Synthesis method of MC-Val-Cit-PAB
By optimizing the synthetic route of MC-Val-Cit-PAB, using L-citrulline and L-valine as raw materials, and combining specific solvent and temperature conditions, the problems of moisture absorption and low purity of intermediates were solved, and efficient and low-cost industrial production was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-25
- Publication Date
- 2026-04-03
AI Technical Summary
In the existing MC-Val-Cit-PAB synthesis method, the intermediate compound is prone to moisture absorption, the post-processing operation is complicated, the purity is low, the starting material cost is high, and the conversion rate is low, resulting in high industrial production costs.
Using L-citrulline and L-valine as raw materials, compounds M2, M4 and M5 are formed through amide condensation reaction. By combining specific solvent and temperature conditions and optimizing the post-processing steps, the purity and yield are improved.
It significantly reduced production costs, improved the conversion rates of compounds M2, M4 and M5, with an overall conversion rate of over 85%, a yield increase of over 25%, and a product purity of 99.4%.
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Figure CN121779487A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of antibody-drug conjugate linker synthesis technology, and more specifically, to a method for synthesizing MC-Val-Cit-PAB. Background Technology
[0002] Antibody-drug conjugates (ADCs) are an innovative therapy that combines the targeting capabilities of antibodies with the cytotoxic effects of drugs. Their core structure consists of a monoclonal antibody, a linker, and a cytotoxic payload. By precisely recognizing tumor surface antigens, the antibody delivers the toxin directly into cancer cells, reducing damage to normal tissues, representing a significant breakthrough in cancer treatment.
[0003] The key to ADC drug development lies in the construction of linkers. Before the drug enters the target cell, the integrity of the antibody and the chemical drug in the in vivo environment must be ensured, and after approaching or entering the target cell, the chemical drug can be accurately released. Achieving this process depends on the "stability" of the antibody and the toxic compound after being linked by the linker. Furthermore, the amount of chemical drug loaded on the antibody in an ADC drug also depends on the linker; therefore, the linker directly affects the safety and efficacy of the ADC drug.
[0004] MC-Val-Cit-PAB, as a structurally mature and well-defined antibody-drug conjugate linker, has been successfully applied to several blockbuster ADC drugs, such as Brentuximab Vedotin for the treatment of Hodgkin's lymphoma and systemic anaplastic large cell lymphoma, Polatuzumab Vedotin for the treatment of diffuse large B-cell lymphoma, and vedicetuzumab. Choosing MC-Val-Cit-PAB as a linker significantly reduces the uncertainty and risk of research and development, and its market application prospects are broad.
[0005] The synthesis of MC-Val-Cit-PAB mainly follows the following path: The synthetic method has a short procedure and simple post-processing. However, the intermediate compound IV is hygroscopic, which increases the difficulty of post-reaction processing. Furthermore, the resulting MC-Val-Cit-PAB has low purity and cannot meet the quality standards of APIs in the pharmaceutical industry.
[0006] To address the shortcomings of the above-mentioned synthesis route, the following synthesis method is adopted for improvement:
[0007] This synthetic route protects the hydroxyl group of S4, improving hygroscopicity and simplifying post-processing, resulting in a product with high purity. However, its starting material, S1 (Fmoc-Val-Cit-OH), has high procurement costs and poor solubility in the reaction solvent, leading to low reaction rate and efficiency, and making post-processing difficult. Furthermore, the steric hindrance between S1 and S2 results in a low conversion rate of S1 to S3, only slightly over 60%, further increasing the difficulty of purification in post-processing. Therefore, industrial production costs are high.
[0008] In view of this, the present invention is proposed. Summary of the Invention
[0009] The purpose of this invention is to provide a method for synthesizing MC-Val-Cit-PAB. The synthesis method provided by this invention improves upon existing synthesis methods by addressing issues such as low product purity, high starting material costs, and low reaction conversion rates.
[0010] This invention is implemented as follows: In a first aspect, the present invention provides a method for synthesizing MC-Val-Cit-PAB, comprising: reacting L-citrulline with M1 via an amide condensation reaction to form M2, wherein the structural formula of M1 is shown below: The structural formula of M2 is shown below: ; L-valine is reacted with M3 via an amide condensation reaction to form M4, wherein the structural formula of M3 is shown below: The structural formula of M4 is shown below: ; M2 and M4 undergo an amide condensation reaction to form M5, the structural formula of which is shown below: .
