A method for preparing lanamivir

Abstract

This invention discloses a method for preparing lanamivir I. The method includes: starting with N-acetylneuraminic acid, esterification to obtain II, followed by acetylation and cyclization without separation to obtain III; then deacetylation to obtain IV, followed by carbonate esterification to obtain V; etherification to obtain VI, followed by azide ring-opening to obtain VII; azide reduction to obtain VIII, followed by guanidinolation to obtain IX; then ester hydrolysis to obtain X; and finally deprotection of the Boc protecting group to obtain lanamivir I, with an overall yield of 25-46%. This invention uses thionyl chloride to catalyze methyl esterification, ensuring complete reaction without separation and allowing direct use in the next step; it utilizes trimethylsilyl trifluoromethanesulfonate as a catalyst to achieve a one-step acetylation and cyclization reaction, simplifying the operation; the azide ring-opening reaction conditions are mild, requiring no strict temperature control, and the yield can reach 88.9%; simultaneously, a crystallization purification step is introduced after guanidinolation, making the intermediate quality controllable, and the final product yield can reach over 85%.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis technology and relates to methods for preparing drugs and intermediates, specifically to a method for preparing the anti-influenza virus drug lanamivir and its intermediates. Background Technology

[0002] Among numerous viral infectious diseases, influenza is one of the most prevalent, geographically widespread, and fatal infectious diseases. Caused by the influenza virus, influenza is a respiratory infection. The H1N1 influenza outbreak in April 2009 cast a shadow over the entire world. It is evident that influenza, as a viral infectious disease, not only seriously threatens public health but also imposes a heavy economic burden on nations and societies. Therefore, the development of anti-influenza drugs has become a hot research topic.

[0003] Lanamivir is a novel neuraminidase inhibitor with significant inhibitory effects against both influenza A and B viruses, and can be used for the treatment and prevention of influenza. Compared with traditional anti-influenza drugs, lanamivir has advantages such as longer duration of action, significant efficacy, and lower likelihood of developing drug resistance, and has broad market prospects. Clinical studies have shown that lanamivir has stronger inhibitory activity against influenza viruses than oseltamivir, and also has some inhibitory effect on oseltamivir-resistant virus strains. In addition, lanamivir has a long half-life, requiring only once-daily dosing, resulting in better patient compliance. The chemical structure of lanamivir is as follows:

[0004] Currently, several synthetic routes for lanamivir have been reported both domestically and internationally. These routes start from different raw materials and construct the complete lanamivir skeleton and multiple chiral centers through a series of reactions. For example, the synthetic route reported by Daiichi Sankyo's research group: starting from acetylglucosamine, lanamivir is synthesized in 16 steps with an overall yield of approximately 20% (see Bioorg. Med. Chem. Lett. 2002, 12, 1921-1924); the synthetic route reported by Ma Dawei's research group: using (Z)-BocNH-substituted nitroolefins as raw materials, lanamivir is synthesized in more than 12 steps through cyclization reactions under the action of Jacobsen chiral thiourea catalyst, with an overall yield of 18% (see Angew. Chem. 2014, 126, 1-5). Both of these synthetic routes are too long and have low overall yields, making them unsuitable for industrial production. Naturally occurring sialic acid (N-acetylneuraminic acid) is inexpensive and readily available. Furthermore, the route for synthesizing lanamivir using sialic acid as a starting material through functional group transformation and modification is relatively short, making it suitable for industrial production. For example, the synthetic route disclosed in patent WO 2008 / 126943 is shown below:

[0005] This synthetic route starts with sialic acid 1, esterifies it to obtain compound 2, fully acetylates it, then cyclizes it to obtain compound 3, deacetylates it to obtain compound 4, carbonates it to obtain compound 5, etherifies it to obtain compound 6, azido ring-opening it to obtain compound 7, azido reduction and ester hydrolysis to obtain compound 8, guanidinolation it to obtain compound 9, and deprotecting it to obtain lanamivir. The entire synthetic route involves 9 transformations with an overall yield of approximately 37%. However, this synthetic route still has some limitations and needs to be improved to make it more suitable for industrial production. For example: (a) When sulfuric acid is used as a catalyst, the esterification of sialic acid 1 is difficult to complete, and dibutyl ether is used in the post-processing. Dibutyl ether is highly flammable and volatile, and its vapor poses an explosion risk when mixed with air; (b) When compound 2 is converted into compound 3, concentrated sulfuric acid is used as a catalyst, which will produce oxazole ring-opening byproducts, and toluene, which is highly toxic, is required in the post-processing; (c) The reaction of compound 4 to compound 5 is difficult to complete; (d) The process of compound 5 to compound 7 requires strict control of the reaction temperature; (e) In the process of compound 7 to compound 9, intermediate product 8 is fed in the form of an aqueous solution, and its quality is difficult to control, resulting in more side reactions. Summary of the Invention

