Method for preparing tetrahydro-beta-carboline alkaloid through electrochemical reduction of C / N
The electrochemical reduction method adsorbs dihydro-β-carboline imine compounds on the cathode to produce tetrahydro-β-carboline alkaloids, solving the problems of risk and low yield of traditional chemical methods, and achieving a safe and efficient synthesis method, which facilitates industrial production.
Patent Information
- Application Number
- CN202510612221.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-13
- Publication Date
- 2025-08-08
AI Technical Summary
The existing chemical methods of synthesis of tetrahydro-β-carboline alkaloids have problems such as high risk, low yield and limited application range, and it is difficult for traditional chemical reducing agents and catalysts to achieve large-scale industrial production.
Electrochemical reduction method is used to adsorb dihydro-β-carboline imine compounds on the cathode, and tetrahydro-β-carboline alkaloids are generated through the electrostatic reduction reaction, avoiding the use of flammable and explosive chemical reducing agents and catalysts. The reaction conditions are mild, the yield is high, and it is suitable for automated continuous operation.
It realizes safe, green and efficient synthesis of tetrahydro-β-carboline alkaloids, which is simple to operate, is convenient for industrial production, has few by-products, and is controllable in reaction speed, which is suitable for the production of fine chemical products.
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Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of organic electrochemistry, and in particular to a method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N. Background Art
[0002] Tetrahydro-β-carboline alkaloids are widely distributed in nature, primarily in the roots, leaves, and seeds of plants such as the Simsariaceae and Veratrum families, as well as in marine organisms such as plants, cyanobacteria, mushrooms, marine life, animals, and even human tissues and blood. They exhibit a wide spectrum of biological activities, including anti-tumor, antiviral, antifungal, anti-inflammatory, antimalarial, algicidal, and antioxidative activities, and are widely used in pharmaceuticals and pesticides. Tetrahydro-β-carboline alkaloids can be purified from natural products or obtained through fermentation, but these processes suffer from long production cycles, complex operations, low production capacity, a limited range of alkaloid species, and difficulties in freely modifying and derivatizing their chemical structures.
[0003] There are two main chemical methods for synthesizing tetrahydro-β-carboline alkaloids: the Pictet-Spengler synthesis and the Bischler-Napieralski synthesis. The Pictet-Spengler synthesis involves the reaction of the corresponding tryptamines with aldehydes. However, aldehydes have the following disadvantages: odor, high toxicity, instability, and storage difficulties. Furthermore, electron-rich aromatic rings favor the ring closure reaction, while electron-deficient aromatic rings are unfavorable, resulting in low yields or even no reaction, limiting their application. The Bischler-Napieralski synthesis involves the cyclization of amide derivatives of the corresponding tryptamines. However, this requires a hydrogenation reduction reaction, such as with a metal hydride, sodium borohydride, or hydrogen catalysis, to reduce the C=N imine bond. However, this hydrogenation reduction reaction is one of the 18 hazardous chemical processes under key regulatory scrutiny, making it relatively dangerous and difficult to scale up for industrial production. Therefore, there is an urgent need for a safe, reliable, and environmentally friendly chemical synthesis method that allows for flexible chemical structure modification and large-scale, targeted synthesis of specific tetrahydro-β-carboline alkaloids.
[0004] Electrochemical synthesis, a green, efficient and versatile synthesis method, has been favored by researchers in academia and industry in recent years. Since the discovery of Kolbe's anodic oxidation reaction in 1848, electrochemical synthesis technology has gradually developed into an indispensable application technology. It uses electrons as reagents and realizes the reduction and oxidation of substances through the gain and loss of electrons, eliminating the disadvantages of using oxidants or reducing agents in traditional chemical reactions. It shows green, environmentally friendly and mild characteristics, and is therefore hailed as a model of "green sustainable" chemistry. Organic electrochemical synthesis has the following advantages: (1) clean, the redox reaction is completed by the gain and loss of electrons, and no additional oxidants and reducing agents are required; (2) mild conditions, organic synthesis can be completed at room temperature and pressure; (3) good selectivity and few by-products; (4) energy saving and easy control, the reaction rate can be completely achieved by adjusting the current, and can be automated and continuous operation, with small scale effects, which is particularly beneficial for the production of fine chemical products.
