Hexoses dehydration

Thermal treatment, applied to honey, may destroy vitamins and bionutrients, and produce a simultaneous decrease in diastase activity and an increase in HMF content. Honey treatment temperature and time must be limited when pasteurising and stabilising it both diastase activity and HMF content are national and international parameters used as controls so as to limit thermal treatment application. HMF can be formed by hexose dehydration in acid media or by the Maillaid reaction [11,12]. According to Ibarz et al., HMF formation can be described by a second order kinetics (auto-catalytic), with the following equation as expression model [13] ... 

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

The products arising from such condensations of the hexoses readily lose the elements of water in a way similar to the dehydration of 7-hydroxy acids, when heated in aqueous solution the hydroxyl groups corresponding to those at C3 and C6 of the original sugar are involved, and a substituted tetrahydrofuran ring results. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

A probable pathway for the degradation of hemicelluloses via free-radical intermediates has been proposed by Fengel and Wegener (1989) and is shown in Figure 5.1. Hemicellulose polymers are depolymerized to form oligosaccharides and monosaccharides, which are dehydrated to form furfural (pentoses) and hydroxymethyl furfural (hexoses). 

It is well known that heating of honey resrrlts in HMF, which is formed dming acid-catalysed dehydration of hexoses [6]. The presence in honey of simple sugars (glucose and fructose) and mar r acids is a favotrrable condition for the production of this substance. 

The initial HMF content in all honey samples was lower than the allowed maximum limit of 40 mg/kg as recommended by Turkish Alimentarus Codex [17], for honey in general. These results contradict the observation made by some authors that the types of honey produced in subtropical climates have high HMF exceeding 40 mg/kg [18]. However, the European Union council directive also allows for a maximum of 80 mg/kg for honey from tropical climates. The HMF level in honey is said to depend on the type of sugar present in honey and the fructose glucose ratio [19]. The HMF formation results from the acid catalyzed dehydration of hexose... 

D-glucose and D-fructose in acidified deuterium oxide, and acid conversion of D-[2- H]glucose were conducted, in order to determine the importance of 39 as an intermediate from the proportion of deuterium incorporated at C-3 of 5-(hydroxymethyl)-2-furaldehyde. However, the 2-furaldehyde formed in the reactions contained no deuterium. Thus, an essentially irreversible sequence that involves hexose, 36, 38, 40, and 11 best explains the acid-catalyzed, dehydration reaction. 

Catalytic dehydration of hexose polysaccharides to 5-hydroxymethylfurfural (HMF) is a highly valuable reaction. Indeed, from HMF, new generations of biofuel (e.g., dimethylfurane) and a wide range of intermediates and fine chemicals can be obtained [55-59]. Comprehensive reviews related to the utilization of HMF can be found in the literature [60-64] see also Chap, by M. Moser. 

HMF has been discovered for the first time in 1895 by Diill and Kiermeyer who independently introduced a method of synthesis of HMF that they named oxy-methylfurfurol [65, 66]. Later, Haworth and Jones studied the mechanism of this reaction and showed that the formation of HMF involved a triple dehydration of hexoses [67]. Other studies performed by Van Dam, Kuster and Antal showed that the dehydration of hexoses (especially fructose and glucose) involved two possible pathways (Scheme 5) [63, 68, 69]. The path 1 involves the dehydration of ring systems (fructopyranose or glucopyranose), while the path 2 is based on acyclic derivatives (glucose and fructose open chain). 

Scheme 5). However, this reaction is not as easy as it looks since numerous side reactions may occur. Antal et al. showed that four different classes of reactions can take place from hexoses (1) their dehydration leading to the formation of HMF, (2) their fragmentation, (3) their isomerization, and (4) their condensation leading to the formation of insoluble polymers and humins [58, 69]. Later, Van Dam and Cottier showed that at least 37 products can be produced, thus showing the complexity of this reaction. In particular, they highlighted that HMF can react with water, leading to the formation of undesirable side products such as levulinic and formic acids [68, 70]. 

The major products formed from hexoses that react in aqueous acidic solution are 5-(hydroxymethyl)-2-furaldehyde, levulinic acid, and polymeric materials. In addition, many minor dehydration products are found. In a study41 of D-fructose, 2-(2-hydroxyacetyl)furan (13), 2-acetyl-3-hydroxyfuran (isomaltol 16), 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one, and 3,4,5-trihydroxy-3,5-hexadien-2-one (acetylformoin) were identified. Products not formed solely by dehydration mechanisms include acetone,56 formaldehyde, acetalde-... 