[0006] This invention provides a synthetic method for preparing lanamivir from N-acetylneuraminic acid as a starting material. Compared with the synthetic route disclosed in patent WO 2008 / 126943, it has the following advantages: (a) using thionyl chloride as a catalyst, the esterification reaction is almost completely converted, and the crude product can be directly proceeded to the next step without additional post-treatment; (b) using trimethylsilyl trifluoromethanesulfonate as a catalyst and reacting with excess acetic anhydride at a higher temperature can achieve acetylation and cyclization in one step, and the post-treatment uses the safer ethyl acetate; (c) using a mixed solvent of anhydrous methanol and tetrahydrofuran, the starting material is completely converted during carbonate esterification; (d) when using the experimental method of this invention for azido ring-opening, the reaction can be carried out at a wider reaction temperature without strict temperature control, and the yield can reach 88.9%; (e) after azido amination, this invention first performs guanidinolation and then hydrolysis ring-opening. The guanidinolation product can be purified by crystallization, making the quality of the product at each step of the route controllable. The yield of lanamivir obtained using this intermediate is above 85%, which is higher than that of patent WO 2008 / 126943. 71.4% of 2008 / 126943. The route provided by this invention has the advantages of inexpensive and readily available reagents, mild reaction conditions, simple and easy operation, and good economic benefits, thus providing a new method for the synthesis of lanamivir.

[0007] To achieve the objectives of this invention, the following technical means are employed:

[0008] Adopting attachment Figure 1 The illustrated technical route for preparing lanamivir includes: using N-acetylneuraminic acid as a starting material, methylating it under thionyl chloride conditions, followed by a one-step reaction with acetic anhydride and trimethylsilyl trifluoromethanesulfonate to obtain an oxazoline compound; removing the acetyl group from this compound with sodium methoxide, then protecting the hydroxyl groups at positions 8 and 9 with dimethyl carbonate, and simultaneously introducing a methyl group with dimethyl sulfate while removing the hydroxyl hydrogen at position 7 using sodium hydride; introducing an azide group with trimethylsilane, then reducing the azide group with triphenylphosphine to generate an amino compound, then introducing a guanidine group with N,N'-di-tert-butoxycarbonyl-1-guanidinopyrazole, followed by removing the hydroxyl and carboxyl groups at positions 8 and 9 under alkaline conditions, and finally removing the tert-butoxycarbonyl group from the guanidine group to obtain lanamivir.

[0009] The specific implementation steps of this invention are described as follows:

[0010] 1. N-acetylneuraminic acid is methyl esterified to yield N-acetyl-D-neuraminic acid methyl ester (i.e., compound II):

[0011] First, N-acetylneuraminic acid was suspended in 5–20 (w / v) times its volume of anhydrous methanol and stirred under ice-salt bath cooling. Then, 0.10–0.50 equivalents of thionyl chloride were slowly added dropwise to the reaction solution, and the mixture was stirred for 10 minutes while maintaining the original temperature. The temperature was then raised to 40–60 °C and reacted for 2–5 hours until N-acetylneuraminic acid was completely converted to compound II. After the reaction was complete, methanol was removed by vacuum distillation to obtain a white solid, compound II. This crude product can be used directly in the next step.

[0012] 2. Compound II was acetylated and eliminated to yield (3aS,4R,7aR)-4-[(1S,2R)-1,2,3-(triacetoxy)propyl]-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester (i.e., compound III):

[0013] The crude compound II obtained in step 1 above was suspended in 1-5 (w / v) times the volume of n-heptane. 6-10 equivalents of acetic anhydride were added to the reaction solution, followed by 1.0-2.5 equivalents of trimethylsilyl trifluoromethanesulfonate. The reaction was carried out at 30-60°C for 2-5 hours. After the reaction was complete, the temperature was lowered to -5 to 0°C in an ice-salt bath, and the reaction was quenched by adding triethylamine dropwise. After the addition was complete, the mixture was stirred for 10 minutes. The reaction solution was then added dropwise to a saturated sodium bicarbonate aqueous solution cooled to 0°C. The mixture was extracted with ethyl acetate, and purified by column chromatography after vacuum distillation to obtain a colorless oily substance, compound III, with a yield of 79-88%. It is noteworthy that if a saturated sodium bicarbonate aqueous solution is added directly to the reaction solution after this step to quench the reaction, it is prone to hydrolysis, generating ring-opening byproducts.

[0014] 3. Deacetylating compound III yields (3aR,4R,7aR)-2-methyl-4-((1R,2R)-1,2,3-trihydroxypropyl)-3a,7a-dihydro-4H-pyrano[3,4-d]oxazol-6-carboxylic acid methyl ester (i.e., compound IV):

[0015] Compound III obtained in step 2 above is added to 2 to 6 (w / v) times the amount of anhydrous methanol, and 0.1 to 0.5 times the equivalent amount of sodium methoxide is added at room temperature. The mixture is reacted at 25 to 30°C for 0.1 to 0.5 hours to obtain compound IV, which can be used directly in the next step.

[0016] 4. Compound IV was reacted with carbonates to yield (3aS,4R,7aR)-4-{(S)-hydroxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester (i.e., compound V):

[0017] Compound IV, obtained in step 3 above, was dissolved in 2 volumes of anhydrous methanol and 4 volumes of dry tetrahydrofuran. Then, 0.1–0.5 equivalents of sodium methoxide were added, followed by 8 volumes of dimethyl carbonate. The mixture was stirred at room temperature for 1 hour, and then reacted at 45–65°C for 4–7 hours. After the reaction was complete, the temperature was lowered to -5–0°C, and crystallization was allowed to occur for 0.1–0.5 hours. The crystals were then filtered to obtain a white solid, which is compound V. The two-step yield was 76–85%.

