A preparation method of proanthocyanidin B2

Abstract

The invention discloses a method for preparing proanthocyanidin B2. The raw material preparation of the invention is simple and easy to obtain, economical and affordable, and high-yield and high-purity raw materials can be obtained without column chromatography purification, which greatly reduces the material, labor and time costs and is suitable for large-scale preparation. The invention selects a suitable protic solvent so that the coupling reaction of the raw material 1 and the intermediate 3 and the deprotection of the silyl ether are carried out simultaneously, which can greatly reduce the reaction time and operation steps. The synthesis cycle is greatly shortened. The invention realizes the equimolar feeding high-yield catalytic coupling of the raw material 1 and the intermediate 3, and can greatly inhibit the self-coupling side reaction of the intermediate 3.

Description

Technical Field

[0001] The invention relates to a method for preparing proanthocyanidin B2. Background Art

[0002] Proanthocyanidin B2, CAS No.: 29106-49-8, Molecular formula: C 30 H 26 O 12 , molecular weight: 578.52, the structural formula of proanthocyanidin B2 is as follows:

[0003]

[0004] Because proanthocyanidin B2 has the effects of protecting cardiovascular system, preventing hypertension, anti-tumor, anti-radiation, anti-mutation and beauty, it is widely used in biomedicine and other fields.

[0005] The raw material of proanthocyanidin B2 in the market is extracted from natural product grape seeds and then repeatedly purified. Although proanthocyanidin B2 is widely distributed, its relative content is low, resulting in very expensive extraction and purification costs. Traditional organic synthesis can achieve laboratory-scale synthesis of proanthocyanidin B2, but the synthesis process involves multiple steps such as multi-step hydroxyl protection, benzyl oxidation and deprotection, with extremely low yields and produces a large amount of solid and liquid waste, making it difficult to use for large-scale production.

[0006] US9133176B discloses the following preparation route 1:

[0007]

[0008] "Condensation of catechin and epicatechin incorporating a TBS-protecting group, DOI: 10.3987 / com-11-s(p)58" discloses the following preparation route 2:

[0009]

[0010] "An Improved Synthesis of Procyanidin Dimers:Regio-and Stereocontrolof the Interflavan Bond, DOI:10.1002 / ejoc.200600668" discloses the following preparation route 3:

[0011]

[0012] The traditional method of preparing proanthocyanidin B2 requires multiple steps of column purification, which cannot be prepared on a large scale and will produce a large amount of solid and liquid waste. In addition, dimerization and deprotection need to be carried out separately, which prolongs the preparation cycle.

[0013] The preparation routes disclosed in the above-mentioned prior art all have some defects. Specifically, both route one and route two require an excess of one of the raw materials (4-5eq), and it is impossible to achieve high-yield dimerization with equimolar feeding of the raw materials. The yield of route one is extremely low, with only about 30% yield in the coupling and deprotection steps, and a large amount of Pd catalyst is required for debenzylation protection, which is expensive and costly. In order to control the coupling racemization problem, route two introduces an acetoxy protecting group into 3-OH, resulting in extremely harsh final deprotection conditions and inability to effectively remove it. After the intermediate 3 is synthesized in route three, due to its high activity, it is easy to self-couple to produce self-polymer 4, and additional C-8 protection is required to inhibit self-coupling.

[0014] Therefore, a more efficient, environmentally friendly and simple method for preparing proanthocyanidin B2 is urgently needed. Summary of the invention

[0015] The technical problem to be solved by the present invention is to provide a method for preparing proanthocyanidin B2, thereby improving the purity and yield, reducing the cost of raw materials, and shortening the preparation time.