Because, on treatment with the anthrone reagents,224,225 hexoses and 5-(hydroxymethyl)-2-furaldehyde give solutions having identical spectral characteristics, dehydration is indicated to be the important reaction in this analysis. This conclusion is further supported by the reported isolation228 of 10-furfurylidene-9,10-dihydro-9-oxoanthra-cene (121) after reaction of 2-furaldehyde with anthrone, and by the fact that 121 has an absorption maximum of 600 nm, a value close to that used for pentose estimations. In similar studies,227 9,10-dihydro-10-(5-methylfurfurylidene)-9-oxoanthracene (122) was reported to have been isolated after the reaction of either L-rhamnose or 5-methyl-... 

Hemicelluloses (cellulosans) is the family name of polysaccharides that includes pentosans (C3H804) , made up of the pentose units and hexosans (C6H,0O5) made up of hexose units. The pentosans include such substances as xylan and araban which are hydrolysed to xylose and arabinose respectively. On dehydration, furfuraldehyde is formed ... 

It is necessary to postulate the existence of a previously unknown, modification reaction of glycosyl nucleotides, that is, deoxygenation at C-4, in order to explain the formation of 4-deoxy-D-arafc//io-hexose, a monosaccharide constituent of Citrobacter lipopolysaccharide.202 4-Deoxy-L-threo-hex-4-enuronic acid, found in Klebsiella K22 polysaccharide,203 may be formed through dehydration of the D-galacturonic acid group or residue, either in the glycosyl nucleotide, or in the polysaccharide. 

In vivo, pyruvate lyases perform a catabolic function. The synthetically most interesting types are those involved in the degradation of sialic acids or the structurally related octulosonic acid KDO, which are higher sugars typically found in mammalian or bacterial glycoconjugates [62-64], respectively. Also, hexose or pentose catabolism may proceed via pyruvate cleavage from intermediate 2-keto-3-deoxy derivatives which result from dehydration of the corresponding aldonic acids. Since these aldol additions are freely reversible, the often unfavourable equilibrium constants require that reactions in the direction of synthesis have to be driven by an excess of one of the components, preferably pyruvate for economic reasons, in order to achieve a satisfactory conversion. 

Hough and Richardson58 explored the reaction of unacetylated hexose dithioacetals with peroxypropanoic acid the sulfone 15 initially formed is only marginally stable, and undergoes dehydration to mixtures of the unsaturated disulfone (18) and the 2,6-anhydro-l-deoxy-l,l-bis(ethylsulfo-nyl)alditol (19). All three products are degraded by aqueous ammonia to the next lower sugar (16) and bis(ethylsulfonyl)methane (17). This degradation is much slower for 18 than for 15, and it was concluded that hydration of 18 to 15 precedes disproportionation (Scheme 8). 

Two basic nonpetroleum chemicals readily accessible from renewable resources, furfural arising from the acid-catalysed dehydration of pentoses, and 5-hydroxy-methylfurfural arising from the acid-catalysed dehydration of hexoses, are suitable starting materials for the preparation of further monomers required for polymer applications. Whereas the former is industrially available (200000 tons year-1), the latter is only produced on a pilot plant scale.[26]... 

The pathways for hexose and pentose are differentiated mainly through the relative chemical stability of the homologues, glucosepane and pentosinane. The former is a proper AGE under physiological conditions, but the latter is smoothly oxidised to an intermediate, which is subsequently dehydrated to the advanced glycoxidation product, pentosidine. At this stage, it is not known whether oxidation or dehydration is rate determining the role of metal ions needs to be clarified as well. 

Wheat pentosans were dehydrated to furfural with concentrated HC1, and the furfural was dissolved in dibutyl ether in preparation for GC (Folkes, 1980). This method is claimed to be specific, accurate, and precise for pentoses and pentosans (Folkes, 1980), because 5-hydroxymethylfurfural is not produced, as is the case in hexose decomposition. Previously, furfural was distilled and analyzed colorimetrically. Alkalization increases the yield of 1-, 2-, and 3-carbon, vapor-phase compounds at the expense of anhydro sugars and furans (Ponder and Richards, 1993). Hydrolysis-GC is adaptable to the estimation of DP (Morrison, 1975) applicability to carbohydrates is limited to oligomers with DP = 6 (Traitler et al., 1984). 

Seri, K., Inoue, Y., and Ishida, H., Catalytic activity of Lanthanide(III) ions for the dehydration of hexose to 5-hydroxymethyl-2-fiiraldehyde in water. Bull Chem Soc Japan 2001, 74 (6), 1145-1150. 

Extensive kinetic investigations have also indicated that the activation energy of the overall thermal-decomposition process is substantially lowered by the addition of sodium chloride and sodium carbonate. Madorsky and coworkers have, therefore, proposed that these salts catalyze the dehydration of cellulose by scission of the C —O bonds (bonds a, b, and c in 1 see p. 438), and that this results in destruction of the hexose units and increases the yield of water and char at the expense of levoglucosan. This theory has found substantial support in subsequent experiments and publications however, it may be noted here that Golova and associates" consider that inorganic salts promote the cleavage of C—C, rather than C—O, bonds in the macromolecule. 

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