Aldoses aldehydic form

Oxidation of the aldehyde group of an aldose to form a carboxyUc acid or carboxyUc acid anion is often used analytically to determine the amount of reducing sugar. The Benedict and Fehling methods measure the amount of reducing sugar present in a fluid. In these reactions, the oxidant, Cu ", is reduced to Cu". Cu" precipitates as CU2O, which can be measured in a variety of ways. In the ToUens test, Ag" is reduced to Ag. ... 

The Kiliani-Fischer synthesis lengthens a carbohydrate chain hy adding one carhon to the aldehyde end of an aldose, thus forming a new stereogenic center at C2 of the product. The product consists of epimers that differ only in their configuration about the one new stereogenic center. For example, the Kiliani-Fischer synthesis converts D-arabinose into a mixture of D-glucose and D-mannose. 

The preceding explanation of the failure of a 2-0-substituted aldose to form saccharinic acids (on treatment with alkali) finds substantiation in the results of a study of the action of lime-water, at room temperature, on 2,3-di-O-methyl-D-glucose. The presence, in this latter reaction mixture, of an fl[,/3-unsaturated aldehyde(IV, R = CH3) was established by its further, facile conversion to 5-(hydroxymethyl)-2-furaldehyde upon acidification. 

The other mechanisms for the formation of 5-(hydroxymethyl)-2-furaldehyde involve the aldehyde form of aldoses or the keto form of ketoses, in contrast to Nef s mechanism, which required the furanose form of a ketose. Hurd and Isenhour, Isbell, and Wolfrom and coworkers all based their suggestions on the well-known elimination of a hydroxyl group in a position /3 to a carbonyl group. They applied their mechanisms to aldoses only. However, Isbell proposed that the /3-elimination proceeds by a consecutive electron-displacement involving an enediol intermediate (63), the enolic form of the aldose (62). Later, Wolfrom and coworkers also proposed the enediol as a possible intermediate. 

The substrate specificity of glucose-phosphate isomerase illuminates additional stereochemical subtleties of the isomerase reaction (JOO). In the aldose to ketose direction, the enzyme potentially operates on an equilibrium mixture of substrate forms composed of two cyclic hemiacetals (the a- and /3-anomers, of glucose 6-phosphate) and trace quantities of the acyclic aldehyde form lEq. (17)] ... 

The hydroxyamine (f) readily loses NH3 on the heating to give the aldehyde. This sequence achieves the removal of Cl from an aldose to form a new aldose with one less carbon, just like the Wohl and Ruff degradations. 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

Up to this point all our attention has been directed toward aldoses carbohydrates hav ing an aldehyde function in their open chain form Aldoses are more common than ketoses and their role m biological processes has been more thoroughly studied Nev ertheless a large number of ketoses are known and several of them are pivotal inter mediates m carbohydrate biosynthesis and metabolism Examples of some ketoses include d nbulose l xylulose and d fructose... 

Aldoses are reducing sugars because they possess an aldehyde function m then-open chain form Ketoses are also reducing sugars Under the conditions of the test ketoses equilibrate with aldoses by way of enediol intermediates and the aldoses are oxidized by the reagent... 

Derivatives of aldoses in which the terminal aldehyde function is oxidized to a car boxylic acid are called aldonic acids Aldonic acids are named by replacing the ose ending of the aldose by omc acid Oxidation of aldoses with bromine is the most com monly used method for the preparation of aldonic acids and involves the furanose or pyranose form of the carbohydrate... 

The presence of an aldehyde function m their open chain forms makes aldoses reactive toward nucleophilic addition of hydrogen cyanide Addition yields a mixture of diastereo meric cyanohydrins... 

Aldonic acid (Section 25 19) Carboxylic acid obtained by oxi dation of the aldehyde function of an aldose Aldose (Section 25 1) Carbohydrate that contains an aldehyde carbonyl group in its open chain form Alicyclic (Section 2 15) Term describing an a/iphatic cyclic structural unit... 

Ketose (Section 25 1) A carbohydrate that contains a ketone carbonyl group in its open chain form Kiliam-Fischer synthesis (Section 25 20) A synthetic method for carbohydrate chain extension The new carbon-carbon bond IS formed by converting an aldose to its cyanohydnn Reduction of the cyano group to an aldehyde function com pletes the synthesis... 

Oxidation to Sugar Acids and Lactones. When the aldehyde group of an aldose is oxidized, the resulting compound is an aldonic acid (salt form = aldonate) (11)4. Some aldonic acids are products of carbohydrate metaboHsm. 

Aldose (Section 25.1) Carbohydrate that contains an aldehyde carbonyl group in its open-chain form. 

Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol called an alditol. The reduction occurs by reaction of the open-chain form present in the aldehyde/ketone hemiacetal equilibrium. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced, more is produced by opening of the pyranose form, that additional amount is reduced, and so on, until the entire sample has undergone reaction. 

Discovery of the chain-lengthening sequence was initiated by the observation of Heinrich Kiliani in 1886 that aldoses react with HCN to form cyanohydrins (Section 19.6). Emil Fischer immediately realized the importance of Kiliani s discovery and devised a method for converting the cyanohydrin nitrile group into an aldehyde. 

Fischer s original method for conversion of the nitrile into an aldehyde involved hydrolysis to a carboxylic acid, ring closure to a cyclic ester (lactone), and subsequent reduction. A modern improvement is to reduce the nitrile over a palladium catalyst, yielding an imine intermediate that is hydrolyzed to an aldehyde. Note that the cyanohydrin is formed as a mixture of stereoisomers at the new chirality center, so two new aldoses, differing only in their stereochemistry at C2, Tesult from Kiliani-Fischer synthesis. Chain extension of D-arabinose, for example, yields a mixture of D-glucose and o-mannose. 

Chart I. Trivial names (with recommended three-letter abbreviations in parentheses) and structures (in the aldehydic, acyclic form) of the aldoses with three to six carbon atoms. Only the D-forms are shown the L-forms are the mirror images. The chains of chiral atoms delineated in bold face correspond to the configurational prefixes given in italics below the names... 

The names of the individual compounds of this type are formed by replacing (a) the -ose of the systematic or trivial name of the aldose by -uronic acid , (b) the -oside of the name of the glycoside by -osiduronic acid or (c) the -osyl of the name of the glycosyl group by -osy luronic acid . The carbon atom of the (potential) aldehydic carbonyl group (not that of the carboxy group as in normal systematic nomenclature [13,14]) is numbered 1 (see 2-Carb-2.1, note 1). 

Through extensive screening of compounds, " " " it was revealed that this enzyme accepts a very wide range of substrates. In addition to phosphorylated aldose, which are the native substrate, non-phosphorylated aldose, simple aliphatic, aromatic, heterocyclic and functionalized aldehydes, even with an increased hydropho-bicity, work as substrates. The stereochemical course has been elucidated in Fig. 18. The hydroxyl group on the 2-position of the aldehyde is very important and 2-deoxygenated aldehydes were rather weak substrates. The substrates with d-configuration at the 2-position have a stronger affinity to TKase than L-form. 

Figure 4.17 The trioses D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), the pentose D-ribose, the hexoses D-galactose and D-glucose (aldoses) and the ketohexose D-fructose in their open chain forms. The configuration of the asymmetrical hydroxyl group on the carbon, the furthest away from the aldehyde or ketone group, determines the assignment of D- or L-configuration.

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

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