drying, with a yield of 87.02% and a purity of 99.52% (as Figure 1 shown).

[0054] S2. Preparation of Intermediate II

[0055] Add 35.0 g of Intermediate I, 140 ml of purified water, 36.3 g of Na2SO3, and 2.9 g of NaHSO3 to a 500 ml three-necked flask, heat under reflux (about 100 °C), and react for 3 - 4 h. After the reaction is complete, cool to 50 - 55 °C, add 15.2 g of CaCl2 solution (dissolved in purified water), stir for 1 h, filter, concentrate the filtrate under reduced pressure until a large amount of solid precipitates, cool to 5 - 10 °C and crystallize for 1.5 - 2 h, filter, add methanol to the solid for pulping, filter, and dry to obtain 41.17 g of a white solid, with a yield of 91.99% and a purity of 99.92% (as Figure 2 shown).

[0056] S3. Preparation of Intermediate III

[0057] Add 40 g of Intermediate II, 40.1 g of N,N-diethylaniline, 280 ml of glacial acetic acid, and 5.4 g of urea to a 500 ml three-necked flask. After feeding, heat to reflux (118 °C) and react for 3 - 4 h. After the reaction is complete, cool to 60 - 65 °C, add 280 ml of methanol, stir for 1 h, filter, and wash with an appropriate amount of methanol to obtain the crude product of Intermediate III. Add the filter cake to 280 ml of water, stir at 60 - 65 °C for 1 h, filter, wash with an appropriate amount of water, and dry to obtain 65.3 g of a white solid, with a yield of 92.58% and a purity of 99.39% (as Figure 3 shown).

[0058] S4. Preparation of Isothiocyanine finished product

[0059] Add 65 g of Intermediate III, 55.1 g of silver oxide, and 650 ml of anhydrous methanol to a 1000 ml three-necked flask, stir and react at 25 - 30 °C for 16 - 18 h; after the reaction is completed, add 25.2 g of anhydrous sodium carbonate, stir and react at 50 - 60 °C for 6 - 8 h; after the reaction is completed, cool to 25 - 30 °C, filter, add 1300 ml of ethyl acetate to the filtrate and stir to crystallize for 2 h, filter to obtain 54.1 g of the Isothiocyanine finished product, with a yield of 80.16% and a purity of 99.87% (as Figure 4 shown).

[0060] Example 2

[0061] On the basis of Example 1, for the preparation of Intermediate I, increase the concentration of fuming sulfuric acid. Add 50 g of 40% fuming sulfuric acid to a 250 ml three-necked flask, place it in a cold well and stir to cool down to 5 - 10 °C. Dropwise add 25 g of 2-chlorobenzaldehyde, control the temperature ≤ 15 °C. After the addition is complete, stir at 10 - 15 °C for 2 h. Heat up to 63 - 68 °C and stir to react for 25 ± 1 h. After the reaction is completed, cool down to 20 - 30 °C, and add it to ice with stirring to quench the reaction. Heat the aqueous solution to 50 - 55 °C, slowly add calcium carbonate with stirring. After the addition is complete (pH ~ 7), stir and disperse for 1.5 - 2 h, then filter. Add sodium carbonate to the mother liquor to adjust the pH to 8, filter. Combine the mother liquors and concentrate under reduced pressure until a large amount of solid precipitates. Cool down to 5 - 10 °C and crystallize for 1.5 - 2 h, then filter and dry to obtain 38.23 g of white solid, with a yield of 88.61% and a purity of 99.88% (as Figure 5 shown).

[0062] Example 3

[0063] On the basis of Example 1, for the preparation of Intermediate II, reduce the proportion of sodium sulfite and sodium bisulfite. Add 35.0 g of Intermediate I, 140 ml of purified water, 32.7 g of Na2SO3, and 2.6 g of NaHSO3 to a 500 ml three-necked flask, heat to reflux (about 100 °C) and react for 3 - 4 h. After the reaction is complete, cool down to 50 - 55 °C, add 13.7 g of CaCl2 solution (dissolved in purified water), stir for 1 h, then filter. Concentrate the filtrate under reduced pressure until a large amount of solid precipitates. Cool down to 5 - 10 °C and crystallize for 1.5 - 2 h, then filter. Add methanol to the solid for pulping, filter, and dry to obtain 40.79 g of white solid, with a yield of 91.11% and a purity of 99.95% (as Figure 6 shown).

