Method for synthesizing S-(4-thiophenyl butyl) cysteine methyl ester derivative

Abstract

The invention relates to the technical field of compound synthesis, in particular to a method for synthesizing an S-(4-thiophenyl butyl) cysteine methyl ester derivative. The preparation method comprises the following steps: mixing an aryl tetrahydrothiophene sulfonium salt derivative, a cysteine derivative, an additive and a solvent, and reacting in an inert atmosphere to obtain the S-(4-thiophenyl butyl) cysteine methyl ester derivative, the synthesis method provided by the invention can be smoothly carried out at room temperature under the action of mild carbonate alkali without high-temperature heating, transition metal catalysis and addition of an oxidizing agent or a strong electrophilic / strong alkali system, so that the risk of side reaction and impurity introduction is reduced from the source. In addition, the system is based on acetonitrile and other conventional solvents and cheap and easily available carbonate base, the operation is simple and convenient, the post-treatment burden is light, and greenness and economical efficiency are embodied; meanwhile, according to the method, a target C-S bond is constructed in one step, a key side chain fragment is introduced, the steps are short, conversion is direct, and good economical efficiency and amplification potential are achieved.

Description

Technical Field

[0001] This invention relates to the field of compound synthesis technology, and in particular to a method for synthesizing S-(4-phenylthiobutyl)cysteine ​​methyl ester derivatives. Background Technology

[0002] Selective modification of the S-site of the side chain of sulfur-containing amino acids (especially cysteine) is a key synthetic operation in peptide coupling, prodrug design, chemical biology probe construction, and drug molecule structure optimization. Existing techniques for preparing S-(4-arylthiobutyl) cysteine ​​derivatives mainly employ the following approaches: First, alkylating cysteine ​​thiols with strong electrophilic reagents such as 4-halobutyl or 4-sulfonate butyl groups; second, constructing a butyl chain from 1,4-dihalobutane, then coupling it with an arylthiol to generate a 4-arylthiobutylating agent, which then reacts with cysteine; third, introducing four-carbon chain fragments via indirect routes such as epoxide ring-opening and Michael addition. While these methods can achieve the construction of the target structure, they often reveal significant limitations in amino acid / peptide systems containing multiple functional groups: First, the reactions often depend on highly reactive halogens or harsh conditions, easily initiating N-alkylation, elimination, side reactions, and potential racemization, leading to insufficient selectivity; second, cysteine ​​thiols are easily oxidized in air to form disulfides, causing a decrease in conversion rate and batch-to-batch consistency; third, the use of aryl thiols, haloalkanes, and other raw materials often involves irritating odors, toxicity, and safety risks, and produces a large number of inorganic salt byproducts, resulting in a heavy burden of post-processing and purification; fourth, some routes involve many steps, harsh conditions, or require metal / oxidation systems, which are not conducive to green and scale-up preparation.

[0003] Recent research progress: Luo's group reported in ACS Catalysis (2025) a La(OTf)3 / ortho-quinone co-catalyzed aerobic oxidation system of thiols inspired by rare earth-dependent methanol dehydrogenase (Ln-MDH). This system can rapidly oxidize cysteine ​​derivatives to disulfides at room temperature with O2 / air as the terminal oxidant, and can achieve efficient disulfide bond construction of cysteine-containing peptides in a buffer system, demonstrating good applicability to biologically relevant substrates. Its advantages lie in its mild reaction, fast speed, simple conditions, and certain functional group tolerance; its main limitation is its strong system dependence—usually requiring a synergistic "La salt + quinone + base (such as tBuOK)" approach—and its sensitivity to solvent and oxygen supply; efficiency decreases significantly under milder alkaline or inert atmospheres.

[0004] Congde Huo's group reported a visible light-driven copper-mediated S-arylation method for cysteine ​​in Organic Letters (2025). Using arylthianthaneonium salt as the aryl source, site-selective thioetherification of cysteine ​​derivatives and some peptides was achieved under Cu(acac)2 / K3PO4, MeCN, and room temperature conditions. The method is characterized by mild conditions, no need for external ligands, good substrate / aryl source applicability, and can be extended to water / alcohol or buffer systems. Its main drawbacks are that the system is significantly condition-dependent (requires light, synergy of copper salt and base), relatively high copper dosage, and sensitivity to oxygen (yield decreases in the presence of oxygen, usually requiring an inert atmosphere). Long peptide substrates may also be affected by solubility limitations, which may affect the yield.

