Method for synthesizing myrcene from monoterpenoid allyl acetate

Abstract

The invention provides a method for synthesizing myrcene from monoterpenoid allyl acetate, which comprises the following steps of: reacting monoterpenoid allyl acetate with strong base in the presence of a phase transfer catalyst, and removing one molecule of acetic acid to obtain a myrcene product. The method has the main advantages that the synthesis route is novel, a large amount of easily available monoterpene allyl acetate is used as a raw material, myrcene is obtained through one-step reaction, the reaction condition is mild, high-temperature and high-pressure equipment is not needed, the reaction yield is high, product separation is simple, and high-purity myrcene can be rapidly obtained. Secondly, the phase transfer catalyst is added in the implementation process, so that the reaction time can be effectively shortened, the reaction speed can be increased, and the reaction energy consumption can be reduced. Compared with a common pinene cracking method for synthesizing myrcene at present, the method has good advantages and certain industrial application prospects.

Description

Technical Field

[0001] The present invention belongs to the fields of fine chemicals and flavor and fragrance, and particularly relates to a method for rapidly and efficiently synthesizing myrcene from monoterpene allyl acetate. Background Art

[0002] Myrcene, also known as β-myrcene, geranene, myrcene, etc., with the scientific name 7-methyl-3-methylene-1,6-octadiene. Myrcene is a colorless or light yellow oily liquid at room temperature and naturally exists in natural essential oils such as bay leaf oil and verbena oil, having a certain sweet orange flavor and balsamic flavor.

[0003] In addition to the above uses in flavor and fragrance, myrcene is also a very important synthetic intermediate. Due to the presence of a single double bond and a pair of conjugated double bonds in its molecule, myrcene can undergo various reactions such as cycloaddition, hydrogenation, epoxidation, chlorination, hydration, ammoniation, etc. to synthesize a series of high-value-added flavor and fragrance chemicals and fine chemicals. For example, myrcene and acrolein undergo a Diels-Alder reaction to obtain citronellal, and citronellal can be further hydrated to obtain lyral. Lyral has a good lily of the valley fragrance, and the aroma is stable and long-lasting (Lin Xiao, Huang Fanglü, Yang Zeyu. Synthesis of Lyral from Myrcene. Guangzhou Chemistry, 2002, 3, 42). Under the action of an acidic catalyst, myrcene and 3-methyl-3-penten-2-one undergo a Diels-Alder reaction to obtain a polysubstituted cyclohexene intermediate, and this intermediate can then be cyclized to obtain ambretone. Ambretone has a unique woody and ambergris fragrance. Because its synthesis method is simple and the cost is relatively low, it is widely used as a commonly used ambergris-type fragrance in perfumes, cosmetics, soaps, etc. (Wang Jin'e, Zhu Yuelin, Xiong Changjian. Sources of Myrcene and Its Applications in Flavor Chemistry. Shandong Chemical Industry, 2011, 40, 47.). In addition, myrcene can be directly hydrated to obtain myrcenol; myrcene reacts with hydrogen chloride to obtain lauryl chloride, which can be further synthesized into nerol, geraniol, and linalool; myrcene ammoniation obtains laurylamine, which can be synthesized into chiral citronellal and menthol, etc.

[0004] Although myrcene is a very useful synthetic intermediate, its source is rather single. At present, myrcene is mainly synthesized industrially by the pyrolysis of β-pinene. β-pinene can be pyrolyzed thermally or catalytically at a high temperature of 500-600 °C to obtain a mixture mainly composed of myrcene, and then rectified and purified to obtain qualified myrcene. The overall yield is only 60-80%, and the by-products are mainly dipentene, allo-ocimene, and some polymers, etc. (Luo Jinyue, Wang Hanzhong, Peng Shujing. Study on the process of preparing myrcene by thermal isomerization of β-pinene. Chemistry and Industry of Forest Products, 2000, 3, 47. Stolle A, Ondruschka B. Synthesis of myrcene by pyrolysis of beta-pinene: Analysis of decomposition reactions. J. Anal. Appl. Pyrol. 2008, 81, 136). β-pinene is derived from turpentine. The collection of turpentine is restricted by factors such as season, climate, and labor, with limited production, large price fluctuations, and unable to meet market demand. Moreover, the price of turpentine has been rising year by year in recent years, and the price of myrcene has also increased accordingly, bringing great pressure to downstream industries.