[0016] The present invention provides a method for preparing proanthocyanidin B2, which comprises the following steps:

[0017]

[0018] (1) Raw material 1 and compound R 1 Cl or (R 1 )2NH undergoes substitution reaction to obtain intermediate 2; where R 1 Selected from trimethylsilyl, triethylsilyl, diphenylmethylsilyl;

[0019]

[0020] (2) Intermediate 2 and R 2 H undergoes oxidation reaction in the presence of an oxidant and a nucleophile to obtain intermediate 3; where R 2 is selected from ethylene glycol ethoxy, diethylene glycol ethoxy, methoxy, ethoxy, hydroxy, tert-butyldimethylsilyloxyethoxy, acetoxy; preferably, R 2 Selected from ethylene glycol ethoxy, methoxy, acetoxy;

[0021]

[0022] (3) The raw material 1 and the intermediate 3 are simultaneously subjected to coupling reaction and silyl ether deprotection to obtain proanthocyanidin B2.

[0023] Preferably, the reaction in step (1) is carried out in the presence of a Lewis base; preferably, the Lewis base is selected from one or more of triethylamine, imidazole, sodium bicarbonate, sodium carbonate, and cesium carbonate.

[0024] Preferably, in step (1), raw material 1 and compound R 1 Cl or (R 1 )2NH has a molar ratio of 1:5 to 1:7.

[0025] Preferably, step (1) is carried out in the presence of one or more solvents selected from acetonitrile, tetrahydrofuran, dichloromethane, N,N-dimethylformamide and acetone, and the reaction temperature is 0° C. to 30° C.; and the reaction time is 1-12 hours.

[0026] Preferably, the oxidant in step (2) is selected from one or more of DDQ, MnO2, potassium peroxymonosulfonate, K2S2O8, 2-iodobenzoic acid, silver oxide, ammonium cerium nitrate, and manganese triacetate; and the nucleophilic reagent is selected from water, methanol, ethanol, acetic acid, tert-butyldimethylhydroxyethoxysilane, ethylene glycol monoethyl ether, and diethylene glycol ethyl ether.

[0027] Preferably, in step (2), intermediate 2 and compound R 2 The molar ratio of H is 1:1.5 to 1:20; the molar ratio of intermediate 2 to oxidant is 1:1.5 to 1:6;

[0028] Preferably, step (2) is carried out in the presence of one or more solvents selected from the group consisting of n-hexane, n-heptane, n-pentane, dichloromethane, chloroform, 1,2-dichloroethane, 1,1-dichloroethane, 1,1-dichloroethylene, 1,2-dichloroethylene, cyclohexane, ethylbenzene, toluene, xylene, chlorobenzene, acetonitrile, tert-butyl methyl ether, 1,4-dioxane, and water, at a reaction temperature of 0° C. to 30° C. and a reaction time of 1 to 12 hours.

[0029] Preferably, in step (3), the molar ratio of raw material 1 to intermediate 3 is 1:1 to 1:1.2.

[0030] Preferably, step (3) is carried out in the presence of a solvent; the solvent is at least one of petroleum ether, n-hexane, cyclohexane, n-heptane, dichloromethane, methanol, ethanol, isopropanol, acetone, acetonitrile, tetrahydrofuran, 1,4-dioxane, water and N,N-dimethylformamide; preferably, the solvent is at least one of ethanol, 1,4-dioxane, petroleum ether and isopropanol; more preferably, the solvent is a mixed solvent of petroleum ether, 1,4-dioxane and isopropanol in a volume ratio of (1-2):(1-2):(1-2); more preferably, the solvent is a mixed solvent of petroleum ether, 1,4-dioxane and isopropanol in a volume ratio of 1:1:1;

[0031] Preferably, step (3) is carried out in the presence of a catalyst; the catalyst is at least one of hydrochloric acid, acetic acid, formic acid, camphorsulfonic acid, montmorillonite K-10, lithium bromide, lithium bis(trifluoromethanesulfonyl)imide, silver tetrafluoroborate, phenylboric acid compounds, 2,6-dichloro-3-pyridineboric acid, boron trifluoride ethyl ether, trifluoromethanesulfonic acid, cerium trifluoromethanesulfonate, ytterbium trifluoromethanesulfonate, and tin tetrachloride. Preferably, the catalyst is montmorillonite K-10.

[0032] Preferably, the reaction temperature of step (3) is 0° C.-50° C., preferably 25° C.-45° C. Preferably, the reaction time of step (3) is 0.5-12 hours.

