Efficient one-pot synthesis of deoxyfructosazine and fructosazine from D-glucosamine hydrochloride using a basic ionic liquid as a dual solvent-catalyst

Abstract

An efficient one-pot dehydration process for convert D-glucosamine hydrochloride (GlcNH₂) into 2-(D-arabino-1′,2′,3′,4′-tetrahydroxybutyl)-5-(D-erythro-2′′,3′′,4′′-trihydroxybutyl)pyrazine (deoxyfructosazine, DOF) and 2,5-bis-(D-arabino-1,2,3,4-tetrahydroxybutyl)pyrazine (fructosazine, FZ) was reported. A task-specific basic ionic liquid, 1-butyl-3-methylimidazolium hydroxide ([BMIM]OH), was employed as an environmentally-friendly solvent and catalyst. The products were qualitatively and quantitatively characterized by MALDI-TOF-MS, ¹H NMR and ¹³C NMR spectroscopy. The influences of GlcNH₂ concentrations, reaction temperature, reaction time, additives and co-solvents on the yields of products were studied. The maximum yield of 49% was obtained in the presence of [BMIM]OH and DMSO under optimized conditions (120 °C, 180 min). In addition, a plausible mechanism was proposed. Our project was to develop efficient, atom economical and eco-compatible routes for the synthesis of heterocyclic compounds from marine biomass (or nitrogen-containing biomass). The obtained aromatic heterocyclic compounds showed potential pharmacological action and physiological effects, and they also could be utilized as flavoring agents in the food industry.

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1. Introduction

Diminishing reserves of fossil fuel resources, increasing demand for energy and chemicals and growing concerns about environmental issues have led to significant research efforts on the utilization of renewable, bio-sourced feedstocks.¹ So far, substantial progress has been made in the production of fuels and value-added platform chemicals, such as 5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA), from lignocellulosic biomass.^2,3 However, the investigation and utilization of marine biomass, such as chitin and chitosan, which are the second most abundant non-edible polysaccharides after cellulose has, to our knowledge, not yet been fully explored.^4,5 As a readily available, non-toxic, and environmentally benign natural resources, chitin can be sourced on a large scale annually from crustaceans' shells (the industrial waste material of fisheries) and exoskeletons of insects. Under alkaline conditions, chitin can be deacetylated to produce chitosan, which is a linear copolymer of D-glucosamine (GlcNH₂) and N-acetyl-D-glucosamine (GlcNAc).⁶ Compared with the lignocellulosic biomass, the most remarkable potential application of marine biomass is that it can serve as the source of biologically-fixed nitrogen. Commonly, the incorporation of heteroatoms, such as nitrogen, give rise to various unique properties of chemical compounds, which have advanced applications in the industrial, agricultural, medical, and food fields.^7–10 The guiding principle of Green Chemistry is to achieve atom economy, therefore, the utilization of these N-containing polysaccharides should be fully realized in order to achieve maximum benefits. As a result, it will not only alleviate environmental pollution, but also produce high value-added chemicals.

Deoxyfructosazine (DOF) and fructosazine (FZ) are two non-volatile (polyhydroxyalkyl)pyrazine derivatives with various applications, such as flavoring agents in food industry. DOF has also been found to exhibit potential pharmacological and physiological activities, especially on treatment and prevention of diabetes and resistance of cancers.^11,12 Since increasing interests in their high biological and pharmaceutical activities, several synthetic methods related to the Maillard reaction were developed to produce these N-containing heterocyclic compounds. Recently, selective conversion of mono- and polysaccharides, such as glucose or inulin, to DOF in aqueous solutions with additional ammonium salts were developed.^13,14 Moreover, GlcNH₂, the monomer unit of chitosan, could undergo self-condensation to produce DOF as the sole product catalyzed by phenylboronic acid and sodium hydroxide with relatively high yield.¹⁵ In addition, condensation of two moles of GlcNH₂ or D-mannosamine in hot methanol could produce FZ.¹⁶ However, the distribution of the reaction products could be easily influenced by the additional amino acids, different kinds of ammonium salts and the pH of the reaction mixture. Because of the complexity of the Maillard reaction, species with different molecular weight, such as furfural, volatile and semivolatile pyrazine derivatives, could be observed as by-products in such reactions.^13,15,16 Although high product yield and substrate conversion could be achieved for the catalytic Maillard reactions in aqueous solution, conventional bases or organic acids are commonly employed. Consequently, neutralization and salt removal procedures are necessary, which make reaction processes relatively complex and bring undesired side products to the reaction system.¹⁷ Furthermore, reactions typically obtained high yields often at the expense of long reaction time and fast selectivity drops.¹⁸ So, it is still highly desirable to develop new and efficient catalytic system to prepare these pyrazine derivatives with high yield and selectivity.