[0005] Based on this, selecting a suitable electrochemical synthesis method to prepare tetrahydro-β-carboline alkaloids has become an urgent problem to be solved. Summary of the Invention
[0006] In response to the problem that there is no electrochemical synthesis method for the existing synthesis of tetrahydro-β-carboline alkaloids, the present invention provides a method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N to solve the above problem. The present invention synthesizes tetrahydro-β-carboline alkaloids by applying electricity, avoiding the use of flammable, explosive, and chemically active chemical reducing agents and catalysts. The present invention adsorbs the raw material dihydro-β-carboline imine compound on the cathode, and a reduction reaction occurs to generate the corresponding tetrahydro-β-carboline product. The reaction conditions are mild, the yield is high, the operation is simple, and it is convenient for automated continuous operation. The scale effect of scaled-up production is small, and it is easy to industrialize. The reaction mechanism of the present invention is as follows: .
[0007] The technical solutions of the present invention are as follows: The present invention provides a method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N, which adopts the following route: ; in, R1 to R7 are the same or different and are each independently H, F, Cl, Br, I, OH, SH, NO2, NO, NHOH, NH2, C1-C4 alkyl, N-C1-C3 alkyl, C1-C2 alkoxy, phenyl, benzyl, keto, aldehyde, carboxyl, ester, amide, hydrazide, pyridyl, ethoxycarbonyl or cyclohexylaminocarbonyl; R8 to R9 are the same or different and are each independently H, C1-C3 alkyl, phenyl, benzyl, acyl, sulfonyl or p-toluenesulfonyl; It is a single bond or a double bond, and there are two double bonds between N No. 1 and C No. 2 to C No. 5; The method comprises the following steps: placing a compound of formula II in an aqueous solution of an inorganic acid, reacting the compound under electric conditions, and preparing a compound of formula I.
[0008] It should be noted that when R1 to R7 contain easily reducible groups such as nitro, nitroso, and aldehyde, they will be simultaneously reduced to amino, hydroxymethyl, and other groups under electrical conditions.
[0009] Furthermore, R1 to R7 are the same or different and are each independently H, methyl, amino, bromine, methoxy, 3-pyridyl, ethoxycarbonyl or cyclohexylaminocarbonyl; R8~R9 are the same or different, and are independently H, methyl or p-toluenesulfonyl.
[0010] Furthermore, the electrode material used for power supply is at least one of platinum, palladium, rhodium or nickel.
[0011] Furthermore, the voltage of the reaction is 4~10 V.
[0012] Furthermore, the aqueous solution of the inorganic acid is an aqueous solution of sulfuric acid, an aqueous solution of phosphoric acid, an aqueous solution of nitric acid, an aqueous solution of sodium bisulfate or an aqueous solution of potassium bisulfate.
[0013] Furthermore, the concentration of the aqueous solution of the inorganic acid is 1 wt% to 20 wt%.
[0014] Furthermore, the reaction temperature is 10°C to 50°C.
[0015] Furthermore, after the electrical reaction, a post-treatment process is also included, including the following steps: after the reaction is completed, the reaction solution is first extracted with an organic solvent 1 under acidic conditions to remove impurities, and then the pH of the reaction solution is adjusted to alkaline, an organic solvent 2 is added to extract the product, and the product is concentrated under reduced pressure and recrystallized to obtain the compound of formula I.
[0016] Furthermore, the first organic solvent is one of dichloromethane, dichloroethane, chloroform, dibromomethane, dibromoethane, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate or ethyl butyrate.
[0017] Furthermore, the second organic solvent is one of dichloromethane, dichloroethane, chloroform, dibromomethane, dibromoethane, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate or ethyl butyrate.
[0018] Furthermore, the recrystallization solvent is at least one of dichloromethane, dichloroethane, chloroform, dibromomethane, dibromoethane, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, methanol, ethanol or propanol.
[0019] The beneficial effects of the present invention are: The present invention adopts an organic electrochemical reduction method instead of a chemical method to reduce the C=N imine bond of a dihydro-β-carboline intermediate, thereby avoiding the use of flammable, explosive, and chemically active chemical reducing agents and catalysts. Active hydrogen atoms obtained by electrolytic reduction at the cathode directly react with the dihydro-β-carboline imine intermediate that is positively charged and adsorbed on the cathode due to salt formation with an acid to produce a tetrahydro-β-carboline product. The reaction conditions are mild, the yield is high, the operation is simple, and the process is convenient for automated and continuous operation. The scale effect of scaled-up production is small, and the process is easy for industrial production. DETAILED DESCRIPTION
[0020] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present invention.