[0011] In an optional embodiment, the step of forming M2 includes: mixing the L-citrulline, M1 and the quinoline condensing agent and reacting at 15-35°C for 10-14 hours.
[0012] In an optional implementation, the conditions for forming M2 satisfy the following requirements: (1) The quinoline condensing agent includes 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline; (2) The molar ratio of L-citrulline to M1 is (1.0:1.1) to (1.0:1.2). (3) The molar ratio of the L-citrulline and the quinoline condensing agent is (1.0:2.0) to (1.0:3.0). (4) The reaction temperature is 15-25℃.
[0013] In an optional embodiment, the step of forming M4 includes: mixing the L-valine with M3 and reacting at 15-20°C; after the reaction is completed, mixing the reaction solution with a C1-C3 alcohol solvent to crystallize; and then cooling, filtering and drying.
[0014] In an optional implementation, the conditions for forming M4 satisfy the following requirements: (1) The C1-C3 alcohol solvent includes isopropanol or ethanol, preferably isopropanol; (2) The molar ratio of L-valine to M3 is (1.0:1.1) to (1.0:1.2).
[0015] In an optional embodiment, the step of forming M5 includes: mixing M2, M4 and the condensing agent and reacting them at 0-5°C for 3-8 hours.
[0016] In an optional embodiment, the step of forming M5 includes: mixing M4, an amide solvent and an organic amine at 0-5°C and stirring for 15-45 minutes, and then mixing with a condensing agent and M2 and reacting at 0-5°C for 3-7 hours. Next, crystallization, pulping, and recrystallization are carried out.
[0017] In an optional implementation, the conditions for forming M5 satisfy the following requirements: (1) The amide solvents include N,N-dimethylformamide and N,N-dimethylacetamide; (2) The pulping solvent includes ester solvents, preferably ethyl acetate; (3) The molar ratio of M4 to M2 is (1.0:1.1) ~ (1.0:1.2). The molar ratio of M4 to the organic amine is (1.0:2.0) ~ (1.0:5.0). The molar ratio of M4 to the condensing agent is (1.0:1.5) to (1.0:2.0).
[0018] In an optional implementation, the method further includes: deprotecting M5; Preferably, the method includes: mixing M5 with dilute hydrochloric acid and reacting at 10-20°C for 16-20 hours.
[0019] In an optional embodiment, the amount of dilute hydrochloric acid used is 5-7 times the equivalent of M5.
[0020] The present invention has the following beneficial effects: (1) The embodiments of the present invention use L-citrulline and L-valine as raw materials, both of which are readily available and inexpensive, which can significantly reduce production costs.
[0021] (2) In this invention, L-citrulline and M1 are reacted by amide condensation to obtain compound M2, L-valine and M3 are reacted by amide condensation to obtain compound M4, and compounds M3 and M4 are reacted by amide condensation to obtain compound M5; the overall conversion rate of the three-step reaction is as high as 85% or more, which fully improves the atom utilization rate.
[0022] (3) When the overall yield of the present invention is calculated based on L-citrulline, the yield of the synthesis method provided by the present invention is more than 25% higher than that of S1 to MC-Val-Cit-PAB. Attached Figure Description
[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.
[0024] Figure 1 The high-performance liquid spectrum of MC-Val-Cit-PAB provided in Example 1 of this invention; Figure 2 The LCMS spectrum of MC-Val-Cit-PAB provided in Embodiment 1 of the present invention; Figure 3 The 1H NMR spectrum of the MC-Val-Cit-PAB provided in Example 1 of this invention. Detailed Implementation
[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.
[0026] In a first aspect, the present invention provides a method for synthesizing MC-Val-Cit-PAB, comprising: reacting L-citrulline with M1 via an amide condensation reaction to form M2, wherein the structural formula of M1 is shown below: The structural formula of M2 is shown below: ; L-valine is reacted with M3 via an amide condensation reaction to form M4, wherein the structural formula of M3 is shown below: The structural formula of M4 is shown below: ; M2 and M4 undergo an amide condensation reaction to form M5, the structural formula of which is shown below: ; Remove protection from M5.
[0027] See the following synthesis route for details: .
[0028] The embodiments of this invention utilize L-citrulline and L-valine, both of which are readily available and inexpensive raw materials, significantly reducing production costs. Furthermore, the conversion rate during preparation is high, and the product purity and yield are also high.
