green solvents and catalysts for the preparation of pyrazine derivatives

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DESs: green solvents and catalysts for the preparation of pyrazine derivatives by self-condensation of D-glucosamine Mengjie Wu, Hui Ma, Zhongyi Ma, Yufang Jin, Chunyan Chen, Xiaoya Guo, Yan Qiao, Christian Marcus Pedersen, Xianglin Hou, and Yingxiong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01788 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Sustainable Chemistry & Engineering

DESs: green solvents and catalysts for the preparation of pyrazine derivatives by selfcondensation of D-glucosamine ‖

※

Mengjie Wu,†,‡ Hui Ma,† Zhongyi Ma, Yufang Jin, Chunyan Chen,†,‡ Xiaoya Guo,‡ Yan Qiao,

‖

Christian Marcus Pedersen,§ Xianglin Hou,† Yingxiong Wang*,† †

Shanxi Engineering Research Center of Biorefinery, Institute of Coal Chemistry, Chinese

Academy of Sciences, 27 South Taoyuan Road, Taiyuan, 030001, P. R. China ‡

Department of Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai,

200444, P. R. China ‖

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Sciences, 27 South Taoyuan Road, Taiyuan, 030001, P. R. China ※

Chemical Biology Ministry of Education Key Laboratory of Molecular Engineering; The

institute of Molecular Science, Shanxi University, 580 Wucheng Road, Taiyuan, 030006, P. R. China §

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100

Copenhagen, Denmark

ACS Paragon Plus Environment

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Corresponding Author E-mail: [email protected] ABSTRACT: Deep eutectic solvents (DESs) exhibit similar physicochemical properties to the ionic liquids. They are inexpensive, renewable, non-toxic and environmental benign solvents and have gradually attracted attention in several fields, for example biorefinery. Here choline chloride-based DESs have been used as solvents and catalysts for the preparation of deoxyfructosazine (DOF) through a self-condensation reaction of D-glucosamine (GlcNH2). The catalytic performances of a “green co-catalyst”, amino acids, and the reaction mechanism were also studied. The results displayed that choline chloride/urea was capable to convert GlcNH2 efficiently, with a 13.5% yield of DOF at low temperature and with a short reaction time (100 °C, 150 min). Among the screened amino acids, arginine showed the highest activity and gave the highest yield of DOF (30.1%) under the optimized reaction conditions. Nuclear magnetic resonance (NMR) studies revealed a strong hydrogen bond interaction between GlcNH2 and arginine. Moreover, a detectable intermediate, namely dihydrofructosazine, in the condensation of GlcNH2 to DOF/fructosazine (FZ) was captured by in situ NMR technique. KEYWORDS: Deep eutectic solvents, NMR, glucosamine, arginine, deoxyfructosazine

INTRODUCTION: Chitin biomass is the most common polysaccharide after cellulose and is usually obtained from industrial marine bio-waste.1 Chitin and chitosan (the deacetylation derivative of chitin) are cheap, non-toxic and bio-degradable, with a high biologically-fixed nitrogen content, but they are insoluble in most common solvents (except for acidic aqueous solution, like 3 wt% acetic acid). Extensive studies focusing on the cellulose and xylose biomass have been carried out, whereas chitin biomass conversion has remained somewhat underdeveloped as research area.2-3 In order to convert the chitin biomass into transportation


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fuels and valuable added fine chemicals efficiently, development of chemical processes and an understanding of the reaction mechanism is urgently needed.4-8 Deoxyfructosazine (DOF) and fructosazine (FZ) are nitrogen containing pyrazine derivatives, which are widely used as flavoring agents in the food and tobacco industries. Recently, DOF has been found to exhibit pharmaceutical activity for diabetes, cancer and immunological diseases, and FZ has been used as a reagent for DNA strand cleavage.9-11 These two high-value heterocyclic compounds are naturally formed through the Maillard reaction between reducing sugars (such as glucose) and ammonia-containing compounds.12 The chemical synthesis routes of DOF from monosaccharides (such as glucose and fructose) and polysaccharides (such as cellobiose and inulin) have been developed using ammonium salts as catalysts in weak acid aqueous solution.13-15 Additional nitrogen-resources were required for all these reaction routes, which results in unfavorable atom economy and oppose green chemistry principles. DOF and FZ have also been obtained from self-condensation of D-glucosamine (GlcNH2), the monomer of chitosan, in neutral pH solution with phosphate buffers11 or by using NH3 aqueous solution.10 Phenylboronic acid or boric acid are usually employed as catalysts for these reactions.16 In the process, two GlcNH2 molecules are condensed to DOF or FZ, leaving additional nitrogenresources unnecessary and hence improve the atom economy. Ionic liquids (ILs) have been widely used in material synthesis, dissolution of biomass, biorefinery and catalytic reactions.17-21 Recently, we found that basic ILs could catalyze the selfcondensation of GlcNH2 in a polar aprotic solvent, such as DMSO, to produce DOF and FZ.22-23 However, ILs are limited for industrial use due to their toxicity and high prices.24 In addition, the reaction media, like organic solvents, also bring the risk to food and pharmaceutical applications. Deep eutectic solvents (DESs), which are usually liquids with the melting point lower than


