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Research 409 (2015) 9e19

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Carbohydrate Research

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Reactivity of D- and D-xylose in acidic media in homogeneous phases

* Maxime B. Fusaro, Vincent Chagnault , Denis Postel

Laboratoire de Glycochimie, des Antimicrobiens et des Agroressources (LG2A), 33 rue Saint-Leu, F-80039 Amiens Cedex 1, France article info abstract

Article history: Chemistry development of renewable resources is a real challenge. from biomass are Received 20 January 2015 complex and their use as substitutes for fossil materials remains difficult (European involvement on the Received in revised form incorporation of 20% raw material of plant origin in 2020). Most of the time, the transformation of these 20 March 2015 polyhydroxylated structures are carried out in acidic conditions. Recent reviews on this subject describe Accepted 23 March 2015 homogeneous catalytic transformations of , specifically toward , and also the trans- Available online 31 March 2015 formation of biomass-derived in heterogeneous conditions. To complete these informations, the objective of this review is to give an overview of the structural variety described during the treatment of Keywords: Carbohydrates two (D-Fructose and D-xylose) in acidic conditions in homogeneous phases. The reac- Acidic media tion mechanisms being not always determined with certainty, we will also provide a brief state of the art HMF regarding this. Dehydration reaction © 2015 Elsevier Ltd. All rights reserved. Conversion of monosaccharides

1. Introduction studies aim to clarify mechanisms. Publications about access to levulinic acid, furfural and 5-HMF but also to other small molecules, To our knowledge, research about the reaction of carbohydrates under various conditions, have seen their number grow exponen- in acidic media dates back to the mid 19th centuryespecially with the tially (Fig. 1). reaction of cane in dilute sulfuric or hydrochloric acid and These studies focus mainly on the aspects of selectivity and heat.1 The first publications on this subject report the production of conversion of monosaccharides to the desired product formation.8 heterocyclic such as furfural and 5-hydroxymethylfurfural The variety of products that is provided by these transformations is (5-HMF) by thermal treatment of carbohydrates (starting from however much broader and a multitude of molecules have been or pure monosaccharides) under acidic conditions.2,3 These identified. derivatives have various valorization areas covering applications The data concerning the formation of other structures by the such as solvents, additives for fuels or as precursors for ‘building transformation of carbohydrates in acidic conditions are spread blocks’ in chemical synthesis.4,5 5-HMF is unfortunately not pro- over more than 170 years. Furthermore, these reports have often duced on industrial scale in contrast to furfural. However, furans such been the subject of verifications and modifications. Marcotullio as these are already within the top 10 bio-based building blocks.5 et al. and Fatehi et al. have recently published reviews focused on The important development of plant biorefinery (valorization of the specific production of furfural or hydroxymethylfurfural.8,9 For the whole plant) has among its main objectives, the rapid substi- their part, Zhang et al. have given an overview of the trans- tution of a part of the fossil molecules by bio-based chemicals formation of biomass-derived sugars using heterogeneous condi- (European involvement on the incorporation of 20% raw material of tions.10 The objective of this review is to supplement these plant origin in 20206). Therefore, the behavior of carbohydrates in informations with an overview of the structural variety obtained acidic media is still an important subject of research and many during the treatment of D-fructose () and D-xylose ()

* Corresponding author. Tel.: þ33322828812. E-mail address: [email protected] (V. Chagnault). http://dx.doi.org/10.1016/j.carres.2015.03.012 0008-6215/© 2015 Elsevier Ltd. All rights reserved. 10 M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19

Fig. 1. Number of publications dealing with A) Levulinic acid, B) Furfural, C) 5-HMF.7