[0029] The specific operation process is as follows: The L-citrulline, M1 (4-((tert-butyldimethoxy)methyl)aniline), and quinoline condensing agent are mixed and reacted at 15-35°C for 10-14 hours. The reaction temperature can be any value between 15-35°C, such as 15°C, 20°C, 25°C, 30°C, or 35°C, but is preferably 15-25°C. Using the temperature specified in the examples is beneficial for improving the conversion rate; decreasing or increasing the temperature will lead to a decrease in the product yield.
[0030] Quinoline condensing agents include 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline; using the above condensing agents can further improve the conversion rate. If the condensing agent is changed, the reaction may not be able to proceed, or the conversion rate may be low.
[0031] Further, the molar ratio of L-citrulline to M1 is (1.0:1.1) to (1.0:1.2), preferably 1.0:1.1; the molar ratio of L-citrulline to the quinoline condensing agent is (1.0:2.0) to (1.0:3.0), preferably 1.0:2.0.
[0032] The steps for forming M4 include: mixing L-valine with M3 and reacting at 15-20°C; after the reaction, mixing the reaction solution with a C1-C3 alcohol solvent to crystallize; then cooling, filtering, and drying. Specifically, after the reaction, a C1-C3 alcohol solvent is added to the reaction solution to crystallize, then the mixture is stirred at 0-5°C, filtered, and dried. Filtering and drying are standard procedures and will not be described in detail here.
[0033] The molar ratio of L-valine to M3 is (1.0:1.1) to (1.0:1.2), preferably 1.0:1.1. The C1-C3 alcohol solvent includes isopropanol or ethanol, preferably isopropanol. Using the above solvent as the crystallization solvent can improve the yield and purity of product M4. If the crystallization solvent is changed, the purity and yield of M4 will be significantly reduced.
[0034] The steps to form M5 include: mixing M2, M4, and a condensing agent and reacting them at 0-5°C for 3-8 hours. The specific process is as follows: mixing M4, an amide solvent, and an organic amine at 0-5°C and stirring for 15-45 minutes, then mixing with the condensing agent and M2 and reacting at 0-5°C for 3-7 hours; followed by crystallization, pulping, and recrystallization.
[0035] The amide solvents include N,N-dimethylformamide and N,N-dimethylacetamide; using specific amide solvents is beneficial to promoting the reaction, while changing the solvent will significantly reduce the yield of the product.
[0036] The organic amine includes, but is not limited to, DIPEA; the pulping solvent includes ester solvents, preferably ethyl acetate; the molar ratio of M4 to M2 is (1.0:1.1) to (1.0:1.2), preferably 1.0:1.1; the molar ratio of M4 to the organic amine is (1.0:2.0) to (1.0:5.0), preferably 1.0:2.5; the molar ratio of M4 to the condensing agent is (1.0:1.5) to (1.0:2.0), preferably 1.0:1.5. The above reaction conditions favor the formation of M5.
[0037] Mix M5 with dilute hydrochloric acid and react at 10-20°C for 16-20 hours. The amount of dilute hydrochloric acid used is 5-7 times the equivalent of M5. The molar concentration of the dilute hydrochloric acid is 1.5-2.5 M. These conditions favor the formation of the product MC-Val-Cit-PAB.
[0038] It should be noted that the materials used in this invention and their corresponding abbreviations are as follows: EEDQ: 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline; HOBt: 1-hydroxybenzotriazole; EDCI: 1-ethyl-(3-dimethylaminopropyl)carbodiimide; HATU: 2-(7-azabenzotriazole)-N,N,N',N'-tetramethylurea hexafluorophosphate; HBTU: benzotriazole-N,N,N',N'-tetramethylurea hexafluorophosphate; DCC: dicyclohexylcarbodiimide; CDI: N,N'-carbonyldiimidazole.
[0039] The features and performance of the present invention will be further described in detail below with reference to embodiments.
[0040] Example 1 This embodiment provides a method for synthesizing MC-Val-Cit-PAB, including: S1, synthesize M2; Perform the synthesis according to the following synthesis path: The specific steps are as follows: (1) Feeding: Add 175.0g L-citrulline and 521.6g EEDQ to 4kg DCM and 1kg MeOH and stir for 0.5h. Slowly add 260.7g compound M1 to the system and react at 20℃ for 12h. (2) Filtration: After the reaction is complete, filter the filter cake with about 100 ml of dichloromethane and collect the filter cake.