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100 °C, share similar physicochemical properties with imidazolium-based ILs.25 Moreover, DESs are inexpensive, non-toxic, renewable and convenient for large-scale preparation through heating or grinding mixtures of hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA).20,

26-27

By varying the reagents, the properties of DES can be tailored for different

purposes.28-30 For example, choline chloride (ChCl)-based DESs can be adjusted from acidic to neutral and then basic, through changing the HBDs from oxalic acid to glycerol and urea, respectively.31 Due to this flexibility, DESs are frequently used as a reaction media to replace ILs in separation,32-33 gas capture34 and biorefinery.29,

35

Hu and co-workers investigated 5-

hydroxymethylfurfural (5-HMF) production from fructose or inulin in acidic ChCl-based DESs (ChCl/oxalic acid and ChCl/citric acid).36-37 Prasad et al. reported the unique abilities of ChCl/urea (CCU) and ChCl/thiourea (CCT) to dissolve chitin biomass even at room temperature allowing chitin biomass conversion.38-39 In this report, we developed an economic and environmental-friendly method applying DESs to convert GlcNH2 into DOF and FZ (Scheme 1). The effects of reaction conditions, such as temperature and ratio of HBA to HBD, on the DOF yields were investigated. ChCl-based DESs provided the dual function of both solvents and catalysts. Furthermore, non-toxic amino acids were found to be outstanding co-catalysts for the self-condensation reaction, and arginine is the most effective co-catalyst. Finally, the reaction mechanism was probed by in situ nuclear magnetic resonance (NMR) technique.


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Scheme 1. Self-condensation of GlcNH2 to DOF and FZ in DESs. EXPERIMENTAL SECTION: Materials. GlcNH2 hydrochloride was purchased from GoldenShell Biochemical Co., Ltd. Deuterium oxide (D2O, 99.8 atom% D), dimethyl sulfoxide-d6 (DMSO-d6, 99.9 atom% D) and sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) was supplied by Qingdao Teng long Microwave Technology Co., Ltd. 1-Ethyl-3-methylimidazolium acetate ([C2C1Im]OAc) was purchased from Shanghai Cheng Jie Chemical Co., Ltd. ChCl, N,N'dimethyl urea (DMU), 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), pyrazine and amino acids (arginine, lysine, histidine, proline, glycine, aspartic acid and glutamic acid) were analytical grade and obtained from Aladdin Co., Ltd. Urea, glycerol, thiourea, levulinic acid, sodium acetate and triethylamine were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification. Deionized water was used in all experiments. GlcNH2 used in this study was prepared by mixing GlcNH2 hydrochloride with triethylamine and dichloromethane at room temperature with constant stirring for 2-3 days. Then the mixture was extracted 5 times by dichloromethane to completely remove triethylamine hydrochloride.23 General procedure for the DESs preparation. The mixture of ChCl and HBDs at a defined molar ratio (Table 1) was heated at 80 °C under constant stirring until a homogeneous liquid was


