A liquid phase synthesis method for semaglutide dipeptide fragment

Abstract

The present invention provides a liquid phase synthesis method of a semaglutide dipeptide fragment, and the synthesis route is as follows: #imgabs0#. The present invention provides a new liquid phase preparation method of a semaglutide dipeptide fragment R1-His-Aib-OH, wherein in step (1), a more economical, efficient, non-toxic, and low-residue T3P is used as a condensing agent, the reaction conditions are milder, the product impurity content is lower, and the yield is higher; at the same time, step (1) directly dehydrates and condenses 2-aminoisobutyric acid and N-protected histidine, without the need for unnecessary modification of the carboxyl group of 2-aminoisobutyric acid, shortening the reaction steps, simplifying the operation process, lowering the cost, and being more conducive to industrial production; in step (2), an organic solvent 3 is selected as a reaction medium to directly remove Trt from the intermediate, accelerate the removal of Trt, and reduce the temperature required for the reaction, thereby improving production efficiency and reducing costs.

Description

Technical Field

[0001] The present invention relates to the technical field of pharmaceutical intermediate synthesis, and in particular to a liquid phase synthesis method of a semaglutide dipeptide fragment. Background Art

[0002] Semaglutide, a long-acting glucagon-like peptide-1 (GLP-1) analog developed by Novo Nordisk of Denmark, can play a hypoglycemic role through multiple mechanisms such as stimulating pancreatic synthesis, secreting insulin and inhibiting glucagon secretion. In addition, it can inhibit appetite and reduce food intake by inhibiting the feeding center, while acting on the gastrointestinal tract to delay gastric emptying, thereby increasing satiety. Therefore, semaglutide not only has a significant hypoglycemic effect, but also has a significant weight loss effect. The injection for the treatment of type 2 diabetes in adults was launched in the United States in 2017 and was approved for marketing in China in 2021. In 2021, the FDA approved the indication for long-term weight management, which was approved for marketing in China in 2024. In 2023, Novo Nordisk's semaglutide sales reached US$21.055 billion, and sales in the first quarter of 2024 alone reached US$6.085 billion, and market demand remains strong. The sequence structure of semaglutide is as follows: H-His-Aib-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Val-Ser-Ser-Tyr-Leu-Glu-Gly-Gln-Ala-Ala-Lys(PEG-PEG-γ-Glu-OctadecanedioicAicd)-Glu-Phe-Lle-Ala-Trp-Leu-Val-Arg-Gly-Arg-Gly-OH.

[0003] Semaglutide is composed of a 31-amino acid main chain and a fatty acid side chain, with two amino acids His-Aib at the ends of the main chain (Aib is a non-natural amino acid). There are few reports on the synthesis of semaglutide terminal dipeptide fragments in the reported synthesis literature. The existing synthesis methods of semaglutide dipeptide fragment R1-His-Aib-OH are mainly divided into two categories: solid phase and liquid phase synthesis, as follows:

[0004] Patents CN106928343A, CN101133082A, CN106478806A, and CN109627317A provide solid-phase synthesis methods for semaglutide dipeptide fragments. The specific resins used in the solid-phase synthesis method are expensive, have low yields, many impurities, and are difficult to purify, resulting in extremely high costs. Patent CN112625087A mentions a liquid-phase synthesis method, but it is necessary to use a more expensive condensing agent and activator, which is costly and has residual by-products, posing risks to product quality and is not conducive to industrial production. Patent CN117659159A has an extremely long synthesis step and involves conversions between different N-protecting groups. The process is cumbersome and not conducive to industrial production. CN113667007A makes unnecessary modifications to the carboxyl group of 2-aminoisobutyric acid, complicating simple problems. CN113667006A uses thionyl chloride and high temperature reflux conditions to activate the His carboxyl group, which easily leads to amino acid racemization and increased impurity content, and is not suitable for the synthesis of amino acids.

[0005] In summary, the existing synthesis methods of semaglutide dipeptide fragments have long synthesis steps, long production cycles, low purity and yield, high production costs, and are not conducive to large-scale production; and the condensing agents and activators used will introduce corresponding impurities, posing risks to product quality. Therefore, it is urgent to develop a new synthesis method of semaglutide dipeptide fragments. Summary of the invention

[0006] In view of the above-mentioned shortcomings of the prior art, the object of the present invention is to provide a liquid phase synthesis method of semaglutide dipeptide fragment, which has a short synthesis route, simple operation, mild reaction conditions, high yield, and is suitable for industrial production.