[0018] 5. Compound V was etherified to give (3aS,4R,7aR)-4-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester (i.e., compound VI):

[0019] Compound V obtained in step 4 above was added to a mixed solvent of dried N,N-dimethylformamide (DMF) and tetrahydrofuran in a volume ratio of 1:1 to 1:5. Under nitrogen protection, the mixture was cooled to -5 to 0°C, and 1.0 to 3.0 equivalents of sodium hydride were added. Then, 1.0 to 2.5 equivalents of dimethyl sulfate were added dropwise. The reaction was carried out at 10 to 25°C for 2 to 5 hours. After the reaction was complete, acetic acid was added to quench the reaction, followed by the addition of toluene and a 5% sodium bicarbonate aqueous solution. The mixture was extracted and separated, and the solvent was removed by vacuum distillation to obtain a colorless oily substance, which is compound VI.

[0020] 6. Compound VI was subjected to an azide ring-opening reaction to yield (4S,5R,6R)-5-acetamido-4-azido-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester (i.e., compound VII):

[0021] Compound VI obtained in step 5 was added to a mixed solvent of toluene and dried tert-butanol in a volume ratio of 5:1 to 2:1. Under nitrogen protection, 1.0 to 3.0 equivalents of azidotrimethylsilane were added, the temperature was lowered to -5 to 0°C, and 0.1 to 0.5 equivalents of tetraisopropyl titanate were added. The reaction was carried out at 20 to 30°C for 9 to 11 hours. After the reaction was completed, the temperature was lowered to -5 to 0°C, and the mixture was stirred for 1 hour to crystallize. The crystals were filtered to obtain a pale yellow solid, which is compound VII. The two-step yield was 78 to 89%.

[0022] 7. Compound VII was reduced by azide to give (4S,5R,6R)-5-acetamide-4-amino-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester (i.e., compound VIII):

[0023] Compound VII obtained in step 6 was added to 7-10 (w / v) times its volume of ethyl acetate, and 1.0-1.5 equivalents of triphenylphosphine and 3.0-5.0 equivalents of water were added at room temperature. The mixture was reacted at 70-85°C for 2-4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain an ethyl acetate solution of compound VIII, which could be used directly in the next reaction step.

[0024] 8. Compound VIII was guanidinolated to yield (4S,5R,6R)-5-acetamide-4-[2,3-di(tert-butoxycarbonyl)guanidinyl]-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester (i.e., compound IX):

[0025] Add 1.0 to 1.5 equivalents of N,N'-di-tert-butoxycarbonyl-1-guanidinopyrazole to the ethyl acetate solution of compound VIII obtained in step 7 above, and react at 30 to 50 °C for 4 to 8 hours. After the reaction is complete, remove most of the solvent by vacuum distillation, and then add a mixed solvent of ethyl acetate and isopropyl ether in a volume ratio of 3:1 to 1:1. Cool to 0 to 5 °C to crystallize, and obtain a pale yellow solid, which is compound IX. The two-step yield is 79 to 86%.

[0026] 9. Compound IX is hydrolyzed via esterification to yield (4S,5R,6R)-5-acetamide-4-[2,3-bis(tert-butoxycarbonyl)guanidinyl]-6-[(1R,2R)-2,3-dihydroxy-1-methoxypropyl]-5,6-dihydro-4H-pyran-2-carboxylic acid (i.e., compound X):

[0027] Compound IX obtained in step 8 was dissolved in 4-6 (w / v) times its volume of methanol, and 2.0-5.0 equivalents of potassium carbonate dissolved in 2-4 times its volume of water were added. The mixture was reacted at 20-35°C for 3-6 hours. After the reaction was complete, the mixture was cooled to 0-5°C, and water was added to adjust the pH to 8-9 with 2N HCl. After removing part of the solvent by vacuum distillation, ethyl acetate was added, and the mixture was stirred for 5 minutes. The mixture was then allowed to stand and separated. The organic phase was discarded, and the aqueous phase was adjusted to pH 1-3 with 2N HCl. The aqueous phase was then extracted with ethyl acetate, and the organic phase was distilled under reduced pressure to remove the solvent, yielding a white solid, which is compound X, in 85-94% yield.

[0028] 10. Compound X undergoes a deprotection reaction to yield lanamivir (i.e., compound I):

[0029] The compound X obtained in step 9 was dissolved in 3-5 times (w / v) of water and reacted at 75-85°C for 3-5 hours. After the reaction was complete, the temperature was lowered to -5 to 0°C, and 5-10 times (w / v) of methanol was added. Crystallization was carried out at -5 to 0°C for 1-2 hours to obtain a white solid, which is lanamivir (I), with a yield of 78-85%. Attached Figure Description

[0030] Appendix Figure 1 The synthetic route for lanamivir. Specific Implementation Cases

[0031] The following examples further illustrate the content of the present invention, but the examples do not affect the scope of protection of the present invention.