in acidic conditions in homogeneous phases. The reaction mecha- 2.1. Example of a hexose: D-fructose nisms being not always determined with certainty, we will also provide a brief state of the art regarding this. 2.1.1. Structures obtained D-Fructose remains the predominant hexose for which reac- 2. Reaction of carbohydrates in acidic media tivity in acidic media has been studied and It can be obtained industrially in large quantity by enzymic conversion of Whatever the medium used, characterization of the products from corn starch. It proves to be a prime substrate to access 5- formed by acidic pathways remains difficult. Only few methods HMF. However, many studies have shown that this compound is allow monitoring their appearance and their in situ characteriza- far from being the only molecule formed in such conditions. Most tion without prior treatment to decrease or neutralize the acidity of structures which can be obtained appear in references in the work 13 14 the reaction mixture. This is even more true when the literature is of Antal et al., Dumesic et al. and more recently De Jong, old, analytical techniques then being limited. Over the years, studies have confirmed or disproved molecules already described and helped to identify new ones. Dilute acids are most commonly used to study the reactivity of carbohydrates in acidic media. Taking into account researches described in the literature, experimental conditions are rarely similar and it is not easy to compare the re- sults obtained, even for the same substrate. In fact, each study has different conditions of temperature, sugar concentration, solvent, reaction time, pressure or acidity. The first compounds resulting from the structural modification of a in an acidic medium were described in 1840 by Stenhouse,2 Mulder1 and in 1895 by Düll.11 These are the most stable and easily isolable com- pounds: furfural, levulinic acid and 5-HMF. Since that time, the development of analytical techniques and interest in the valorization of bio-based compounds have both expanded the range of identified molecules and also proposed mechanisms of transformation that may involve different in- termediates and pathways reaction. However, an exhaustive census remains difficult.12 We will focus herein on the description of the structures obtained in the most used acidic conditions, starting from a hexose and a pentose (D-fructose and D-xylose). The mech- anisms proposed to explain their formation are from four types of reactions: isomerizations, dehydrations, fragmentations and condensations. Fig. 2. Example of structures of humic segments described in the literature.27,28

Table 1 13 Products obtained from the reaction of D-fructose in dilute acidic media according to the classification of Antal

Isomerization Dehydration Fragmentation Condensation

17 20 24 - D-Glucose - 5-HMF - Formic acid - Difructose dianhydrides 16 17 17 25 - D- - 5-Methyl-2-furfural - Levulinic acid -(E)-2-[1-(3-hydroxy-2-furanyl)ethylidene]-(2H)furan-3-one - a-Angelica lactone17 - Dihydroxyacetone21 - Isomaltose26 - b-Angelica lactone17 - Glyceraldehyde21 - ‘Humins’27,28 - 2-(2-Hydroxyacetyl)furan17 - Furfural17 - 2-(2-hydroxyacetyl)furan formate17 - Pyruvaldehyde21 - Isomaltol17,18 - Lactic acid13 - 4-Hydroxy-2,3,5-hexanetrione17 - Acetol13 - 2,3-Dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one18 - Glycolaldehyde13 12 17 - 2,6-Anhydro-b-D-fructofuranose - Acetic acid - 1,2,4-Benzenetriol19 - 2,3-Butanedione13 - Acetone22 - Propionaldehyde23 M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19 11

12 31 Scheme 1. Set of products resulting from the reaction of D-fructose in dilute acidic media and for which a reaction mechanism is described (A: Horvath, B: Mednick, C: Bobleter,21 D: Antal,32 E: Antal,13 F: Cammerer,€ 33 G: Anet,34 H: Yoshida,35 I: Brown,26 J: Moreau,16 K: Feather,36 L: Ferrier,37 M: Van Bekkum,38 N: Civelekoglu,39 O: Kiermayer,40 P: Lund,30 Q: Van Bekkum,19 R: Tatum,17 S: Hodge,25 T: Tatum18). 12 M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19

Heeres, De Vries et al.15 According to the authors, they can be classified into categories related by the reactions involved in their formation (Table 1). Humins comprise a mixture of highly colored insoluble mole- cules, with an oligomeric or polymeric structure.27,29 The analysis of their structures is still the subject of research28,30 as well as the intermediates involved in their formation (Fig. 2). Scheme 1 includes a compilation of all the compounds obtained during the reaction of D-fructose in an acidic medium and for which a mechanism has been proposed. It consists of four distinct parts according to the molecules resulting from 2,3-enediol, 1,2-enediol, oxocarbenium ions and retroaldolisation reactions.