[0007] In order to achieve the above-mentioned object and other related objects, the present invention provides a liquid phase synthesis method of semaglutide dipeptide fragment R1-His-Aib-OH, and the synthesis route is as follows:

[0008]

[0009] Wherein R1 is selected from Fmoc or Cbz, and R2 is selected from Trt or H; when R2 is H, the product dipeptide fragment R1-His-Aib-OH can be obtained in only one step;

[0010] The specific preparation steps are as follows:

[0011] (1) Add N-protected histidine (R1-L-His(R2)-OH), 2-aminoisobutyric acid and a base to an organic solvent 1, control the temperature at 0°C~30°C to react for 1 hour or more, then raise the temperature to 40°C~55°C, dropwise add propylphosphoric anhydride (T3P) or an organic solvent containing propylphosphoric anhydride, keep the temperature to react until the reaction is completed as monitored by TLC, cool to room temperature, adjust the pH to 7.5~8.5 with an alkaline solution, and then post-treat to obtain the intermediate R1-His(R2)-Aib-OH;

[0012] When R2 is H, the intermediate is the product dipeptide fragment R1-His-Aib-OH, and step (2) is not required;

[0013] (2) When R2 is Trt, the intermediate obtained in step (1) is added to an organic solvent 3, the temperature is controlled at -10°C to 10°C, trifluoroacetic acid is added, and after a period of reaction, the temperature is raised to room temperature and the reaction is continued until the reaction is completed as monitored by HPLC, and the concentrate is concentrated under reduced pressure. The concentrate is slurried with an organic solvent 4 under temperature control and dried to obtain the product dipeptide fragment R1-His-Aib-OH.

[0014] The preferred solution of step (1) is as follows:

[0015] The molar ratio of N-protected histidine to 2-aminoisobutyric acid is 1:1.0 to 2.0, and 1:1.2 is particularly preferred.

[0016] The base is selected from diisopropylethylamine, triethylamine or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).

[0017] The molar ratio of N-protected histidine to base is 1:1.2~2.0.

[0018] The organic solvent 1 is selected from tetrahydrofuran, acetonitrile, dimethyl sulfoxide or N,N-dimethylformamide, wherein tetrahydrofuran is particularly preferred.

[0019] The mass volume ratio of N-protected histidine to the organic solvent 1 is 1:1-10.

[0020] The molar ratio of N-protected histidine to T3P is 1:1.5 to 3.0, with 1:2.0 being particularly preferred.

[0021] The organic solvent containing propyl phosphoric anhydride is an ethyl acetate solution containing propyl phosphoric anhydride; among them, an ethyl acetate solution of propyl phosphoric anhydride with a concentration of 50% is particularly preferred.

[0022] The post-treatment process is as follows: the mixed solution is concentrated under reduced pressure, the organic solvent 2 is added to the concentrate for dissolution, and then washed once with a 5% citric acid aqueous solution and saturated brine in sequence, the organic phase is collected, the organic phase is dried over anhydrous sodium sulfate and then filtered, and the filtrate is concentrated under reduced pressure to obtain the intermediate R1-His(R2)-Aib-OH.

[0023] The organic solvent 2 is selected from at least one of dichloromethane, chloroform and ethyl acetate, among which dichloromethane is particularly preferred.

[0024] The mass volume ratio of N-protected histidine to the organic solvent 2 is 1:3-10.

[0025] The preferred solution of step (2) is as follows:

[0026] The organic solvent 3 is selected from tetrahydrofuran, dichloromethane, acetonitrile or N,N-dimethylformamide, wherein tetrahydrofuran is particularly preferred.

[0027] The mass volume ratio of the intermediate to the organic solvent 3 is 1:2-5.

[0028] The mass ratio of the intermediate to trifluoroacetic acid is 1:2.5~5.5.

[0029] The organic solvent 4 is at least one selected from n-hexane, n-heptane, cyclohexane, methyl tert-butyl ether, ethyl acetate and isopropanol.

[0030] The mass volume ratio of the intermediate to the organic solvent 4 is 1:1.5~20.

