Sugar, reactions dehydration

When used at room temperature in the presence of an active platinum catalyst in an inert solvent, e.g., acetone or ethyl acetate, oxygen will oxidize nonhindered, saturated hydroxyl groups and exposed allylic alcohols. This reagent has found extensive use in sugar chemistry and is particularly suited for the selective oxidation of either 3a- or 3j -alcohols of steroids. Other hydroxyl groups on the steroid skeleton are much less sensitive to oxidation. As a result, this reaction has been used extensively in research on polyhydroxy cardiac-active principles, e.g., the cardenolides and bufadienolides, where the 3-hydroxyl group is easily oxidized without extensive oxidation or dehydration of other hydroxyl groups. The ordinarily difficult selective oxidation of the... 

FIGURE 1.9 (a) Amino acids build proteins by connecting the n-carboxyl C atom of one amino acid to the n-amino N atom of the next amino acid in line, (b) Polysaccharides are built by combining the C-1 of one sugar to the C-4 O of the next sugar in the polymer, (c) Nucleic acids are polymers of nucleotides linked by bonds between the 3 -OH of the ribose ring of one nucleotide to the 5 -P04 of its neighboring nucleotide. All three of these polymerization processes involve bond formations accompanied by the elimination of water (dehydration synthesis reactions). 

Efforts were made by Garcia Gonzalez and his coworkers to elucidate the mechanism of this reaction. In one of the working hypotheses, it was considered that the aldehydo form of the sugar and the 1,3-dicarbonyl compound undergo an aldol reaction to yield a 2-C-(alditol-l-yl)-l,3-dicar-bonyl compound, which is then dehydrated to form the furan. This hypothesis was supported by the isolation of the aldol-addition product of... 

Recently several pubhcations have examined replacing aqueous solvents with ionic liquids. Since simple and complex sugars are soluble in many imidazolium hahdes, water is not required as a co-solvent and degradation of HMF is minimal. Lansalot-Matras et al. reported on the dehydration of fmctose in imidazolium ionic liquids using acid catalyst (6). Moreau et al. reported that l-H-3-methylimidazolium chloride has sufficient acidity to operate without added acid (7). And we reported that a 0.5 wt% loading (6 mole% compared to substrate) of many metal halides in 1-ethyl-3-methylimidazohum chloride ([EMIM]C1) result in catalytically active materials particularly useful for dehydration reactions (8). 

Earlier studies showed that reactions of sugars with ammonia lead to small molecules such as amines or organic acids. A. L. Weber has reported important autocatalytic processes occurring when trioses are allowed to react with ammonia under anaerobic conditions, such reactions provide products which are autocatalyt-ically active. Their autocatalytic activity was determined directly by investigating their effect on an identical triose-ammonia reaction. Both an increase in the triose degradation rate and an increased rate of synthesis of pyruvate, the dehydration product of the triose, were observed. Such processes may have been of importance for prebiotic chemistry occurring on the primeval Earth (Weber, 2007). 

Heterogeneous catalysts, particularly zeolites, have been found suitable for performing transformations of biomass carbohydrates for the production of fine and specialty chemicals.123 From these catalytic routes, the hydrolysis of abundant biomass saccharides, such as cellulose or sucrose, is of particular interest. The latter disaccharide constitutes one of the main renewable raw materials employed for the production of biobased products, notably food additives and pharmaceuticals.124 Hydrolysis of sucrose leads to a 1 1 mixture of glucose and fructose, termed invert sugar and, depending on the reaction conditions, the subsequent formation of 5-hydroxymethylfurfural (HMF) as a by-product resulting from dehydration of fructose. HMF is a versatile intermediate used in industry, and can be derivatized to yield a number of polymerizable furanoid monomers. In particular, HMF has been used in the manufacture of special phenolic resins.125... 

The third hypothesis (C) is that reaction takes place through an aldol condensation between the ethyl acetoacetate and the carbonyl form of the sugar. There is some precedent indicating that it is possible for /3-ketonic esters to undergo aldol condensation. These compounds react mole to mole with the aliphatic aldehydes and afford unsaturated substances, which could be formed via an aldol condensation with subsequent dehydration. 

For example, the 0,C-trisaccharide 153 was obtained by a Henry reaction of nitro disaccharide 151 and sugar aldehyde 150 followed by the dehydration of (3-nitro alcohol 152 and reduction of the resulting nitroolefin, to give nitro sugar 153. Finally, a radical elimination of the nitro group afforded the target 154 (Scheme 47).105... 

In the presence of bases, monomeric as well as polymeric sugars are converted into various carboxylates salts [14]. The reaction proceeds through a retro-Michael dehydration step to form an unsaturated aldehyde, followed by rehydration of the aldehyde function and isomerization to acid (Fig. 2.3). The same reaction is also responsible for a stepwise depolymerization of polymeric sugar to carboxylate... 

Alkane production from sugars by aqueous phase dehydration/hydrogenation reactions has the advantage that most of the alkane fraction is spontaneously separated from the aqueous phase. Unfortunately, the major compound produced by this process is hexane, which has a low value as a gasoline additive due to its relatively high volatility. This limitation has been partially overcome by promoting a base-catalyzed aldol condensation step which links carbohydrate-derived units via formation C-C bonds to form heavier alkanes ranging from C to C15 [151]. 

The formation of acyclic enediols is, apparently, the initial reaction that leads to dehydration products. Sugar enediols are transitory compounds that have never been isolated. However, because, when treated with either acid or base, an aldose gives rise to its 2-epimer, as well as to its 2-keto isomer, a persuasive argument is provided for the 1,2-enediol as the intermediate common to each of the products. The evidence in favor of these intermediates is based primarily on isotope-exchange experiments, on reactions that involve isomeriza-tions of O-methyl sugars, and on kinetic measurements.10... 

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