A method for synthesizing arabinogalactan core tetrasaccharide fragment

Abstract

The invention discloses a method for synthesizing a core tetrasaccharide fragment of arabinogalactan. 2,3-O-dibenzyl-4,6-O-naphthylidene-D-pyranosyl trichloroacetimidate and 2-O-benzyl-3-O-triisopropylsilyl-4,6-O-naphthylidene-D-pyranosyl trichloroacetimidate are used as glycosyl donors, ZnI2 is used as a catalyst, glycosylation conditions are mild, β selectivity is good, side reactions are few, and target yield is high. 2,3,5-O-tribenzoyl-L-arabinofuranosyl trichloroacetimidate is used as a glycosyl donor, TMSOTf is used as a catalyst, and the glycosylation product is an α configuration without a β configuration byproduct.

Description

Technical Field

[0001] This invention belongs to the field of natural oligosaccharide chain synthesis technology, specifically relating to a method for synthesizing a tetrasaccharide fragment of arachidonic acid core. Background Technology

[0002] Arabinogalactan (AG) is a large class of carbohydrates belonging to hemicellulose, widely found in the primary and secondary cell walls of plant cells. AG is a long, highly branched, neutral polysaccharide composed of arabinose and galactose. The backbone of AG is galactan, and its branches are mainly arabinose side chains linked to the galactose chain via β-1,3 or β-1,6 bonds.

[0003] The effects of AG include: 1. Antioxidant: AG has good iron-reducing antioxidant capacity and DPPH free radical scavenging capacity, indicating that AG has good activity in in vitro antioxidant processes; 2. Immunological activity: AG affects both immunity and hematopoietic inhibition. AG also has a significant effect on cholesterol and blood lipids. AG also affects the immunoreactivity of mononuclear phagocytes in nodular antigens. It can induce immediate-type hypersensitivity reactions in the body, as well as induce cell infiltration in local lesions and inhibit antigen-antibody reactions; 3. Antitumor: AG can stimulate the immune system and block the metastasis of tumor cells to the liver. For cancer treatment, AG may be an effective adjuvant. Tumor cell metastasis to the liver is more common than to other organ sites, possibly because tumor cells in the liver parenchyma are specific for lectin carrier sites. Animal studies have shown that AG has the ability to inhibit or block lectin carrier sites, thus reducing the clone of liver tumor cells and improving the survival time of subjects; 4. Maintaining intestinal homeostasis: AG can increase the content of short-chain fatty acids by promoting the fermentation of intestinal microbiota. AG regulates the structure of the gut microbiota in the host, maintaining fluid and electrolyte balance, thereby delivering nutrients to intestinal cells and protecting intestinal mucosal cells. It increases the density of anaerobic and lactic acid bacteria, lowers cholesterol levels, and maintains and restores intestinal balance; 5. Anti-diabetic: AG can protect against osteoporosis caused by hyperglycemia by inhibiting α-glucosidase activity, improving glucose tolerance, and reducing the accumulation of advanced glycation end products (AGEs); 6. Anti-inflammatory: AG can enhance damage to macrophages and inhibit the activity of pro-inflammatory factors. AG can promote the secretion of pro-inflammatory cytokines and has strong immunomodulatory activity; 7. Antiviral: AG has in vitro activity against herpes simplex virus type 1 (HSV-1) and inhibits bovine herpesvirus type 1 (BoHV-1); 8. Anti-angiogenesis: Anti-angiogenic experiments show that AG can reduce the migration activity and tube-forming ability of endothelial cells on the matrix, but has no effect on endothelial cell growth. Further research has found that AG can exert an anti-angiogenic effect through the BMP2 signaling pathway; 9. Liver protection: AG can increase the levels of antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase, and can also increase the non-enzymatic antioxidant level of liver tissue.

[0004] In recent years, arabinogalactan (AG) has been closely related to important physiological processes and life phenomena such as intercellular communication, cell adhesion and molecular recognition, structural support, signal transduction, and immune defense. Among these, the potential regulatory role of AG in the immune system has attracted widespread attention. The immune system relies heavily on precise cell-cell communication to achieve optimal function, and any damage to the signaling system can lead to impaired immune responsiveness. Therefore, effective and safe immunomodulators are crucial for people with low immunity.

