Hydroxymethylfurfural, from hexoses

Absorption spectra of the phenol-sulfuric acid solutions tested for total sugars show that 5-hydroxymethylfurfural from hexoses is more common in the uppermost Silurian and Devonian samples than in the earlier deposits. Furfurals from pentose sugars evidently form the bulk of the residual carbohydrates in these samples, however. There is no definite evidence as to the marine or terrestrial origin of the hexose products in the samples. 

The formation of furfural from pentoses, of 5-methylfurfuraI from methylpentoses, and of 5-hydroxymethylfurfural from hexoses under acidic conditions has long been known.1 There are only a few more recent investigations to be mentioned. 

Formula 4.59 shows examples of products obtained on the decomposition of 3-deoxy-osones. The best known compounds are 5-hydroxymethylfurfural from hexoses (HMF, II in Formula 4.59) and furfural from pentoses (I in Formula 4.59). Taking the furanoid structures of 3-deoxyosone as a basis (Formula 4.55), 3,4-dideoxyosone is obtained after ring opening, enolization, and water elimination (Formula 4.60). Water elimination from the hemiacetal form of 3,4-dideoxyosone directly yields HMF. Taking into account the water elimination required to form 3-deoxyosone (cf. Formula 4.55), 5-hydroxymethylfurfural is formed from hexose by the stoichiometric elimination of 3 mols of water. 

II. Furan Compounds Derived from Hexoses 1. 5-Hydroxymethylfurfural... 

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. 

Hydroxymethylfurfural is not volatile by steam. It is prepared from hexoses in the presence of an acid catalyst by short heat treatment to avoid further degradation to levulinic acid. After recovery by solvent extraction hydroxymethylfurfural is purified by distillation. Levulinic acid can be prepared in good yield from hexose-based polysaccharides by heating with acids. In this reaction formic acid is liberated and levulinic acid is easily lactonized to form a- and /3-angelica lactones (Fig. 2-31). 

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]. 

Levulinic acid and formic acid are end products of the acidic and thermal decomposition of lignocellulosic material, their multistep formation from the hexoses contained therein proceeding through hydroxymethylfurfural (HMF) as the key intermediate, while the hemicellulosic part, mostly xylans, produces furfural.A commercially viable fractionation technology for the specific... 

Catalytic dehydration of hexose polysaccharides to 5-hydroxymethylfurfural (HMF) is a highly valuable reaction. Indeed, from HMF, new generations of biofuel (e.g., dimethylfurane) and a wide range of intermediates and fine chemicals can be obtained [55-59]. Comprehensive reviews related to the utilization of HMF can be found in the literature [60-64] see also Chap, by M. Moser. 

The most important aspect of the chemistry of the furan ring in 5-hydroxymethylfurfural is its scission under the influence of acidic reagents. In the very earliest work on the acidic degradation of hexoses, levulinic and formic acids were obtained which were shown subsequently to have arisen from the decomposition of 5-hydroxymethylfurfural. This degradation was studied by Teunissen53 87 who measured its rate and showed it to be a unimolecular reaction. He proposed the scheme represented by XLIV-XLIX for the conversion of 5-hydroxymethyl-... 

The hexoses can also be converted directly to levulinic acid and formic acid on acid treatment without isolation of 2-hydroxymethylfurfural. Note that levulinic acid can be synthesized from both pentoses and hexoses, but is usually manufactured from cane sugar or starch by boiling with hydrochloric acid. A process for the manufacture of levulinic acid has also been developed for paper-mill sludge feedstocks (Fitzpatrick and Jarnefeld, 1996). 

Increasing attention has recently been devoted to the selective dehydration of hexoses, particularly fructose, to 5-hydroxymethylfurfural (10, Scheme 8) [14], This compound, which can thus be derived from renewable biomass and agricultural surpluses, finds application as starting material in the preparation of fine chemicals and polymers. 

Foods derived from plant materials contain pentose as well as hexose sugars, which are usually formed under weakly acidic conditions. It is well known that pentose contributes more to browning by the Maillard reaction than hexose does, because the oxo-or reducible form of sugars is higher in pentose than in hexose. Hydroxymethylfurfural (HMF) is one of the major decomposed products of such hexoses as glucose and fructose under acidic conditions, while furfural is the corresponding one of pentose and is also formed by the decomposition of ascorbic acid. 

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