[0032] Example 1 Preparation of N-acetyl-D-neuraminic acid methyl ester II: Weighed N-acetylneuraminic acid (5.000 g, 16.170 mmol) was placed in a dry 250 mL single-necked flask. 70 mL of anhydrous methanol was added at room temperature, and the mixture was stirred for 15 min to suspend it. The reaction flask was then placed in an ice-salt bath and cooled to -5 to 0 °C. Thionyl chloride (0.580 g, 4.850 mmol) was then added dropwise to the reaction mixture. After the addition was complete, the system was heated to 50 °C and the reaction was stirred for 3 hours. Thin-layer chromatography (TLC: n-butanol:acetic acid:water = 3:1:1) confirmed the completion of the reaction. The reaction mixture was concentrated to dryness under reduced pressure to obtain 5.410 g of a white solid, which was compound II, and could be used directly in the next step. The characterization data of compound II are as follows: [α] D 10 = -47.2° ( c= 2.00, MeOH). 1 H NMR (400 MHz, D2O) δ 4.07 (d, J = 10.5 Hz, 2H), 3.92 (t, J = 10.3Hz, 1H), 3.84 (s, 3H), 3.73 (m, J = 9.1, 6.3, 2.7 Hz, 1H), 3.62 (dd, J =11.8, 6.2 Hz, 1H), 3.55 (d, J = 9.2 Hz, 1H), 2.32 (dd, J = 13.1, 4.9 Hz, 1H), 2.05 (s, 3H), 1.97–1.85 (m, 1H). 13 C NMR (101 MHz, D2O) δ 174.80, 171.37,95.31, 70.32, 70.08, 68.17, 66.63, 63.13, 53.47, 52.04, 38.64, 22.05 ppm.

[0033] Example 2 Preparation of (3aS,4R,7aR)-4-[(1S,2R)-1,2,3-(triacetoxy)propyl]-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester III: 10 mL of n-heptane was added directly to a reaction flask containing compound II obtained in Example 1, followed by the addition of acetic anhydride (14.850 g, 145.500 mmol) at room temperature, and the mixture was stirred for 10 min. Trimethylsilyl trifluoromethanesulfonate (5.380 g, 24.250 mmol) was added dropwise. After the addition was complete, the system was heated to 50 °C and the reaction was stirred for 3 hours. Thin-layer chromatography (TLC: ethyl acetate as the developing solvent) showed that the reaction was complete. The reaction solution was cooled to -5 to 0 °C, and triethylamine (5.720 g, 56.580 mmol) was added dropwise, while the mixture was stirred at this temperature for 15 min. In a separate 500 mL single-necked flask, 100 mL of saturated sodium bicarbonate aqueous solution and 60 mL of ethyl acetate were added, respectively. The mixture was cooled to 0 °C, and then the above reaction solution was slowly added dropwise to the mixture. After stirring for 30 min, the mixture was extracted and separated. The aqueous phase was extracted again with 40 mL of ethyl acetate, and the organic phases were combined. The organic phase was dried with anhydrous magnesium sulfate and concentrated. The solution was then subjected to rapid column chromatography (eluting with ethyl acetate:petroleum ether = 1:1) to obtain 5.870 g of a colorless oil, which was compound III. The yield of the two-step reaction was 87.8%. The characterization data of compound III are as follows: [α] D 10 = -8.2° ( c= 1.00, MeOH). 1 H NMR (400 MHz, CDCl3) δ 6.36 (d, J = 4.0 Hz, 1H), 5.61 (dd, J = 6.0,2.6 Hz, 1H), 5.42 (td, J = 6.2, 2.7 Hz, 1H), 4.80 (dd, J = 8.7, 4.0 Hz, 1H), 4.57 (dd, J = 12.5, 2.7 Hz, 1H), 4.20 (dd, J = 12.4, 6.3 Hz, 1H), 3.93 (t, J= 8.8 Hz, 1H), 3.79 (s, 3H), 3.40 (dd, J = 10.1, 2.6 Hz, 1H), 2.13 (s, 3H),2.03 (d, J = 2.0 Hz, 6H), 1.98 (d, J = 1.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ170.67, 169.82, 169.60, 167.26, 161.89, 147.17, 107.56, 72.28, 70.30, 68.86,62.04, 62.00, 52.55, 20.85, 20.78, 20.62, 14.15 ppm.

[0034] Example 3 Preparation of (3aR,4R,7aR)-2-methyl-4-((1R,2R)-1,2,3-trihydroxypropyl)-3a,7a-dihydro-4H-pyrano[3,4-d]oxazol-6-carboxylic acid methyl ester IV: Compound III (10.000 g, 24.190 mmol) obtained in Example 2 was placed in a 250 mL dry single-necked flask. 50 mL of anhydrous methanol was added at room temperature, and the mixture was stirred for 15 min to dissolve it. Sodium methoxide (0.260 g, 4.840 mmol) was then added, and the mixture was stirred at room temperature for 30 min. Thin-layer chromatography (TLC: dichloromethane:methanol = 10:1) was used to monitor the reaction until completion. The solvent was removed under reduced pressure, and the mixture was concentrated to approximately 20 mL to obtain a methanol solution of the product, which could be used directly in the next step. The characterization data for compound IV are as follows: 1 H NMR (400 MHz, D2O) δ 6.38 (d, J = 4.2 Hz, 1H), 5.01 (dd, J = 8.4, 4.2Hz, 1H), 4.22 – 4.16 (m, 1H), 3.89 – 3.81 (m, 2H), 3.77 (s, 3H), 3.71 (d, J =9.0 Hz, 1H), 3.63 (dd, J = 11.9, 6.0 Hz, 1H), 3.56 (d, J = 10.0 Hz, 1H), 1.94 (s, 3H). 13 C NMR (101 MHz, D2O) δ 165.58, 162.55, 147.71, 107.11, 77.55, 72.65,70.44, 69.06, 64.06, 61.22, 52.75, 14.24 ppm.