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obtained. All synthesized DESs were allowed reach room temperature and were dried under vacuum at 55 °C overnight. General reaction procedure. In a typical procedure, 2 mmol (0.358 g) GlcNH2 was dissolved in 20 mmol DES, for example 5.195 g CCU, with or without co-catalysts. Then the mixture was heated and stirred in oil bath with a magnetic stirrer. At certain times, 0.1 ml reaction mixture was taken out and immediately cooled in an ice bath to stop the reaction. When DBU and triethylamine were used as co-catalysts, the following reaction procedure was applied. The mixture made of GlcNH2, DESs and DBU/triethylamine was added in a 10 ml stainless steel vessel with a Teflon lining and a magnetic stirrer, and then sealed by a screw cap. The vessel was immerged in an oil bath with constantly stirring, and at certain times the vessel was taken out and immediately cooled in an ice bath to stop the reaction. NMR sample preparation. To observe the

13

C chemical shift variations (∆δ) of GlcNH2 with

increasing arginine dosage, samples were prepared by adding different amount of arginine and 20 mg GlcNH2 in 0.5 ml DMSO-d6. After ultrasonic treatment, the samples were transferred into 5 mm NMR tubes and measured by NMR spectrometer. Procedure for in situ NMR study. Owing to high viscosity of CCU DES, in situ NMR experiment could not be carried out. Thus, the following procedure was applied. 0.358 g GlcNH2 and 0.174 g arginine were dissolved in 5.195 g CCU. Then, the mixture was heated at 60 °C and stirred. At certain times, 0.1 mL reaction mixture was taken out and mixed with 0.4 mL cold D2O with DSS, which provided a field lock and shift calibration. Then, the sample was immediately transferred to the NMR spectrometer for 1H measurement. These samples were also used to acquire 2D 1H-13C HSQC and 1H-1H COSY spectra.


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Characterization. 1D and 2D NMR spectra were collected on a Bruker AV-III 400 MHz NMR spectrometer (9.39 T). The structures of products were characterized by 1H and 13C NMR using D2O at 400.13 MHz and 100.61 MHz, respectively. Quantitative 1H NMR measurement. The GlcNH2 conversions and product yields were evaluated by quantitative 1H NMR spectroscopy (1H qNMR) (pulse sequence: zg, relaxation delay: 15 s, number of scans: 16). 0.3 mg·mL-1 pyrazine was prepared in D2O, and used as a standard solution for the 1H qNMR measurement.40 The 1H qNMR sample was prepared by mixing 0.10 mL reaction mixture and 0.35 mL standard solution. The sample was added into a 5 mm NMR sample tube and immediately transferred to the spectrometer followed by a 1H qNMR measurement. Based on the 1H qNMR spectra, the product yields were calculated following the procedure described by Rundlöf et al.40 (see Figure S1). To increase the integral accuracy in the 1

H qNMR spectrum, a peak deconvolution method was applied, which effectively separated and

integrated overlapping peaks. The yields of products were calculated by following equation (1): Yield = 2 ×

× 100%

(1)

RESULTS AND DISCUSSION: The effect of nature of DESs on product yields. With easy access to DESs with different properties,41 several ChCl-based DESs were employed as reaction media for the selfcondensation of GlcNH2 at 100 °C, including CCU, ChCl/DMU (CCD), CCT, ChCl/glycerol (CCG) and ChCl/levulinic acid (CCL). These DESs could be classified into amide-based/basic DESs (CCU, CCD and CCT), alcohol-based/neutral DES (CCG) and Brønsted acid-based/acidic DES (CCL). Firstly, GlcNH2 was dissolved in CCU at 100 °C. The process was monitored by 1H NMR in time (see Figure S2). The growth of peaks at 8.37/8.53 ppm and 8.57 ppm are attributed to the formation of products, and the gradually disappearing signals at 5.44 ppm and 4.94 ppm