[0033] The method for preparing proanthocyanidin B2 of the present invention may further include the following step (4), further refining the obtained crude proanthocyanidin B2:

[0034] (4) The crude proanthocyanidin B2 was filtered and concentrated, and then slurried in a mixed solution of dichloromethane and methanol, and the R removed after the reaction in the solution was filtered out. 1 and R 2 The residue was filtered and dried to obtain a light brown crude solid powder of proanthocyanidin B2. Preferably, the volume ratio of dichloromethane to methanol is 5:1 to 30:1.

[0035] The reagents and raw materials used in the present invention are commercially available.

[0036] The positive and progressive effects of the present invention are: the raw material preparation of the present invention is simple, and the intermediate 3 with high purity and high yield is prepared by crystallization and extraction; by using the polarity difference between the raw material 1 and the intermediate 3 and a suitable solvent system, the self-coupling of the intermediate 3 can be inhibited under equimolar feeding conditions, thereby improving the purpose of coupling between the raw material 1 and the intermediate 3.

[0037] The presence of the C-3 protecting group of the intermediate 3 of the present invention results in the generation of only a single chiral proanthocyanidin dimer during the SN1 reaction between the raw material 1 and the intermediate 3 due to the steric hindrance of the hydroxyl protecting group.

[0038] Beneficial effects: The raw material preparation of the present invention is simple and easy to obtain, economical and affordable, and high-yield and high-purity raw materials can be obtained without column purification, which greatly reduces the material, labor and time costs and is suitable for large-scale preparation. The present invention uses a suitable protic solvent so that the coupling reaction of raw material 1 and intermediate 3 and the deprotection of silyl ether are carried out simultaneously, which can greatly reduce the reaction time and operation steps. The synthesis cycle is greatly shortened. The present invention realizes equimolar feeding high-yield catalytic coupling of raw material 1 and intermediate 3, and can greatly inhibit the self-coupling side reaction of intermediate 3. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The proanthocyanidin B2 prepared by Example 4 of the present invention 1 H-NMR spectrum;

[0040] Figure 2 The proanthocyanidin B2 prepared by Example 4 of the present invention 13 C-NMR spectrum;

[0041] Figure 3 HPLC spectrum of pure proanthocyanidin B2 obtained by Example 4 of the present invention;

[0042] Figure 4 is the mass spectrum of proanthocyanidin B2 prepared by Example 4 of the present invention;

[0043] Figure 5 HPLC spectrum of crude proanthocyanidin B2 obtained in Example 4 of the present invention;

[0044] Figure 6 HPLC spectrum of crude proanthocyanidin B2 obtained in Example 5 of the present invention;

[0045] Figure 7 The chiral HPLC spectrum of the proanthocyanidin B2 prepared by the present invention and the natural proanthocyanidin B2 analyzed by Example 8 of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods in the following examples without specifying specific conditions are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0047] In each embodiment, 1HNMR was recorded by BRUKER AVANCE NEO 600 MHz, JNM-ECZ400s NMR instruments, and the chemical shifts were expressed in δ (ppm). Liquid chromatography-mass spectrometry (LCMS) was recorded by Shimadzu LC-20AD, Agilent 1260 and Agilent 1200 mass spectrometers.

[0048] abbreviation

[0049]

[0050]

[0051] Example 1 (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta(diphenylmethylsilyloxy)-4-acetoxyflavane-3, 4-diol

[0052] (1) (2R,3R)-(3',4',3,5,7)-penta(diphenylmethylsilyloxy)flavan-3-ol

[0053] In N 2 Under atmosphere, 20g (2R, 3R)-flavan-3-ol and 47g imidazole (10eq) were dissolved in a mixed solvent of 200mL THF (tetrahydrofuran) and DCM (dichloromethane) (volume ratio 3:1). Then, 73mL DPMSCl (5eq) was added dropwise to the reaction bottle at room temperature, and the reaction was continued for 6h at room temperature until the raw material disappeared. After the reaction was completed, it was concentrated under reduced pressure, and the remaining residue was recrystallized in n-hexane to obtain 93% yield of (2R, 3R)-(3', 4', 3, 5, 7)-penta (diphenylmethylsilyloxy)flavan-3-ol.