[0031] The beating temperature is -10℃~30℃.

[0032] The beating times were twice and the beating time was 0.5 hours.

[0033] As described above, the liquid phase synthesis method of a semaglutide dipeptide fragment of the present invention has the following beneficial effects:

[0034] 1. The present invention provides a new liquid phase preparation method of semaglutide dipeptide fragment R1-His-Aib-OH, which uses T3P, which is more economical, efficient, non-toxic and low in residue, as a condensation agent, and has milder reaction conditions, lower impurity content in the product and higher yield. At the same time, step (1) directly dehydrates and condenses 2-aminoisobutyric acid and N-protected histidine, without the need for unnecessary modification of the carboxyl group of 2-aminoisobutyric acid, shortening the reaction steps, simplifying the operation process, lowering the cost and being more conducive to industrial production.

[0035] 2. In the process of removing Trt in step (2) of the present invention, the organic solvent 3 is selected as the reaction medium, which can provide a milder and more efficient reaction environment, accelerate the removal of Trt, and reduce the temperature required for the reaction, thereby improving production efficiency, reducing costs, and achieving higher product purity and yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Shown is the H NMR spectrum of the dipeptide fragment Fmoc-His-Aib-OH prepared in Example 6 of the present invention.

[0037] Figure 2 Shown is the mass spectrum of the dipeptide fragment Fmoc-His-Aib-OH prepared in Example 6 of the present invention. DETAILED DESCRIPTION

[0038] The following is a description of the implementation of the present invention by means of specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0039] Example 1

[0040] Preparation of intermediate Fmoc-His(Trt)-Aib-OH:

[0041] Fmoc-L-His(Trt)-OH (10 g, 16.14 mmol), 2-aminoisobutyric acid (2.47 g, 23.95 mmol) and triethylamine (3.27 g, 32.32 mmol) were added into a 250 ml three-necked flask, and 30 ml of tetrahydrofuran was added. The temperature was controlled at 10°C~20°C and stirred for 1 h. The temperature was raised to 40°C~55°C, and a 50% ethyl acetate solution of T3P (20.54 g, 32.28 mmol) was added dropwise at a temperature control. The reaction was stirred for 5 h. After the reaction was complete after TLC monitoring, the temperature was lowered to room temperature, and the pH was adjusted to 7.5 with a sodium hydroxide aqueous solution. The concentrate was concentrated under reduced pressure, 40 ml of dichloromethane was added to the concentrate, and stirred to dissolve, and 20 ml of The mixture was washed once with 5% citric acid aqueous solution and 20 ml saturated brine, the organic phase was collected, dried over anhydrous sodium sulfate and filtered, and the filtrate was concentrated to dryness under reduced pressure to obtain 10.59 g of an off-white solid, i.e., the intermediate Fmoc-His(Trt)-Aib-OH, with a yield of 93.1% and a purity of 97.22%.

[0042] Example 2

[0043] Preparation of intermediate Fmoc-His(Trt)-Aib-OH:

[0044] Add Fmoc-L-His(Trt)-OH (10 g, 16.14 mmol), 2-aminoisobutyric acid (2.47 g, 23.95 mmol) and triethylamine (3.27 g, 32.32 mmol) into a 250 ml three-necked flask, add 100 ml of acetonitrile, control the temperature at 10℃~20℃ and stir to react for 1 h; raise the temperature to 40℃~55℃, control the temperature and add dropwise a 50% solution of T3P (30.81 g, 48.42 mmol) in ethyl acetate, stir to react for 3 h, and monitor the reaction completion by TLC. The mixture was cooled to room temperature and the pH was adjusted to 7.0 with aqueous sodium hydroxide solution; the mixture was concentrated under reduced pressure, 100 ml of ethyl acetate was added to the concentrate and stirred to dissolve, the mixture was washed once with 20 ml of 5% aqueous citric acid solution and 20 ml of saturated brine respectively, the organic phase was collected, dried over anhydrous sodium sulfate and filtered, and the filtrate was concentrated to dryness under reduced pressure to obtain 10.33 g of an off-white solid intermediate Fmoc-His(Trt)-Aib-OH with a yield of 90.8% and a purity of 95.17%.