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are assigned to the α/β anomers of GlcNH2. The chemical structures of the main products and CCU are characterized by 1H, 13C NMR and 1H-13C HSQC NMR spectra (Figure S3-S5). These compounds are pyrazine derivatives, based on the authentic NMR spectra previously reported.22 As no catalyst was employed in GlcNH2-CCU reaction system, CCU presumable acts as the promotor for this self-condensation reaction. The reaction was carried out in the above listed DESs at 100 °C for 150 min. The GlcNH2 conversions, DOF and FZ yields, were calculated and listed in Table 1. The results showed that DOF was the main product, whereas FZ was only observed in trace amounts. CCU was found to be the most effective DES with 13.5% DOF yield and 1.3% FZ yield at 100% GlcNH2 conversion. Despite CCD and CCT being basic DESs, the corresponding yields of DOF were only 4.3% and 2.6%, respectively. Upon changing the reaction media to CCG and CCL, 42% and 94% conversions of GlcNH2 were achieved. However, DOF and FZ were almost not detectable by 1H NMR. Thus, neutral and acidic DESs could not efficiently convert GlcNH2 to DOF.42 Three basic HBDs, herein urea, DMU and thiourea, provide a slightly basic reaction environment for the DOF formation, however only CCU could provide an acceptable DOF yield. The molar ratio of ChCl to urea in the eutectic solvents is usually 1:2 (melting point: 12 °C).25 The urea content in CCU changes its basicity strength, which is a key factor for the selfcondensation of GlcNH2 to DOF. Thus, the effect HBD to HBA molar ratio of CCU on the selfcondensation reaction was screened. The obtained conversions and yields at 100 °C after 150 min reaction are presented in Table 1. An initial test showed that when the ratio of ChCl to urea was increased from 1:2 to 1:1, the DOF yield decreased from 13.5% to 9.3%. On the other hand, when the ratio of ChCl to urea decreased from 1:2 to 1:4, the DOF yield decreased to 10.9%. For


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all the screened CCU mixtures, the catalytic performance of 1:2 was the best, whether in HBD types or the molar ratio of components, thus it was used for the further investigations. Table 1. Conversions and yields for the self-condensation of GlcNH2 in different DESs.a ChCl: HBD GlcNH2 DOF yield FZ yield DES HBD (molar ratio) conversion (%) (%) (%)

a

CCU

Urea

1:2

100

13.5

1.3

CCD

Dimethylurea

1:2

44

4.3

0.8

CCT

Thiourea

1:2

57

2.6

1.7

CCG

Glycerol

1:2

42

0.5

0.4

CCL

Levulinic acid

1:2

94

0

0

CCU

Urea

1:1

100

9.3

0.7

CCU

Urea

1:1.5

100

13.1

0.7

CCU

Urea

1:3

100

11.9

0.6

CCU

Urea

1:4

100

10.9

0.5

Reaction conditions: GlcNH2 (2 mmol), DES (20 mmol), 100 °C, 150 min. The effect of reaction temperature. A range of temperature ranging from 80 °C to 120 °C

were screened in order to examine the influence on self-condensation of GlcNH2 in CCU (Figure 1). When the reaction was carried out at 80 °C for 90 min, the GlcNH2 conversion was 35.1% and the DOF yield was only 6.3%. When the reaction time was prolonged to 210 min, the DOF increased slightly to 9.1%. However, upon increasing the temperature to 90 °C or 100 °C and leaving the reaction for 90 min the yield increased to 11.2% or 13.3% respectively. Increasing the reaction temperature further, i.e. to 110 °C or 120 °C, enhanced the reaction rate, but the yields of DOF were slightly lowered. These results indicated that reaction temperature has great influence on both reaction rate and DOF yield. The maximum yield of DOF was 13.5%, which was achieved at 100 °C for 150 min without adding additional catalyst.


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Figure 1. The effect of reaction temperature on the self-condensation of GlcNH2 to DOF in CCU (a) GlcNH2 conversion, (b) DOF yield. Reaction conditions: GlcNH2 (2 mmol), DES (20 mmol). The effect of co-catalysts. Amino acids have recently been used as basic co-catalysts in biomass conversion.43 Herein, several amino acids, including basic amino acids (arginine, lysine and histidine), neutral amino acids (proline and glycine) and acidic amino acids (aspartic acid and glutamic acid), were employed as co-catalyst in CCU for the synthesis of DOF from GlcNH2. All the studied amino acids screened increased the reaction rate resulting in a 94%-100% GlcNH2 conversion and improved the yields of DOF to 15.1-26.5% (Figure 2).

Figure 2. The effect of amino acids on the yield of DOF. Reaction conditions: GlcNH2 (2 mmol), CCU (20 mmol), co-catalyst (1 mmol), 100 °C.