2.1.2. Mechanisms of formation The formation of 5-HMF has been the subject of many studies. Two mechanisms are described, via intermediates of the linear enediol type or via a cyclic oxocarbenium ion. The formation of this compound has been reported by Anet,34 Tatum et al.17 et Moreau et al.16 who described 1,2- and 2,3- enediol as reaction intermediates (Scheme 2). The enediol system is obtained by the Lobry De BruyneAlberda Van Ekenstein trans- formation, usually conducted under basic conditions, which helps to explain the enolization of to and conversely.41 However, these enediols can also be formed in an acidic medium as reported by the works of Speck.42 Regardless of the enediol formed, it is followed by a dehydration involving the atom 16 Scheme 2. Formation of 5-HMF via acyclic intermediates, according to Moreau et al. C-4, and the formation of an additional double bond. A last

12 31 Scheme 3. Products formed from oxocarbenium ions using D-fructose (A: Horvath, B: Mednick ). M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19 13

a b Fig. 3. Structures of the difructose dianhydrides (DFAs). Old notation system. D-fructose-D-glucose mixed dianhydrides.

cyclization step involves the site either 3,6 (leading to 2-(2- involvement of oxocarbenium ions in the formation of compounds hydroxyacetyl)furan) or 2,5 (leading to 5-HMF). These enediol in- including furfural derivatives, anhydrides or humins. These authors termediates can also lead to fragmentations by retroaldolisation emphasize the contribution of cyclic intermediates rather than a and enable access to furfural.17 sequence implying acyclic structures by relying on studies of dehydration of D-fructose in D2O that highlight less than 1% of 2.1.2.1. Mechanisms involving intermediates of oxocarbenium type. deuterium integration in 5-HMF at the C-3 position. This result Various studies (Feather,36 Antal et al.,13 Horvath et al.12) report the casts doubt on mechanisms involving linear intermediates. As the 14 M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19

Fig. 4. Different trisulfated compounds obtained by the reaction of concentrated sul- furic acid on free carbohydrates at 5 C during 2.5 h.

existing equilibrium between 3-deoxyglucos-2-ene and the 3- deoxyglucosone (Scheme 2)43 requires, by tautomerism, the incorporation of a deuterium atom in position C-3. Similarly, the passage through the 1,2-enediol should lead to the incorporation of deuterium at C-1. The low percentage of deuterium incorporated tends to favor the involvement of cyclic intermediates (Scheme 3). These results have been corroborated by in situ NMR analyzes.12 In summary, two different pathways may be advanced which involve oxocarbenium intermediates based on a scaffold either (17) or (18). These can explain the formation of the major products identified (5-HMF; humins; 2,6-anhydro-b-D- Scheme 4. General scheme proposed by García Fernandez et al. explaining the DFAs fructofuranose 19; difructose dianhydrides). formation.46 Even if these new results12 tend to confirm the pathway implying cyclic intermediates, no mechanism has been definitively 15 refuted. To date, both are accepted by the scientific community. problematic their identification, quantification and evaluation of Difructose dianhydrides (DFAs) are spiroketals resulting from their biological properties. the cyclodimerization of two molecules of D-fructose. They were Some of these compounds (16 known in the literature, Fig. 3) initially observed (at the beginning of the 1920s) by reaction of D- were identified few years later as natural molecules produced by 44 45 fructose or ( mixture mainly consisting of plants47,48 (like for DFAs 1, 10 et 15) or by microorganisms49 (DFAs 1, fructose units) in concentrated hydrochloric acid. However, the 10, 15 et 16). They are found in artichokes and Jerusalem arti- yields obtained in these conditions remain modest due to chokes,48 chicory and roasted coffee beans, tequila,50 and especially concomitant degradation reactions. In addition, the purification in commercial where they represent up to 18% by even partial of the mixture took several years and involved several weight.51 research groups. Even if a slight improvement, in term of yields, One of the most effective synthetic methods is the use of Lew- 46 ® þ was achieved using trifluoroacetic acid, purification of these atit S2328 (H ), a strongly acidic ion exchange resin having sul- compounds remained extremely complex. This difficulty makes fonic acid functional groups;50 García Fernandez's coworkers have been able to get a more accurate analysis of the resultant mixture using GC/FID (Chart 1). Moreover, it is the first description (which still constitutes a reference) of a precise relative distribution of DFAs obtained starting from D-fructose. However, a different relative distribution has been observed during thermal treatment of inulin.52 Defaye, García Fernandez et al. have proposed a general scheme for the DFAs formation (Scheme 4). This mechanism goes through the activation of D-fructose resulting in the formation of an oxocarbenium ion (step A), which will lead to fructodisaccharides after dimerization (step B). DFAs are then obtained by intramolecular (step C). While the mechanism may seem simple, many factors can influence the mixture composition.50

