Furan Derivatives from Hexose Acids

In contrast to gluconic acid, 5-ketogluconic acid (VIII) is readily transformed into methyl 5-formylfuroate (IX) in almost quantitative 

As would be expected by analogy with the foregoing examples, the saccharic acids on treatment with acidic reagents are converted into furan 2,5-dicarboxylic acid (X) (known to earlier workers as dehydro- 

An interesting biochemical oxidation was observed recently when it was found that the inclusion of either D-glucuronic lactone or D-galactu-ronic acid in the human diet causes a great increase in the amount of furan 2,5-dicarboxylic acid excreted in the urine.37 

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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. 

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 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... 

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