[0005]

[0006] Isoprene is a by-product of petroleum pyrolysis, with rich sources, low price and easy availability. In recent years, there have been many reports on the dimerization of isoprene to myrcene. Takabe et al. first found that sodium metal can catalyze the dimerization of isoprene to obtain myrcene, but the reaction yield is only 5% (Takabe K., Katagiri T., Tanaka J. Formation of myrcene in the sodium-catalyzed oligomerization of isoprene. Bulletin of the Chemical Society of Japan, 1972, 45, 2662). Patent DE2451575C3 discloses that adding DIPEA in the isoprene dimerization reaction can increase the myrcene yield to about 9%. Patent DE2542798C3 discloses that adding TMEDA in the reaction system can also increase the myrcene yield. Fan Cunliang et al. in China further improved the isoprene dimerization process by using a mixed catalyst of sodium and potassium, and the myrcene yield can reach 40% (Fan Cunliang, Xu Peiruo. Synthesis of myrcene from isoprene. Fine Chemicals, 2002, 19, 137.). Generally speaking, the yield of the dimerization of isoprene to myrcene is still low, and the catalyst cost is expensive. Compared with the synthesis of myrcene by β-pinene pyrolysis, there is no cost advantage, and there is basically no report on large-scale production at present.

[0007]

[0008] Some monoterpene allyl acetates such as linalyl acetate, neryl acetate, geranyl acetate, etc. can not only be isolated from natural essential oils, but also have been produced on a large scale through petrochemical routes. These esters have a structure similar to that of myrcene, so myrcene can be synthesized through an elimination reaction. For example, Tsuji et al. used palladium acetate - triphenylphosphine as a catalyst to catalyze the elimination reactions of linalyl acetate, neryl acetate, and geranyl acetate, and myrcene could be obtained in a yield of 60 - 74%*(Tsuji J., Yamakawa T., Kaito M., et al. Formation of a terminal conjugated diene system by the palladium - catalyzed elimination reactions of allylic acetates and phenyl ethers. Tetrahedron Lett., 1978, 19, 2075.). Barry et al. used molybdenum carbonyl and N,O - bis(trimethoxy)acetamide as catalysts to catalyze the elimination reactions of linalyl acetate and neryl acetate, and myrcene was obtained in a yield of about 75% (Trost B.M., Lautens M., Peterson B.M. Molybdenum - catalyzed eliminations of allylic acetates; New diene synthesis. Ibid, 1983, 24, 4525.). Adriano et al. used Pd(PPh3)4 as a catalyst and triethylamine as a solvent to catalyze the elimination reaction of linalyl acetate, and the selectivity of myrcene was increased to over 95%, but the yield was only 66% (Carpita A., Bonaccorsi F., Rossi R. Synthesis of 2 - substituted 1,3 - butadienyl compounds by palladium - catalyzed regioselective 1,2 - elimination reaction of methylvinylcarbinol acetates. Ibid, 1984, 25, 5193.).

[0009]

[0010] In summary, the current main synthesis method of myrcene is the pyrolysis of β-pinene. The pyrolysis reaction conditions are harsh and the yield is not high. Moreover, in recent years, the price of β-pinene has been rising year by year, and the economy of this route is getting worse and worse. Other synthetic routes of myrcene, such as the dimerization of isoprene and the synthesis of myrcene from monoterpene allyl acetates catalyzed by precious metals, basically have the disadvantages of expensive catalysts and poor selectivity of myrcene, making it difficult to produce on a large scale. Therefore, if a new method can be discovered to improve the yield of myrcene synthesis from monoterpene allyl acetates, while having low catalyst cost and simple operation, the disadvantages of the existing synthetic routes can be overcome, and myrcene products can be obtained more efficiently, environmentally friendly and with atomic economy. Summary of the Invention

[0011] The object of the present invention is to provide a method for simply, efficiently and low-cost synthesizing myrcene using monoterpene allyl acetate as a raw material.