[0005] Arabinogalactan (AG) has gradually gained widespread attention as a natural product for safeguarding human health. AG possesses various biological activities and can be used as an antioxidant, anti-tumor drug, and immune enhancer. Plant-derived active polysaccharides exist within cell walls, intertwined with cellulose and lignin, increasing the difficulty of polysaccharide extraction. Simultaneously, plants contain a large number of soluble pigments, proteins, and other soluble impurities, which dissolve in the extract along with the polysaccharides during extraction. Polysaccharide structures are complex and diverse; polysaccharides extracted using conventional methods are usually multi-component mixtures. The separation and purification of high-purity, homogeneous polysaccharides is a prerequisite for polysaccharide structural analysis and the study of structure-activity relationships. Furthermore, extracted polysaccharides are difficult to modify at specific sites.

[0006] Given the current difficulties in extracting arabinogalactan, there is an urgent need for a synthetic method that facilitates polysaccharide extraction, yields the target product in high quantities, and minimizes side reactions. Summary of the Invention

[0007] The purpose of this invention is to overcome at least one deficiency of the prior art and provide a method for synthesizing the core tetrasaccharide fragment of arachidonic acid.

[0008] The technical solution adopted in this invention is:

[0009] A method for synthesizing the core tetrasaccharide fragment of arachidonic acid, the synthetic route of which is as follows:

[0010]

[0011] Includes the following steps:

[0012] S1) The glycosyl donor compound a and the acceptor compound b are mixed in a solvent, a glycosylation reaction catalyst is added, the reaction is stirred until complete, and compound c is obtained by separation and purification.

[0013] S2) Dissolve compound c in a solvent, add BH3–THF and Cu(OTf)2, and after the reaction is complete, quench the reaction and separate and purify to obtain compound d;

[0014] S3) Mix the glycosyl donor compound e and the acceptor compound d in a solvent, add the glycosylation reaction catalyst, stir until the reaction is complete, and separate and purify to obtain compound f;

[0015] S4) Dissolve compound f in a solvent, add BH3–THF and Cu(OTf)2, and after the reaction is complete, quench the reaction and separate and purify to obtain compound g;

[0016] S5) The glycosyl donor compound a and the acceptor compound g were mixed in a solvent, a glycosylation reaction catalyst was added, the reaction was stirred until complete, and the compound h was separated and purified.

[0017] S6) Dissolve compound h in a solvent, add TABF and stir until the reaction is complete, then separate and purify to obtain compound i;

[0018] S7) Mix the glycosyl donor compound j and the acceptor compound i in a solvent, add TMSOTf to the reaction solution, and continue stirring until the reaction is complete. Separate and purify to obtain compound k.

[0019] S8) Dissolve intermediate k in a solvent, add NaOMe, stir until the reaction is complete, adjust the pH of the reaction system to neutral, and separate and purify to obtain compound L;

[0020] S9) Compound L is dissolved in a solvent, catalytically hydrogenated, and then separated and purified to obtain the final product m.

[0021] In some synthetic methods, the glycosylation reaction catalyst is ZnI2. Studies have shown that ZnI2 has better catalytic efficiency while requiring milder reaction conditions. Of course, other known glycosylation reaction catalysts can also be selected.

[0022] In some examples of synthesis methods, the synthesis method satisfies at least one of the following conditions:

[0023] In step S1), the molar ratio of donor a, acceptor b, and ZnI2 is (1.0–2.0):(1.0–2.0):(0.1–1.0).

[0024] In step S2), the molar ratio of compound c, BH3–THF, and Cu(OTf)2 is (1.0–2.0):(16.0–32.0):(0.4–0.8).

[0025] In step S3), the molar ratio of donor e, acceptor d, and ZnI2 is (1.0–2.0):(1.0–2.0):(0.1–1.0).

[0026] In step S4), the molar ratio of compound f, BH3–THF, and Cu(OTf)2 is (1.0–2.0):(16.0–32.0):(0.4–0.8).

[0027] In step S5), the molar ratio of donor a, acceptor g, and ZnI2 is (1.0–2.0):(1.0–2.0):(0.1–1.0).

[0028] In step S6), the molar ratio of compound h to TABF is (1.0~2.0):(4.0~8.0).