Chart 1. DFAs relative abundance in a crude reaction mixture resulting from the re- 2.1.2.2. Retroaldolization products. Whatever the favored mecha- ® þ 50 action of Lewatit S2328 (H ) on the D-fructose, at 90 C. nism for the reactivity of oses in acidic conditions, they can also M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19 15

21 32 13 33 Scheme 5. Retroaldolization products of D-fructose (C: Bobleter, D: Antal, E: Antal, F: Cammerer€ ).

lead to low molecular weight products resulting from retro- has been suggested. It consists of three distinct parts, from acyclic aldolization reactions. The retroaldolization products obtained intermediates, cyclic intermediates and retroaldolization reactions. starting from D-fructose are described in Scheme 5. These com- pounds can be detected at the end of the reaction as final products, or, as reported by Cammerer€ et al., be involved in the formation of 2.2.2. Mechanisms of formation 5-HMF by aldolization.33 The formation of furfural has been the subject of many discus- sions. Two mechanisms are proposed: from open chain (enediol type) or cyclic intermediates. 2.2. Example of a pentose: D-xylose

2.2.1. Structures obtained 2.2.2.1. Mechanisms involving open chain intermediates (enediol While D-fructose remains the hexose for which the reactivity in type). The production of furfural involving an 1,2-enediol inter- acidic media has been the most studied, it is the reactivity of D- mediate has been proposed in several works, including those of xylose which has been widely explored in the pentose series. In Isbell56 and Anet34 (Scheme 7). Once again, formation of this in- fact, this allows an easy access to furfural. However, this is not the termediate is initiated by the transformation of Lobry De only product to be formed under these acidic conditions and almost BruyneAlberda Van Ekenstein. This is followed by a double dehy- all of the compounds obtained were identified by Antal et al.53 As dration involving sites 3 and 4, adding two additional conjugated with D-fructose, these products can be divided into several cate- unsaturations, and leading to intermediate 20. Finally, a cycload- gories depending on reactions allowing their formation (Table 2). dition step will allow the formation of furfural (involvement of sites Scheme 6 summarizes all the molecules obtained by the reac- 2,5) or of 2,3-dihydroxycyclopent-2-ene-1-one (involvement of tion of D-xylose in diluted acidic media and for which a mechanism sites 1,5).

Table 2 Products obtained from the reaction of D-xylose in diluted acidic media, according to Antal's review

Isomerization Dehydration Fragmentation Condensation

53 55 - D- - 2-Furaldehyde - Formaldehyde - ‘Humins’ - 2,3-Dihydroxy-2-cyclopenten-1-one54 - Formic acid - Acetaldehyde55 - Crotonaldehyde55 - Lactic acid53 - Dihydroxyacetone53 - Glyceraldehyde53 - Pyruvaldehyde53 16 M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19

56 53 57 Scheme 6. Set of products resulting from the reaction of D-xylose in dilute acidic media and for which a reaction mechanism is described (A: Isbell, B: Antal, C: Zeitsch, D: Bobleter,21 E: Antal32).