[0012] To achieve the above object and technical effects, the present invention adopts the following technical solution: in the presence of a phase transfer catalyst, the monoterpene allyl acetate reacts with a strong base to remove one molecule of acetic acid to obtain a myrcene product.

[0013] In the present invention, the monoterpene allyl acetate is linalyl acetate, geranyl acetate, neryl acetate, etc.

[0014] In the present invention, the phase transfer catalyst can be, but is not limited to, one or more of quaternary ammonium salts, quaternary phosphonium salts, crown ethers, etc., such as tetrabutylammonium chloride, tetrabutylammonium bromide, benzyltriethylammonium chloride, trioctylmethylammonium chloride, tetrabutylphosphonium bromide, 18-crown-6 ether, etc. Among them, tetrabutylammonium bromide is preferred. The dosage of the phase transfer catalyst is 1.0-10.0 mol% of the molar amount of the monoterpene allyl acetate, preferably 2.0-5.0 mol%.

[0015] In the present invention, the strong base can be, but is not limited to, one or more of sodium carbonate, potassium carbonate, cesium carbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, sodium methoxide, sodium ethoxide, sodium tert-butoxide, potassium tert-butoxide, sodium hydride, calcium hydride, etc. Among them, sodium tert-butoxide and potassium tert-butoxide are preferred; the dosage of the strong base is 100-500 mol% of the molar amount of the monoterpene allyl acetate, preferably 200-400 mol%.

[0016] In the present invention, the deprotection reaction is carried out in a polar aprotic solvent, and the solvent is selected from one or more of diethyl ether, propyl ether, isopropyl ether, methyl tert-butyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, dichloromethane, 1,2-dichloroethane, chlorobenzene, dimethyl sulfoxide, N,N-dimethylformamide, and among them, tetrahydrofuran and 2-methyltetrahydrofuran are preferably used as the reaction solvents; preferably, the amount of the solvent used is 3 to 5 times the mass of the monoterpene allyl acetate.

[0017] In the present invention, the reaction temperature of the deprotection reaction is 20 to 60 °C, preferably 40 to 60 °C; and / or: the reaction pressure is normal pressure, and the reaction time is 4 to 8 hours.

[0018] In the present invention, after the deprotection reaction is completed, the reaction solution is directly filtered to remove the excessive strong base and salt, and a crude myrcene product is obtained.

[0019] In the present invention, the crude myrcene product can be purified by distillation or rectification, and the purity of the final product is 93.0% or higher.

[0020] Adopting the above technical solutions, the present invention has the following positive effects:

[0021] 1. The raw material monoterpene allyl acetate can be synthesized through a petrochemical route, is readily available in large quantities, and has a stable cost;

[0022] 2. Under the action of strong bases such as sodium tert-butoxide and potassium tert-butoxide, the monoterpene allyl acetate selectively removes acetic acid with high chemical selectivity to obtain myrcene, and there are basically no isomers such as allo-ocimene, facilitating the obtaining of high-purity myrcene;

[0023] 3. This method has simple operation, mild reaction conditions such as temperature and pressure, and is suitable for the scale-up production of myrcene. Detailed Embodiments

[0024] The present invention will be described in detail below through examples, but the present invention is not limited to the following examples.