[0005] Existing cysteine ​​(Cys) thiols derivatization generally follows two main routes: electrophilic alkylation and Michael addition. The former (typically haloalkanes / iodoacetamides, etc.) often requires alkaline conditions, longer reaction times, and even higher temperatures and excess reagents to achieve "complete conversion." However, these "stronger conditions" significantly amplify the risk of nonspecific alkylation (such as misalkylation of N-terminal / lysine / histidine nuclide sites), leading to purification burdens and batch-to-batch variations. On the other hand, while maleimide has advantages such as fast kinetics and simple operation, its thiols addition products suffer from stability issues such as retro-Michael-triggered thiols exchange and hydrolysis in physiological / sulfur-containing environments, resulting in "reversible / easily transferable" coupling bonds. This is one of the core driving forces behind the continued research by organic synthetic chemists.

[0006] Therefore, there is an urgent need to develop a mild, highly site-selective, functional group-compatible, safe, and easily scalable S-functionalization synthesis method to meet the practical needs of sulfur-containing amino acid derivatives in medicinal chemistry and peptide modification, and to shift Cys-S functionalization from a "strongly electrophilic / strongly conditional / unstable adduct" to a "mild, controllable, functional group-compatible, and more stable product bond structure" synthetic paradigm. Summary of the Invention

[0007] The purpose of this invention is to provide a method for synthesizing S-(4-phenylthiobutyl)cysteine ​​methyl ester derivatives. The traditional multi-step construction of 4-(arylthio)butyl is transformed into a single-step strategy using an aryl-substituted thioonium salt (thiocyclic thioonium salt) as the centralized "butylation / ring-opening" reagent. This method achieves selective nucleophilic ring-opening / substitution of the thioonium salt by generating sulfonium anions in situ in a polar aprotic solvent using a mild carbonate base. This allows for the efficient construction of the target S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative under metal-free and oxidant-free conditions at room temperature. This method specifically addresses problems in existing technologies such as insufficient selectivity, yield fluctuations due to thiol oxidation, raw material safety concerns, and heavy post-processing burdens.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution: One of the technical solutions of this invention provides a method for preparing an S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative, comprising the following steps: Aryltetrahydrothiophene sulfonium salt derivative, cysteine ​​derivative, additive and solvent are mixed and reacted under an inert atmosphere to obtain S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative. The structural formula of the aryltetrahydrothiophene sulfonium salt derivative is shown in Formula I: Formula I; Among them, R 1 Selected from 4-methyl, 2,4-dimethyl or 4-tert-butyl; The structural formula of the cysteine ​​derivative is shown in Formula II: Formula II; Among them, R 2 It is selected from either acetyl or tert-butyloxycarbonyl.

[0009] The second technical solution of the present invention provides an S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative prepared by the above preparation method, characterized in that it has the structure shown in Formula III: Formula III; Among them, R 1 Selected from one of 4-methyl, 2,4-dimethyl, or 4-tert-butyl, R 2 It is selected from either acetyl or tert-butyloxycarbonyl.

[0010] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a method for synthesizing S-(4-phenylthiobutyl)cysteine ​​methyl ester derivatives via a base-promoted ring-opening coupling reaction. An aryl-substituted thioonium salt is used as the electroactivated 4-(arylthio)butylating agent. The reaction can proceed smoothly at room temperature under mild carbonate alkali conditions, without the need for high-temperature heating, transition metal catalysis, or the addition of external oxidants or strong electrophilic / strong base systems, thus reducing the risk of side reactions and impurity introduction from the outset. Since the thioanion formed by the deprotonation of cysteine ​​thiols is the key active species, nucleophilic ring-opening / substitution of the thioonium salt preferentially occurs, exhibiting excellent S-site selectivity and functional group compatibility. It shows good tolerance to common sensitive groups such as ester groups and amide bonds, and can effectively suppress thiol oxidation side reactions under an inert atmosphere, improving yield and batch consistency. Furthermore, the system is based on conventional solvents such as acetonitrile and inexpensive and readily available carbonate bases, making it simple to operate and requiring minimal post-processing. It reduces metal residues and oxidation byproducts, demonstrating its greenness and economy. At the same time, this method constructs the target C–S bond and introduces key side chain fragments in one step, resulting in a short and direct conversion. It has good atom economy and scale-up potential, making it suitable for the efficient synthesis and functionalization modification of sulfur-containing amino acids and related derivatives. Attached Figure Description

