Sugars furan derivatives

Deoxy-sugars. Part XXIV. Conversion of D-Gluca into Furan Derivatives, F. Shafizadeh and M. Stacey,/. Chem. Soc., (1952) 3608-3610. 

The well-known acid-catalyzed conversion of sugars into furan derivatives obviously consists of a complex sequence of reactions, and the industrial heterophasic conversion of pentosans in plant tissues has been discussed in detail.11 The reactions themselves are still not well understood, although xylose and glucuronic acid in deuterium oxide afford 2-furaldehyde without uptake of isotope thus limiting the mechanistic possibilities to those not permitting reversible enolization.12 The bacterial sugar streptose yields... 

A comparative analytical study, by means of paper chromatography, has confirmed the conclusion that, in the condensation of aldoses with ethyl acetoacetate, the yield of the furan derivatives increases as the length of the sugar chain is decreased.17... 

Furan-2-carbaldehyde 8 is commonly referred to as furfural. It is produced on a large scale by the action of acids on sugars and is a commercially important raw material used in furfural-phenol resins and as a synthetic intermediate . The 2-furylmethyl radical is called furfuryl and, for example, the alcohol 9 is commonly known as furfuryl alcohol. The furan derivative 10 (ranitidine) was one of the first H2-antagonists for the treatment of gastric ulcers and a major contribution to modern medicine. 

Furans. The last statement is certainly true of furans derived from sugars (142,143), particularly furfural and furfuryl alcohol, which is readily derived from furfural (144)- Dr. McKillip of QO Chemicals discusses furan resin chemistry and furan polymers in Chapter 29. Dr. Stanford and his colleagues at the University of Manchester Institute of Science and Technology discuss the use of a diisocyanate derived from furfural for polyurethane production in Chapter 30. 

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. 

Since the enolization is not restricted to the 2 and 3 positions, a number of products are formed that undergo subsequent aldol condensations and the Cannizzaro oxidation. They are all 2-hydroxy-3-methyl, 3,4-dimethyl-2-hydroxy, 3,5-dimethyl-2-hydroxy, and 3-ethyl-2-hydroxy-2-cyclopenten-l-ones sugar acids acetic acid hydroxyacetone three isomeric hydroxy-2-butanones y-butyrolactone and such furan derivatives as furyl alcohol, 5-methyl-2-furyl alcohol, and 2,5-dimethyl-4-hydroxy-3(2H)-furanone. They are food flavoring agents. 

Ever since the work of Cemiani in 1951, the pyrolysis of sugars has been studied at temperatures ranging from 200 to 1000°. Above 200°, or 250°, deep changes occur in sugars, and a great variety of products is formed, such as carbon mon-, and di-oxide, hydrocarbons (among them alkanes, alkenes, and aromatic hydrocarbons), alcohols, aldehydes, ketones, and several furan derivatives. The number, and character, of the products depend, at least, on both the temperature and the time of reaction. Controversy exists in respect to the influence of the atmosphere in which the pyrolysis takes place. 

Where, as illustrated in the above described example, the Diels-Alder reaction provides avenues into the formation of C-glycosides from furan-derived dienes, the hetero Diels-Alder reaction allows for the direct formation of sugar rings from carbonyl groups. As shown in Scheme 7.5.2, Schmidt, et al.,25 effected reactions between conjugated carbonyl compounds and olefins. The illustrated reaction proceeded in 81% yield giving an adduct which, after further manipulations, was converted to a C-aryl glycoside. 

Whereas natural monomers like terpenes and sugars constitute building blocks for a limited number of macromolecular structures, associated with their own peculiar chemical features, the realm of furan polymers bears a qualitatively different connotation in that it resembles the context of petrol refinery, that is, it is open to a whole domain of monomers, whose only specificity is the fact that they all incorporate the furan heterocycle in their structure. This state of affairs stems from the fact that, as in petroleum chemistry, saccharide-based renewable resources are used to produce two first-generation furan derivatives, which constitute the substrates capable of being converted into a vast array of monomers and hence a correspondingly large number of macromolecular structures associated with materials possessing different properties and applications [34]. 

The second furan derivative which can be prepared from the appropriate C6 polysaccharides or sugars is hydroxymethylfuraldehyde (HMF). The mechanism of its formation from hexoses is entirely similar to that of F, but difficulties associated with the recovery of the product have delayed its industrial production, despite, again, the ubiquitous character of its natural precursors. A very substantial effort has been devoted in recent years to investigate and optimize novel processes and/or catalysts [36] and the ensuing results suggest that HMF will be a commercial commodity very soon. Interestingly, because of its relative fragility, some of these approaches consider the in situ conversion of HMF into its very stable dialdehyde (FCDA) or diacid (FDCA) derivatives. 

On comparing the reactions of 2-amino-2-deoxyaldoses and 1-amino-l-deoxyketoses with 8-dicarbonyl compounds, which yield pyrroles (Scheme A, X = NH, A -alkyl, or A -aryl), with the reactions of the nonnitrogenous aldoses and ketoses with the same compounds, which yield furan derivatives (Scheme A, X = 0), it may be noted that the changes of bonds are the same. It seems reasonable, therefore, to assume that both processes proceed through the same, or at least similar, mechanisms. From the experimental point of view, the main differences between these two reactions concern rates and catalysts. Aldoses and ketoses react very slowly in the absence of such acidic catalysts as zinc chloride and ferric chloride. The reactions of the amino sugars are much faster and are usually performed under neutral or slightly basic conditions. 

The used BSG hydrolyzate conqjosition is shown in Table 2. It has approximately 26 g F of monosaccharides and a low level of microbial inhibitors, specially aliphatic acids, and furan derivatives compared to similar hemicellulosic hydrolyzates used for SCP production, e.g., eucalyptus wood [28] and sugar cane bagasse [7,8]. Also, it has a low content of phenolic compounds. Crude protein has a concentration of about 1.2 g F. Much of this nitrogen (about half) is in ammonia form, the rest as amino acids (data not shown). 

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