[0035] Example 4 Preparation of (3aS,4R,7aR)-4-{(S)-hydroxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester V: 40 mL of tetrahydrofuran (dried by molecular sieve), 80 mL of dimethyl carbonate, and sodium methoxide (0.130 g, 2.420 mmol) were added directly to a reaction flask containing approximately 20 mL of a methanol solution of compound IV obtained in Example 3. The mixture was stirred at room temperature for 1 hour, then the system was slowly heated to 60 °C and stirred for another 5.5 hours. Thin-layer chromatography (TLC, with dichloromethane and methanol as the developing solvents at 10:1) confirmed the completion of the reaction. The reaction was cooled to 0 °C in an ice-water bath and stirred at 0 °C for another 30 min, during which solid precipitation was observed. The solid was filtered off, washed with 5 mL of methanol, and dried under reduced pressure to obtain 6.430 g of a white solid, which was compound V. The yield of the two-step reaction was 84.8%. The characterization data for compound V are as follows: [α] D 25 = -22.8° (c = 1.00, DMSO). 1 H NMR (400 MHz, DMSO- d 6 ) δ 6.31 (d, J = 4.0 Hz, 1H), 6.19 (d, J =6.5 Hz, 1H), 5.02 (ddd, J = 8.3, 7.1, 3.0 Hz, 1H), 4.93 (dd, J = 8.5, 4.1 Hz,1H), 4.57 (dd, J = 1.93 (s,3H). 13 C NMR (101 MHz, DMSO- d 6 ) δ 166.19, 162.09, 155.27, 146.77, 108.09,78.71, 78.20, 72.22, 67.41, 65.83, 60.94, 52.95, 14.22 ppm.

[0036] Example 5 Preparation of (3aS,4R,7aR)-4-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester VI: Compound V (7.000 g, 22.350 mmol) obtained in Example 4 was placed in a 100 mL dry three-necked flask. 21 mL of tetrahydrofuran (dried with molecular sieves) and 7 mL of N,N-dimethylformamide (dried with molecular sieves) were added, and the mixture was stirred for 10 min. The mixture was then cooled to -5 to 0 °C under nitrogen protection and stirred for 15 min. 60% sodium hydride (1.180 g, 49.160 mmol) (gray in appearance) was added in portions, and the mixture was stirred for 10 min after each addition. Dimethyl sulfate (3.950 g, 31.280 mmol) was added dropwise, and the mixture was heated to approximately 10 °C and stirred for 3 hours. Thin-layer chromatography (TLC: dichloromethane:methanol = 20:1) confirmed the completion of the reaction. Acetic acid (1.340 g, 22.350 mmol) and 70 mL of toluene were added to the reaction solution. The mixture was washed with 5% sodium bicarbonate aqueous solution (70 mL × 3) to separate the organic and aqueous layers. The aqueous layer was extracted with toluene (70 mL × 3). The organic layers were combined and the solvent was removed by vacuum evaporation to obtain 8.100 g of a pale yellow-white oily substance, which is compound VI, and can be used directly in the next step. The characterization data for compound VI are as follows: 1 H NMR (400 MHz, CDCl3) δ 6.41 (d, J = 4.1 Hz, 1H), 4.93 (td, J = 8.1,2.0 Hz, 1H), 4.85 (dd, J = 8.5, 4.1 Hz, 1H), 4.70 (p, J = 9.1 Hz, 2H), 4.30 -4.21 (m, 1H), 4.10 (t, J = 2.3 Hz, 1H), 3.82 (s, 3H), 3.74 (s, 3H), 3.18 (dd,J = 10.7, 2.4 Hz, 1H), 2.03 (s, 3H). 13 C NMR (101 MHz, CDCl3) δ 167.53,161.63, 154.93, 146.90, 107.68, 78.86, 78.65, 77.10, 72.47, 65.81, 62.37,60.96, 52.63, 29.66, 14.12ppm.

