Pentose dehydration

The mechanism of pentose dehydration has been a matter of study for several years. The accepted pathway (see Scheme 1) to 2-furaldehyde from a pentose, in this case D-xylose (1), involves the reversible formation of a 1,2-enediol (2) followed by dehydration to the enolic form (3) of a 3-deoxypentosulose, which is further dehydrated to the 3,4-dideoxypent-3-enos-2-ulose (4) prior to cyclization to afford 2-furaldehyde 5. This mechanism, initially suggested by Isbell,has been substantiated by later work. This confirmation required incorporation of deuterium or tri-tium into the furaldehyde at various ring positions. However, when... 

Danon B, Marcotullio G, de Jong W (2014) Mechanistic and kinetic aspects of pentose dehydration towards furfural in aqueous media employing homogeneous catalysis. Green Chem 16(l) 39-54... 

Acetic acid, formed through treatment of wood with steam and uniformly distributed in the particles, catalyzes the hydrolysis of pentosans to monosaccharides. Further conversion of pentoses into furfural directly in the cell wall does not occur because acetic acid is too weak as a catalyst for dehydration at a low temperature. The pentoses formed in the process diffuse to the surface of particles where their dehydration to furfural occurs under the influence of sulfuric acid. Studies on the process kinetics (6) have confirmed that a higher concentration of sulfuric acid increases the rate of pentoses dehydration more... 

Table I. Kinetic rate constant values as a function of catalyst concentration. ki pentosan hydrolysis ki - pentoses dehydration (6)...

Furfural is obtained industrially (200000 t a-1) by dehydration of pentoses produced from hemicelluloses. Furfurylic alcohol is obtained by selective hydrogenation of the C=0 bond of furfural, avoiding the hydrogenation of the furan ring. Liquid phase hydrogenation at 80 °C in ethanol on Raney nickel modified by heteropolyacid salts resulted in a 98% yield of furfuryl alcohol [31]. 

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). 

Figure 11.4 Condensation, dehydration and reduction reactions in fatty add synthesis. These reactions constitute the major components of the pathway of fatty acid synthesis and are all catalysed by fatty acid synthase. The reduction reactions, indicated by addition of 2H in the diagram, involve the conversion of NADPH to NADP . (The re-conversion of NADP back to NADPH occurs in the pentose phosphate pathway.) The condensation reaction results in an increase in size of acyl-ACP by two carbon units in each step. The two carbons for each extension are each provided by malonyl-CoA. ACP - acyl carrier protein.

A number of lower volume chemicals can be obtained from wood hydrolysis. Furfural is formed from the hydrolysis of some polysaccharides to pentoses, followed by dehydration. This process is still used in the Soviet Union. Furfural is used in small amounts in some phenol plastics it is a small minor pesticide and an important commercial solvent. It can be converted into the common solvent tetrahydrofuran (THF) and an important solvent and intermediate in organic synthesis, furfuryl alcohol. 

The answer is A. This patient exhibits several signs of acute arsenic exposure, including the cholera-like gastrointestinal symptoms and probable dehydration. He may currently be in hypovolemic shock and beginning chelation therapy is the only recourse. Arsenic is a metabolic toxin because it inhibits enzymes that require lipoic acid as a coenzyme the PDH complex, the a-ketoglutarate dehydrogenase complex, and trans-ketolase of the pentose phosphate pathway. 

Xylose. Catalytic dehydration of xylose, which is the most abundantly available pentose monomer in hemicellulose, has been known for a long time (Scheme 5). In fact, as early as 1922, an industrial process involving sulfuric acid catalyzed dehydration of xylose to produce furfural was developed by the Quaker Oats Co. 

Furfural. Furfural is readily obtainable from dehydration of pentoses. Reduction of furfural can lead to a variety of products that are more volatile, more stable and possibly also more useful than furfural itself. Selective reduction of the aldehyde moiety leads to furfuryl alcohol (Scheme 15), whereas further reduction of the furan core will lead to tetrahydrofurfuryl alcohol. Reductive deoxygenation can result in the formation of either 2-methylfuran or 2-methyltetrahydrofuran, which can be used as liquid fuels or solvents. 

This compound is one of the more important furan derivatives, and is commercially available from pentosans (polysaccharides) which are present in rice husks, oats and corn residues (furfur is the Latin name for bran ). When treated with sulfuric acid, pentosans decompose into pentoses, which then undergo dehydration to the aldehyde (Scheme 6.31). 

The analogous product from the pentose glycals and 2-deoxy-pentoses would be furfuryl alcohol, but, as it is unstable and is readily converted into levulinic acid under the conditions of formation, it is difficult to isolate. The spectral data52 and the fact that levulinic acid is the common product from 2-deoxy-D-erythro-pentose, D-ara-binal, and furfuryl alcohol19 substantiate the supposition that the mechanism is analogous to that just described. Other (unknown) products are formed in significant yield from the dehydration of 2-deoxy-D-ert/fhro-pentose.52,84 The mechanism of formation of levulinic acid is discussed in Section V (see p. 212). 