[0021] Example 1 Synthesis of baldness amine:
[0022] Harmalan is obtained from tryptamine and acetyl chloride via a Bischler-Napieralski reaction. 5% dilute sulfuric acid is added to an H-type electrolytic cell with a Pt electrode at the anode and a Pd electrode at the cathode. Harmalan (18.4 g, 0.1 mol) is then added. The temperature is maintained at 25°C, the DC electrolysis switch is turned on, and the DC voltage is adjusted to 5 V for electrolysis. HPLC is used to monitor the reaction. After completion of the reaction, dichloromethane is added to extract impurities. The organic phase is discarded, and the aqueous phase is adjusted to an alkaline pH with 30% liquid caustic soda. Extraction with dichloromethane is then added, and the organic phase is dried and concentrated to yield a light brown solid. Recrystallization from dichloromethane-methanol yields 17.2 g of balsamina as an off-white solid with a purity of 98.7% and a yield of 92.5%.
[0023] HNMR: (400Hz, DMSO-d6) δ10.7(s, 1H), 7.3(d, 1H), 7.2(d, 1H), 7.0(m,2H), 4.0(m, 1H), 3.1(m, 1H), 2.8(m, 1H), 2.6(m, 2H), 1.3(d, 3H). MS: [M+H] + 187.11.
[0024] Example 2 Synthesis of tetrahydro-β-carboline:
[0025] 5% dilute phosphoric acid was added to an H-type electrolytic cell with a Pt electrode as the anode and a Ni electrode as the cathode. Dihydro-β-carboline (17.0 g, 0.1 mol) was then added. The temperature was controlled at 40°C, the DC electrolysis switch was turned on, and the DC voltage was adjusted to 10 V for electrolysis. The reaction was monitored and tracked by HPLC. After the reaction was completed, ethyl acetate was added to extract impurities, and the organic phase was discarded. 20% aqueous sodium carbonate solution was added to the aqueous phase to adjust the pH to alkaline, and ethyl acetate was added for extraction. The organic phase was dried and concentrated under reduced pressure to obtain an off-white solid. Ethyl acetate-ethanol was added for recrystallization to obtain 16.4 g of tetrahydro-β-carboline as a white solid product with a purity of 98.4% and a yield of 95.3%.
[0026] HNMR: (400Hz, DMSO-d6) δ11.4(s, 1H), 7.4(d, 1H), 7.2(d, 1H), 6.9(m,2H), 4.3(m, 2H), 3.3(m, 1H), 3.1(m, 1H), 2.8(m, 2H).
[0027] MS: [M+H] +173.1.
[0028] Example 3 Synthesis of dihydroharmine
[0029] 8% sodium bisulfate aqueous solution was added to an H-type electrolytic cell, the anode and cathode of the electrolytic cell were both Pt electrodes, and then harmine (21.4 g, 0.1 mol) was added, the temperature was controlled at 50°C, the DC electrolysis switch was turned on, the DC voltage was adjusted to 8V for electrolysis, and HPLC was used to monitor and track the reaction. After the reaction was completed, chloroform was added to extract impurities, the organic phase was discarded, 20% potassium carbonate aqueous solution was added to the aqueous phase to adjust the pH to alkaline, chloroform was added for extraction, the organic phase was dried, and concentrated under reduced pressure to obtain a light yellow solid, which was recrystallized by adding chloroform-isopropanol to obtain 19.1 g of an off-white solid product, dihydroharmine, with a purity of 97.9% and a yield of 88.4%.
[0030] HNMR: (400Hz, CDCl3) δ7.6(s, 1H), 7.3(d, 1H), 6.9(d, 1H), 6.7(m, 1H),4.1(m, 1H), 3.8(s, 3H), 3.4(m, 1H), 3.1(m, 1H), 2.7(m, 2H), 1.4(d, 3H).
[0031] MS: [M+H] + 217.13.
[0032] Example 4 Synthesis of 4,9-dimethyl-2-p-toluenesulfonyl-tetrahydro-β-carboline
[0033] 7% dilute sulfuric acid was added to an H-type electrolytic cell with a Pt electrode at the anode and a Rh electrode at the cathode. 4,9-dimethyl-2-(p-toluenesulfonyl)-dihydro-β-carboline (35.2 g, 0.1 mol) was then added. The temperature was maintained at 35°C, the DC electrolysis switch was turned on, and the DC voltage was adjusted to 7 V. The reaction was monitored and tracked using HPLC. After completion of the reaction, dichloroethane was added to extract impurities. The organic phase was discarded, and the aqueous phase was adjusted to an alkaline pH by adding 30% liquid caustic soda. Dichloroethane was then added to extract the organic phase. The organic phase was dried and concentrated under reduced pressure to yield an off-white solid. This was recrystallized from isopropyl alcohol to obtain 32.4 g of 4,9-dimethyl-2-(p-toluenesulfonyl)-tetrahydro-β-carboline as a white solid with a purity of 98.8% and a yield of 91.5%.