[0045] Example 3

[0046] Preparation of intermediate Fmoc-His(Trt)-Aib-OH:

[0047] Fmoc-L-His(Trt)-OH (10 g, 16.14 mmol), 2-aminoisobutyric acid (3.32 g, 32.20 mmol) and diisopropylethylamine (3.34 g, 25.84 mmol) were added to a 250 ml three-necked flask, and 30 ml of N,N-dimethylformamide was added. The temperature was controlled at 0°C~10°C and stirred for 2 h. The temperature was raised to 40°C~55°C, and a 50% T3P (15.41 g, 48.43 mmol) ethyl acetate solution was added dropwise at a temperature control. The reaction was stirred for 7 h. After the reaction was complete after TLC monitoring, the temperature was lowered to room temperature, and the pH was adjusted to 8.5 with a sodium hydroxide aqueous solution. The concentrate was concentrated under reduced pressure, 30 ml of dichloromethane was added to the concentrate, and stirred to dissolve, and 20 ml of The mixture was washed once with 5% citric acid aqueous solution and 20 ml saturated brine, the organic phase was collected, dried over anhydrous sodium sulfate and filtered, and the filtrate was concentrated to dryness under reduced pressure to obtain 10.19 g of off-white solid intermediate Fmoc-His(Trt)-Aib-OH with a yield of 89.6% and a purity of 96.58%.

[0048] Example 4

[0049] Preparation of intermediate Fmoc-His(Trt)-Aib-OH:

[0050] Add Fmoc-L-His(Trt)-OH (10 g, 16.14 mmol), 2-aminoisobutyric acid (2.47 g, 23.95 mmol) and DBU (2.95 g, 19.38 mmol) into a 250 ml three-necked flask, add 30 ml of dimethyl sulfoxide, control the temperature at 20℃~30℃ and stir to react for 1.5 h; raise the temperature to 40℃~55℃, control the temperature and add dropwise a 50% T3P (20.54 g, 32.28 mmol) ethyl acetate solution, stir to react for 4 h, monitor the reaction by TLC, cool to room temperature after completion, and adjust the pH to 8.0 with sodium hydroxide aqueous solution. The mixture was concentrated under reduced pressure, 30 ml of dichloromethane was added to the concentrate and stirred to dissolve, the mixture was washed once with 20 ml of 5% citric acid aqueous solution and 20 ml of saturated brine respectively, the organic phase was collected, dried over anhydrous sodium sulfate and then filtered, and the filtrate was concentrated to dryness under reduced pressure to obtain 10.15 g of an off-white solid intermediate Fmoc-His(Trt)-Aib-OH with a yield of 89.2% and a purity of 95.62%.

[0051] Example 5

[0052] Preparation of intermediate Cbz-His(Trt)-Aib-OH:

[0053] Cbz-L-His(Trt)-OH (10 g, 18.81 mmol), 2-aminoisobutyric acid (2.85 g, 27.91 mmol) and triethylamine (3.80 g, 37.55 mmol) were added into a 250 ml three-necked flask, and 30 ml of tetrahydrofuran was added. The temperature was controlled at 10°C~20°C and stirred for reaction for 1 h; the temperature was raised to 40°C~55°C, and a 50% concentration of T3P (23.94 g, 37.62 mmol) ethyl acetate solution was added dropwise at a temperature controlled rate. After stirring for 5 h, the reaction was completed after TLC monitoring, and then the temperature was lowered to room temperature, and the pH was adjusted to 7.5 with aqueous sodium hydroxide solution. The mixture was concentrated under reduced pressure, 40 ml of dichloromethane was added to the concentrate and stirred to dissolve, and the mixture was washed once with 20 ml of 5% citric acid aqueous solution and 20 ml of saturated brine respectively. The organic phase was collected, dried over anhydrous sodium sulfate and filtered, and the filtrate was concentrated to dryness under reduced pressure to obtain 10.61 g of an off-white solid intermediate Cbz-His(Trt)-Aib-OH with a yield of 91.5% and a purity of 94.17%.