[0011] Figure 1 The proton NMR spectrum of compound 3aa prepared in Example 1; Figure 2 The carbon spectrum of compound 3aa prepared in Example 1; Figure 3 The proton NMR spectrum of compound 3ba prepared in Example 1; Figure 4 The carbon spectrum of compound 3ba prepared in Example 1; Figure 5 The proton NMR spectrum of compound 3ca prepared in Example 1; Figure 6 The carbon spectrum of compound 3ca prepared in Example 1; Figure 7 The proton NMR spectrum of compound 3ab prepared in Example 1; Figure 8 The carbon spectrum of compound 3ab prepared in Example 1. Detailed Implementation

[0012] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0013] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0014] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0015] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0016] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0017] All room temperatures mentioned in this invention are calculated as 25±1℃.

[0018] All raw materials used in this invention can be obtained commercially or prepared using existing technologies.

[0019] This invention provides a method for preparing an S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative, comprising the following steps: Aryltetrahydrothiophene sulfonium salt derivative, cysteine ​​derivative, additive and solvent are mixed and reacted under an inert atmosphere to obtain S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative. The structural formula of the aryltetrahydrothiophene sulfonium salt derivative is shown in Formula I: Formula I; Among them, R 1 Selected from 4-methyl, 2,4-dimethyl or 4-tert-butyl; The structural formula of the cysteine ​​derivative is shown in Formula II: Formula II; Among them, R 2 It is selected from either acetyl or tert-butyloxycarbonyl.

[0020] The innovation of this invention lies in the fact that the cyclic thioonium salt is designed as a programmable transfer reagent for the 4-(phenylthio)butyl fragment. By utilizing the high electroactivation characteristics brought by the positive charge of the thioonium salt, it can undergo selective nucleophilic ring opening / substitution of thiol sites under the action of mild carbonate base. The target Cys-S-(CH2)4-SPh bond type can be directly constructed at room temperature, without metal catalysis and without external oxidant. The key advantages of this strategy compared to traditional routes are: ① The reaction driving force comes from the intrinsic reactivity of thioonium salts, which are characterized by "excellent leaving groups" and "highly attackable by nuclides." Thionium salts themselves typically possess good thermodynamic stability and operability, thus achieving a "low energy consumption + high selectivity" S-functionalization window; ② The formed thioether bond is more structurally stable than the maleimide addition bond, which can mechanistically avoid unstable sources such as retro-Michael / thiol exchange; ③ The target side chain fragment is introduced in one step, reducing the resource consumption caused by multiple steps of constructing electrophilic reagents and multiple purifications, forming a clearly articulated patentable advantage in terms of "step economy / atom utilization / green process"; at the same time, since the system does not depend on light / metal / oxidation systems, it naturally has better scale-up controllability, reduces metal residues, and avoids oxidation side reactions.

[0021] In this invention, the cysteine ​​derivative was purchased from Energie Chemicals.

[0022] In this invention, the reaction equation for the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative is as follows: .

[0023] In this invention, the additives include sodium bicarbonate, potassium carbonate, cesium carbonate, or sodium hydroxide, preferably potassium carbonate.

[0024] In this invention, the solvent includes 1,2-dichloroethane, nitromethane, or acetonitrile, preferably acetonitrile.

[0025] In this invention, the reaction temperature is 25~50℃, for example, it can be 25℃, 30℃, 35℃, 40℃, 45℃ or 50℃, etc., and the time is 2~5h, for example, it can be 2h, 3h, 4h or 5h.

[0026] In this invention, the molar ratio of the aryltetrahydrothiophene sulfonium salt derivative, the cysteine ​​derivative and the additive is 1:1 to 3:1 to 4, for example, it can be 1:1:1, 1:2:3:1:2:1, 1:2:4, 1:3:1, 1:3:2, 1:3:4 or 1:2:2, etc.

[0027] In this invention, the inert atmosphere includes nitrogen.

[0028] In this invention, the ratio of aryltetrahydrothiophene sulfonium salt derivative to solvent is 0.2 mmol: 1~5 mL, preferably 0.2 mmol: 2 mL.