Room temperature ionic liquids (RTILs) have attracted growing interests in green synthesis and process due to their unique chemical and physical properties, such as non-volatile, non-flammable, good stability and high solubility for polymeric materials.^19,20 For example, Kerton et al. reported that good yield of 3-acetamido-5-acetylfuran could be obtained by the dehydration of GlcNAc in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) with boric acid as additive.²¹ Today applications of ionic liquids are far beyond the fields of being just as alternative solvents. Catalysis in ionic liquids is an ongoing and promising area of research. For example, ionic liquids containing imidazolium cations could efficiently promote reactions as catalyst.²² Basic ionic liquids could replace conventional bases and be used in based-catalyzed processes as environmentally-friendly solvents and catalysts with high activity and selectivity.²³ For example, 1-butyl-3-methylimidazolium hydroxide, [BMIM]OH, has been applied to catalyze a number of reactions including Michael addition, Markovnikov addition and Knoevenagel condensation.^24,25 Alkylated chitosan could be achieved with basic ionic liquid [BMIM]OH as alkaline reagent.²⁶ Although basic ionic liquids have been successfully employed as a catalyst for above mentioned reactions, to our knowledge, there have been no reports of the utilization of [BMIM]OH as an efficient catalyst for the conversion of amino sugars into nitrogen containing compounds.

The work reported herein represents an efficient method for the synthesis of two nonvolatile hydroxyalkyl pyrazine derivatives, DOF and FZ, from the self-condensation of GlcNH₂ (Scheme 1). The basic ionic liquid, [BMIM]OH, was employed as both solvent and catalyst. GlcNH₂ was served as both substrate and the source of biologically-fixed nitrogen from the viewpoint of atom economy. Moreover, in this reaction process, DOF and FZ were achieved as major components without any conventional by-products produced in previous research work. To the best of our knowledge, no reports have shown the highly selective production of pyrazine derivatives from GlcNH₂ under basic imidazolium ionic liquid conditions.

Scheme 1 Direct conversion of GlcNH₂ to DOF and FZ under basic imidazolium ionic liquid.

2. Materials and methods

2.1. Materials

Practical grade D-glucosamine hydrochloride (designated as GlcNH₂, white crystalline powder) was obtained from Golden-Shell Biochemical Co. Ltd. Deuterium oxide (D₂O, 99.9%) and 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) were supplied by Qingdao Tenglong Microwave Technology Co. Ltd. 1-Butyl-3-methylimidazolium hydroxide ([BMIM]OH, 12%) was purchased from Shanghai Cheng Jie Chemical Co. Ltd. Dimethyl sulfoxide (DMSO), pyrazine and all other chemicals (analytical grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were used without further purification. Deionized water was used in all experiments.

2.2. General reaction procedure

In typical experimental procedures, 0.2 g of GlcNH₂ was dissolved in 2 g alcoholic solution of 12% alkaline [BMIM]OH with different volume ratio of co-solvent (GlcNH₂ : [BMIM]OH, molar ratio 0.6 : 1). Reaction mixture system was loaded in a 10 ml stainless steel vessel with a Teflon lining and sealed by a screw cap following a magnetic bar. The reaction vessel was placed into a pre-heated oil bath and stirred at a speed of 300 rpm at 120 °C for 180 min. A thermostatic oil bath as heating source reaching the desired temperature, zero time was taken. In addition, experiments were conducted at 25, 80, 90, 100, 120, 150 and 180 °C to investigate the effect of reaction temperature. After the reaction, the reaction vessel was taken out from the oil bath and immediately put into ice bath to quench the reaction. The obtained products were diluted by distilled water for further characterization.