[0054] Example 6

[0055] Preparation of Fmoc-His-Aib-OH:

[0056] Take 10g of the intermediate Fmoc-His(Trt)-Aib-OH and 35ml of tetrahydrofuran and add them to a 250ml three-necked flask, cool to -5℃~5℃, add 40g of trifluoroacetic acid and react for 1h, warm to room temperature, react for 2h and monitor the reaction completion by HPLC; concentrate the system to dryness under reduced pressure, add 100ml of methyl tert-butyl ether, cool to -10℃~-5℃ and slurry for 0.5h; filter, slurry the filter cake with 100ml of methyl tert-butyl ether for 0.5h at -10℃~-5℃; filter, and vacuum dry the filter cake to obtain 6.24g of white powder, i.e., the product Fmoc-His-Aib-OH, with a yield of 95.09% and a purity of 99.72%.

[0057] Example 7

[0058] Preparation of Fmoc-His-Aib-OH:

[0059] Take 10g of the intermediate Fmoc-His(Trt)-Aib-OH and 20ml of dichloromethane and add them to a 250ml three-necked flask, cool to -10℃~10℃, add 25g of trifluoroacetic acid and react for 1.5h, warm to room temperature, react for 6h and monitor the reaction completion by HPLC; concentrate the system to dryness under reduced pressure, add 15ml of isopropanol, cool to -10℃~-5℃ and slurry for 0.5h; filter, slurry the filter cake with 15ml of isopropanol for 0.5h at -10℃~-5℃; filter, and vacuum dry the filter cake to obtain 6.06g of white powder product Fmoc-His-Aib-OH with a yield of 92.35% and a purity of 99.41%.

[0060] Example 8

[0061] Preparation of Fmoc-His-Aib-OH:

[0062] Take the intermediate Fmoc-His(Trt)-Aib-OH (10g, 0.014mol) and 50ml acetonitrile, add them into a 250ml three-necked flask, cool to 0℃~10℃, add 55g trifluoroacetic acid and react for 1h, warm to room temperature, react for 1.5h and monitor the reaction completion by HPLC; concentrate the system to dryness under reduced pressure, add 200ml n-hexane, cool to -10℃~0℃ and slurry for 0.5h; filter, slurry the filter cake with 200ml n-hexane for 0.5h at -10℃~0℃; filter, and vacuum dry the filter cake to obtain 6.11g white powder product Fmoc-His-Aib-OH with a yield of 93.11% and a purity of 98.53%.

[0063] Example 9

[0064] Preparation of Cbz-His-Aib-OH:

[0065] Take 10g of Cbz-His(Trt)-Aib-OH prepared in Example 5 and 35ml of tetrahydrofuran, add them into a 250ml three-necked flask, cool to -5℃~5℃, add 40g of trifluoroacetic acid and react for 1h, warm to room temperature, react for 2h and monitor the reaction completion by HPLC; concentrate the system to dryness under reduced pressure, add 100ml of methyl tert-butyl ether, cool to -10℃~-5℃ and beat for 0.5h; filter, beat the filter cake with 100ml of methyl tert-butyl ether for 0.5h, temperature -10℃~-5℃; filter, and vacuum dry the filter cake to obtain 5.69g of white powder product Cbz-His-Aib-OH, with a yield of 93.73% and a purity of 98.25%.

[0066] Example 10

[0067] One-step preparation of Fmoc-His-Aib-OH:

[0068] Fmoc-L-His-OH (5 g, 13.25 mmol), 2-aminoisobutyric acid (2.11 g, 20.46 mmol) and triethylamine (2.68 g, 26.48 mmol) were added into a 250 ml three-necked flask, and 30 ml of tetrahydrofuran was added. The temperature was controlled at 10°C~20°C and stirred for 1 h. The temperature was raised to 40°C~55°C, and 50% T3P ethyl acetate solution (13.49 g, 21.20 mmol) was added dropwise at a temperature of 13.49 g, 21.20 mmol. The temperature was stirred for 4 h. After the reaction was complete after monitoring by TLC, the temperature was lowered to room temperature, and the pH was adjusted to 7.5 with aqueous sodium hydroxide solution. The concentrate was concentrated under reduced pressure, 40 ml of dichloromethane was added to the concentrate, and stirred to dissolve, and 20 ml of each was added. The mixture was washed once with 5% citric acid aqueous solution and 20 ml saturated brine, the organic phase was collected, dried over anhydrous sodium sulfate and filtered, the filtrate was concentrated to dryness under reduced pressure, 100 ml of methyl tert-butyl ether was added to the concentrate, the temperature was lowered to -10°C ~ -5°C and slurrying was performed for 0.5 h; filtered, the filter cake was slurried with 100 ml of methyl tert-butyl ether for 0.5 h at a temperature of -10°C ~ -5°C; filtered, and the filter cake was vacuum dried to obtain 5.51 g of off-white solid Fmoc-His-Aib-OH with a yield of 89.92% and a purity of 96.71%.