[0034] HNMR: (400Hz, CDCl3) δ7.8(d, 2H), 7.5(d, 1H), 7.3(d, 2H), 7.2(d, 1H),7.1(m, 2H), 4.4(d, 1H), 4.2(d, 1H), 3.5(s, 3H), 3.2(m, 3H), 2.4(s, 3H), 1.4(d, 3H).
[0035] MS: [M+H] + 355.15.
[0036] Example 5 Synthesis of 6-Bromo-1-methyl-tetrahydro-β-carboline
[0037] A 6% aqueous potassium bisulfate solution was added to an H-type electrolytic cell, wherein both the anode and the cathode of the electrolytic cell were Pt electrodes. Then, 6-bromo-1-methyl-dihydro-β-carboline (26.3 g, 0.1 mol) was added. The temperature was controlled at 30°C, the DC electrolysis switch was turned on, and the DC voltage was adjusted to 6V for electrolysis. The reaction was monitored and tracked by HPLC. After the reaction was completed, dibromomethane was added to extract impurities, and the organic phase was discarded. 10% liquid caustic soda was added to the aqueous phase to adjust the pH to alkaline, and dibromomethane was added for extraction. The organic phase was dried and concentrated under reduced pressure to obtain an off-white solid. Chloroform-isopropanol was added for recrystallization to obtain 21.9 g of an off-white solid product, 6-bromo-1-methyl-tetrahydro-β-carboline, with a purity of 95.4% and a yield of 82.6%.
[0038] HNMR: (400Hz, CDCl3) δ8.4(s, 1H), 7.7(m, 1H), 7.2(d, 1H), 6.9(m, 1H),4.2(m, 1H), 3.2(m, 1H), 3.1(m, 1H), 2.6(m, 2H), 1.3(d, 3H).
[0039] MS: [M+H] + 265.03 / 267.02.
[0040] Example 6 Synthesis of 1-Methyl-8-amino-tetrahydro-β-carboline
[0041] 5% dilute sulfuric acid was added to an H-type electrolytic cell, the anode and cathode of the electrolytic cell were both Pt electrodes, and then 8-nitro-1-methyl-dihydro-β-carboline (22.9 g, 0.1 mol) was added. The temperature was controlled at 20°C, the DC electrolysis switch was turned on, the DC voltage was adjusted to 4V for electrolysis, and HPLC was used to monitor and track the reaction. After the reaction was completed, ethyl acetate was added to extract impurities, and the organic phase was discarded. 10% liquid caustic soda was added to the aqueous phase to adjust the pH to strong alkalinity, and ethyl acetate was added for extraction. The organic phase was dried and concentrated to obtain a light yellow solid. Isopropanol was added for recrystallization to obtain 17.2 g of a light yellow solid product, 1-methyl-8-amino-tetrahydro-β-carboline, with a purity of 96.8% and a yield of 85.6%.
[0042] HNMR: (400Hz, CDCl3) δ7.5(s, 1H), 7.0(m, 1H), 6.6(m, 2H), 5.4(brs,2H), 4.2(m, 1H), 3.3(m, 1H), 3.1(m, 1H), 2.7(m, 2H), 1.6(brs, 1H), 1.2(d,3H).
[0043] MS: [M+H] + 202.12.
[0044] Example 7 Synthesis of 1-m-pyridyl-tetrahydro-β-carboline-3-carboxylic acid ethyl ester
[0045] 3% dilute phosphoric acid was added to an H-type electrolytic cell with a Pt electrode at the anode and a Ni electrode at the cathode. Ethyl 1-m-pyridyl-dihydro-β-carboline-3-carboxylate (31.9 g, 0.1 mol) was then added. The temperature was maintained at 10°C, the DC electrolysis switch was turned on, and the DC voltage was adjusted to 10V. The reaction was monitored and tracked using HPLC. After completion of the reaction, ethyl acetate was added at 10°C to extract impurities. The organic phase was discarded, and a 10% aqueous sodium carbonate solution was added to the aqueous phase at 10°C to adjust the pH to alkaline. Ethyl acetate was then added for extraction. The organic phase was dried and concentrated under reduced pressure to obtain an off-white solid. Ethyl 1-m-pyridyl-tetrahydro-β-carboline-3-carboxylate was recrystallized from ethyl acetate-isopropanol to obtain 27.9 g of ethyl 1-m-pyridyl-tetrahydro-β-carboline-3-carboxylate as a white solid with a purity of 97.1% and a yield of 86.9%.
