Sugars, aldehyde form

Condensation of A/-alky lade nine with 2-deoxyribose (654) in a phosphorus pentoxide mixture gave 655 via a probable Michael-type addition to an a,)3-unsaturated sugar aldehyde formed in situ from 2-deoxyribose. Reduction of 655 gave 656(92JHC511). Neither 655 nor 656showed any significant activity against HSV-1,... 

In fact, it has been found (52) that in unbuffered solution, at room temperature, authentic 2-deoxy ribose 5-phosphate reduces more than 4 molar equivalents of periodate, but. that there is no noticeable slowing down of the reaction rate after the reduction of the first molar equivalent. This may be owing to the fact that only the aldehydo form (76) of 2-deoxy ribose 5-phosphate has a free vicinal diol group as the acyclic 2-deoxy ribitol 5-phosphate reduces one molar equivalent of periodate quite fast (58), it is probable that the time-curve of periodate uptake by the phosphorylated sugar reflects the rate of formation of the aldehyde form from the furanose form. 

Maltose and cellobiose are both reducing sugars because the anomeric carbons on the right-hand glucopyranose units have hemiacetal groups and are in equilibrium with aldehyde forms. For a similar reason, both maltose and cellobiose exhibit mutaiotation of a and /3 anomers of the glucopyranose unit on the right. 

The configuration around the Cj of glucose (i.e. the anomeric C) is not stable and can readily change (mutarotate) from the a- to the / -form and vice versa when the sugar is in solution as a consequence of the fact that the hemiacetal form is in equilibrium with the open chain aldehyde form which can be converted into either of the two isomeric forms (Figure 2.2). 

When there are several chiral carbons in a molecule, the configuration at one center usually is related directly or indirectly to glyceraldehyde, and the configurations at the other centers are determined relative to the first. Thus in the aldehyde form of the important sugar, (+)-glucose, there are four chiral centers, and so there are 24 = 16 possible stereoisomers. The projection formula of the isomer that corresponds to the aldehyde form of natural glucose... 

Monosaccharides Cyclize to Form Hemiacetals Aldehydes can add hydroxyl compounds to the carbonyl group. If a molecule of water is added, the product is an aldehyde hydrate, as shown in figure 12.4. If a molecule of alcohol is added, the product is a hemiacetal the addition of a second alcohol results in an acetal. Sugars readily form intramolecular hemiacetals in cases in which the resulting compound has a five- or six-member ring. 

All of these forms of sulfur are known as free sulfur dioxide. The bisulfite ion (HS03 ) can react with aldehydes, dextrins, pectic substances, proteins, ketones, and certain sugars to form addition compounds. 

The situation was clarified by the isolation of a, )8, and y isomers of D-glucose by Tanret in 1895. He showed that the a and y isomers mutarotate in opposite directions and, at equilibrium, each have the same rotation as the y3 form. The isomers were also found to have the same molecular weight. Tanret s three isomers of D-glucose were considered to be ring and free aldehyde forms by Lobry de Bruyn and Alberda van Ekenstein in 1895, von Lipp-mann in 1896, and Simon in 1901. Fischer in 1893, and von Lipp-mann pointed out that ring formation would produce a new asymmetric carbon atom, and thus the existence of isomeric sugars, glycosides, and acetates was clarified. 

Hemiacetals in sugars are formed in the same way that other hemiacetals are formed—that is. by cyclization of hydroxy aldehydes. Thus, the hemiacetal of glucose is formed by cyclization of an acyclic po/yhydroxy aldehyde (A), as shown in the accompanying equation. This process illustrates two important features. 

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