Pentoses furan derivatives

Scheme 1 Synthesis of trihydroxyalkyl-substituted furans derivatives 3 or 4 from D-pentose and C-furyl glycosides 5 or 6 from D-hexoses...

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 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... 

The transformation of pentoses and hexoses into 2-furaldehyde and 5-(hydroxymethyl)-2-furaldehyde, respectively, by the action of acids is a well-known reaction. Professor Bognar was long interested in ascertaining whether this reaction is reversible. With both a theoretical and a practical goal, the Bogn r group then synthesized the dl forms of several important monosaccharides (xylose, ribose, and arabinose) from the aforementioned furan derivatives. 

Although the higher recovery of pentoses was obtained at CS 1.28, if the dilution effect (owing to the addition of acid) is accounted for, the recovery of pentoses obtained at CS 1.10 becomes similar to that obtained at CS 1.28, whereas only slightly lower glucose recovery was achieved. Since aliphatic acid and furan derivative recoveries were also similar, the least severe condition should be considered the most adequate condition for acid posthydrolysis, once it decreases acid and later also alkali requirements, both with a positive impact on process economics. 

There are several classes of compounds formed from rapid pyrolysis of carbohydrates. Besides anhydrosugars, they are carbonyl compounds, furan derivatives, lactones, pyran derivatives, phenols, acids and acid esters, and other compounds. In general, the presence of a substantial quantity of 5-hydroxymethylfuraldehyde in the pyrogram indicates that a hexose is present. Substantial amounts of furaldehyde and the absence of 5-hydroxymethylfuraldehyde in the pyrolysis products indicate the presence of a pentose. However, these markers are not diagnostic for a specific hexose or pentose. 

In strongly acidic media, saccharides produce furan derivatives in a sequence of reactions that are rearrangements and dehydrations followed by cyclization. Similar products are available thermally. Pentoses and hexoses give furan-2-aldehyde and 5-hydroxymethylfuran-2-aldehyde, respectively. Both of these products are responsible for the specific aroma of caramel and burnt sugar. 

Experiments showed that furan derivatives, which are frequently found to occur in the products of hydrolysis of pentose-containing polysaccharides with mineral acids, are not formed on solvolysis with hydrogen fluoride. ... 

As furan derivatives, both furfural and 5-hydroxymethylfurfural (HMF) are readily prepared from renewable biomass. Furfural can be easily obtained from a variety of biomass containing pentoses, mainly including com cobs, oats and rice hulls, sugar cane bagasses, cotton seeds, ohve husks and stones, and wood chips. Furfuryl was first produced in the early nineteenth century and right now the annual production is 300,000 tons [101]. On the other hand, HMF is another major promising furan derivative due to its rich chemistry and potential availability from hexose carbohydrates or from their precursors such as fructose, glucose, sucrose, cellulose, and inulin [14]. 

In carbohydrate-containing foods, furan-2-carbaIdehyde is often present, arising from pentoses and ascorbic acid as a dehydration product. Other common furan-derived aldehydes are 5-hydroxymethyIfuran-2-carbaIdehyde (from hexoses) and 5-methyIfuran-2-carbaIdehyde (resulting from 6-deoxyhexoses). Other heterocyclic aldehydes derived from pyrrole, thiophene, pyridine, pyrazine and other heterocyclic compounds are formed as products of the Maillard reaction. 

A large number of furan derivatives are produced in non-enzymatic browning reactions as dehydration products of carbohydrates and related compounds, such as ascorbic acid (y-lactones can also be considered furan derivatives). Most furans are common to many thermally processed foods. Higher levels of these compounds are also present in acid protein hydrolysates. Furan-2-carbaldehyde has the characteristic pleasant woody aroma resembling nuts, which arises from pentoses and ascorbic acid, and 5-methylfuran-2-carbaldehyde, which is a product of 6-deoxyhexoses. A fatty herbal type flavour is provided by 5-hydroxymethylfuran-2-carbaldehyde arising from hexoses. 2-Acetylfuran arising from hexoses and pentoses is an important component of a number... 

Reducing sugars are dehydrated in the presence of hot mineral acids with simultaneous cyclization to furan derivatives pentoses give fural, methyl-pentoses give 5-methylfural, and hexoses yield 5-hydroxymethylfural ... 

Like the pentoses, mucic acid loses three molecules of water and is converted, as described on p. 386, into derivatives of furane with concentrated hydrochloric acid the product is furane-a d-dicarboxylic acid. Dry distillation yields furane-a-carboxylic or pyromucic acid. 

Figure 9.7 Six- and five-membered cyclic ethers. The stable ring structures which are adopted by hexoses and pentoses are five- or six-membered and contain an oxygen atom. They are named as derivatives of furan or pyran, which are the simplest organic compounds with similar ring structures, e.g. glucofuranose or glucopyranose for five-or six-membered ring structures of glucose respectively.

Xylose-based chemicals analogous to some of the glucose derivatives discussed in the next section are commercially available. An example is xylitol, the pentahydric alcohol that is manufactured by hydrogenation of the aldehyde group in xylose. One of the unique reactions of the pentoses known since the nineteenth century is cyclodehydration to yield furfural. Xylose, for example, is dehydrated by acid treatment through one or two intermediates to form the furan-substituted aldehyde in quantitative yield, or 64.0 wt % of the xylose... 

An acyclic precursor devised by Bogndr and Herczegh, prepared via oxidative cleavage of the furane nucleus, has been applied in the synthesis of racemic pentoses. 2-(2-Furyl)-4,4,S,S-tetramethyl-l,3-dioxolane 257 obtained in reaction of furfural with 2,3-dimethylbutane-2,3-diol was chosen as a substrate, owing to its stability in acidic media (at pH > 3). Oxidation of 257 with bromine water gave the unstable endialone derivative 258, which was reduced with sodium borohydride to 259 (isolated as crystalline dibenzoate). Epoxidation of 259 gave... 

Delmas M., Biomass refining and furanic chemistry. Book of Abstracts, Biomass Derived Pentoses Conference, Reims,... 

In the presence of sulfur compounds (e.g. cysteine) pentoses and methylpentoses produce, in addition to 4-hydroxy-5-methyl-2H-furan-3-one and 4-hydroxy-2,5-dimethyl-2ff-furan-3-one, respectively, related compounds containing sulfur in the molecule. Many of these compounds resemble the aroma of cooked or roasted meat. In the presence of primary amines, 1-deoxyhexo-2,3-diuloses derived from disaccharides yield pyridinium betaines and, further, pyrid-4-ones and isomeric 2-acetylpyrroles. The reaction with secondary amines leads to N-substituted... 

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