2.3. Analytic methods

2.3.1. Characterization. The chemical structures of products were characterized by ¹H NMR, ¹³C NMR, and MALDI-TOF-MS techniques. The ¹H NMR and ¹³C NMR spectra of DOF and FZ were recorded in D₂O with DSS as internal standard on a Bruker AV-III 400 for frequency at 400.13 and 100.61 MHz, respectively. The MALDI-TOF-MS spectra were performed on a Biflex III reflectron time-of-flight (TOF) mass spectrometer (Bruker Daltonics, USA) equipped with a nitrogen laser as the excitation source and N-(1-naphthyl)ethylenediamine dihydrochloride (NEDC) as a matrix.

2.3.2. Quantitative ¹H NMR measurement. Quantitative ¹H NMR was applied to calculate the substrate conversion and yields of products using pyrazine as an internal standard (Fig. S1†). Pyrazine with concentration of 0.3 mg ml⁻¹ in D₂O as standard solution was prepared following the procedure described by Rundlöf et al.²⁷ To prepare the ¹H NMR sample, 0.1 ml of the reaction mixture was mixed with 0.4 ml of standard solution in a 5 mm NMR tube. The measurements were performed on a Bruker AV-III 400 at 400.13 MHz with scan times ns = 16. The yields of products were calculated as:

3. Results and discussion

3.1. Characterization of the products

From the viewpoint of atom economy, the catalytic dehydration of non-edible marine biomass to nitrogen containing compounds has become an attractive topic for carbohydrate-based biomass conversion.^5,9,21 We aimed to find milder and more efficient methods for the transformation of amino sugars into organic N-containing platform molecules. Herein, we studied the synthesis of pyrazine derivatives, DOF and FZ, through the self-condensation of GlcNH₂ catalyzed by a basic ionic liquid [BMIM]OH via an one-pot one-step reaction route.

In a typical reaction, 200 mg of GlcNH₂ and 2 g IL(OH)/C₂H₅OH (12%) were mixed and stirred at 120 °C for 180 min (GlcNH₂ : [BMIM]OH, molar ratio 0.6 : 1). A brown mixture reaction system was achieved and the obtained products were identified by ¹H NMR, ¹³C NMR, and MALDI-TOF-MS techniques, which were shown in the ESI.† Fig. S2† showed a typical mass spectrum of products in negative ion modes using of N-(1-naphthyl) ethylenediamine dihydrochloride (NEDC) as a matrix for MALDI MS analysis of products with a time-of-flight (TOF) mass spectrometer.^28,29 Besides the peaks originated from matrix, two strong peaks were observed at m/z 339.132 and 355.129, corresponding to the chloride adduct ions of products, (DOF + ³⁵Cl)⁻ and (FZ + ³⁵Cl)⁻, respectively. The molecular weights were 304 g mol⁻¹ and 320 g mol⁻¹ respectively, which are in agreement with the molecular weight of DOF and FZ.¹⁸ Furthermore, ¹H NMR and ¹³C NMR were further applied to confirm the chemical structures of products, including spectra of pure ionic liquid, which were shown in Fig. S3 and S4.† Compounds were found to be disubstituted pyrazine derivatives identical with authentic sample that had been prepared and characterized in our group (Fig. S5†). Remarkably, typical by-products observed in GlcNH₂ conversion, such as 5-HMF, LA and low molecule weight volatile pyrazine derivatives were not detected in our experiments.^15,18 In previous research, pyrolysis of solid-state GlcNH₂ at high temperature (200 °C) or thermal degradation of GlcNH₂ in aqueous solution above 100 °C yielded a mixture of products, such as furans, pyrazines and pyridines.³⁰ Severe reaction conditions and unsuitable reaction systems may be responsible for the production of unwanted by-products so that these methods cannot achieve a selective production of desired compounds. In this report, introducing the application of basic ionic liquid as a dual solvent-catalyst, high efficiency of GlcNH₂ conversion could be obtained under this relatively milder condition without any traditional side products. Previous research have already focused on the reaction efficiency of basic ionic liquids, for example, researchers investigated to transform carbohydrates into 5-HMF under alkaline ionic liquids as catalysts.³¹ Furthermore, Michael addition can be easily achieved with high yield and conversion using basic ionic liquid as catalyst and reaction medium.^24,25 As far as we aware, there have been no reports of the application of basic ionic liquids as a dual solvent and catalyst for the conversion of marine biomass into N-containing compounds. Herein, the work represented a feasible way to prepare nitrogen containing compounds from amino sugars catalyzed by basic ionic liquid under milder and more efficient conditions.