[0037] Example 6 Preparation of (4S,5R,6R)-5-acetamide-4-azido-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester VII: The pale yellow-white oily compound VI obtained in Example 5 was placed in a 100 mL three-necked flask, and 22 mL of toluene and 7 mL of tert-butanol dried with molecular sieves were added. The mixture was stirred for 15 min. Under nitrogen protection, trimethyl azidosilane (TMSN3) (5.150 g, 22.350 mmol) was added dropwise at room temperature. After the addition was complete, the reaction was cooled to -5 to 0 °C, and tetraisopropyl titanate (1.909 g, 6.700 mmol) was added. The mixture was then brought back to room temperature and stirred for 9 hours. Thin-layer chromatography (TLC: dichloromethane:methanol = 10:1) showed that the reaction was complete. The reaction solution was cooled to 0 °C and stirred at this temperature for 1 hour, during which solid precipitation was observed. The solid was filtered, and the filter cake was washed with 6 mL of toluene at 0 °C. The filter cake was dried under reduced pressure to obtain 7.360 g of pale yellow-white powdery solid, which was compound VII. The yield of the two-step reaction was 89.0%. The characterization data for compound VII are as follows: [α] D 15 = +68.0° ( c= 1.00, MeOH). 1 H NMR (400 MHz, DMSO- d 6 ) δ 8.20 (d, J = 9.1 Hz, 1H), 5.83 (d, J = 2.5Hz, 1H), 5.08 (td, J = 7.9, 2.3 Hz, 1H), 4.57 (dd, J = 7.9, 2.8 Hz, 2H), 4.31(dd, J = 9.2, 2.6 Hz, 1H), 4.22 (dd, J = 10.7, 1.7 Hz, 1H), 4.02 (q, J = 9.6Hz, 1H), 3.91 (t, J = 2.0 Hz, 1H), 3.73 (s, 3H), 3.38 (s, 3H), 1.91 (s, 3H). 13 C NMR (101 MHz, DMSO- d 6) δ 170.01, 161.64, 154.90, 144.64, 108.56, 79.02,77.58, 76.57, 66.04, 61.64, 58.95, 52.96, 46.86, 23.30 ppm.

[0038] Example 7 Preparation of (4S,5R,6R)-5-acetamide-4-amino-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester VIII: Compound VII (2.000 g, 5.400 mmol) obtained in Example 6 was placed in a 100 mL dry single-necked flask, and 20 mL of ethyl acetate was added. The mixture was stirred for 15 min. Triphenylphosphine (1.560 g, 5.940 mmol) and water (389 mg, 21.600 mmol) were added at room temperature. After the addition was complete, the reaction mixture was slowly heated to 80 °C and stirred for 2.5 h. Thin-layer chromatography (TLC: dichloromethane:methanol = 10:1) showed that the reaction was complete. The reaction solution was cooled to room temperature to obtain an ethyl acetate solution of compound VIII, which could be used directly for the next step without separation. The characterization data for compound VIII are as follows: [α] D 15 = +49.9° ( c= 1.00, MeOH). 1 H NMR (400 MHz, CDCl3) δ 6.94 (d, J = 8.9 Hz, 1H), 5.87 (d, J = 2.4 Hz, 1H), 4.91 (td, J = 8.2, 1.8 Hz, 1H), 4.73 (t, J = 8.7 Hz, 1H), 4.63 (t, J = 8.9 Hz, 1H), 4.19 (dd, J = 10.3, 2.0 Hz, 1H), 3.94 (s, 1H), 3.84 (q, J = 9.4 Hz, 1H), 3.72 (s, 3H), 3.61 (dd, J = 9.2, 2.5 Hz, 1H), 3.49 (s, 3H), 2.03 (s, 3H).13 C NMR (101 MHz, CDCl3) δ 171.18, 162.01, 155.32, 142.74, 114.11, 79.71, 77.57,76.95, 66.28, 61.94, 52.40, 51.07, 50.24, 23.35 ppm.

[0039] Example 8 Preparation of (4S,5R,6R)-5-acetamide-4-[2,3-di(tert-butoxycarbonyl)guanidinyl]-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester IX: 10 mL of ethyl acetate was added directly to the ethyl acetate solution of compound VIII obtained in Example 7, followed by the addition of N,N'-di-tert-butoxycarbonyl-1-guanidinopyrazole (1.760 g, 5.670 mmol) at room temperature, and the mixture was stirred for 15 min. The reaction was then heated to 50 °C and stirred for 6.5 h. Thin-layer chromatography (TLC: ethyl acetate:petroleum ether = 6:1) showed that the reaction was complete. The reaction solution was concentrated to remove most of the solvent, resulting in a viscous substance. 3 mL of a mixed solvent of ethyl acetate:isopropyl ether = 2:1 was added to this viscous substance, and the mixture was stirred at 0–5 °C for 30 min to induce crystallization. The solid was filtered, washed with a small amount of the mixed solvent, and evaporated under reduced pressure to dryness, yielding 2.720 g of a pale yellow solid, which was compound IX. The two-step yield was 85.8%. The characterization data for compound IX are as follows: 1 H NMR (400 MHz, CDCl3) δ 11.37 (s, 1H), 8.53 (d, J = 8.6 Hz, 1H), 6.44(d, J = 8.6 Hz, 1H), 5.84 (d, J = 2.4 Hz, 1H), 5.15 (t, J = 9.1 Hz, 1H), 4.87 (t, J =8.1 Hz, 1H), 4.78 (t, J = 8.6 Hz, 1H), 4.63 (t, J = 8.7 Hz, 1H), 4.25 (q, J = 9.6Hz, 1H), 4.10 (dd, J = 10.5, 1.7 Hz, 1H), 3.94 (t,J = 1.8 Hz, 1H), 3.76 (s, 3H), 3.56 (s, 3H), 1.96 (s, 3H), 1.47 (d, J = 5.5 Hz, 18H). 13 C NMR (101 MHz, CDCl3) δ171.05, 162.88, 161.45, 157.13, 154.79, 152.72, 144.41, 109.37, 83.99, 79.79,79.65, 78.60, 76.54, 65.98, 61.95, 52.50, 48.61, 48.32, 28.24, 28.03, 23.24ppm.

