Carbohydrate furans

C. Moreau, M. N. Belgacem, and A. Gandini, Recent catalytic advances in the chemistry of substituted furans from carbohydrates and in the ensuing polymers, Top. Catal, 27 (2004) 11-30. 

The reaction with ethyl acetoacetate has been extended to glycolaldehyde, and to carbohydrates other than n-glucose, by employing different experimental conditions it is probably applicable to aldoses in general. With d-fructose, yields are lower, but two molar proportions of water are liberated and a crystalline product results. This has a constitution similar to that of II but with the D-omhfno-tetrahydroxybutyl chain at the /3-position on the furan ring. The reaction has been applied successfully to other ketoses and... 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

Furan Derivatives Catalytic processes used to obtain furan derivatives from carbohydrates and the catalytic routes from furan intermediates to chemicals and polymers have been reviewed by Moreau et al. [27]. Some of the main reactions are summarized in Fig. 3.2. From fructose or carbohydrates based on fructose (sucrose, inulin), the first transformation step is dehydration to 5-hydroxy methylfur-fural (HMF). Fructose dehydration at 165 °C was performed in the presence of... 

D-Erythrose has also been exposed to a boiling solution in pH 4.5 buffer. Low yields (<0.2%) of a number of products were obtained, as shown in Scheme 3. These included 5-(hydroxymethyl)-2-furaldehyde (11), 2-acetyl-5-(hydroxymethyl)furan (12), 3,4-dihydroxyacetophenone (13), 3,4-dihydroxy be nzaldehyde (14), 3,4-dihydroxybenzoic acid (15), 2,3-dihydroxytoluene (16), and 1,2-benzenediol (pyrocatechol) (17). Also detected were formic, hydroxyacetic, and 3-hydroxypropanoic acids. Pyrocatechol seems to be a product formed from all carbohydrates boiled in aqueous solutions at pH 4-10 it may constitute a statistical product arising from retro-aldol and re-aldol reactions. It has been shown that the aldol reaction may operate at a pH as low as 4. An aldol reaction... 

The formation of oxygen-containing heterocyclic compounds is also a consequence of the Maillard reaction. Amines and amino acids have a catalytic effect upon the formation of 2-furaldehyde (5), 5-(hydroxy-methyl)-2-furaldehyde (11),2-(2-hydroxyacetyl)furan (44),2 and 4-hy-droxy-5-methyl-3(2//)-furanone (111) (see Ref. 214). This catalytic effect can be observed with several other non-nitrogenous products, including maltol. The amino acid or amine catalysis has been attributed to the transient formation of enamines or immonium ions, or the 1,2-2,3 eno-lization of carbohydrates. Of interest is the detection of A -(2-furoyl-... 

Several enzymatic procedures have been developed for the synthesis of carbohydrates from acyclic precursors. Aldolases appear to be useful catalysts for the construction of sugars through asymmeteric C-C bond formation. 2-deoxy-KDO, 2-deoxy-2-fluoro-KDO, 9-0-acetyl sialic acid and several unusual sugars were prepared by a combined chemical and enzymatic approach. Alcohol dehydrogenases and lipases have been used in the preparation of chiral furans, hydroxyaldehydes, and glycerol acetonide which are useful as building blocks in carbohydrate synthesis. 

Enzymatic reduction of carbonyl compounds and enzymatic enantioselective transformation of racemic or meso alcohols (25,43.) are two methodologies that have proven to be beneficial in the preparation of optically active hydroxyl compounds, key chiral building blocks used in carbohydrate and natural product syntheses (44-45. Our interest in this area is to develop enzymatic routes to optically active glycerol and furan derivatives, and hydroxyaldehydes. 

Numerous furan and pyran derivatives, many of which originate from heat treatment of carbohydrates, largely determine the odor of processed foods. Of this group, 2,5-dimethyl-4-hydroxy-277-furan-3-one and maltols are used in fairly large quantities in flavors. The following compounds are used in relatively small amounts in flavor compositions ... 

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... 

Two approaches, based on furan, have found wide application in carbohydrate synthesis. Cycloaddition reactions of furan with 2-substituted acrylonitrile or acrolein lead to oxabicycloheptanes which, in tnm, can be transformed to monosaccharides. On the other hand, furfuryl alcohols can be converted—either by the Clauson-Kaas reaction or by mild oxidation—into 5,6-dihydro-4-pyrones, suitable for easy functionalization to sugars. 

The popular approach to tetrahydrofurans involves an electrophilic process and the commonly used electrophiles for the cyclization are acids, oxygen, halogen, mercury (see Section 3.11.2.2.9) and selenium. The ionic hydrogenation of furans with excess triethyl-silane in trifluoroacetic acid affords high yields, e.g. 2-methylfuran is reduced to 2-methyl-tetrahydrofuran and 2-ethylfuran to 2-ethyltetrahydrofuran (see Section 3.11.2.5). The synthesis of several dihydro and tetrahydrofurans containing natural products by chirality transfer from carbohydrates has been used successfully for total synthesis, e.g. (-)-nonactic acid. A reasonable yield of 2-alkyltetrahydrofuran was prepared from 4-alkylbut-l-en-4-ol by hydroboration followed by cyclization with p-toluenesulfonic acid. 

The terms furanose and pyranose arise from attachment of carbohydrate endings to the names of the cyclic compounds, furan and pyran. 

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. 

Myriad natural products such as carbohydrates, ionophore antibiotics and many others possess polysubstituted oxygenated rings, such as furans, pyrans and lactone units, within their complex structure. Pyran and dihydropyran units are par-... 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Although exact mechanisms have not been described for the formation of other furanoid compounds through amine-carbohydrate interactions, probably many of the 32 furans described by Ohloff and Flament (23) from meat aroma mixtures are from this source. Shiba-moto (24) described many of the same components from mixtures producing meat odors. 

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