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Arginine gave the highest yield of DOF (26.5%) which was almost a doubling compared to using CCU alone (13.3%). Aspartic acid (24.5%), glutamic acid (24.1%), lysine (22.2%) and histidine (20.4%) also showed increased the DOF yield, whereas the neutral amino acids gave DOF yields in the range between the basic/acidic amino acids. Amino acids with basic or acidic side chains are bifunctional catalysts,43 and show more effective catalysis for the selfcondensation of GlcNH2. The carboxylic acid groups of amino acids interacts with the α-amine group in GlcNH2 through hydrogen bonding (H-bond), which likely also contribute to the conversion. For comparison, several readily available basic catalysts were examined under the same reaction conditions. As shown in Figure 3, triethylamine (17.3%), DBU (22.4%), CH3COONa (22.4%) and [C2C1Im][OAc] (23.2%) achieve comparable DOF yields with that of lysine, albeit lower than arginine. Moreover, the yields of FZ were measured and the results were also shown in Table S1. The FZ yields were only slightly increased by arginine (1.8%), lysine (1.7%) related to the control experiment (1.3%).

Figure 3. Self-condensation of GlcNH2 to DOF with or without co-catalysts added: Control, no co-catalyst; Arg, arginine; Lys, lysine; His, histidine; Pro, proline; Gly, glycine; Asp, aspartic


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acid; Glu, glutamic acid; TEA, triethylamine; AcONa, CH3COONa; IL, [C2C1Im][OAc]. Reaction conditions: GlcNH2 (2 mmol), CCU (20 mmol), co-catalyst (1 mmol), 100 °C, 90 min. Effect of arginine dosage. The catalytic performances of arginine with different amounts of arginine (0.25-2.0 equivalents relative to GlcNH2) were investigated. As shown in Figure 4, adding 0.25 equivalents of arginine (relative to GlcNH2), the DOF yield increased to 25.5% relative to the control experiment (13.3%). When increasing the amount of arginine to 0.75 equivalents (relative to GlcNH2), the DOF yield was further improved to 30.1%. However, increasing the amount further caused a decrease of the yield of DOF.

Figure 4. The effect of arginine dosage on the yield of DOF. Reaction conditions: GlcNH2 (2 mmol) and arginine in CCU (20 mmol), 100 °C, 90 min. Thus, the GlcNH2/CCU/arginine system is an alternative process for the preparation of DOF due to the following merits: atom efficiencies for both nitrogen and carbon are 100% in theory from the term of atom economy; DES and arginine have been widely known as non-toxic, cheap and sustainable reagents, which could meet green chemistry and pharmaceutical application demand. Proposed reaction mechanism. To enlighten the catalytic interaction between GlcNH2 and arginine, 1H and

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C-NMR analyses were carried out in DMSO-d6, which is a weakly


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coordinating solvent and thus employed in interaction studies by NMR.23, 44 DMSO-d6 does not catalyze the mutarotation of GlcNH2, and only the α-anomer is observed without catalyst present.23 The signals of the hydroxyl and amino groups in GlcNH2 are clearly identified in 1HNMR, and

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C-NMR shows the signals of the α-anomer (Figure S6-S7). When adding 0.0625

equivalents of arginine (relative to GlcNH2) to the GlcNH2/DMSO-d6 solution, the clearly distinct proton resonance signals of the hydroxyl and amino groups merged together to a new broad signal in the 1H NMR spectrum (Figure S8). The addition of 0.25 equivalents arginine resulted in slight shifts of the resonances of the broad signal as well as the methylene and methine signals in the GlcNH2 backbone, the former moving downfield and the latter upfield. We suppose that the broad signal results from quickly intermolecular proton exchange caused by basic catalyst, herein arginine.23,

45

H-bond interactions are probably catalyzing the proton

exchange.23 The chemical shift variances of methylene and methine signals are also ascribed to the H-bond formation.46 This 1H NMR result was further supported by the changes in the 13C chemical shifts (∆δ) in αGlcNH2. The 13C-NMR chemical shifts and ∆δ values of α-GlcNH2 anomer with different molar ratio of arginine and GlcNH2 are listed in Table S2 and Figure 5. The chemical shifts of C1 and C4 of α-GlcNH2 are more sensitive to the addition of arginine than the other four carbons, see by a larger downfield shift (Figure 5a). The H-bond formed between GlcNH2 and arginine mainly affects the carbon atoms at the interaction site. A similar phenomenon of arginine has also been proposed by Karton et al. for the interaction with methanol (a model of aliphatic hydroxyl groups in saccharides).47 The same group also proposed that the guanidine group of arginine played the dominant role by forming strong H-bond during biomass pretreatment and in catalysis.