[0029] In step S7), the molar ratio of donor j, acceptor i, and TMSOTf is (1.0–2.0):(1.0–2.0):(0.1–0.3).

[0030] In step S8), the molar ratio of compound k to NaOMe is (1.0-2.0):(0.1-0.3).

[0031] In some examples of synthetic methods, in step S9), the catalyst for catalytic hydrogenation is Pd / C. Pd / C is widely available, lower in cost, and has good deprotection effects.

[0032] In some examples of synthetic methods, in step S9), the molar ratio of compound L to Pd / C is (1.0–2.0):(1.0–2.0).

[0033] In some examples of synthetic methods, the solvent used in steps S1), S3), and S5) is Et2O.

[0034] In some synthetic methods, molecular sieves are also added independently to the reaction systems in steps S1), S3), S5), and S7). Molecular sieves help maintain anhydrous conditions in the reaction system, facilitating the reaction.

[0035] In some examples of the synthesis methods, the solvent used in steps S2), S4), and S7) is dichloromethane; and / or the solvent used in step S6) is THF; and / or the solvent used in steps S8) and S9) is a mixed solution of dichloromethane and methanol.

[0036] In some synthetic methods, the glycosylation reaction temperature in steps S1), S3), and S5) does not exceed 10°C. This helps to reduce the formation of impurities.

[0037] In some examples of the synthesis methods, the reaction temperature in steps S2) and S4) does not exceed 10°C; and / or the reaction temperature in step S6) is 40–80°C; and / or the reaction temperature in step S8) does not exceed 40°C. This ensures the reaction rate while avoiding the generation of excessive impurities.

[0038] The beneficial effects of this invention are:

[0039] The synthetic methods of some examples of this invention use 2,3-O-dibenzyl-4,6-O-naphthene-D-galactopyranosyltrichloroacetilime and 2-O-benzyl-3-O-triisopropylsilyl-4,6-O-naphthene-D-galactopyranosyltrichloroacetilime as glycosyl donors and ZnI2 as a catalyst. The glycosylation conditions are mild, the β-selectivity is good, the side reactions are few, and the target yield is high.

[0040] The synthesis methods of some examples of the present invention use 2,3,5-O-tribenzoyl-L-furanarabinose trichloroacetylimide as a glycosyl donor and TMSOTf as a catalyst. The glycosylation product is in the α configuration and there are no β configuration byproducts.

[0041] In some examples of the synthesis methods of this invention, the protecting groups of the intermediates are easily removed under the conditions of sodium methoxide and catalytic hydrogenation. Detailed Implementation

[0042] As used in this article:

[0043] "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0044] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0045] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0046] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0047] "Parts by mass" refers to the basic unit of measurement that expresses the mass ratio of multiple components. One part can represent any unit mass, such as 1g or 2.689g. If we say that component A has "a" parts by mass and component B has "b" parts by mass, it means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, it can mean that the mass of component A is aK and the mass of component B is bK (where K is any number representing a multiplier). It is important to understand that, unlike parts by mass, the sum of the mass parts of all components is not limited to 100 parts.

[0048] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0049] It should be noted that the synthetic route in this invention is the key innovation, and the ratio of reactants in each step is secondary. Therefore, the ratio of reactants in each step does not constitute a factor affecting the synthetic product.

[0050] The embodiments of the present invention will be described in detail below with reference to specific examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0051] In the following examples, compounds a through m are respectively:

[0052] Compound a: 2,3-O-dibenzyl-4,6-O-naphthene-D-pyranogalactosyltrichloroacetimines, its chemical structural formula is as follows: .

[0053] Compound b: .

[0054] Compound C: .

[0055] Compound d: .

[0056] Compound e: 2-O-benzyl-3-O-triisopropylsilyl-4,6-O-naphthene-D-pyranogalactosyltrichloroacetimines, its chemical structural formula is as follows: .

[0057] Compound f: .

[0058] Compound g: .

[0059] Compound h: .

[0060] Compound i: .

[0061] Compound j: 2,3,5-benzoyl-L-arabinofuranyltrichloroacetimines, its chemical structural formula is as follows: .

