Glucose cyclic hemiacetal form

Glycolysis is a ten-step process that begins with isomerization of glucose from its cyclic hemiacetal form to its open-chain aldehyde form—a reverse nucleophilic addition reaction. The aldehyde then undergoes tautomerixa-tion to yield an enol, which undergoes yet another tautomerization to give the ketone fructose. 

To explain the above results, glucose is assumed to exist as a cyclic hemiacetal in equilibrium with a small amount of the open-chain aldehyde. In the hemiacetal, C is chiral and two diastereomers (anomers) are possible they are called a- and jS-glucosides. The C of C=0 is always the anomeric position. Cyclic hemiacetals form when an OH and a CHO group in the same molecule can form a five- or six-membered ring that is more stable than the open chain. 

The halogen in the acylglycosyl halide is reactive and may be readily displaced, for example, by an alkoxy group on reaction with an alcohol under anhydrous conditions in the presence of a silver or mercury(n) salt. In this case the products are glycosides which are the mixed cyclic acetals related to the cyclic hemiacetal forms of the monosaccharides. In the case of the D-glucose derivative shown below (and of other 1,2-cis acylglycosyl halides) the replace-... 

Two structures for the sugar glucose are shown on page 858. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base. 

The Cyclic Hemiacetal Form of Glucose Aldoses contain an aldehyde group and several hydroxyl groups. The solid, crystalline form of an aldose is normally a cyclic hemiacetal. In solution, the aldose exists as an equilibrium mixture of the cyclic hemiacetal and the open-chain form. For most sugars, the equilibrium favors the cyclic hemiacetal. 

Glucose exists almost entirely as its cyclic hemiacetal form. 

Draw the cyclic hemiacetal forms of D-mannose and D-galactose both as chair conformations and as Haworth projections. Mannose is the C2 epimer of glucose, and galactose is the C4 epimer of glucose. 

Allose is the C3 epimer of glucose. Draw the cyclic hemiacetal form of D-allose, first in the chair conformation and then in the Haworth projection. 

We are now set to draw the cyclic hemiacetal formed by nucleophilic attack of the OH group on C5 on the aldehyde carbonyl. Because cyclization creates a new stereogenic center, there are two cyclic forms of D-glucose, an a anomer and a P anomer. All the original stereogenic centers maintain their configuration in both of the products formed. 

The Cyclic Hemiacetal Form of Glucose Aldoses contain an aldehyde giuup and... 

Most simple sugars (Chapter 22) exist primarily in a cyclic hemiacetal form. Glucose is an example ... 

FIGURE 22.3 Formulas 1-3 are used for the open-chain structure of D-(-F)-glucose. Formulas 4-7 are used for the two cyclic hemiacetal forms of D-(-F)-glucose. 

Studies of the structures of the cyclic hemiacetal forms of D-(-+-)-glucose using X-ray analysis have demonstrated that the actual conformations of the rin are the chair forms represented by conformational formulas 6 and 7 in Fig. 22.3. This shape is exactly what we would expect from our studies of the conformations of cyclohexane (Chapter 4), and it is especially interesting to notice that in the j8 anomer of D-glucose all of the large substituents, —OH and —CH2OH, are equatorial. In the a anomer, the only bulky axial substituent is the —OH at Cl. 

FIGURE 22.4 Haworth formulas for the cyclic hemiacetal forms of D-(+)-glucose and their relation to the open-chain polyhydroxy aldehyde structure. (Reprinted with... 

As you study the open chain and cyclic hemiacetal forms of D-glucose, note that, in converting from a Fischer projection to a Haworth structure,... 

Enzyme-catalyzed oxidation of the primary alcohol at carbon 6 of a hexose yields a uronic acid. Enzyme-catalyzed oxidation of D-glucose, for example, yields D-glucuronic acid, shown here in both its open-chain and cyclic hemiacetal forms ... 

Although aldoses exist primarily in cyclic hemiacetal forms, these structures are in equilibrium with a small but finite amount of the open-chain aldehyde. These aldehyde groups can be easily oxidized to acids (review Sec. 9.13). The products are called aldonic acids. For example, D-glucose is easily oxidized to D-gluconic acid. 

D-Glucose, the most important carbohydrate in mammalian metabolism, exists as a six-membered cyclic hemiacetal form as both a and j8 anomers. 

With D-glucose in the cyclic hemiacetal form, the a-anomer is the one with the anomeric —OH group axial, while for the j8-anomer, the anomeric —OH group is equatorial. 

PROBLEM 16.12 Note how odd the cyclic structure in Rgure 16.39 looks. Construct a three-dimensional picture of D-glucose that better represents its structure in the cyclic hemiacetal form. Note that the position of the OH on the carbon marked in red in Rgure 16.39 can be either axial or equatorial. 

Figure 22.4 Haworth formulas for the cyclic hemiacetal forms of D-(+)-glucose and their relation to the open-chain polyhydroxy aldehyde structure. (Reprinted with permission of John Wiley Sons, Inc., from Holum, J. R., Organic Chemistry A Brief Course, p. 316. Copyright 1975.)...

Although many of the properties of D-(-l-)-glucose can be explained in terms of an open-chain structure (1,2, or 3), a considerable body of evidence indicates that the open-chain structure exists, primarily, in equilibrium with two cyclic forms. These can be represented by structures 4 and 5 or 6 and 7. The cyclic forms of D-(+)-glucose are hemiacetals formed by an intramolecular reaction of the —OH group at C5 with the aldehyde group (Fig. 22.4). Cyclization creates a new chirality center at Cl, and this chirality center explains how two cyclic forms are possible. These two cyclic forms are diastereomers that differ only in the configuration of C1. 

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