[0025] The information of the main raw materials is as follows:

[0026] Linalyl acetate, geranyl acetate, neryl acetate, self-made, 99% (GC);

[0027] Sodium tert-butoxide, potassium tert-butoxide, Aladdin Chemistry, AR; sodium hydroxide, potassium hydroxide, Sinopharm Reagent, AR;

[0028] Tetrabutylammonium chloride, tetrabutylammonium bromide, tetrabutylphosphonium bromide, 18-crown-6 ether, Aladdin Chemistry, 98%;

[0029] Anhydrous tetrahydrofuran, anhydrous 2-methyltetrahydrofuran, anhydrous methyl tert-butyl ether, n-hexane, Aladdin Reagent, chromatographically pure;

[0030] The gas chromatography test conditions of the present invention are as follows:

[0031] Instrument model: Shimadzu GC; chromatographic column: Agilent cyclodex - B (30m×0.25mm×0.25μm); column temperature: initial temperature 60°C, heated to 120°C at a rate of 5°C / min, then heated to 200°C at a rate of 10°C / min, and finally heated to 220°C at a rate of 20°C / min and held for 6 min; injection port temperature: 280°C; FID detector temperature: 300°C; split injection, split ratio 60:1; injection volume: 2.0 μL; H2 flow rate: 40 mL / min; air flow rate: 360 mL / min.

[0032] Example 1

[0033] In a nitrogen atmosphere and at room temperature, anhydrous tetrahydrofuran (393 g), linalyl acetate (98.14 g, 0.5 mol), and tetrabutylammonium bromide (3.22 g, 0.01 mol) were successively added to a 2 - L three - necked flask. Stirring was started. After the tetrabutylammonium bromide was completely dissolved, potassium tert - butoxide (112.21 g, 1.0 mol) was added to obtain a suspension. The three - necked flask was placed in an oil bath at 50°C and reacted under rapid stirring at atmospheric pressure. After 4 hours, GC detection showed that the raw material linalyl acetate had completely reacted. The three - necked flask was removed from the oil bath and allowed to stand. After the reaction solution cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary - evaporated to remove the solvent tetrahydrofuran to obtain a light - yellow crude product. The crude product was dissolved in n - hexane (150 mL). The obtained myrcene solution was successively washed with saturated aqueous sodium bicarbonate solution and saturated brine, phase - separated, dried over anhydrous sodium sulfate, filtered to remove sodium sulfate, and then rotary - evaporated to remove n - hexane to obtain a crude myrcene product. Finally, the product myrcene was obtained by vacuum distillation, with a mass of 63.74 g, a purity of 95%, and a yield of 88.9%.

[0034] Example 2

[0035] In a nitrogen atmosphere, at room temperature, anhydrous tetrahydrofuran (491 g), linalyl acetate (98.14 g, 0.5 mol), and tetrabutylammonium bromide (1.61 g, 0.005 mol) were successively added to a 2 L three-necked flask. Stirring was started, and after the tetrabutylammonium bromide was completely dissolved, potassium tert-butoxide (224.42 g, 2.0 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 30 °C and reacted under rapid stirring at atmospheric pressure. After 8 hours, GC detection showed that the raw material linalyl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution had cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary evaporated to remove the solvent tetrahydrofuran to obtain a light yellow crude product. The crude product was dissolved in n-hexane (150 mL), and the resulting myrcene solution was successively washed with saturated aqueous sodium bicarbonate and saturated brine, phase-separated, dried over anhydrous sodium sulfate, filtered to remove the sodium sulfate, and then rotary evaporated to remove the n-hexane to obtain a crude myrcene product. Finally, the product myrcene, 62.48 g, with a purity of 93% and a yield of 85.3% was obtained by vacuum distillation.

[0036] Example 3

[0037] In a nitrogen atmosphere, at room temperature, anhydrous tetrahydrofuran (294 g), linalyl acetate (98.14 g, 0.5 mol), and tetrabutylammonium bromide (16.12 g, 0.05 mol) were successively added to a 2 L three-necked flask. Stirring was started, and after the tetrabutylammonium bromide was completely dissolved, potassium tert-butoxide (84.16 g, 0.75 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 60 °C and reacted under rapid stirring at atmospheric pressure. After 4 hours, GC detection showed that the raw material linalyl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution had cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary evaporated to remove the solvent tetrahydrofuran to obtain a light yellow crude product. The crude product was dissolved in n-hexane (150 mL), and the resulting myrcene solution was successively washed with saturated aqueous sodium bicarbonate and saturated brine, phase-separated, dried over anhydrous sodium sulfate, filtered to remove the sodium sulfate, and then rotary evaporated to remove the n-hexane to obtain a crude myrcene product. Finally, the product myrcene, 63.06 g, with a purity of 93% and a yield of 86.1% was obtained by vacuum distillation.