[0062] Compound k: 5-(N-benzylcarbonylamino)pentyl2,3-di-O-benzyl4,6-O-(2'-naphthyl)-β-D-galactopyranosyl-(1→6)-2-O-benzyl-3-O-[2,3,5-tri-O-benzoyl-α-L-arabinofuranyl]-4-O-(2'-naphthyl)methyl-β-D-galactopyranosyl-(1→6)-2,3-di-O-benzyl-4-O-(2'-naphthyl)methyl-β-D-galactopyranoside, with the following chemical structural formula: .

[0063] Compound L: .

[0064] Compound m: 5-Aminopentylβ-D-galactopyranosyl-(1→6)-β-D-galactopyranosyl-3-O-[α-L-arabinofuranosyl]-(1→6)-β-D-galactopyranoside, with the following chemical structural formula: .

[0065] The abbreviations used are those commonly used in this field, and the meanings of some of the abbreviations are as follows:

[0066] THF: Tetrahydrofuran, TMSOTf: Trimethylsilyl trifluoromethanesulfonate.

[0067] Example 1

[0068] The specific method for synthesizing the core tetrasaccharide fragment of arachidonic acid includes the following steps:

[0069] (1) Glycosyl donor compound a (547 mg, 0.85 mmol) and acceptor compound b (135 mg, 0.57 mmol) were mixed in a flask, azeotropically three times with toluene (3 mL), and dissolved in 57 mL of Et2O. Freshly activated molecular sieve powder (100 mg / mL solvent) was added, and the mixture was stirred at –10 °C for 30 min. At -10 °C, an Et2O solution of ZnI2 (0.50 equiv, 0.1 M Et2O solution) was added to the mixture, and stirring was continued until TLC showed the disappearance of the glycosyl donor (48 h). The reaction was quenched by adding Et3N and diluted with EtOAc, then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure to obtain a residue, which was then purified by simple preparative thin-layer chromatography on silica gel (acetone-toluene = 1:10) to obtain the desired product c (306.6 mg, 0.43 mmol), with a yield of 75%.

[0070] (2) At 0 °C, the product compound c (717 mg, 1.0 mmol) obtained in the previous step was dissolved in anhydrous DCM (100 mL), and BH3–THF (16.0 mL, 1M in THF, 16.0 mmol, 16 equiv.) was added. Cu(OTf)2 (144.4 mg, 0.4 mmol, 0.4 equiv.) was added at 0 °C, and the reaction mixture was stirred at room temperature for 1 hour. When TLC showed the disappearance of the starting material, the reaction was quenched by a saturated aqueous solution of NaHCO3. The reaction solution was washed with a saturated aqueous solution of NaHCO3 and saturated brine, dried over Na2SO4, concentrated under vacuum, and then purified by silica gel chromatography (petroleum ether–ethyl acetate = 1:1) to give compound d (431.4 mg, 0.60 mmol) in a yield of 60%.

[0071] (3) The glycosyl donor compound e (53.0 mg, 0.075 mmol) and the acceptor compound d (36.0 mg, 0.05 mmol) were mixed in a flask, azeotropically three times with toluene (3 mL), and dissolved in 5 mL of Et2O. Freshly activated molecular sieve powder (100 mg / mL solvent) was added, and the mixture was stirred at -10 °C for 30 min. At -10 °C, an Et2O solution of ZnI2 (0.50 equiv, 0.1 M Et2O solution) was added to the mixture, and stirring was continued until TLC showed the disappearance of the glycosyl donor (48 h). The reaction was quenched by adding Et3N and diluted with EtOAc, then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure to obtain a residue, which was then purified by simple preparative thin-layer chromatography on silica gel (acetone–toluene = 1:10) to obtain the desired product f (43.7 mg, 0.03 mmol), with a yield of 69%.

[0072] (4) At 0 °C, the product compound f (250 mg, 0.2 mmol) obtained in the previous step was dissolved in anhydrous DCM (20 mL), and BH3–THF (3.6 mL, 1 M in THF, 3.6 mmol, 16 equiv.) was added. Cu(OTf)2 (28.9 mg, 0.08 mmol, 0.4 equiv.) was added at 0 °C, and the reaction mixture was stirred at room temperature for 1 hour. When TLC showed the disappearance of the starting material, the reaction was quenched by a saturated aqueous solution of NaHCO3. The reaction solution was washed with a saturated aqueous solution of NaHCO3 and saturated brine, dried over Na2SO4, concentrated under vacuum, and then purified by silica gel chromatography (petroleum ether–ethyl acetate = 1:1) to give compound g (134.4 mg, 0.1 mmol) in a yield of 53%.