[0054] 1 H NMR (600MHz, CDCl3) δ7.57-7.61(m,20H),7.35-7.38(m,30H),6.67(dd,J=8. 2,2.1Hz,1H),6.66(d,J=2.1Hz,1H),6.65(d,J=8.2Hz,1H),6.01(d,J=2.3Hz ,1H),5.85(d,J=2.3Hz,1H),4.88(d,J=2.2Hz,1H),4.76(dd,J=2.6,1.2Hz,1 H), 2.85 (dd, J=16.2, 4.6Hz, 1H), 2.60 (dd, J=16.2, 4.6Hz, 1H), 0.66 (s, 15H).

[0055] (2) (2R,3R,4S)-(3',4',3,5,7)-penta(diphenylmethylsilyloxy)-4-acetoxyflavan-3,4-diol

[0056] Under N2 atmosphere, (2R, 3R)-(3', 4', 3, 5, 7)-penta (diphenylmethylsilyl) flavan-3-ol synthesized in step (1) was dissolved in 500 mL of dichloromethane. Then, 29 g DDQ (2.0 eq) was added to the reaction bottle in batches at 0°C. Subsequently, 23 mL of acetic acid solution was added dropwise, and the reaction was continued at room temperature for 3 hours until the raw material disappeared. At 0°C, saturated sodium bicarbonate aqueous solution was slowly dripped into the reaction solution, the pH of the reaction solution was adjusted to neutral, 9.4 g DMAP (1.2 eq) was added to the reaction bottle, and stirring was continued for 10 min. After filtering through a diatomaceous earth pad and concentrating under reduced pressure to remove the solvent, ethyl acetate was added for extraction, the organic phase was extracted, washed with brine, dried, and concentrated under reduced pressure to obtain a yellow-brown oil (purity 90%, yield 47%).

[0057] 1 H NMR (600MHz, CDCl3) δ7.63-7.51(m,20H),7.38-7.35(m,30H),6.88(dd,J=8.2,2.1Hz,1H),6.79(d,J=2.1Hz,1H),6.75( d,J=8.2Hz,1H),6.11(d,J=2.3Hz,1H),5.93(d,J=2.3Hz,1H),5.42(d,J=2.2Hz,1H),4.92(dd,J=2.6,1.2Hz,1H),4.86(d J=2.3Hz,1H),2.19(s 3H),0.66(s,15H).

[0058] Example 2 (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta(trimethylsilyloxy)-4-(2-ethylene glycol)flavane-3, 4-diol

[0059]

[0060] (1) (2R,3R)-(3',4',3,5,7)-penta(trimethylsilyloxy)flavan-3-ol

[0061] Under N2 atmosphere, 20g (2R, 3R)-flavan-3-ol was dissolved in 200mL acetonitrile solution. Then, 72mL HMDS (5eq) was added dropwise into the reaction bottle at 0°C and reacted at room temperature for 3h until the raw material disappeared. After the reaction was completed, the mixture was concentrated under reduced pressure, and the remaining residue was recrystallized in petroleum ether to obtain 98% yield of (2R, 3R)-(3', 4', 3, 5, 7)-penta (trimethylsilyl) flavan-3-ol.

[0062] 1 H NMR (600MHz, CDCl3) δ6.93 (dd, J=8.2, 2.1Hz, 1H), 6.88 (d, J=2.1Hz, 1H), 6.81 ( d,J=8.2Hz,1H),6.14(d,J=2.3Hz,1H),5.97(d,J=2.3Hz,1H),4.89–4.85(m,1H ),4.14(td,J=4.3,2.0Hz,1H),2.80(dd,J=16.5,4.5Hz,1H),2.58(dd,J=16.6, 4.0Hz,1H),0.26(s,9H),0.25(s,9H),0.24(s,9H),0.23(s,9H),-0.11(s,9H); 13 C NMR (125MHz, CDCl3) δ155.84,154.64,154.37,146.21,145.95,132.93,120.74,120.62 ,119.86,105.21,104.24,101.87,78.82,67.48,29.08,0.58,0.52,0.43,0.41,-0.08.