[0038] Example 4

[0039] In a nitrogen atmosphere, at room temperature, anhydrous tetrahydrofuran (393 g), linalyl acetate (98.14 g, 0.5 mol), and tetrabutylammonium chloride (2.78 g, 0.01 mol) were successively added to a 2 L three-necked flask. Stirring was started. After the tetrabutylammonium chloride was completely dissolved, sodium tert-butoxide (96.10 g, 1.0 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 50 °C and reacted under rapid stirring at normal pressure. After 6 hours, GC detection showed that the raw material linalyl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary evaporated to remove the solvent tetrahydrofuran to obtain a light yellow crude product. The crude product was dissolved in n-hexane (150 mL). The obtained myrcene solution was successively washed with saturated aqueous sodium bicarbonate solution and saturated brine, phase-separated, dried over anhydrous sodium sulfate, filtered to remove sodium sulfate, and then rotary evaporated to remove n-hexane to obtain a crude myrcene product. Finally, vacuum distillation was carried out to obtain 66.23 g of the product myrcene, with a purity of 94% and a yield of 91.4%.

[0040] Example 5

[0041] In a nitrogen atmosphere, at room temperature, anhydrous 2-methyltetrahydrofuran (393 g), linalyl acetate (98.14 g, 0.5 mol), and 18-crown-6 ether (2.64 g, 0.01 mol) were successively added to a 2 L three-necked flask. Stirring was started. After the 18-crown-6 ether was completely dissolved, sodium hydroxide (60 g, 1.5 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 60 °C and reacted under rapid stirring at normal pressure. After 8 hours, GC detection showed that the raw material linalyl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary evaporated to remove the solvent 2-methyltetrahydrofuran to obtain a light yellow crude product. The crude product was dissolved in n-hexane (150 mL). The obtained myrcene solution was successively washed with saturated aqueous sodium bicarbonate solution and saturated brine, phase-separated, dried over anhydrous sodium sulfate, filtered to remove sodium sulfate, and then rotary evaporated to remove n-hexane to obtain a crude myrcene product. Finally, vacuum distillation was carried out to obtain 54.57 g of the product myrcene, with a purity of 94% and a yield of 75.3%.

[0042] Example 6

[0043] In a nitrogen atmosphere, at room temperature, anhydrous methyl tert-butyl ether (393 g), linalyl acetate (98.14 g, 0.5 mol), and tetrabutylphosphonium bromide (3.39 g, 0.01 mol) were successively added to a 2 L three-necked flask. Stirring was started. After tetrabutylphosphonium bromide was completely dissolved, potassium hydroxide (70.13 g, 1.25 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 60 °C and reacted under rapid stirring at atmospheric pressure. After 8 hours, GC detection showed that the raw material linalyl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary evaporated to remove the solvent methyl tert-butyl ether to obtain a light yellow crude product. The crude product was dissolved in n-hexane (150 mL). The resulting myrcene solution was successively washed with saturated aqueous sodium bicarbonate solution and saturated brine, phase-separated, dried over anhydrous sodium sulfate, filtered to remove sodium sulfate, and then rotary evaporated to remove n-hexane to obtain a crude myrcene product. Finally, the product myrcene, 56.03 g, with a purity of 93% and a yield of 76.5% was obtained by vacuum distillation.