2.2.2.2. Mechanisms implying cyclic intermediates. Several studies reaction mechanisms have been suggested before the products reported in the literature (Antal et al.,53 Zeitsch57) have examined have been observed experimentally. Thus, if Antal et al.13 assume the involvement of cyclic intermediates in the formation of the formation of a , coproduct of the D-fructose retro- furfural (Scheme 8). According to these works, two different aldolization leading to , the confirmation of this pathways involving cyclic intermediates can explain the formation hypothesis and the identification of D-, were done of furfural. The first pathway (B) is also divided into two parts, one thanks to the works of Yoshida et al.,35 sixteen years later. of which goes through a cyclic oxocarbenium ion, both leading to Similarly, while epimerization of D-xylose to D-lyxose is sug- the formation of 2,5-anhydroxylose (compound 21) via an intra- gested in the Anet mechanism,34 it has been detected only 53 molecular rearrangement. Furfural is then obtained after a double twenty-five years later by Antal. Its ketopentose form (D- dehydration. The second pathway (C), suggested in 2000 by ) has not yet been identified. Zeitsch, goes through a successive formation of a pyranosidic oxocarbenium ion, a ring opening, a serie of protonations/de- 2.2.2.4. A special case: nitric acid. Finally, it is noted that nitric acid, hydrations and a final cyclization leading to furfural. However, one of the common mineral acids, is not studied in the case of sugar 14 according to a study carried out on D-xylose-1- C, the labeled decomposition in an acidic medium. Indeed, the acid property of carbon is found almost exclusively at the carbonyl of furfural, this one is in competition with its oxidizing power. Thus, in 1888, which is in contradiction with pathway C (for which the carbonyl Sohst and Tollens showed that it is possible to form D-mannaric acid 58 59 is in position C-5). from D-mannose using nitric acid as an oxidizer (Scheme 10). 60 Fischer did the same with D-glucaric acid from D-glucose. 2.2.2.3. Retroaldolization products. As for D-fructose, D-xylose in Their work has been taken up by Kiely et al. to improve the diluted acidic media can lead to low molecular weight products protocols used, and make them more attractive from a commercial resulting from retroaldolization reactions (Scheme 9). point of view.61,62 As we have seen, both with D-fructose and D-xylose, the structure and the mechanisms of formation of certain com- 2.2.2.5. Another special case: sulfuric acid. In addition to self- pounds are still not understood. For some other products, their condensation products, different authors have reported sulfation M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19 17

Scheme 7. Furfural formation via acyclic intermediates (A: Isbell,56 B: Antal53).

reactions during the treatment of D-glucose by cold sulfuric acid. they have not been able to determine an exact structure for any of Turvey in 196563 and Takiura et al. in 1970,64 report the production these polymers. of variously substituted compounds. The first one mentioned the possibility of obtaining a complex mixture of monosulfated (at C-6) and polysulfated D-glucose by dissolving the monosaccharide in 3. Conclusion concentrated sulfuric acid at 0 C. Takiura et al. describe the pro- duction of mono-, di- and trisulfated D-glucose by diluting D- Carbohydrate chemistry is a research field that is still of in- glucose in concentrated sulfuric acid at 5 C. The ratio between terest. Environmental challenges are such important that it is the various final products depends on the reaction time. The necessary to find alternatives to the scarcity of fossil substances. exclusive formation of the D-glucose 1,3,6-trisulfate (compound 22, Only a few of molecules resulting from the conversion of sugars in Fig. 4) is reached after 2.5 h. The D-mannose 1,3,6-trisulfate (com- acidic media can be produced on an industrial scale. However, pound 23), D- 1,3,6-trisulfate (compound 24) and the D- these reactions enable access to a very important structural vari- fructose 1,2,4-trisulfate (compound 25) have been obtained in the ety. This allows envisaging the production of a whole range of same conditions. reagents that could substitute or complete those obtained from Other authors, in the example of Nagasawa et al., also reported non-renewable resources. However further studies are needed the formation of sulfated during polymerization of before exploiting the structures obtained from biomass. These monosaccharides in concentrated sulfuric acid.65 Nevertheless, scientific advances must result in a better understanding of the 18 M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19

Scheme 8. Furfural formation via cyclic intermediates (B: Antal,53 C: Zeitsch57).

Scheme 10. Oxidation of D-mannose and D-glucose with nitric acid.

mechanisms involved in the conversion of carbohydrates into acidic environments.

Author contributions 53 21 Scheme 9. Retroaldolization reactions from D-xylose (B: Antal, D: Bobleter, E: Antal32). All authors have given approval to the final version of the manuscript. M.B. Fusaro et al. / Carbohydrate Research 409 (2015) 9e19 19

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