[0063] (2) (2R,3R,4S)-(3',4',3,5,7)-penta(trimethylsilyloxy)-4-(2-ethylene glycol)flavan-3,4-diol

[0064] Under N2 atmosphere, (2R,3R)-(3',4',3,5,7)-penta(trimethylsilyloxy)flavan-3-ol synthesized in step (1) was dissolved in 500 mL of dichloromethane. Then, DDQ (2.0 eq) was added to the reaction bottle in batches at 0°C. Subsequently, 67 mL of ethylene glycol ethyl ether (10 eq) was added dropwise and the reaction was continued at room temperature for 3 hours until the starting material disappeared. Subsequently, 10 g of DMAP (1.2 eq) was added to the reaction bottle at 0°C, and after stirring for 10 min, it was filtered through a diatomaceous earth pad. After the filtrate was concentrated, it was diluted with ethyl acetate and extracted with water. The organic phase was extracted, washed with brine, dried, and concentrated under reduced pressure to obtain a yellow-brown oil (purity 93%, yield 48%).

[0065] 1 H NMR (600MHz, CDCl3) δ6.97(dd,J=8.2,2.1Hz,1H),6.92(d,J=2.1Hz,1H),6.84(d,J=8.2Hz,1 H),6.12(d,J=2.3Hz,1H),5.98(d,J=2.3Hz,1H),5.06(s,1H),4.25(d,J=2.8Hz,1H),3.91(d d,J=2.8,1.4Hz,1H),3.85(dd,J=6.0,4.2Hz,2H),3.62(dd,J=5.7,4.3Hz,2H),3.56(qt,J=7 .0,3.3Hz,2H),1.23(t,J=7.0Hz,3H),0.31(s,9H),0.25(s,18H),0.24(s,9H),-0.21(s,9H); 13 C NMR (125MHz, CDCl3) δ156.8,156.6,156.2,146.3,146.0,132.9,120.9,120.8,120.1 ,105.7,103.6,101.6,75.2,71.8,70.2,69.4,69.1,66.8,15.4,0.7,0.6,0.4,-0.21.

[0066] Example 3 (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta(triethylsilyloxy)-4-methoxyflavan-3, 4-diol

[0067]

[0068] (1) (2R,3R)-(3',4',3,5,7)-penta(triethylsilyloxy)flavan-3-ol

[0069] Under N2 atmosphere, 20g (2R, 3R)-flavan-3-ol and 47g imidazole (10eq) were dissolved in a mixed solvent of 200mL THF (tetrahydrofuran) and DCM (dichloromethane) (volume ratio 3:1). Then, 58mL TESCl (5eq) was added dropwise to the reaction bottle at room temperature, and the reaction was continued for 6h at room temperature until the raw material disappeared. After the reaction was completed, it was concentrated under reduced pressure, and the remaining residue was recrystallized in n-hexane to obtain 98% yield of (2R, 3R)-(3', 4', 3, 5, 7)-penta (triethylsilyl) flavan-3-ol.

[0070] 1H NMR (600MHz, CDCl3) δ6.87 (dd, J=8.4, 2.3Hz, 1H), 6.79 (d, J=2.3Hz, 1H), 6.65 (d, J=8.4Hz, 1H), 5.98 (d, J= 2.3Hz,1H),5.95(d,J=2.3Hz,1H),4.95(s,1H),4.45(td,J=4.3,1.3Hz,1H),2.97(dd,J=16.6,4.4Hz,1H), 2.63(dd,J=16.6,4.5Hz,1H),1.21(t,J=7.8Hz,9H),1.09(t,J=7.9Hz,18H),1.06(t,J=8.0Hz,9H),1.01(t ,J=7.9Hz,9H),0.83(q,J=7.8Hz,6H),0.80(q,J=7.9Hz,12H),0.78(q,J=8.0Hz,6H),0.76(q,J=7.9Hz,6H); 13 C NMR (125MHz, CDCl3) δ156.3,155.3,155.2,147.2,144.4,131.3,122.3,120.8,119.1,10 7.7,104.1,99.5,86.7,84.5,29.1,11.2,10.8,10.7,10.6,10.5,7.0,6.9,6.8,6.7,6.6.