[0064] Example 4

[0065] On the basis of Example 1, for the preparation of the finished product, increase the reaction temperature and crystallize with isopropyl ether. Add 65 g of Intermediate III, 55.1 g of silver oxide, and 650 ml of anhydrous methanol to a 1000 ml three-necked flask, stir and react at 25 - 30 °C for 16 - 18 h; after the reaction is completed, add 25.2 g of anhydrous sodium carbonate, heat up to reflux (65 - 68 °C) and stir to react for 4 - 6 h; after the reaction is completed, cool down to 25 - 30 °C, filter. Add 1300 ml of isopropyl ether to the filtrate and stir to crystallize for 2 h, then filter to obtain 55.7 g of the finished product of isothiocyanine blue, with a yield of 82.53% and a purity of 99.86% (as Figure 7 shown).

[0066] Comparative Example 1

[0067] Based on Example 1, for the preparation of Intermediate I, the desalting operation to remove calcium carbonate was carried out. 100 g of 20% fuming sulfuric acid was added to a 250 ml three-necked flask, and 25 g of 2-chlorobenzaldehyde was added dropwise. After the addition was complete, the mixture was stirred at 80 - 85 °C for 4 h. After the reaction ended, the temperature was lowered to 20 - 30 °C, and it was quenched by adding it to 75 g of ice with stirring. 60 g of sodium chloride was slowly added to the aqueous solution with stirring. After the addition was complete, the mixture was stirred for crystallization for 1.5 - 2 h, filtered, and dried to obtain 48.9 g of inorganic salts with a purity of 89.14% (content 65.13%) (as Figure 8 shown).

[0068] Comparative Example 2

[0069] Based on Example 1, for the preparation of Intermediate II, the desalting operation to remove calcium chloride was carried out. 48.0 g of Intermediate I and 140 ml of purified water were added to a 500 ml three-necked flask. The pH was adjusted to 7 with 40% sodium hydroxide, 22.4 g of Na2SO3 was added, and the mixture was heated to reflux (about 100 °C) and reacted for 12 h. After the reaction was complete, the solvent was removed by concentration under reduced pressure to obtain 76 g of inorganic salt solid with a purity of 91.09% (content 47.23%) (as Figure 9 shown).

[0070] Comparative Example 3

[0071] Based on Example 1, for the preparation of Intermediate III, the reaction solvent was changed to hydrochloric acid solution. A mixture of the above-produced Intermediate II and 81.46 g (3.4 eq, based on 2-chlorobenzaldehyde) of N,N-diethylaniline was stirred evenly at 30 °C. 25 g of concentrated hydrochloric acid was added, and the temperature was raised to reflux and reacted for 12 h. After the reaction ended, it was filtered, and the filter cake was washed with water and methanol respectively. 80 ml of 95% ethanol was added to the filter cake and stirred for 30 min. The pH was adjusted to 12 with 40% sodium hydroxide, and then it was filtered. 80 ml of n-hexane was added to the filter cake and slurried at 60 - 70 °C for 1 h, filtered, and the filter cake was dried under vacuum to obtain 25.4 g of Intermediate III with a purity of 88.40% (as Figure 10 shown).

[0072] Comparative Example 4

[0073] Based on Example 1, for the preparation of the finished product of isothiocyanine blue, the oxidant was changed to ammonium dichromate. 25 g of Intermediate III was dissolved in 100 ml of methanol, and the temperature was lowered to 0 - 5 °C. 4.1 g of ammonium dichromate was added in five equal portions. 100 ml of 40% sulfuric acid was added dropwise to the reaction mixture, and the addition was completed in about 10 min. Then the reaction mixture was stirred at 0 - 5 °C for 2 h. After the reaction ended, it was filtered, and the filter cake was washed with methanol. 31.3 g of sodium carbonate was added to the filtrate and stirred for 12 h. Filtered, the filter cake was washed with methanol. The filtrate was concentrated under reduced pressure until there was no solvent left to obtain 19.6 g of the finished product of isothiocyanine blue with a purity of 98.97% (as Figure 11 shown).

