Monosaccharides furans

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

The study of monosaccharides subjected to conditions of methanolysis is considered for two reasons. First, the decomposition of monosaccharides is indicative of the decomposition of monosaccharides liberated during methanolysis second, the ratio of methyl glycosides of a particular standard monosaccharide is the same for the same monosaccharide released during methanolysis (for a particular set of methanolysis conditions), provided that the concentration of sugars is relatively low. Up to four methyl glycosides (the a and )8 anomers of the pyranoside and furan-oside forms) of a particular monosaccharide may be formed the acyclic dimethyl acetal is an additional possibility. The ratios of methyl glycosides of 10 monosaccharides subjected to methanolysis with M hydrogen chloride for 24 h at 80° has been reported. Similar information is also available in another study."... 

The common anomers have either a five- or a six-membered ring that contains an oxygen atom. A six-membered ring is a derivative of pyran therefore, the monosaccharide is a pyranose. A five-membered ring is a derivative of furan. 

Considerable effort has heen directed towards the synthesis of monosaccharide derivatives, and, in particular, C-glycosyl compounds, from Diels-Alder adducts offuran. Just and his group investigated the reaction of furan with a number of dienophiles, and described many interesting transformations of the adducts obtained from... 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

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. 

Monosaccharides react with a variety of 1,3-dicarbonyl compounds in the presence of zinc chloride in ethanolic or aqueous solution to yield substituted furans (Scheme 69) (56MI31200). The reaction of ethyl acetoacetate with D-glucose and D-mannose yielded the trisubstituted furan (252) in 20% yield, while D-fructose under similar conditions yielded (253 7%). These products have been used for the synthesis of dehydromuscarones (63HCA1259). Oxidation of the tetrahydroxybutyl side chains with lead tetraacetate gives the aldehyde, which can be converted to the corresponding acid with alkaline silver oxide. 

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. 

Monosaccharide, aliphatic acids, furan derivatives, and phenolic compound recoveries after posthydrolysis were calculated as the ratio between the concentration determined in the reaction media and the concentration that resulted from the quantitative acid hydrolysis (29) of oligosaccharides into monosaccharides and other compounds. In enzymatic treatments, the concentrations obtained were corrected by subtracting the corresponding concentration in the respective control assays, since the commercial enzymes contain mono-, di-, and oligosaccharides. The dilution factor introduced by adding the dilute enzyme preparation or the different volumes of sulfuric acid in posthydrolysis were also accounted for. 

Operational Conditions, CS, and Relative Composition of Posthydrolysis Liquors (Measured by Monosaccharides and Furan Derivatives) Expressed as Percentages of Concentrations Obtained on Quantitative Acid Hydrolysis (CS = 2.01)fl... 

Gas chromatographic analysis allows the determination of the monosaccharides present by comparing the retention times with those of reference compounds. It is noteworthy to mention that the chromatographic profile is quite simple, because the deuterated compounds are not separated from the non-deuterated, even if a small difference in retention time can be evidenced by comparison of specific ion chromatograms. Each monosaccharide can give a maximum of four peaks, corresponding to the cd(3 and the pyran/furan isomers. 

Surface Activation. Acid Activation. Acid treatment of cellulose and hemicelluloses generally leads to hydrolysis to monosaccharides, which can subsequently dehydrate and condense to form furan-type compounds such as furfural and its 5-hydroxymethyl adduct. Further reactions lead to polymeric materials of dark color as well as to monomers such as levulinic acid, formic acid, and angelica lactones. Various condensation and solvolysis reactions also accompany the acid treatment of lignin 123). The hydrolysis, dehydration, and condensation reactions have been used to explain formation of covalent bonds between surfaces (85), increase in water resistance (85, 124), and weakening of wood (75) in nonconventional plywood or particle board production. However, very little factual information is available on how far, in terms of the consecutive reactions mentioned, and in what direction, in terms of the parallel reactions mentioned, does the surface of lignocellulosic materials actually change... 

L-Arcanose and L-olimycose have been prepared in enantiomerically pure forms and with high stereoselectivity by Lewis-acid promoted addition of (5)-2-benzyloxypropanal to 1-tri-methylsilyl-2,3-butadiene. Depending on the nature of the Lewis acid either the syn (with TiCl4) or the anti adduct (with BF3 Et20) can be obtained. Epoxidation with lateral control by the allylic alcohol moieties and standard reactions lead to the unprotected monosaccharides [334]. Total syntheses of 2,3-dideoxy-3-C-methyl-D-maw o-heptose and of 2,3-dideoxy-2,3-di-C-methyl-D-gfycero-D-ga(acto-heptose have been realized by addition of 2-(trimethylsiloxy)furan to 2,3-0-isopropylidene-D-glyceraldehyde ((R)-37) [335]. 

Pyrolysis of peracetylated pyranose monosaccharides has been studied, and found to proceed via initial loss of the 1-acetoxy group followed in some cases by ally lie rearrangement. In contrast, peracetylated furanoses lose two acetoxy groups to give substituted furans, e.g. equation (59). ... 

If not utilized in the pulp industry, hemicelluloses are hydrolyzed in the acid-catalyzed process, mainly to monosaccharides and to furan-2-aldehyde (pen-tosanes) and 5-hydroxymethylfuran-2-aldehyde (5.62) (hexosanes). Monosaccharide-containing syrups, after purification, are either fermented or utilized as wood molasses for feeding ruminants. In another approach, xylose, the least soluble component of syrup, is allowed to crystallize. Separated xylose is then hydrogenated over an Ni/Al catalyst at 120°C under 6 x 106 Pa into xylitol. Hemicelluloses, together with proteins, are capable of the Maillard reaction and may contribute to the overall secondary aroma of processed foodstuffs (Tomasik and Zawadzki, 1998). 

De Novo Synthesis in Carbohydrate Chemistry From Furans to Monosaccharides and Oligosaccharides... 

Amino-2-deoxyaldohexoses, 1-amino-l-deoxyketohexoses, and their V-alkyl and JV-aryl derivatives react with /3-dicarbonyl compounds, giving substituted (tetrahydroxybutyl)pyrroles. It is, most probably, a general reaction of 2-amino-2-deoxyaldoses and 1-amino-l-deoxyketoses which parallels the formation of (polyhydroxyalkyl)furans from nonnitrogenous monosaccharides. In the reactions with amino sugars in alkaline media, simpler pyrrole compounds lacking the tetrahydroxybutyl chain are also obtained. 

---