[0040] Example 9 Preparation of (4S,5R,6R)-5-acetamide-4-[2,3-bis(tert-butoxycarbonyl)guanidinyl]-6-[(1R,2R)-2,3-dihydroxy-1-methoxypropyl]-5,6-dihydro-4H-pyran-2-carboxylic acid X: Compound IX (1.850 g, 3.150 mmol) obtained in Example 8 was placed in a 100 mL dry single-necked flask, and 10 mL of methanol was added. Potassium carbonate (1.740 g, 12.610 mmol) was dissolved in 6 mL of water and added dropwise. After the addition was complete, the mixture was stirred at 30 °C for 5 hours. Thin-layer chromatography (TLC: n-butanol:acetic acid:water = 3:1:1) showed that the reaction was complete. The reaction solution was cooled to 0–5 °C, 18 mL of water was added, and the pH was adjusted to 8–9 with 2N hydrochloric acid. The solvent was distilled under reduced pressure to about 16 mL, and the aqueous layer was washed with ethyl acetate (30 mL × 3). The organic layer was discarded. The pH of the aqueous layer was adjusted to 2–3 with 2N hydrochloric acid and washed with ethyl acetate (60 mL × 3). The organic layers were combined and evaporated to dryness under reduced pressure to give 1.630 g of white solid, which was compound X, with a yield of 93.5%. The characterization data for compound X are as follows: [α] D 10 = +31.9° ( c= 1.00, MeOH). 1 H NMR (400 MHz, MeOD) δ 5.78 (d, J = 2.5 Hz, 1H), 4.92 (dd, J = 9.5, 2.5Hz, 1H), 4.30 (dd, J= 10.7, 1.3 Hz, 1H), 4.18 (dd, J = 10.8, 9.5 Hz, 1H), 3.84(ddd, J = 8.3, 4.5, 2.9 Hz, 1H), 3.74 (dd, J = 11.5, 3.0 Hz, 1H), 3.56 (dd, J =11.5, 4.6 Hz, 1H), 3.38 (dd, J = 8.5, 1.4 Hz, 1H), 3.33 (s, 3H), 1.85 (s, 3H), 1.40 (d, J = 24.4 Hz, 18H). 13 C NMR (101 MHz, MeOD) δ 173.75, 165.29, 164.52,158.36, 154.10, 146.79, 110.41, 85.10, 80.89, 79.52, 78.55, 71.62, 64.35,61.41, 51.57, 48.98, 28.87, 28.57, 23.17 ppm.

[0041] Example 10 Preparation of lanamivir I: Compound X (0.800 g, 1.460 mmol) obtained in Example 9 was placed in a 50 mL dry single-necked flask, and 5 mL of water was added. After stirring for 10 min, the mixture was slowly heated to 80 °C and stirred for 4 hours. The reaction solution was cooled to -5 to 0 °C, and 8 mL of methanol was added. After maintaining the temperature and stirring for 1 hour, the crystals were filtered to obtain 0.430 g of a white solid, which was the product lanamivir I, with a yield of 85.3%. The characterization data of compound I are as follows: 1H NMR (400 MHz, D2O) δ 5.52 (d, J = 2.3 Hz, 1H), 4.31 (d, J = 11.4Hz, 2H), 4.11 (t, J = 9.7 Hz, 1H), 3.89 (ddd, J = 8.6, 5.5, 2.8 Hz, 1H), 3.78(dd, J = 12.0, 2.9 Hz, 1H), 3.58 (dd, J = 11.9, 5.5 Hz, 1H), 3.46 (d, J = 8.7Hz, 1H), 3.31 (s, 3H), 1.95 (s, 3H). 13 C NMR (101 MHz, D2O) δ 174.21, 169.05,157.09, 149.44, 104.03, 77.90, 75.78, 69.62, 62.54, 60.57, 51.83, 47.78,22.13 ppm。