[0071] (2) (2R,3R,4S)-(3',4',3,5,7)-penta(triethylsilyloxy)-4-methoxyflavan-3,4-diol

[0072] Under N2 atmosphere, the (2R,3R)-(3',4',3,5,7)-penta(triethylsilyl)flavan-3-ol synthesized in step (1) was dissolved in 500 mL of dichloromethane solvent. Then, 15.6 g DDQ (1.0 eq) was added to the reaction bottle in batches at 0°C. Subsequently, 2.8 mL methanol was added dropwise, and the reaction was continued at room temperature for 3 hours until the raw material disappeared. Subsequently, 10 g DMAP (1.2 eq) was added to the reaction bottle at 0°C, and after stirring for 10 min, it was filtered through a diatomaceous earth pad. After the filtrate was concentrated, it was diluted with ethyl acetate and extracted with water. The organic phase was extracted, washed with brine, dried, and concentrated under reduced pressure to obtain a light brown oil (purity 92%, yield 48%).

[0073] 1H NMR(600MHz, CDCl3) δ7.03(d,J=2.2Hz,1H),6.91(dd,J=8.3,2.2Hz,1H),6.85(d,J=8.2Hz,1H),6.14 (d,J=2.2Hz,1H),6.01(d,J=2.2Hz,1H),5.92(d,J=2.7Hz,1H),4.95(s,1H),4.01(dd,J=5.8,2.6Hz, 2H),3.17(s,3H),1.24(t,J=7.8Hz,9H),1.12(t,J=7.9Hz,18H),1.09(t,J=8.0Hz,9H),1.05(t,J=7. 9Hz, 9H), 0.83 (q, J = 7.8Hz, 6H), 0.80 ( q, J = 7.9Hz, 12H), 0.77 ( q, J = 8.0Hz, 6H), 0.75 ( q, J = 7.9Hz, 6H); 13 C NMR (125MHz, CDCl3) δ156.3,155.3,155.2,147.2,144.4,131.3,122.3,120.8,119.1,10 7.7,104.1,99.5,86.7,84.5,29.1,11.2,10.8,10.7,10.6,10.5,7.0,6.9,6.8,6.7,6.6.

[0074] Example 4 Synthesis of proanthocyanidin B2

[0075] Take a 500mL round-bottomed double-necked flask, dissolve 0.5mL of concentrated hydrochloric acid in 200mL of a mixed solvent of petroleum ether / 1,4-dioxane / isopropanol (volume ratio 1:1:1), then add 10g of epicatechin and 25.5g of (2R,3R,4S)-(3',4',3,5,7)-penta(trimethylsilyloxy)-4-(2-ethylene glycol)flavane-3,4-diol (prepared in Example 2) to dissolve, and stir at room temperature under nitrogen protection for 1 hour to react until the raw material (2R ,3R,4S)-(3',4',3,5,7)-penta(trimethylsilyloxy)-4-(2-ethylene glycol)flavan-3,4-diol disappeared. After the reaction was completed, the reaction solution was concentrated under reduced pressure and slurried with 100 mL of dichloromethane / methanol (volume ratio of 20:1) to obtain the crude proanthocyanidin B2. The crude product was purified by silica gel column chromatography using a mixed solvent of dichloromethane:methanol (volume ratio of 10:1) as an eluent to obtain proanthocyanidin B2 (13.2 g, purification yield 66%).