3.2. Reaction parameter optimization

3.2.1. Effect of the reaction time and temperature. All reactions were conducted in a similar manner, we started systematically to investigate the effects of different reaction parameters, such as reaction time, temperature, substrate concentrations, additives and co-solvents. The effects of reaction time and temperature on the cyclocondensation of GlcNH₂ to DOF and FZ were firstly examined. By varying the temperature from room temperature to 180 °C, the total yields of products changed significantly. The DOF and FZ yields and substrate conversion were additionally determined by quantitative ¹H NMR. When the reaction was conducted at 25 °C for 37 h, no products were detected under such conditions keeping a heterogeneous reaction system. However, in contrast to room temperature reaction, the amount of products showed an approximately linear increase to 36% with increasing temperature from 25 to 80 °C, indicating that high temperature could efficiently enhance the rate of reaction conversion. The yields of products did not proportionally increase with increasing reaction temperature from 80 to 150 °C at a given reaction time. When further raising temperature to 180 °C, the yields rapidly decreased, which can be seen in Fig. 1. Higher reaction temperature was not favorable for this reaction, presumably as a result of side reactions. Therefore, it could be concluded that temperature has a significant effect on the yields of products and the conversions of substrates and choosing a suitable temperature is particularly significant for such reactions. In addition, the influence of reaction time on reactions was also investigated at 120 °C. As can be seen in Fig. 2, a linear increase in DOF and FZ production as the reaction time increased from 0 to 30 min. A maximum yield of products was obtained at a reaction time of 180 min. However, after reaching the peak values, the yields started to decrease when further increasing reaction time, which was probably a result of side reactions. In addition, it can also be concluded that DOF has relatively higher yields than FZ over basic ionic liquid. In the end, we choose 120 °C, 3 h as the optimal reaction conditions to further investigate such transformation processes.

Fig. 1 The effect of reaction temperature on the yields of products catalyzed by basic ionic liquid. Reaction conditions: GlcNH₂ 0.2 g, ionic liquid 2 g, 3 h.

Fig. 2 The effect of reaction time on the yields of products catalyzed by basic ionic liquid. Reaction conditions: GlcNH₂ 0.2 g, ionic liquid 2 g, 120 °C.

3.2.2. Effect of substrate concentration. The effect of substrate concentration on the condensation of GlcNH₂ into DOF and FZ was also investigated, and results were shown in Fig. 3. 100% GlcNH₂ conversions were achieved with increasing substrate concentration from 100 to 400 mg. The highest DOF and FZ yields obtained were 45% with 500 mg of substrate amounts. However, when further increasing the amounts of substrate from 500 to 800 mg, the results were disappointing. Products yields and reaction conversions rapidly decreased. As increasing the amount of substrates, the concentrations of hydrochloride from substrates increased at the same time. This was attributed to the presence of dissolved hydrochloride in the substrate which coordinate to the OH center of ionic liquid through ion-exchange reaction and deactivate the catalyst.³² The chloride ion as a strong nucleophilic anion could partially replace the hydroxyl group of the basic ionic liquid [BMIM]OH to form neutral ionic liquid [BMIM]Cl and the existence of [BMIM]Cl could be verified by NMR spectra, shown in Fig. S6.† We also found that ionic liquids with acidic or neutral properties cannot catalyze such reactions to prepare nitrogen containing compounds. The undesired transformation of [BMIM]OH into [BMIM]Cl in the presence of hydrochloride was responsible for the low yields and conversions. Therefore, substrate concentration had a significant effect on the condensation reaction. These results clearly demonstrated that suitable amounts of catalysts and substrates concentrations with an appropriate ratio should be considered to improve the efficiency of reactions.

Fig. 3 Deoxyfructosazine and fructosazine yields and glucosamine conversion as a function substrate concentrations. Reaction conditions: 120 °C, 2 g IL(OH)/C₂H₅OH (12%), 3 h.

