Aldohexoses D-glucose

Hausoul et al. [60] also reported on telomerization with aldopentoses (D-xylose, L-arabinose), aldohexoses (D-glucose, D-mannose, D-galactose), ketohexoses (d-fructose, L-sorbose) and the disaccharides D-sucrose and cellobiose, using Pd/ TOMPP as catalyst without the addition of base in /V,/V-di methyl acetamide as the solvent (Fig. 15). The Pd/TOMPP combination had previously been shown to be highly active in the telomerization of various polyols (vide supra). Good conversion... 

Application of the Kiliani-Fischer synthesis to the aldopentose D-arahinose produces aldohexoses D-glucose and D-mannose. Because the Kiliani-Fischer synthesis incorporates the stereocenters of D-arabinose without changes, D-glucose and D-mannose must have the same configurations at carbons 3,4, and 5 as does D-arabinose at carbons 2, 3, and 4, respectively. Furthermore, D-glucose and D-mannose must differ only in their configuration at carbon 2. 

Later in this chapter we shall see how the great carbohydrate chemist Emil Fischer was able to establish the stereochemical configuration of the aldohexose D-(+)-glucose, the most abundant monosaccharide. In the meantime, however, we can use D-(+)-glucose as an example illustrating the various ways of representing the structures of monosaccharides. 

The second step of this sequence illustrates the use of a bacterial oxidation the microorganism A. suboxydans accomplishes this step in 90% yield. The overall result of the synthesis is the transformation of a D-aldohexose (D-glucose) into an L-ketohexose (L-sorbose). What does this mean about the specificity of the bacterial oxidation ... 

The aldohexose D-glucose in the open-chain Fischer formula (I), the Fischer-Tollens hemiacetal ring formula (II), and the Haworth formula (III). The numbering system is shown. 

Of the eight d aldohexoses, D-glucose is the most common and most important, and the rest of this chapter will focus primarily on D-glucose. Its structure should be at your fingertips. 

If protonation occurs at the enolate oigrgen, a double end shown in Rgure 22.26 is formed. This compound can re-form a carbon-oxygen double bond in two ways. The double enol can form either an aldehyde or a ketone. If an aldehyde is generated, the epimeric aldohexoses D-glucose and D-mannose shown in Rgure 22.25 are produced. If a ketone is formed, it is D-fmctose that is the product. 

In the third step, the position of the last OH in D-arabinose is fixed by the observation that both aldohexoses, D-glucose and D-mannose, give optically active diacids when oxidized wath nitric acid. There are two possibilities for the OH at C(4) in the aldohexoses It is either on the left or on the right. If it is on the right, both sugars give an optically active diacid, in agreement with the experimental data (Rg. 22.47). 

So, the correct position of the OH at C(3) in D-arabinose and at C(4) in the two D-aldohexoses, D-glucose and D-mannose, must be on the right. D-Glucose and D-mannose share the two structures in Figure 22.49. The next problem is to tell which is which. 

We wll use lactose as an example of how the structure of a disaccharide can be unraveled. The first important observation is that (-l-)-lactose, a disaccharide haring the formula C12H22O11, can be hydrolyzed in acid to a pair of simple aldohexoses D-glucose and D-galactose (Fig. 22.59). This observation identifies the two monosaccharides making up (-l-)-lactose. 

Like aldopentoses aldohexoses such as d glucose are capable of forming two fura... 

Six of the 18 aldohexoses, namely, D-glucose, d- and L-mannose, D-galac-tose, D-allose, and L-altrose have been found in bacterial polysaccharides. 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

CW, continuous wave Cys, cysteine DFT, density functional theory ENDOR, electron nuclear double resonance ehba, 2-ethyl-2-hydroxybutanoate2 EPR, electron paramagnetic resonance Glc6P, D-glucose 6-phosphate GSH, reduced glutathione HEPES, 4-(2-hydroxyethyl)-l-piperazineethanesulfonic acid Hex, aldohexose ... 

A distinction must be made between septanosides that lack (6-unsubstituted) or contain (6-substituted) an exocyclic hydroxymethyl group at C-6. In the same way that D-glucose can form a septanose ring by connecting the 6-hydroxyl group to the anomeric center as in Fig. 1, all of the common aldohexoses can adopt a septanose... 

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