[0076] Proanthocyanidin B2 has multiple rotational isomeric conformations around the C4-C8′ single bond, resulting in 1 NMR and 13 NMR spectrum shows broad peak characteristics:1 H NMR (600MHz, CD3OD) 7.10 (1H, br s), 6.90 (1H, br s), 6.78-6.62 (4H, m), 6.05-5.80 (3H, m), 5.06-4.95 (1H, m), 5.00-4.90 (1H, m), 4.62 (1H, br s), 4.35-4.20 (1H, m), 3.90 (1H, br s), 2.90-2.83 (1H, m), 2.82-2.70 (1H, m); 13 CNMR: 158.36, 157.84, 156.45, 154.44, 145.79, 145.78, 145.53, 145.52, 132.51, 132.06, 119.22, 119.21, 115.92, 115.91 , 115.21, 115.20, 107.25, 101.37, 100.46, 97.36, 96.48, 96.08, 96.07, 79.64, 77.02, 73.46, 66.91, 61.59, 37.06, 29.67.

[0077] Example 5 Synthesis of proanthocyanidin B2

[0078] Take a 2L round-bottomed double-necked flask, dissolve 100g epicatechin and 255g (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta (trimethylsilyl)-4-(2-ethylene glycol) flavan-3, 4-diol (1eq) (prepared in Example 2) in 1L of a mixed solvent of petroleum ether / 1, 4-dioxane / isopropanol (volume ratio 1: 1: 1), add 195g montmorillonite K-10 (2eq), and stir at room temperature under nitrogen protection for 1 hour to react until the raw material (2R, 3R ,4S)-(3',4',3,5,7)-penta(trimethylsilyloxy)-4-(2-ethylene glycol)flavane-3,4-diol disappears. After the reaction is completed, montmorillonite K-10 is removed by filtration, and the filtrate is concentrated under reduced pressure. The crude proanthocyanidin B2 is obtained after pulping with 600mL of dichloromethane / methanol (volume ratio 20:1). The crude proanthocyanidin B2 is purified by silica gel column chromatography using a mixed solvent of dichloromethane:methanol (volume ratio 10:1) as an eluent to obtain proanthocyanidin B2 (143g, purification yield 72%). The structural detection data is basically the same as that of Example 4.

[0079] Example 6 Synthesis of proanthocyanidin B2

[0080] Take a 2L round-bottomed double-necked flask, dissolve 50g epicatechin and 230g (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta (diphenylmethylsilyloxy)-4-acetoxyflavan-3, 4-diol (1eq) (prepared in Example 1) in 1L methanol / tetrahydrofuran / dichloromethane (volume ratio 1: 1: 1), add 30g lithium bromide (2eq), and stir at 45°C under nitrogen protection for 3 hours until the raw material (2R, 3R, 4S)-(3', 4', 3, 5, 7) · -penta (diphenylmethylsilyloxy) -4-acetoxyflavan-3, 4-diol disappears. After the reaction, the filtrate is concentrated under reduced pressure and purified by silica gel column chromatography with a mixed solvent of dichloromethane: methanol (volume ratio 10: 1) as an eluent to obtain proanthocyanidin B2 (47.5g, purification yield 48%). The structural detection data is basically the same as that of Example 4.

[0081] Example 7 Synthesis of proanthocyanidin B2

[0082] Take a 1L round-bottomed double-necked flask, dissolve 50g epicatechin and 153g (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta (triethylsilyl)-4-methoxyflavane-3, 4-diol (1eq) (prepared in Example 3) in 800mL methanol / 1, 4, dioxane (volume ratio 1: 1), add 21.4g ytterbium trifluoromethanesulfonate (0.2eq), stir at room temperature under nitrogen protection for 1 hour, until the raw material (2R, 3R, 4S)-(3', 4', 3, 5, 7)-penta (triethylsilyl)-4-methoxyflavane-3, 4-diol disappears, after the reaction, concentrate the filtrate under reduced pressure, and purify by silica gel column chromatography with a mixed solvent of dichloromethane: methanol (volume ratio 10: 1) as an eluent to obtain proanthocyanidin B2 (62.5g, purification yield 63%). The structural detection data is basically the same as that of Example 4.