3.2.3. Effect of basic additives. It is noteworthy that selective formation of DOF and FZ can be influenced by the concentration of the substrates as well. To further the study, substrates with equimolar amounts of basic additives, such as sodium hydroxide, were studied in the hope of neutralizing hydrochloride and increasing products yields. However, basic additives proved ineffective at increasing the yields of products under the reaction conditions employed, which can be seen in Fig. 4. Han et al. have already confirmed that higher yields can be obtained at pH of 6 and 8, respectively, indicating that the reaction can be carried out effectively at less acid or neutral conditions.¹⁴ Therefore, a more basic reaction condition is not suitable for such reaction system. In addition, basic additives may react with hydrochloride from substrates to form inorganic salts. According to relative literature, the common way of preparing room temperature ionic liquids is by the anion exchange of halide salts with metal salts.³² This may be the second reason leading to the low yields and low conversions. The nature of the DOF and FZ formed was strongly dependent on the reaction conditions, particularly on the pH, which required further investigation.

Fig. 4 The effect of additives on the production of deoxyfructosazine and fructosazine. Reaction conditions: 200 mg GlcNH₂, 2 g IL(OH)/C₂H₅OH (12%), 120 °C, 3 h.

3.2.4. Effect of co-solvents. In order to find suitable co-solvents to improve the reaction yields, we systematically evaluated the performance of dipolar aprotic solvents for GlcNH₂ conversion to DOF and FZ under basic ionic liquid conditions. Experiments with different content of dipolar aprotic solvents were carried out at 120 °C with 180 min, and results were shown in Fig. 5. DOF and FZ yields increased with the DMSO content increased from 1 to 3 ml. Products yields of 46% was obtained at 3 ml of DMSO as co-solvent. After reaching the peak values, further increasing the content of DMSO to 4 ml did not produce additional increase in products yields. Results have shown that DMSO indeed played a positive role in the conversion of GlcNH₂. However, other co-solvents employed were slightly less effective in our experiments. Compared with the control reaction, the highest yields of products achieved was 49% in the presence of 3 ml DMSO as co-solvent with 300 mg substrate concentration and 2 g IL(OH)/C₂H₅OH (12%) (GlcNH₂ : [BMIM]OH, molar ratio 0.9 : 1). It is worth noting that DMSO, in particular, was a much more effective co-solvents. DMSO, as a weak base solvent, could provide good solubility in this condensation reaction of GlcNH₂ and also could decrease side reactions in sugars conversion, which were in accordance with previous investigation.³³ In addition, it was also found that the signal intensity of chemical shifts of [BMIM]Cl in the ¹H NMR gradually decreased with adding DMSO into reaction system, which were shown in Fig. S7.† To some extent, DMSO could decrease the transformation of [BMIM]OH to [BMIM]Cl. These results are in accord with the present scientific judgment of the effect of DMSO on the chemical reactions in general.

Fig. 5 The effect of co-solvents on the production of deoxyfructosazine and fructosazine. Reaction conditions: 200 mg GlcNH₂, 2 g IL(OH)/C₂H₅OH (12%), 120 °C, 3 h.