[0074] The isothiocyanine intermediates I and II prepared by the method of Example 1 have high purity and high content, and the finished products have high purity and high yield; in Examples 2-4, changing the concentration of concentrated sulfuric acid and the amount of sodium sulfite used results in no obvious change in the yield and purity of the intermediates and the finished products; in Comparative Examples 1-2, the desalting step is omitted compared with the method of Example 1, and the purity and content of the intermediates are significantly decreased; in Comparative Example 3, the reaction solvent in Example 1 is changed from acetic acid to hydrochloric acid, and the purity of the prepared intermediate III is significantly decreased; in Comparative Example 4, the oxidizing reagent is changed from silver oxide to ammonium dichromate, and the impurities in the prepared isothiocyanine finished product are significantly increased. In summary, it can be seen that the isothiocyanine preparation process of the present invention is stable, and the isothiocyanine intermediates and finished products prepared have high yield and high purity, which is more conducive to industrial production.

[0075] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the present invention; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing high-purity isothiocyanine blue, characterized in that, It includes the following steps: S1. Preparation of intermediate I: 2-chlorobenzaldehyde is subjected to a sulfonation reaction with fuming sulfuric acid. After the reaction is completed, ice water is added for quenching. After the temperature is raised, calcium carbonate is added to adjust the pH to 6-7. After filtration, sodium carbonate is added to the filtrate to adjust the pH to 8-9. After filtration, the filtrate is concentrated under reduced pressure, cooled for crystallization, filtered, and dried to obtain intermediate I; S2. Preparation of intermediate II: Intermediate I undergoes a reflux reaction with sodium sulfite in water under the action of sodium bisulfite. Calcium chloride solution is added to precipitate calcium sulfite. After filtration, the filtrate is concentrated under reduced pressure, cooled for crystallization, slurried with methanol, filtered, and dried to obtain intermediate II; S3. Preparation of intermediate III: Intermediate II and N,N-diethylaniline are heated under reflux in glacial acetic acid. After cooling, methanol is added and stirred, and intermediate III is obtained by filtration; S4. Preparation of isothiocyanine finished product: Intermediate III undergoes an oxidation reaction with silver oxide in methanol. Anhydrous sodium carbonate is added to displace it into the sodium salt. After filtration, ethyl acetate is added to the filtrate for crystallization, filtered, and dried to obtain a high-purity isothiocyanine finished product.

2. The preparation method according to claim 1, characterized in that, In step S1, the concentration of fuming sulfuric acid is 20%-60%, the sulfonation reaction temperature is controlled at 10-15°C, and after the sulfonation is completed, the temperature is raised to 63-68°C and stirred for reaction for 25±1 hour.

3. The preparation method according to claim 3, characterized in that, In step S1, after the sulfonation reaction, ice water is added for quenching, the temperature is raised to 50-60°C, the mass ratio of calcium carbonate to fuming sulfuric acid is greater than 1:1, and the pH is adjusted to 6-7; the weight ratio of sodium carbonate to 2-chlorobenzaldehyde is greater than 0.6:1, and the pH is adjusted to 8-9.

4. The preparation method according to claim 1, characterized in that, In step S2, the molar ratio of intermediate I: sodium sulfite: sodium bisulfite = 1: (1.5 - 2.5): (0.15 - 0.25 ) , and the heating reflux reaction time is 3 - 4 hours.

5. The preparation method according to claim 4, wherein In step S2, the addition amount of the calcium chloride solution is the same as the molar ratio of the remaining sodium sulfite after reflux until the precipitation is complete.

6. The preparation method according to claim 1, characterized in that, In step S3, N,N-diethylaniline:intermediate II (molar ratio) = (2.0-2.5):1 until the reaction is complete, the heating reflux reaction temperature is 115-120°C, and the time is 3-4 hours.

7. The preparation method according to claim 1, characterized in that, In step S4, silver oxide:intermediate III (molar ratio) = (1.8-2.2):1, the oxidation reaction temperature is 25-30°C, and the time is 16-18 hours.

8. The preparation method according to claim 7, characterized in that, In step S4, anhydrous sodium carbonate:intermediate III (molar ratio) = (1.8-2.2):1, the reaction temperature is 50-60°C, and the reaction time is 6-8 hours.

9. The preparation method according to claim 1, characterized in that, In the crystallization process of step S4, ethyl acetate is used as the solvent, the crystallization temperature is 25-30°C, and the crystallization time is 1-2 hours.

10. The preparation method according to claim 1, characterized in that, The purity of the obtained isothiocyanine finished product is not less than 99.8%, and the residual inorganic salt content is not higher than 0.1%.

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