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Figure 5. The trend of the chemical shift for difference carbons in

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13

C NMR GlcNH2 anomers

(α-, β-pyranose) with increasing arginine dosage (∆δ = δobs-δ0) in DMSO-d6, (a) α-pyranose; (b) β-pyranose. The chemical shift scale is in units of δ, δ0 is obtained in DMSO-d6 without arginine. Furthermore, the mutarotation of GlcNH2 in the presence of arginine was observed in the 1H NMR spectra (Figure S8). After addition of 0.5 equivalents of arginine, the integral of the βanomer rapidly increased at the expense of the α-anomer (for a mechanistic suggestion see Figure S9). The ring-opened form of GlcNH2 suggested in the mechanism is important for the formation of DOF and FZ, and hence this could explain the catalytic abilities of arginine, which promotes the acyclic formation of GlcNH2 and activation of GlcNH2. The effect of ChCl for the formation of strong H-bond in combination with arginine has been proposed recently.47 It is therefore suggested that arginine and CCU play a synergistic role in the catalysis. To get more insight into a possible reaction mechanism, an in situ 1H NMR study was conducted. As shown in Figure 6, peaks around δ = 8.37-8.57 ppm are attributed to DOF and FZ and their intensities increase with the decrease of GlcNH2 (5.44 ppm and 4.94 ppm) in time. Interestingly, a new singlet peak appeared at 7.59 ppm after ca. 20 min, and its intensity reaches a maximum at 40 min. As the reaction proceeded, the signal became gradually weaker again and it almost disappeared after 110 min. Early reports suggest a self-condensation intermediate,


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formed through intermolecular cyclization and dehydration of GlcNH2.10-11,

48, 49

2D NMR

techniques were applied to further assign and confirm the structure of this intermediate (Figure S10~S11). From the NMR data the signal was assigned to be from dihydrofructosazine, which has earlier been identified as an intermediate in the synthesis of DOF/FZ from GlcNH2 in IL.23, 49

Figure 6. In situ 1H NMR spectra as a function of reaction time for the self-condensation of GlcNH2 catalyzed by arginine in CCU and D2O at 60 °C. Based on these results and previous studies, a mechanism has been suggested (Scheme 2). The nitrogen in arginine interacts with the hydrogen atoms of hemiacetal in GlcNH2 via H-bonds. This catalyze ring opening of GlcNH2. An intermolecular nucleophilic addition and dehydration lead to the detectable intermediate, dihydrofructosazine. This is finally further dehydrated to give DOF.


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Scheme 2. Possible reaction pathway for the formation of DOF catalyzed by arginine in CCU from GlcNH2. CONCLUSION: In summary, the low-cost and biocompatible DES, CCU, was shown to dissolve and catalyze the transformation of GlcNH2 to DOF under mild conditions. DOF was the main product with a yield of 13.5% without co-catalysts. Amino acids, especially arginine, contributed significant to the catalysis and increased the yield of DOF to 30.1% under optimum conditions. NMR studies confirmed the existence of H-bond interactions between GlcNH2 and arginine and dihydrofructosazine has been detected and confirmed as the key intermediate in the reaction pathway. The present research provides an insight into the mechanism of GlcNH2 selfcondensation in DESs, paving the way for a sustainable method for conversion of chitin biomass.


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ASSOCIATED CONTENT Supporting Information. Quantitative 1H NMR calculation details; NMR spectra of products, GlcNH2, CCU and the possible intermediate; the GlcNH2 conversions and yields of DOF and FZ by various co-catalysts; the proposed mutarotation process of GlcNH2; AUTHOR INFORMATION Corresponding Author *Prof. Yingxiong Wang: [email protected] ACKNOWLEDGMENTS The authors thank for the Key Research and Development Program of Shanxi Province (international cooperation) (201703D421041) for financial support. Christian Marcus Pedersen acknowledges the CAS President’s International Fellowship Initiative (2015VMB052).


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TOC:

SYNOPSIS: The sustainable and environmental-friendly DESs and amino acids are employed for preparation of pyrazine compounds by self-condensation of chitin biomass.


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green solvents and catalysts for the preparation of pyrazine derivatives

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