3.3. Proposed reaction mechanism

GlcNH₂, α-amino carbonyl structure from the degradation of chitosan, which is generally considered to be the precursors of pyrazine compounds, can self-react to produce heterocyclic products. The condensation of GlcNH₂ is a complex multistep process, and there are several reports on the mechanistic investigation on this industrially significant reaction. All these studies were carried out in aqueous solution with neutral or less acidic conditions. Moreover, all the earlier mechanistic studies were related to the dehydration and keto–enol tautomerization process involved in the reactions.^11,13,17,18 However, the mechanism of the condensation reaction in basic ionic liquids has not been fully investigated. Based on previous research, we proposed a plausible reaction mechanism in order to demonstrate the catalysis process with this specific basic ionic liquid. The main reaction pathway from GlcNH₂ to pyrazines is a combination of self-condensation and dehydration reactions. [BMIM]OH remained intact in ¹H NMR and ¹³C NMR spectra that obviously indicated that this basic ionic liquid indeed played an important role in the reaction as a catalyst. Pyrazine heterocycles are probably formed by way of cyclization of two acyclic forms molecules through a condensation reaction involving the aldehyde and amino groups. Two possible reaction paths are proposed, which could explain the formation of DOF and FZ respectively. The formation of open chain aldose form of the amino sugars is the first step in the reaction. Mechanistically, the formation of an imine involves two steps in path one, which is used to explain the formation of DOF. First, the nitrogen electron pair acts as a nucleophile, attacking electrophilically activated carbonyl carbon.³⁶ Next step is that the nitrogen is deprotonated, and the electrons from this N–H bond ‘push’ the oxygen off of the carbon, leaving with a C=N double bond (an imine) and displaced two water molecules. The end result of this reaction is a compound in which the C=O double bond is replaced by a C=N double bond. Both the cation and anion of [BMIM]OH were involved in the formation of Schiff base.^23,34,37 After that, six-membered heterocycle was formed via an intermolecular nucleophilic cyclization of intermediate, typically called dihydropyrazine.³⁵ The dihydropyrazine intermediate may further eliminate two water molecules to form conjugated polyene system. However, such system is contrary to the rules of Hückel with high energy and cannot exist stably. In fact, closed loop conjugated system tend to be stable with lower energy. Therefore, the dihydropyrazine intermediate only eliminate one water molecule followed by electronic rearrangement process to form a stable six-membered conjugated ring product, called DOF.¹⁸

Path two is described to explain the formation of FZ. At first, the amine nitrogen acts as a nucleophile, attacking the carbonyl carbon. Contrary to the formation of Schiff base, next step is that the ring carbon is deprotonated to form a C=C double bond. Six-membered heterocycle, called dihydropyrazine, is also formed.³⁵ Then the N–H bond with weak acid properties is attacked by the hydroxyl anion of [BMIM]OH as base through loss one water molecule. A six-membered conjugated ring is formed through electronic rearrangement. Furthermore, under alkaline conditions, the nitrogen is deprotonated. In the end, another stable six-membered conjugated ring product, called FZ, is formed.¹⁸ The conversion of GlcNH₂ into DOF and FZ with high atom economy via isomerization and dehydration process in the presence of [BMIM]OH demonstrated that basic ionic liquid could efficient catalyze this reaction. The proposed reaction pathway can be seen in Scheme 2. In addition, conversion of GlcNH₂ to pyrazine derivatives under basic conditions often produced side products. The significant side reaction is the condensation of the GlcNH₂ to form polymeric caramel colors, which are soluble in aqueous solution. Therefore, undesired side products also produced under basic ionic liquids conditions, which is a major challenge in the synthesis of pyrazines from amino sugars. The structure and amount of water-soluble by-products were under the detection limit of our instrument. However, it is worth noting that insoluble humins, typical side products produced in carbohydrates conversion, were not detected under basic ionic liquids conditions. Moreover, many nitrogenous heterocyclic compounds, such as imidazoles, pyridines, and pyrroles, which have been found in sugar-amine browning reaction, were also not detected in our reaction system.

Scheme 2 Proposed reaction pathway for the formation of deoxyfructosazine and fructosazine from GlcNH₂.

4. Conclusions

Direct conversion of GlcNH₂ into nitrogen containing compounds (DOF and FZ) under basic imidazolium based ionic liquid in good yield was systematically investigated for the first time. Enhanced reaction rate and improved selectivity were achieved using [BMIM]OH as a dual solvent and catalyst. A maximum yield of 49% was obtained with catalytic amount of ionic liquid. This investigation contrasted with recently published work from our group where 5-HMF was obtained as the primary product through conversion of marine biomass in concentrated ZnCl₂ aqueous solution. [BMIM]OH and DMSO were identified as the effective catalyst and the best co-solvent respectively. A possible mechanism was proposed. Nitrogen and carbon atom efficiency were quite good up to 100% from the viewpoint of atom economy.

Acknowledgements

This work was financially supported by the Major State Basic Research Development Program of China (973 Program) (no. 2012CB215305), the Natural Science Foundation of China (no. 21106172) and Science Foundation of Shanxi Province (2012021009-2). Special thanks to Prof. Zongxiu Nie (Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences) for Mass Spectrometry analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06832g

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