Furan Compounds Derived from Hexoses

Furan Compounds Derived from Hexoses 1. 5-Hydroxymethylfurfural 

A few years later it was confirmed that the degradation of carbohydrates with acids gave rise also to furfural (II). This had been obtained fifty years earlier by Dobereiner by the action of sulfuric acid and manganese dioxide on sugar but its significance was not realized at the time. Furfural however was obtained only from pentoses, under normal acid conditions. 

In 1895 Dull,9 who was studying inulin and its products of hydrolysis, found that when either fructose or sorbose was treated with an aqueous solution of oxalic acid under pressure, a substance was obtained which had the formula CeHeOa and resembled furfural in its properties. This substance was further investigated by Kiermayer4 who found that fructose and sucrose were the best sources when they were heated with 0.3% aqueous oxalic acid at 120°. It was however only the fructose portion of the sucrose molecule which was transformed since the glucose moiety was recovered unchanged. Kiermayer prepared several derivatives of CeH Os and from its reactions concluded that its structure was probably /3-hydroxy-S-methylfurfural (III). Van Ekenstein and 

Blanksma5 continued the investigation of the formation of hydroxy-methylfurfural and confirmed Kiermayer s results that it was formed from hexoses by elimination of three molecules of water on acidic degradation and showed that ketoses reacted more readily than aldoses, a fact which was observed by Kiermayer when sucrose was treated with aqueous oxalic acid. Kiermayer had observed also that levulinic acid was obtained when hydroxymethylfurfural was treated with aqueous oxalic acid under pressure and this received further confirmation by Van Ekenstein and Blanksma.6 It was these authors who first pointed out that the complete degradation of hexoses to levulinic acid took place through the intermediate formation of hydroxymethylfurfural. 

Further work by V an Ekenstein and Blanksma7 led to a revision of the formulation of hydroxymethylfurfural. It was found that chitose on dehydration gave hydroxymethylfurfural which when oxidized was converted into hydroxymethylpyromucic acid, identical with the compound obtained by Fischer and Andreae from chitonic and chitaric acids.8 The nature of chitose (a 2,5-anhydrohexose) was such that Kiermayer s original formulation of /J-hydroxy-S-methyl-furfural could not obtain and the accepted formulation, -hydroxymethylfurfural (5-hydroxymethyl-2-furaldehyde (IV)) was assigned to the compound. 

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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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