[0044] Example 7

[0045] In a nitrogen atmosphere, at room temperature, anhydrous tetrahydrofuran (393 g), neryl acetate (98.14 g, 0.5 mol), and tetrabutylammonium chloride (2.78 g, 0.01 mol) were successively added to a 2 L three-necked flask. Stirring was started. After tetrabutylammonium chloride was completely dissolved, sodium tert-butoxide (96.10 g, 1.0 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 50 °C and reacted under rapid stirring at atmospheric pressure. After 6 hours, GC detection showed that the raw material neryl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was rotary evaporated to remove the solvent tetrahydrofuran to obtain a light yellow crude product. The crude product was dissolved in n-hexane (150 mL). The resulting myrcene solution was successively washed with saturated aqueous sodium bicarbonate solution and saturated brine, phase-separated, dried over anhydrous sodium sulfate, filtered to remove sodium sulfate, and then rotary evaporated to remove n-hexane to obtain a crude myrcene product. Finally, the product myrcene, 65.58 g, with a purity of 94% and a yield of 90.5% was obtained by vacuum distillation.

[0046] Example 8

[0047] Under a nitrogen atmosphere, at room temperature, anhydrous tetrahydrofuran (393 g), geranyl acetate (98.14 g, 0.5 mol), and tetrabutylammonium chloride (2.78 g, 0.01 mol) were successively added to a 2 L three-necked flask. Stirring was started, and after the tetrabutylammonium chloride was completely dissolved, sodium tert-butoxide (96.10 g, 1.0 mol) was added to obtain a suspension. The three-necked flask was placed in an oil bath at 50 °C and reacted under rapid stirring at atmospheric pressure. After 6 hours, GC detection showed that the raw material geranyl acetate had completely reacted. The three-necked flask was removed from the oil bath and allowed to stand. After the reaction solution had cooled to room temperature, it was filtered to remove the insoluble salts. The filtrate was concentrated by rotary evaporation to remove the solvent tetrahydrofuran, yielding a light yellow crude product. The crude product was dissolved in n-hexane (150 mL), and the resulting myrcene solution was washed successively with saturated aqueous sodium bicarbonate and saturated brine, then phase-separated, dried over anhydrous sodium sulfate, filtered to remove the sodium sulfate, and then concentrated by rotary evaporation to remove the n-hexane to obtain a crude myrcene product. Finally, the product myrcene (64.39 g) with a purity of 95% and a yield of 89.8% was obtained by vacuum distillation.

Claims

1. A method for synthesizing myrcene from monoterpene allyl acetate, the method comprising: In the presence of a phase transfer catalyst, monoterpene allyl acetate reacts with a strong base to obtain myrcene.

2. The method according to claim 1, wherein The phase transfer catalyst includes one or more of tetrabutylammonium chloride, tetrabutylammonium bromide, benzyltriethylammonium chloride, trioctylmethylammonium chloride, tetrabutylphosphonium bromide, and 18-crown-6 ether.

3. The method according to claim 2, wherein The amount of the phase transfer catalyst used is 1.0 to 10.0 mol%, preferably 2.0 to 5.0 mol%, based on the molar amount of monoterpene allyl acetate.

4. The method according to claim 1, characterized in that The strong base includes one or more of sodium carbonate, potassium carbonate, cesium carbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, sodium methoxide, sodium ethoxide, sodium tert-butoxide, potassium tert-butoxide, sodium hydride, and calcium hydride.

5. The method according to claim 4, wherein The amount of the strong base used is 100 to 500 mol%, preferably 200 to 400 mol%, based on the molar amount of monoterpene allyl acetate.

6. The method according to any one of claims 1-5, characterized in that, The reaction is carried out in a polar aprotic solvent, and the solvent is preferably selected from one or more of diethyl ether, propyl ether, isopropyl ether, methyl tert-butyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, dichloromethane, 1,2-dichloroethane, chlorobenzene, dimethyl sulfoxide, and N,N-dimethylformamide.

7. The method according to claim 6, characterized in that, The amount of the solvent used is 3 to 5 times the mass of monoterpene allyl acetate.

8. According to the synthesis method described in claim 1, the reaction temperature is 20 to 60 °C, preferably 40 to 60 °C; and / or: the reaction pressure is atmospheric pressure, and the reaction time is 4 to 8 hours.

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