[0029] In this invention, the reactants are obtained after the reaction, and the reactants are separated and purified by silica gel column chromatography to obtain S-(4-phenylthiobutyl)cysteine ​​methyl ester derivatives.

[0030] In this invention, the mobile phase used for silica gel column chromatography separation and purification is petroleum ether and ethyl acetate; the volume ratio of petroleum ether to ethyl acetate is 10:1.

[0031] The present invention also provides an S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative prepared by the above preparation method, having the structure shown in Formula III: Formula III; Among them, R 1 Selected from one of 4-methyl, 2,4-dimethyl, or 4-tert-butyl, R 2 It is selected from either acetyl or tert-butyloxycarbonyl.

[0032] The S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative prepared in this invention has broad application prospects in key synthetic operations such as peptide coupling, prodrug design, chemical biological probe construction, and drug molecule structure optimization.

[0033] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0034] Example 1 In a ground glass joint reaction tube, 1a (0.2 mmol), potassium carbonate (0.4 mmol), acetonitrile (2 mL), a magnetic ball, and 2a (0.4 mmol) were added sequentially. Nitrogen gas was introduced, and the reaction was carried out at room temperature for 4 h. The reaction progress was monitored by thin-layer chromatography until the reaction was complete. Finally, the mixture was purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1) to obtain compound 3aa in 78% yield.

[0035] The reaction equation is as follows: .

[0036] Spectral data of the product: 1 H NMR (400 MHz, CDCl3, ppm): δ 7.24 (d, J = 8.3 Hz, 2 H), 7.09 (d, J= 8.4 Hz, 2 H), 6.49 (d, J = 7.6 Hz, 1 H), 4.83-4.78 (m, 1 H), 3.74 (s, 3 H),2.97-2.94 (m, 2 H), 2.86 (t, J = 7.2 Hz, 2 H), 2.50 (t, J = 2.8 Hz, 2 H),2.31 (s, 3 H), 2.03 (s, 3 H), 1.70-1.66 (m, 4 H); 13 C NMR (100 MHz, CDCl3, ppm): δ 171.4, 169.9, 136.1, 132.5, 130.1, 129.7, 52.6, 51.9, 34.1, 33.9,32.1, 28.4, 28.1, 23.1, 21.0.

[0037] Example 2 In a ground glass joint reaction tube, 1b (0.2 mmol), potassium carbonate (0.4 mmol), acetonitrile (2 mL), a magnetic ball, and 2a (0.4 mmol) were added sequentially. Nitrogen gas was introduced, and the reaction was carried out at room temperature for 4 h. The reaction progress was monitored by thin-layer chromatography until the reaction was complete. Finally, the compound 3ba was purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1) with a yield of 90%.

[0038] The reaction equation is as follows: .

[0039] Spectral data of the product: 1H NMR (600 MHz, CDCl3, ppm): δ 7.21 (d, J = 7.8 Hz, 1 H), 7.03 (s, 1H), 6.99 (d, J = 8.4 Hz, 1 H), 6.28 (d, J = 7.2 Hz, 1 H), 4.85-4.82 (m, 1 H),3.79 (s, 3 H), 3.04-2.96 (m, 2 H), 2.86 (t, J = 6.6 Hz, 2 H), 2.54 (t, J =6.6 Hz, 2 H), 2.37 (s, 3 H), 2.31 (s, 3 H), 2.07 (s, 3 H), 1.74-1.71 (m, 4H); 13 C NMR (150 MHz, CDCl3, ppm): δ 171.5, 169.9, 138.3, 136.1, 132.0, 131.0,129.4, 127.3, 52.8, 52.0, 34.4, 33.2, 32.4, 28.7, 28.2, 23.3, 21.0, 20.5.

[0040] Example 3 In a ground glass joint reaction tube, 1c (0.2 mmol), potassium carbonate (0.4 mmol), acetonitrile (2 mL), a magnetic ignition element, and 2a (0.4 mmol) were added sequentially. Nitrogen gas was introduced, and the reaction was carried out at room temperature for 4 h. The reaction progress was monitored by thin-layer chromatography until the reaction was complete. Finally, the compound 3ca was purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1) with a yield of 90%.

[0041] The reaction equation is as follows: .