[0069] In summary, the present invention provides a new liquid phase preparation method of semaglutide dipeptide fragment R1-His-Aib-OH. In step (1), a more economical, efficient, non-toxic and low-residue T3P is used as a condensing agent, the reaction conditions are milder, the product impurity content is lower and the yield is higher; at the same time, step (1) directly dehydrates and condenses 2-aminoisobutyric acid and N-protected histidine, without unnecessary modification of the carboxyl group of 2-aminoisobutyric acid, shortening the reaction steps, simplifying the operation process, lowering the cost and being more conducive to industrial production; in step (2), an organic solvent 3 is selected as a reaction medium to directly remove Trt from the intermediate, accelerate the removal of Trt, and reduce the temperature required for the reaction, thereby improving production efficiency and reducing costs. Therefore, the present invention effectively overcomes various shortcomings in the prior art and has a high industrial utilization value.

[0070] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A liquid phase synthesis method of semaglutide dipeptide fragment, characterized in that: The synthetic route is as follows: wherein R1 is selected from Fmoc or Cbz, and R2 is selected from Trt or H; The specific preparation steps are as follows: (1) Add N-protected histidine, 2-aminoisobutyric acid and a base to an organic solvent 1, control the temperature at 0°C to 30°C to react for 1 hour or more, then raise the temperature to 40°C to 55°C, dropwise add propyl phosphoric anhydride or an ethyl acetate solution containing propyl phosphoric anhydride, keep the temperature to react until the reaction is completed as monitored by TLC, cool to room temperature, adjust the pH to 7.5 to 8.5 with an alkaline solution, and then concentrate the mixed solution under reduced pressure. Add organic solvent 2 to the concentrate to dissolve it, and then wash it once with a 5% citric acid aqueous solution and saturated brine in sequence, collect the organic phase, dry the organic phase over anhydrous sodium sulfate, and then filter it. The filtrate is concentrated under reduced pressure to obtain the intermediate R1-His(R2)-Aib-OH; Wherein: the molar ratio of N-protected histidine to 2-aminoisobutyric acid is 1:1.0-2.0; the base is selected from diisopropylethylamine, triethylamine or 1,8-diazabicyclo[5.4.0]undec-7-ene, and the molar ratio of N-protected histidine to the base is 1:1.2-2.0; the organic solvent 1 is selected from tetrahydrofuran, acetonitrile, dimethyl sulfoxide or N,N-dimethylformamide, and the mass volume ratio of N-protected histidine to organic solvent 1 is 1:1-10; the molar ratio of N-protected histidine to propylphosphoric anhydride is 1:1.5-3.0; the organic solvent 2 is selected from at least one of dichloromethane, chloroform and ethyl acetate, and the mass volume ratio of N-protected histidine to organic solvent 2 is 1:3-10; When R2 is H, the intermediate is the product dipeptide fragment R1-His-Aib-OH, and step (2) is not required; (2) When R2 is Trt, the intermediate obtained in step (1) is added to an organic solvent 3, the temperature is controlled at -10°C to 10°C, trifluoroacetic acid is added, and after a period of reaction, the temperature is raised to room temperature and the reaction is continued until the reaction is completed as monitored by HPLC, and the concentrate is concentrated under reduced pressure. The concentrate is slurried with an organic solvent 4 under temperature control and dried to obtain the product dipeptide fragment R1-His-Aib-OH; Wherein: the organic solvent 3 is selected from tetrahydrofuran, dichloromethane, acetonitrile or N,N-dimethylformamide, and the mass volume ratio of the intermediate to the organic solvent 3 is 1:2-5; the mass ratio of the intermediate to trifluoroacetic acid is 1:2.5-5.5; the organic solvent 4 is selected from at least one of n-hexane, n-heptane, cyclohexane, methyl tert-butyl ether, ethyl acetate and isopropanol, and the mass volume ratio of the intermediate to the organic solvent 4 is 1:1.5-20.

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