[0073] (5) Glycosyl donor compound a (48 mg, 0.075 mmol) and acceptor compound g (63.4 mg, 0.05 mmol) were mixed in a flask, azeotropically three times with toluene (3 mL), and dissolved in 5 mL of Et2O. Freshly activated molecular sieve powder (100 mg / mL solvent) was added, and the mixture was stirred at –10 °C for 30 min. At -10 °C, an Et2O solution of ZnI2 (0.50 equiv, 0.1 M Et2O solution) was added to the mixture, and stirring was continued until TLC showed the disappearance of the glycosyl donor (48 h). The reaction was quenched by adding Et3N and diluted with EtOAc, then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure to obtain a residue, which was then purified by simple preparative thin-layer chromatography on silica gel (acetone-toluene = 1:10) to obtain the desired product h (52.4 mg, 0.03 mmol), with a yield of 60%.

[0074] (6) Compound h (300 mg, 0.17 mmol) was dissolved in 8.5 mL of anhydrous THF at room temperature, and TABF (1 M THF solution, 4.0 equiv) was added. The mixture was then stirred overnight at 60 °C. The mixture was diluted with EtOAc (50 mL), washed with saturated NaHCO3 aqueous solution and brine, dried over Na2SO4, concentrated under vacuum, and then purified by rapid chromatography on silica gel (petroleum ether–ethyl acetate = 1:1) to give the desired product i (184 mg, 0.12 mmol) in 68% yield.

[0075] (7) Glycosyl donor compound j (27.2 mg, 0.045 mmol) and acceptor compound i (50 mg, 0.03 mmol) were mixed in a flask, azeotropically reacted three times with toluene (3 mL), and then dissolved in 5 mL of DCM. Powdered freshly activated molecular sieve (100 mg / mL solvent) was added, and the mixture was stirred at room temperature for 30 minutes. 2 μL of TMSOTf was added to the reaction mixture at room temperature, and stirring continued until TLC showed the disappearance of the glycosyl donor (12 hours). The reaction was quenched by adding Et3N and diluted with EtOAc, then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure to obtain the residue, which was then purified by simple preparative thin-layer chromatography on silica gel (acetone-toluene = 1:6) to obtain the desired product k (31.8 mg, 0.015 mmol) in 52% yield. The chromatographic information of the product is as follows:

[0076] 1H NMR (C6D6, 400 MHz): δ 8.14–6.61 (m, 66H, ArH), 6.08–6.07 (m, 2H,ArCH, and C2 Ara -H), 5.74 (d, 1H, J = 3.00 Hz, C3 Ara -H), 5.42 (d, 1H, J = 7.36 Hz,O CH2 Ph), 5.40 (s, 1H, C1 Ara -H), 5.11–5.08 (m, 4H, O CH2 Ph), 5.02–4.84 (m, 6H,C5 Ara –H, O CH2 Ph), 4.78–4.67 (m, 6H, C5 Ara –H, C4 Ara –H, and O CH2 Ph), 4.62 (d, 1H, J =7.96 Hz, O CH2 Ph), 4.49 (d, 1H, J = 8.00 Hz, O CH2 Ph), 4.45 (d, 1H, J =5.12 Hz,C1 Gal -H), 4.36 (d, 2H, J = 5.08 Hz, C1 Gal –H, and C1 Gal –H), 4.32 (dd, 1H, J = 5.16,6.56 Hz, C2 Gal -H), 4.28–4.23 (m, 2H, C6 Gal –H, and C6 Gal –H), 4.18–4.16 (m, 1H, O(CH2)5NHCbz), 4.12 (dd, 1H, J = 5.08, 6.08 Hz, C2 Gal -H), 4.09–4.05 (m, 2H, C2 Gal –H, and C4 Gal –H), 4.00 (dd, 1H, J = 4.56, 6.72 Hz, C6 Gal-H), 3.96–3.88 (m, 4H,C6 Gal -H, C3 Gal -H , C6 Gal -H and O(CH2)5NHCbz), 3.66–3.64 (m, 2H, C4 Gal –H, and C4 Gal –H), 3.57–3.55 (m, 1H, C5 Gal –H), 3.46–3.44 (m, 1H, C5 Gal –H), 3.39–3.32 (m, 4H,C3 Gal -H, C3 Gal -H , C6 Gal -H and O(CH2)5NHCbz), 2.88–2.85 (m, 2H, O(CH2)5NHCbz),2.35 (s, 1H, C5 Gal -H), 1.54–1.17 (m, 6H, O(CH2)5NHCbz); 13 C{1H} NMR (C6D6, 400MHz): δ 165.78, 165.07, 139.59, 139.56, 139.15, 139.04, 136.73, 135.96,133.74, 133.52, 133.46, 133.18, 133.12, 132.90, 129.97,129.76, 129.68,128.36, 128.27, 128.24, 128.20, 128.17, 128.14, 128.10, 128.08, 128.02,127.98, 127.85, 127.77, 127.69, 127.61, 127.53, 127.41, 127.36, 127.24,127.17, 126.85, 126.72, 126.58, 125.83, 125.77, 125.74, 125.59, 124.21,107.81 (ArCH), 104.18 (C1 Gal ), 103.75 (C1 Gal ), 100.96 (C1 Ara), 82.52, 82.47, 82.29, 80.28, 79.81, 79.66, 78.88, 78.70, 77.66, 75.02, 74.82, 74.72, 74.26, 73.96, 73.69, 72.90, 71.70, 68.72, 66.20, 63.65, 40.78, 33.31, 31.99, 29.85, 29.78, 29.57, 29.48, 29.32, 24.71, 23.10, 22.77, 22.52, 22.36, 22.01, 14.02.

[0077] (8) Compound k (38 mg, 18.4 µmol) was dissolved in a mixed solution of CH2Cl2 (0.5 mL) and MeOH (2.5 mL), NaOMe (1 mg, 18.4 µmol) was added, and the mixture was stirred at room temperature for 2 h. After the starting material was completely consumed, the reaction mixture was rinsed with Amberlite 120 H2O. + The resin was neutralized to adjust pH to neutral, filtered and concentrated, and purified by rapid column chromatography (petroleum ether–ethyl acetate = 1:2) to give the desired debenzoylated product l (25 mg, 14.5 µmol) in 79% yield.

[0078] (9) Compound l (25 mg, 14.5 µmol) was dissolved in a mixed solution of CH2Cl2 (0.5 mL) and MeOH (3 mL). Pd / C (25 mg) was added to the solution and the mixture was stirred in hydrogen for 48 hours. The reaction mixture was filtered through diatomaceous earth, washed with methanol, and concentrated under reduced pressure to give the desired product m (10 mg, 13.7 µmol) in 95% yield. The chromatographic information of the product is as follows:

[0079] 1 H NMR (D2O, 400 MHz): δ 5.21 (s, 1H, C1 Ara -H), 4.51 (d, 1H, J = 7.60 Hz, C1 Gal -H), 4.42 (d, 1H, J = 7.84 Hz, C1 Gal -H), 4.39 (d, 1H, J = 7.80 Hz, C1 Gal-H),4.19–4.10 (m, 4H), 4.05–3.88 (m, 14H), 3.82–3.63 (m, 13H), 3.54–3.49 (m,6H),3.41–3.33 (m, 2H), 3.01–2.97 (m, 2H), 1.75–1.66 (m, 6H); 13 C NMR (D2O, 400MHz): δ 109.28 (C1 Ara ), 103.42 (C1 Gal ), 103.12 (C1 Gal ), 102.75 (C1 Gal ), 83.92,81.35, 80.19, 76.60, 75.20, 73.01, 72.77, 71.64, 70.78, 70.17, 68.68, 68.49,64.79, 61.06,32.00, 30.33, 30.02, 29.72, 26.46, 23.32, 22.69, 22.13, 18.92,13.92, 13.41, 13.38.