[0042] Spectral data of the product: 1H NMR (600 MHz, CDCl3, ppm): δ 7.33 (d, J = 8.4 Hz, 2 H), 7.29 (d, J= 8.4 Hz, 2 H), 6.29 (s, 1 H), 4.85-4.82 (m, 1 H), 3.78 (s, 3 H), 3.04-2.97(m, 2 H), 2.91 (t, J = 6.6 Hz, 2 H), 2.54 (t, J = 7.2 Hz, 2 H), 2.06 (s, 3H), 1.73 (t, J = 6.6 Hz, 4 H), 1.33 (s, 9 H); 13 C NMR (150 MHz, CDCl3, ppm): δ171.5, 169.9, 149.5, 132.9, 129.6, 126.1, 52.8, 52.0, 34.6, 34.3, 33.8, 32.3,31.4, 28.6, 28.3, 23.3.

[0043] Example 4 In a ground glass joint reaction tube, 1a (0.2 mmol), potassium carbonate (0.4 mmol), acetonitrile (2 mL), a magnetic ball, and 2b (0.4 mmol) were added sequentially. Nitrogen gas was introduced, and the reaction was carried out at room temperature for 4 h. The reaction progress was monitored by thin-layer chromatography until the reaction was complete. Finally, the mixture was purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1) to obtain compound 3ab in 77% yield.

[0044] The reaction equation is as follows: .

[0045] Spectral data of the product: 1 H NMR (400 MHz, CDCl3, ppm): δ 7.24 (d, J = 8.4 Hz, 2 H), 7.10 (d, J= 7.6 Hz, 2 H), 5.36 (d, J = 8.0 Hz, 1 H), 4.54-4.50 (m, 1 H), 3.75 (s, 3 H),2.94 (t, J = 4.8 Hz, 2 H), 2.87 (t, J = 4.2 Hz, 2 H), 2.52 (t, J = 7.2 Hz, 2H), 2.32 (s, 3 H), 1.71-1.67 (m, 4 H), 1.45 (s, 9 H); 13C NMR (100 MHz, CDCl3,ppm): δ 171.7, 155.2, 136.2, 132.6, 130.2, 129.8, 80.2, 53.3, 52.6, 34.5,34.0, 32.2, 28.5, 28.4, 28.2, 21.1.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing an S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative, characterized in that, Includes the following steps: Aryltetrahydrothiophene sulfonium salt derivative, cysteine ​​derivative, additive and solvent are mixed and reacted under an inert atmosphere to obtain S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative. The structural formula of the aryltetrahydrothiophene sulfonium salt derivative is shown in Formula I: Formula I; Among them, R 1 Selected from 4-methyl, 2,4-dimethyl or 4-tert-butyl; The structural formula of the cysteine ​​derivative is shown in Formula II: Formula II; Among them, R 2 It is selected from either acetyl or tert-butyloxycarbonyl.

2. The method for preparing the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative according to claim 1, characterized in that, The additives include sodium bicarbonate, potassium carbonate, cesium carbonate, or sodium hydroxide.

3. The method for preparing the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative according to claim 1, characterized in that, The solvent includes 1,2-dichloroethane, nitromethane, or acetonitrile.

4. The method for preparing the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative according to claim 1, characterized in that, The reaction is carried out at a temperature of 25-50°C for 2-5 hours.

5. The method for preparing the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative according to claim 1, characterized in that, The molar ratio of the aryltetrahydrothiophene sulfonium salt derivative, the cysteine ​​derivative, and the additive is 1:1~3:1~4.

6. The method for preparing the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative according to claim 1, characterized in that, The reactants were obtained after the reaction and purified by silica gel column chromatography to obtain S-(4-phenylthiobutyl)cysteine ​​methyl ester derivatives.

7. The method for preparing the S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative according to claim 6, characterized in that, The mobile phase used for silica gel column chromatography separation and purification was petroleum ether and ethyl acetate; the volume ratio of petroleum ether to ethyl acetate was 10:

1.

8. The S-(4-phenylthiobutyl)cysteine ​​methyl ester derivative prepared by the method according to any one of claims 1 to 7 is characterized in that, It has the structure shown in Equation III: Formula III; Among them, R 1 Selected from one of 4-methyl, 2,4-dimethyl, or 4-tert-butyl, R 2 It is selected from either acetyl or tert-butyloxycarbonyl.

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