[0083] Example 8 Chiral Configuration Analysis of Proanthocyanidin B2 Prepared by the Present Invention

[0084] In order to further verify that the proanthocyanidin B2 prepared by the present invention has the same absolute configuration as the natural proanthocyanidin B2 (purchased from Yunnan Xili Biotechnology Co., Ltd.), the following method was used for analysis: Instrument model: Agilent 1120; Chromatographic column: IA; chromatographic column specifications: 250mm×4.6mm; chromatographic column filler particle size: 5μm; mobile phase ratio: 0.1% TFA n-hexane: ethanol = 20:80; flow rate: 1.000mL / min; column temperature: 25°C; detection wavelength: 280nm; injection volume: 20μL. Under the above chiral analysis conditions, it was confirmed through repeated experiments that the elution time of the synthesized proanthocyanidin B2 of the present invention and the natural proanthocyanidin B2 were consistent.

Claims

1. A method for preparing proanthocyanidin B2, characterized in that: The steps include: (1) Raw material 1 and compound R 1 Cl or (R 1 )2NH undergoes substitution reaction to obtain intermediate 2; where R 1 Selected from trimethylsilyl, triethylsilyl, diphenylmethylsilyl; (2) Intermediate 2 and R 2 H undergoes oxidation reaction in the presence of an oxidant and a nucleophile to obtain intermediate 3; where R 2 Selected from ethylene glycol ethoxy, methoxy, acetoxy; (3) The raw material 1 and the intermediate 3 are simultaneously subjected to coupling reaction and deprotection of the silyl ether to obtain proanthocyanidin B2; The reaction in step (1) is carried out in the presence of a Lewis base; the Lewis base is selected from one or more of triethylamine, imidazole, sodium bicarbonate, sodium carbonate, and cesium carbonate; The oxidant in step (2) is selected from one or more of DDQ, MnO2, potassium peroxymonosulfonate, K2S2O8, 2-iodobenzoic acid, silver oxide, ammonium cerium nitrate, and manganese triacetate; the nucleophilic reagent is selected from water, methanol, ethanol, acetic acid, tert-butyldimethylhydroxyethoxysilane, ethylene glycol monoethyl ether, and diethylene glycol ethyl ether; Step (3) is carried out in the presence of a catalyst; the catalyst is at least one of hydrochloric acid, acetic acid, formic acid, camphorsulfonic acid, montmorillonite K-10, lithium bromide, lithium bis(trifluoromethanesulfonyl)imide, boron trifluoride ethyl etherate, trifluoromethanesulfonic acid, cerium trifluoromethanesulfonate, and ytterbium trifluoromethanesulfonate; In step (1), raw material 1 and compound R 1 Cl or (R 1 ) The molar ratio of 2NH is 1:5 to 1:7; In step (2), intermediate 2 and compound R 2 The molar ratio of H is 1:1.5 to 1:20; the molar ratio of intermediate 2 to oxidant is 1:1.5 to 1:6; In step (3), the molar ratio of raw material 1 to intermediate 3 is 1:1 to 1:1.2; Step (1) is carried out in the presence of one or more solvents selected from acetonitrile, tetrahydrofuran, dichloromethane, N,N-dimethylformamide, and acetone, at a reaction temperature of 0° C. to 30° C. and a reaction time of 1 to 12 hours; Step (2) is carried out in the presence of one or more solvents selected from the group consisting of n-hexane, n-heptane, n-pentane, dichloromethane, chloroform, 1,2-dichloroethane, 1,1-dichloroethane, 1,1-dichloroethylene, 1,2-dichloroethylene, cyclohexane, ethylbenzene, toluene, xylene, chlorobenzene, acetonitrile, tert-butyl methyl ether, 1,4-dioxane, and water, the reaction temperature being 0° C. to 30° C. and the reaction time being 1 to 12 hours; Step (3) is carried out in the presence of a solvent; the solvent is at least one of petroleum ether, n-hexane, cyclohexane, n-heptane, dichloromethane, methanol, ethanol, isopropanol, acetone, acetonitrile, tetrahydrofuran, 1,4-dioxane, water and N,N-dimethylformamide; the reaction temperature is 0°C-50°C, and the reaction time is 0.5-12 hours.

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