The fact that 27 is produced by dehydration both of uronic acids and of pentoses has led to the suggestion112 that pentoses may be intermediates in decarboxylation reactions of uronic acids, and that treatment of such glycuronans as pectin with strong acids results in the production of pentosans.113 Little evidence supports this theory, be-... 

Kurata and Sakurai124 have investigated the mechanism of dehydration of L-ascorbic acid by examining the products from it and from L-xyZo-hexulosonic acid after treatment for 1 hour at 100° at pH 2.2, or with 5% sulfuric acid at 100°. In both experiments, 2-furaldehyde and 3-deoxy-L-threo-pentosulose (isolated as the phenylhydrazone) were the major products no pentose was detected. 

During the dehydration of pentoses in acidic solution, the cis and trans 2-enes (homologs of 44 and 46 see p. 177) react rapidly to form the 3-enes, 94a and 94b. Anet8 has presented evidence for the existence of cis and trans forms of the 6-carbon 2-enes. The relative proportion of the geometrical isomers 94a and 94b is controlled by the ratio of the cis and trans forms of the reacting 2-enes and by the rate of interconversion through the tautomer, 72. The trans form (94a) is unable to cyclize, whereas the cis isomer (94b) could readily cyclize to 28 and this could be dehydrated to 2-furaldehyde (27). Formed by extended enolization of 94, the diene 72 has the electronic arrangement needed for cyclization to afford 75a, a tautomer of re-... 

Scott and coworkers52 have investigated some of the parameters of the dehydration reaction in concentrated sulfuric acid solution, in order to use it as an analytical method. The concentration of the sulfuric acid was found to be the most important variable in the reaction it influences the position and the intensity of the ultraviolet-absorption maximum, the stabilities of the reaction products, and, for certain pentoses and hexuronic acids, the course of the reaction. In general, an increase in the concentration of the sulfuric acid causes a shift of the 2-furaldehyde absorption to higher wavelength, and an increase in the molar absorptivity. 

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-... 

The routes involved in the formation of the various furan sulphides and disulphides involve the interaction of hydrogen sulphide with dicarbonyls, furanones and furfurals. Possible pathways are shown in Scheme 12.8. Furanthiols have been found in heated model systems containing hydrogen sulphide or cysteine with pentoses [56-58]. 2-Methyl-3-furanthiol has also been found as a major product in the reaction of 4-hydroxy-5-methyl-3(2H)-furanone with hydrogen sulphide or cysteine [21, 59]. This furanone is formed in the Maillard reaction of pentoses alternatively it has been suggested that it may be produced by the dephosphorylation and dehydration of ribose phosphate, and that this may be a route to its formation in cooked meat [21, 60]. 

The formation of furan derivatives in acid-catalyzed dehydrations of carbohydrate substrates is a well known reaction, first reported by Dobereiner186 in 1832. Among the plethora of compounds formed, 2-furaldehyde is the main product obtained from all of the pentoses, whereas 5-(hydroxymethyl)-2-furaldehyde is the major product... 

Furan is synthesized by decarbonylation of furfural (furfuraldehyde), which itself can be prepared by acidic dehydration of the pentose sugars found in oat hulls, corncobs and rice hulls. 

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 ... 

PENTOSAN. A complex carbohydrate (hemicellulose) present with the cellulose in many woody plant tissues, particularly cereal straws and brans, characterized by hydrolysis to give five-carbon-atom sugars (pentoses). Thus the pentosan xylan yields the sugar xylose (HOH,C CHOH CHOH CHOH CHO) that is dehydrated with sulfuric acid to yield furfural (C5H4O2). 

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. 

Furoic acid (furan-2-carboxylic acid, or pyromucic acid) is used as a bactericide, and the furoate esters are used as flavoring agents, as antibiotic and corticosteroid intermediates. It is obtained by the enzymatic or chemical/catalytic aerobial oxidation of furfural (2-furalaldehyde) the latter is the only unsaturated large-volume organic chemical prepared from carbohydrates today. D-Xylose and L-ara-binose, the pentoses contained in the xylan-rich portion of hemicelluloses from agricultural and forestry wastes, under the conditions used for hydrolysis undergo dehydration to furfural. 

With acid degradation, the first step appears to involve the formation of 1,2-enols from the aldose or ketose (7), followed by a series of dehydration reactions resulting in the formation of 5-hydroxymethyl-2-furfuraldehyde. If the initial sugar is a pentose, the final product is 2-furfuraldehyde. 

Furfural identified in beef diffusate appears to be a prominent meat flavor intermediate. It is a dehydration product of pentoses similar to formation of hydroxy methyl furfural from hex-oses. These compounds are formed by dehydration of 1,2-enediols derived from deamination of Amadori compounds (51). 

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]... 

As previously stated, furfural is obtained through dehydration of pentoses, xylose in particular, or hemicelluloses, at high temperatures (200-250 °C), and in the presence of mineral acids as catalysts, mainly sulfuric acid.[31] Under these... 

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