[0046] HNMR: (400Hz, CDCl3) δ8.6(m, 2H), 7.9(s, 1H), 7.6(m, 2H), 7.2(m, 2H),7.1(m, 2H), 5.3(s, 1H), 4.3(q, 2H), 4.0(m, 1H), 3.2(m, 1H), 3.0(m, 1H), 1.3(t, 3H).
[0047] MS: [M+H] + 322.15.
[0048] Example 8 Synthesis of N-cyclohexyl-1-methyl-tetrahydro-β-carboline-3-carboxamide
[0049] A 4% aqueous potassium hydrogen sulfate solution was added to an H-type electrolytic cell, wherein the anode and the cathode of the electrolytic cell were both Pt electrodes, and then N-cyclohexyl-1-methyl-dihydro-β-carboline-3-carboxamide (30.9 g, 0.1 mol) was added. The temperature was controlled at 20°C, the DC electrolysis switch was turned on, and the DC voltage was adjusted to 5V for electrolysis. The reaction was monitored and tracked by HPLC. After the reaction was completed, chloroform was added to extract impurities, and the organic phase was discarded. A 10% aqueous potassium hydroxide solution was added to the aqueous phase to adjust the pH to alkaline, and dichloroethane was added for extraction. The organic phase was dried and concentrated under reduced pressure to obtain an off-white solid. Isopropyl alcohol was added for recrystallization to obtain 28.4 g of a white solid product, N-cyclohexyl-1-methyl-tetrahydro-β-carboline-3-carboxamide, with a purity of 99.2% and a yield of 91.3%.
[0050] HNMR: (400Hz, CDCl3) δ7.8(s, 1H), 7.5(d, 1H), 7.3(d, 1H), 7.1(m, 2H), 6.9(d, 1H), 4.2(m, 1H), 3.8(m, 1H), 3.6(m, 1H), 3.3(m, 1H), 2.7(m, 1H), 1.9(m, 2H), 1.6(m, 4H), 1.5(d, 3H), 1.3(m, 4H).
[0051] MS: [M+H] + 312.2 Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and substance of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be readily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.
Claims
1. A method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N, characterized in that: Use the following route: ； in, R1 to R7 are the same or different and are each independently H, F, Cl, Br, I, -OH, -SH, -NO2, -NO, -NHOH, -NH2, C1-C4 alkyl, N-C1-C3 alkyl, C1-C2 alkoxy, phenyl, benzyl, keto, aldehyde, carboxyl, ester, amide, hydrazide, pyridyl, ethoxycarbonyl or cyclohexylaminocarbonyl; R8 to R9 are the same or different and are each independently H, C1-C3 alkyl, phenyl, benzyl, acyl, sulfonyl or p-toluenesulfonyl; It is a single bond or a double bond, and there are two double bonds between N No. 1 and C No. 2 to C No. 5; The method comprises the following steps: placing a compound of formula II in an aqueous solution of an inorganic acid, and preparing a compound of formula I under an electric condition.
2. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 1, characterized in that: The electrode material used for the current flow is at least one of platinum, palladium, rhodium or nickel.
3. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 1, wherein: The aqueous solution of the inorganic acid is an aqueous solution of sulfuric acid, an aqueous solution of phosphoric acid, an aqueous solution of nitric acid, an aqueous solution of sodium bisulfate or an aqueous solution of potassium bisulfate.
4. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 1, wherein: The voltage of the power supply is 4~10V.
5. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 1, wherein: The concentration of the aqueous solution of the inorganic acid is 1 wt % to 20 wt %.
6. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 1, wherein: The reaction temperature is 10 ℃ ~ 50 ℃.
7. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 1, characterized in that: After the electrical reaction, a post-treatment process is also included, including the following steps: after the reaction is completed, the reaction solution is first extracted with an organic solvent 1 under acidic conditions to remove impurities, and then the pH of the reaction solution is adjusted to alkaline, an organic solvent 2 is added to extract the product, and the product is concentrated under reduced pressure and recrystallized to obtain the compound of formula I.
8. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 7, characterized in that: The first organic solvent is one of dichloromethane, dichloroethane, chloroform, dibromomethane, dibromoethane, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate or ethyl butyrate.
9. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 7, characterized in that: The second organic solvent is one of dichloromethane, dichloroethane, chloroform, dibromomethane, dibromoethane, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate or ethyl butyrate.
10. The method for preparing tetrahydro-β-carboline alkaloids by electrochemical reduction of C=N according to claim 7, characterized in that: The recrystallization solvent is at least one of dichloromethane, dichloroethane, chloroform, dibromomethane, dibromoethane, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, methanol, ethanol or propanol.