[0041] (3) Pulping: Add the filter cake to 1L of ethyl acetate and stir for 2.5h.
[0042] (4) Filtration: Filter by suction, rinse the filter cake with about 100 ml of ethyl acetate, and collect the filter cake.
[0043] (5) Pulping: Add the filter cake to 1L of purified water and stir for 2.5h.
[0044] (6) Filtration: Filter by suction, rinse the filter cake with about 100ml of purified water, and collect the filter cake.
[0045] (7) Drying: The filter cake was placed in a vacuum drying oven and dried at 38°C for 24 hours to obtain 343.4 g of compound M2, with a yield of 90.8% and a purity of 97.7% (based on the mass of L-citrulline).
[0046] S2, synthesize M4; Perform the synthesis according to the following synthesis path: The specific steps are as follows: (1) Feeding: Add 200g of L-valine to 2L of acetonitrile and stir until dissolved. Add 578.8g of compound M3 to the system and react at 20℃ for 5h.
[0047] (2) Crystallization: After the reaction was complete, 6 L of isopropanol was added to the system with stirring. A large amount of white solid precipitated out. The temperature was lowered to 2 °C and stirred for 2 h.
[0048] (3) Filtration: Filter the filter cake with about 500ml of isopropanol and collect the filter cake.
[0049] (4) Drying: The filter cake was placed in a forced-air drying oven and dried at 58°C for 20 hours to obtain 443.5g of compound M4, with a yield of 86.3% (based on the mass of L-valine) and a purity of 98.6%.
[0050] S3, synthesize M5; Perform the synthesis according to the following synthesis path: The specific steps are as follows: (1) Feeding: Add 3L of N,N-dimethylformamide to 251.3g of compound M4 and stir until dissolved. Then add 314g of DIPEA and stir for 10min. Control the temperature at 2℃. Add 369.6g of HATU and react for 0.5h. Then add 343.4g of compound M2 and react for 5h.
[0051] (2) Crystallization: After the reaction was complete, 9 L of acetonitrile was poured into the system with stirring. A large amount of white solid precipitated out. The temperature was lowered to 2 °C and stirred for 2 h.
[0052] (3) Filtration: Filter the filter cake with about 500 ml of ethyl acetate and collect the filter cake.
[0053] (4) Pulping: Add the filter cake to 6L of ethyl acetate and stir for 5 hours, keeping the temperature at 20℃.
[0054] (5) Filtration: Filter the filter cake with about 500 ml of ethyl acetate and collect the filter cake.
[0055] (6) Drying: Place the filter cake in a forced-air drying oven and dry at 58°C for 22 hours.
[0056] (7) Recrystallization: Add the dried material to 3L of N,N-dimethylformamide and stir until basically dissolved. Filter out a very small amount of insoluble matter, and add 9L of ethanol to the filtrate while stirring. Cool to -5℃ and stir for 7h.
[0057] (8) Filtration: Filter the filter cake with about 500ml of ethanol and collect the filter cake.
[0058] (9) Drying: The filter cake was placed in a forced-air drying oven and dried at 58°C for 22 hours to obtain 509.2 g of compound M5, with a yield of 81.1% (based on the mass of M2) and a purity of 98.8%.
[0059] S4, Synthesis of MC-Val-Cit-PAB; Perform the synthesis according to the following synthesis path: The specific steps are as follows: (1) Feeding: Add 509.2g of compound M5 to 5L of 95% ethanol and stir for 1h, keeping the temperature at 15℃. Add 162.4g of 2M hydrochloric acid (6.0 equivalents) dropwise to the system and react for 18h at 15℃.
[0060] (2) Crystallization: After the reaction is complete, the temperature is lowered to 0℃ and stirred for 8 hours. A large amount of solid is clearly precipitated.
[0061] (3) Filtration: Filter the filter cake with about 500ml of ethanol and collect the filter cake.
[0062] (4) Washing: Add the filter cake to 5L of purified water, stir at 20℃ for 4 hours, and then cool down to 0℃ and stir for 2 hours.
[0063] (5) Filtration: Filter the filter cake with about 500ml of purified water and collect the filter cake.
[0064] (6) Filtration: Filter by suction, rinse the filter cake with about 100ml of purified water, and collect the filter cake.
[0065] (7) Drying: The filter cake was placed in a vacuum drying oven and dried at 55°C for 24 hours to obtain 381.6g of product MC-Val-Cit-PAB, with a yield of 90.8% (based on the mass of M5) and a purity of 99.4%.