[0080] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. A method for synthesizing a core tetrasaccharide fragment of alagalactan, wherein the synthetic route is as follows: The steps include: S1) mixing the glycosyl donor compound a and the acceptor compound b in a solvent, adding a glycosylation reaction catalyst, stirring until the reaction is complete, and separating and purifying to obtain a compound c; S2) dissolving compound c in a solvent, adding BH3–THF and Cu(OTf)2, quenching the reaction after the reaction is complete, and separating and purifying to obtain compound d; S3) mixing the glycosyl donor compound e and the acceptor compound d in a solvent, adding a glycosylation reaction catalyst, stirring until the reaction is complete, and separating and purifying to obtain a compound f; S4) dissolving compound f in a solvent, adding BH3–THF and Cu(OTf)2, quenching the reaction after the reaction is complete, and separating and purifying to obtain compound g; S5) mixing the glycosyl donor compound a and the acceptor compound g in a solvent, adding a glycosylation reaction catalyst, stirring until the reaction is complete, and separating and purifying to obtain a compound h; S6) dissolving compound h in a solvent, adding TABF and stirring until the reaction is complete, and isolating and purifying to obtain compound i; S7) mixing the glycosyl donor compound j and the acceptor compound i in a solvent, adding TMSOTf to the reaction solution, and continuing to stir until the reaction is complete, separating and purifying to obtain compound k; S8) dissolving the intermediate k in a solvent, adding NaOMe, stirring until the reaction is complete, adjusting the pH of the reaction system to neutral, and isolating and purifying to obtain compound L; S9) dissolving compound L in a solvent, catalytically hydrogenating, separating and purifying to obtain the final product m.

2. The synthesis method according to claim 1, characterized in that The glycosidation reaction catalyst is ZnI2.

3. The synthesis method according to claim 2, characterized in that The synthesis method satisfies at least one of the following conditions: In the step S1), the molar ratio of the donor compound a, the acceptor compound b and ZnI2 is (1.0-2.0): (1.0-2.0): (0.1-1.0); In the step S2), the molar ratio of the compound c, BH3–THF and Cu(OTf)2 is (1.0-2.0): (16.0-32.0): (0.4-0.8); In the step S3), the molar ratio of the donor compound e, the acceptor compound d and ZnI2 is (1.0-2.0): (1.0-2.0): (0.1-1.0); In the step S4), the molar ratio of the compound f, BH3–THF and Cu(OTf)2 is (1.0-2.0): (16.0-32.0): (0.4-0.8); In the step S5), the molar ratio of the donor compound a, the acceptor compound g and ZnI2 is (1.0-2.0): (1.0-2.0): (0.1-1.0); In the step S6), the molar ratio of the compound h to TABF is (1.0-2.0): (4.0-8.0); In the step S7), the molar ratio of the donor compound j, the acceptor compound i and TMSOTf is (1.0-2.0): (1.0-2.0): (0.1-0.3); In the step S8), the molar ratio of the compound k to NaOMe is (1.0-2.0): (0.1-0.3).

4. The synthesis method according to claim 1, characterized in that In step S9), the catalyst for catalytic hydrogenation is Pd / C.

5. The synthesis method according to claim 4, characterized in that In step S9), the molar ratio of compound L to Pd / C is (1.0-2.0): (1.0-2.0).

6. The synthesis method according to any one of claims 1 to 5, characterized in that: The solvent used in steps S1), S3) and S5) is independently Et2O.

7. The synthesis method according to any one of claims 1 to 5, characterized in that: In the reaction systems of steps S1), S3), S5) and S7), molecular sieves are also added independently.

8. The synthesis method according to any one of claims 1 to 5, characterized in that: The solvent used in steps S2), S4) and S7) is independently dichloromethane; and / or the solvent used in step S6) is THF; and / or the solvent used in steps S8) and S9) is independently a mixed solution of dichloromethane and methanol.

9. The synthesis method according to any one of claims 1 to 5, characterized in that: The glycosidation reaction temperature of steps S1), S3) and S5) does not exceed 10°C.

10. The synthesis method according to any one of claims 1 to 5, characterized in that: The reaction temperature of steps S2) and S4) does not exceed 10°C; and / or the reaction temperature of step S6) is 40-80°C; and / or the reaction temperature of step S8) does not exceed 40°C.

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