Claims

1. A method for preparing lanamivir (compound I) from N-acetylneuraminic acid as a starting material via the synthetic route shown in Figure 1, characterized in that... This method includes the following chemical synthesis steps: (1) N-acetylneuraminic acid was suspended in anhydrous methanol, and 0.10-0.50 equivalents of thionyl chloride were slowly added dropwise as a catalyst at -5℃ to 0℃. The reaction was carried out at 40-60℃ for 2-5 hours to obtain N-acetyl-D-neuraminic acid methyl ester, which was labeled as compound II. (2) Compound II obtained in step (1) above can be used directly in the next step without separation and purification. It is mixed with n-heptane at room temperature, and then 6-10 equivalents of acetic anhydride are added. Trimethylsilyl trifluoromethanesulfonate is then added as a catalyst and reacted at 30℃~60℃ for 2~5 hours. After the reaction is complete, the reaction is quenched with a suitable tertiary amine to obtain (3aS,4R,7aR)-4-[(1S,2R)-1,2,3-(triacetoxy)propyl]-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester, labeled as compound III. (3) The compound III obtained in step (2) above was dissolved in anhydrous methanol, and 0.10~0.50 equivalents of sodium methoxide were added at room temperature. The mixture was reacted at 25~30℃ for 0.1~0.5 hours to obtain (3aR,4R,7aR)-2-methyl-4-((1R,2R)-1,2,3-trihydroxypropyl)-3a,7a-dihydro-4H-pyrano[3,4-d]oxazol-6-carboxylic acid methyl ester, which was labeled as compound IV. (4) The compound IV obtained in step (3) above was dissolved in a mixture of anhydrous methanol and dry tetrahydrofuran. 0.10 to 0.50 equivalents of sodium methoxide were added at room temperature, followed by the addition of dimethyl carbonate (DMC). The mixture was reacted at 45 to 65 °C for 4 to 7 hours to obtain (3aS,4R,7aR)-4-{(S)-hydroxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester, labeled as compound V; (5) The compound V obtained in step (4) above was suspended in a mixed solvent of dried N,N-dimethylformamide (DMF) and tetrahydrofuran (THF). Under nitrogen protection, 1.0-3.0 equivalents of sodium hydride and 1.0-2.5 equivalents of dimethyl sulfate were added, and the mixture was reacted at 10-20°C for 2-5 hours to obtain (3aS,4R,7aR)-4-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-2-methyl-3a,7a-dihydro-4H-pyrano[3,4-d][1,3]oxazol-6-carboxylic acid methyl ester, labeled as compound VI; (6) The compound VI obtained in step (5) above was dissolved in a mixed solvent of toluene and dried tert-butanol. 0.1 to 0.5 equivalents of tetraisopropyl titanate were added as a catalyst, followed by 1.0 to 3.0 equivalents of azidotrimethylsilane. The mixture was reacted at 20 to 30 °C for 9 to 11 hours to obtain (4S,5R,6R)-5-acetamide-4-azido-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester, labeled as compound VII; (7) Dissolve compound VII obtained in step (6) above in ethyl acetate, add 1.0 to 1.5 equivalents of triphenylphosphine and 3.0 to 5.0 equivalents of water at room temperature, reflux for 2 to 4 hours to obtain (4S,5R,6R)-5-acetamido-4-amino-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester, labeled as compound VIII; (8) The compound VIII obtained in step (7) above was dissolved in ethyl acetate, and 1.0 to 1.5 equivalents of N,N'-di-tert-butoxycarbonyl-1-guanidinylpyrazole were added. The mixture was reacted at 30 to 50 °C for 4 to 8 hours. After the reaction was completed, the crude product was crystallized in a mixed solvent of ethyl acetate and isopropyl ether to obtain (4S,5R,6R)-5-acetamido-4-[2,3-di(tert-butoxycarbonyl)guanidinyl]-6-{(S)-methoxy[(4R)-2-oxo-1,3-dioxolane-4-yl]methyl}-5,6-dihydro-4H-pyran-2-carboxylic acid methyl ester, which was labeled as compound IX. (9) Dissolve compound IX obtained in step (8) above in a mixed solvent of methanol and water, add 2.0 to 5.0 equivalents of potassium carbonate, and react at 20 to 35°C for 3 to 6 hours to hydrolyze and obtain (4S,5R,6R)-5-acetamido-4-[2,3-di(tert-butoxycarbonyl)guanidinyl]-6-[(1R,2R)-2,3-dihydroxy-1-methoxypropyl]-5,6-dihydro-4H-pyran-2-carboxylic acid, labeled as compound X; (10) The compound X obtained in step (9) above is mixed with 3 to 5 times the amount of water and reacted at 75 to 85°C for 3 to 5 hours to obtain lanamivir, which is labeled as compound I.

2. The preparation method according to claim 1, characterized in that... The catalyst used in step (1) is thionyl chloride, and its dosage is 0.10~0.50 equivalents, preferably 0.3 equivalents.

3. The preparation method according to claim 1, characterized in that... The equivalent of trimethylsilyl trifluoromethanesulfonate used in step (2) is 1.0 to 2.5 equivalents, preferably 1.5 equivalents.

4. The preparation method according to claim 1, characterized in that... The tertiary amine used in the quenching reaction in step (2) is one of pyridine, triethanolamine, and triethylamine, with triethylamine being preferred.

5. The preparation method according to claim 1, characterized in that... In step (4), the volume ratio of anhydrous methanol / tetrahydrofuran / dimethyl carbonate is 1:1:3 to 1:2:4, preferably 1:2:

4.

6. The preparation method according to claim 1, characterized in that... The solvent used in step (5) is a mixture of DMF and THF in a volume ratio of 1:1 to 1:5, preferably 1:

3.

7. The preparation method according to claim 1, characterized in that... In step (6), the amount of tetraisopropyl titanate used is 0.1 to 1.0 equivalents, preferably 0.5 equivalents.

8. The preparation method according to claim 1, characterized in that... The crystallization solvent in step (8) is a mixture of ethyl acetate and isopropyl ether in a volume ratio of 3:1 to 1:1, preferably 2:

1.

9. The preparation method according to claim 1, characterized in that... The solvent used in step (9) is a mixture of methanol and water in a volume ratio of 5:1 to 5:4, preferably 5:3.

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