Reaction Mechanism And Process Study Of Acid-catalyzed Conversion Of Sesamin To Asarinin

Asarinin is a kind of furofuran lignan with various physiological activities such as anti-inflammatory,antibacterial and antioxidant,which has received more and more attention.Due to the limited source of asarinin and the complicated operation and low yield of the traditional asarinin preparation method,it is urgent to develop a novel and efficient method for the preparation of asarinin.Sesamin,the most abundant sesame lignan in sesame,was found to be converted into more physiologically active asarinin.Therefore,a systematic study on the chemically catalyzed conversion of sesamin into asarinin was conducted in this project using sesamin as raw material and the main works and conclusions are as follows:(1)Mechanistic study of Br(?)nsted acid-catalyzed conversion of sesamin to asarinin:The effects of acid type and acid strength on the conversion of sesamin to asarinin were investigated,and the reaction mechanism of the conversion of sesamin to asarinin was studied by kinetic and theoretical calculations.The results showed that strong Br(?)nsted acid(B acid)sites played an important role in the conversion of sesamin to asarinin,and the catalytic activity was positively correlated with the acid strength.Phosphotungstic acid(HPW)with super strong B acidity could efficiently promote the conversion of sesamin to asarinin with a turnover number as high as 19.70 mol.The first-level kinetic model was fitted with the experimental results,and the activation energy E_a of phosphotungstic acid-catalyzed conversion of sesamin to asarinin in the anhydrous ethanol system was obtained as105.25±0.05 k J/mol.Through theoretical calculations and isotope tracing NMR experiments,it was determined that the B-acid proton only triggered the formation of a carbon-positive ion intermediate,and sesamin was catalyzed by the B-acid proton through two transition states and the reaction mechanism of the conversion of sesamin to asarinin is a"four-ligand"transition state pathway.(2)Process study on the conversion of sesamin to asarinin catalyzed by phosphotungstic acid:The process of converting sesamin to asarinin was optimized by response surface methodology,and the kinetics of converting sesamin to asarinin was investigated,and finally,the reusability and stability of phosphotungstic acid were explored.The results showed that tetrahydrofuran has excellent performance and is suitable as a reaction solvent for sesamin conversion.The response surface experiments showed that the dosage of phosphotungstic acid,reaction time and sesamin initial concentration had significant effects on the yield of asarinin,and the order of influence was HPW dosage>reaction time>sesamin initial concentration.The optimum process conditions were 0.50%initial sesamin concentration,3.70%phosphotungstic acid dosage,and 3.50 h reaction time,under which the yield of asarinin was53.33%±0.35.The reaction activation energy E_a=114.32±1.50 k J/mol for the phosphotungstic acid-catalyzed sesamin conversion in tetrahydrofuran was obtained by the first-stage reaction kinetic fitting and calculation.Repeated utilization experiments showed that phosphotungstic acid exhibited good stability in this isomerization reaction,and the catalytic activity remained84.29%after six repetitions.(3)Conversion of sesamin to asarinin catalyzed by phosphotungstic acid in a solvent-free system:The effect of grinding,heating and microwaving on the yield of asarinin was investigated in a solvent-free system to study the reaction kinetics of the reaction.The results showed that grinding,heating and microwave can promote the conversion of sesamin to asarinin,and by grinding,the conversion of sesamin to asarinin can be catalyzed by phosphotungstic acid at room temperature with mild reaction conditions.In the measured heating temperature range(80～110~oC),the higher the temperature,the higher the yield of asarinin,and when the temperature was 110~oC and the reaction time was 30 min anyway,the yield of asarinin was 50.02%.When the microwave power was 540 W and the reaction time was 7 min,the yield of asarinin was 50.09%,which significantly shortened the reaction time.The kinetic model was fitted with the experimental results to obtain the activation energy E_aof 292.99±4.25 k J/mol for the conversion of sesamin to asarinin catalyzed by phosphotungstic acid in the solvent-free heating system.(4)Study on the separation and purification of sesamin and asarinin:The mixture of sesamin and asarinin was separated by thin-layer chromatography and recrystallization method.The experimental results showed that sesamin and asarinin could be successfully separated by thin-layer chromatography,and the purity of sesamin and asarinin reached97.65±0.66%and 97.27±0.29%,respectively,at a spot sample size of 80 mg.The separation of sesamin and asarinin could be effectively carried out by recrystallization method,under the conditions of using glacial acetic acid as the crystallization solvent,crystallization temperature of 45~oC and crystallization time of 40 min,high contents of sesamin(69.46%)and asarinin(71.32%)were obtained in unrestricted amounts.In summary,this study investigated the effect of reaction conditions on the conversion of acid-catalyzed sesamin into asarinin and the reaction kinetics in different systems,elucidated the reaction mechanism of sesamin into asarinin,optimized the process conditions for acid-catalyzed conversion of sesamin into asarinin,explored the method of separating sesamin and asarinin in large quantities,and provided a new way for the preparation ofasarinin,as well as a theoretical basis and technical support for the industrial generation of sesamin oil rich in asarinin.