[0066] For the high-performance liquid chromatography (HPLC) spectrum of MC-Val-Cit-PAB, please refer to [link / reference needed]. Figure 1 See LCMS diagram. Figure 2 See the 1H NMR spectrum. Figure 3 .
[0067] Process optimization 1 The operation of step (1) of S1 in the synthesis method provided in Example 1 was the same as that of step (1) in Example 1, except that the condensing agent was changed. The other operations and conditions were the same as those of step (1) of S1 in Example 1. After the reaction was completed (reacted for 12 hours respectively), the content of product M2 in the reaction system was detected. At this time, the reaction system was not subjected to the subsequent post-processing (2)-(7). The detection method was the external standard method. The results are shown in Table 1.
[0068] Table 1 Selection of condensing agents
[0069] N / A indicates that the reaction was not detected, meaning no reaction occurred. The product is M2.
[0070] As shown in Table 1, changing the condensing agent may prevent the reaction from proceeding, thus preventing the formation of M2, or may significantly reduce the yield of M2. The conversion rate of L-citrulline is highest when EEDQ is used as the condensing agent.
[0071] Process optimization 2 The operation of step (1) of S1 in the synthesis method provided in Example 1 was the same as that in Example 1, except that the reaction temperature was changed. The other operations and conditions were the same as those in step (1) of S1 in Example 1. The content of product M2 in the reaction system was detected after the corresponding reaction time. At this time, the reaction system was not subjected to the subsequent post-processing (2)-(7). The detection method was the external standard method. The results are shown in Table 2.
[0072] Table 2 Reaction Temperature
[0073] The raw material is L-citrulline; the product is M2.
[0074] As shown in Table 2 above, the conversion rate of L-citrulline is highest at a reaction temperature of 20℃ and a reaction time of 12h. Further extending the reaction time leads to partial degradation of product M2. Therefore, the preferred reaction temperature is 20℃ and the preferred reaction time is 12h. Changing the reaction temperature will result in incomplete reaction of the raw materials, leading to more residual raw materials and less product.
[0075] Process optimization 3 The operation in step S2 of the synthesis method provided in Example 1 was the same as that in Example 1, except that the crystallization solvent was changed. The other operations and conditions were the same as those in step S2. The results are shown in Table 3.
[0076] Table 3 Crystallization Solvents
[0077] As shown in Table 3, the crystallization solvent provided by this invention can guarantee the yield and purity of M4. If the crystallization solvent is changed, the purity and yield of product M4 will be significantly reduced, and some products may even fail to crystallize.
[0078] Process optimization 4 The operation of step (1) of S3 in the synthesis method provided in Example 1 was followed, except that the reaction solvent was changed. All other operations and conditions were the same as step (1) of S1 in Example 1. After the reaction was completed, the content of product M5 in the reaction system was detected. At this time, the reaction system was not subjected to the subsequent post-processing (2)-(9). The detection method was the external standard method. The results are shown in Table 4.
[0079] Table 4 Reaction Solvents
[0080] As shown in Table 4 above, changing the reaction solvent will lead to a significant decrease in the content of M5 in the reaction system, further demonstrating that the solvent provided in the embodiments of the present invention can improve the purity and yield of the product.
[0081] Process optimization 5 The operation of step (1) of S3 in the synthesis method provided in Example 1 was followed, except that the reaction temperature was changed. All other operations and conditions were the same as step (1) of S1 in Example 1. After the reaction was completed, the content of product M5 in the reaction system was detected. At this time, the reaction system was not subjected to the subsequent post-processing (2)-(9). The detection method was the external standard method. The results are shown in Table 5.
[0082] Table 5 Reaction Temperature
[0083] As shown in Table 5, changing the reaction temperature will cause raw material M2 to not react completely and the product content to decrease. However, using the temperature provided in the embodiments of the present invention can make raw material M2 react almost completely, and the content of product M5 in the reaction system will increase significantly, thereby increasing the product yield.
[0084] Process optimization 6 The operation in S3 of the synthesis method provided in Example 1 was the same as that in Example 1, except that the pulping solvent in (4) was changed. The other operations and conditions were the same as those in S3 of Example 1. The results are shown in Table 6.
[0085] Table 6 Pulping Solvents
[0086] As shown in Table 6, using the pulping solvent provided in this embodiment of the invention can significantly improve the purity and yield of product M5. If the pulping solvent is changed, the purity or yield of product M5 will decrease. Here, the yield refers to the yield of the pulping step.
[0087] Process optimization 7 The operation of step (1) in S4 of the synthesis method provided in Example 1 was followed, except that the amount of hydrochloric acid was changed. All other operations and conditions were the same as those in step (1) of S4 in Example 1. The content of product MC-Val-Cit-PAB in the reaction system was detected after the corresponding reaction time. At this time, the reaction system was not subjected to the subsequent post-processing (2)-(7). The detection method was the external standard method. The results are shown in Table 7.
[0088] Table 7 Hydrochloric Acid Dosage
[0089] Based on the above results, it can be seen that when the hydrochloric acid equivalent is too low, a large amount of raw material will still remain after the extended time. When the equivalent is too high, the system becomes slightly impure and the purity of the product decreases slightly. Therefore, the hydrochloric acid equivalent is 5-7, preferably 6.0 eq.
[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for synthesizing MC-Val-Cit-PAB, characterized in that, include: L-citrulline is reacted with M1 via an amide condensation reaction to form M2, wherein the structural formula of M1 is shown below: The structural formula of M2 is shown below: ; L-valine is reacted with M3 via an amide condensation reaction to form M4, wherein the structural formula of M3 is shown below: The structural formula of M4 is shown below: ; M2 and M4 undergo an amide condensation reaction to form M5, the structural formula of which is shown below: 。 2. The method for synthesizing MC-Val-Cit-PAB according to claim 1, characterized in that, The steps for forming M2 include: mixing the L-citrulline, M1 and quinoline condensing agent and reacting at 15-35°C for 10-14 hours.
3. The method for synthesizing MC-Val-Cit-PAB according to claim 2, characterized in that, The conditions for the formation of M2 must meet the following requirements: (1) The quinoline condensing agent includes 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline; (2) The molar ratio of L-citrulline to M1 is (1.0:1.1) to (1.0:1.2). (3) The molar ratio of the L-citrulline and the quinoline condensing agent is (1.0:2.0) to (1.0:3.0). (4) The reaction temperature is 15-25℃.
4. The method for synthesizing MC-Val-Cit-PAB according to claim 1, characterized in that, The steps for forming M4 include: mixing the L-valine with M3 and reacting at 15-20°C; after the reaction is complete, mixing the reaction solution with a C1-C3 alcohol solvent to crystallize; and then cooling, filtering and drying.
5. The method for synthesizing MC-Val-Cit-PAB according to claim 4, characterized in that, The conditions for forming M4 must meet the following requirements: (1) The C1-C3 alcohol solvent includes isopropanol or ethanol, preferably isopropanol; (2) The molar ratio of L-valine to M3 is (1.0:1.1) to (1.0:1.2).
6. The method for synthesizing MC-Val-Cit-PAB according to claim 1, characterized in that, The steps to form M5 include: mixing M2, M4 and the condensing agent and reacting them at 0-5°C for 3-8 hours.
7. The method for synthesizing MC-Val-Cit-PAB according to claim 6, characterized in that, The steps to form M5 include: mixing M4, amide solvent and organic amine at 0-5°C and stirring for 15-45 minutes, and then mixing with condensing agent and M2 and reacting at 0-5°C for 3-7 hours; Next, crystallization, pulping, and recrystallization are carried out.
8. The method for synthesizing MC-Val-Cit-PAB according to claim 7, characterized in that, The conditions for forming M5 must meet the following requirements: (1) The amide solvents include N,N-dimethylformamide and N,N-dimethylacetamide; (2) The pulping solvent includes ester solvents, preferably ethyl acetate; (3) The molar ratio of M4 to M2 is (1.0:1.1) ~ (1.0:1.2). The molar ratio of M4 to the organic amine is (1.0:2.0) ~ (1.0:5.0). The molar ratio of M4 to the condensing agent is (1.0:1.5) to (1.0:2.0).
9. The method for synthesizing MC-Val-Cit-PAB according to claim 1, characterized in that, Also includes: Remove protection from M5; Preferably, the method includes: mixing M5 with dilute hydrochloric acid and reacting at 10-20°C for 16-20 hours.
10. The method for synthesizing MC-Val-Cit-PAB according to claim 9, characterized in that, The amount of dilute hydrochloric acid used is 5-7 times the equivalent of M5.