Cyclic Forms Anomers

When a monosaccharide exists in the heterocyclic intramolecular hemiacetal form, the size of the ring is indicated by the suffixes -furanose, -pyranose, and -septanose for five-, six-, and seven-membered rings, respectively. 

Two configurations known as anomers may result from the formation of the ring. These are distinguished by the anomeric prefixes a- and p-, which relate the configuration of the anomeric carbon atom to the configuration at a reference chiral carbon atom (normally the highest-numbered chiral carbon atom). For example, consider the glucopyranoses  

Suffixes used in carbohydrate nomenclature to indicate cyclic forms are as follows  

Similar suffixes can be constructed for dicarbonyl sugars and other modifications, for example  

The suffixes for the acids can be modified to indicate the corresponding amide, nitriles, acid halides, etc., e.g., -uronamide, -ononitrile, and -ulosonyl chloride. 

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Anomers are enantiomers formed when the symmetric carbonyl carbon is made asymmetrical by the formation of the cyclical form of a carbohydrate. 

The cyclic form of glucose is termed glucopyra-nose, since the new ring system is a reduced form of the oxygen heterocycle pyran. Nucleophilic attack onto the planar carbonyl may occur from either of its two faces, generating two different stereochemistries at this new chiral centre, designated as a or p. This new chiral centre is termed the anomeric centre. Since there are other chiral centres in the molecule, the mixture of a- and -anomeric forms is not a racemate, but a mixture of diastereoisomers (see Section 3.4.4). The mixture does not contain 50% of each anomer (see below). Although both forms are produced, the form with the equatorial hydroxyl is thermodynamically favoured (see Section 3.3.2). 

It follows that, when we dissolve a sugar such as glucose or ribose in water, we create a mixture of various equilibrating structures. The relative proportions of pyranose and furanose forms, and of their respective anomers for the eight aldohexoses, are shown in Table 12.1. In each case, the proportion of non-cyclic form is very small (<1%). 

Mutarotation produces two types of cyclic forms called anomers (a and P), which differ in their arrangement about the anomeric carbon atom (originally the carbonyl carbon atom). If the -OH on the anomeric carbon atom is down, then the structure represents the a anomer if it s up, the structure represents the p anomer. Due to the equilibrium present, one anomer rapidly converts to the other. 

For most of the reactions of monosaccharides that involve the aldehyde or ketone functional group, the presence of open chain form is crucial, as only in this form do these functional groups exist. A sugar solution contains two cyclic anomers and the open chain form in an equilibrium. Once the aldehyde or ketone group of the open chain form is used up in a reaction, the cyclic forms open up to produce more open chain form to maintain the equilibrium. 

FIGURE 7-6 Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-l and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the a and fi anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of a and fi anomers is called mutarotation. 

Figure 20-1 Structure and configuration of the D-aldoses from C3 to C6l showing the configurational relationship to D-glyceraldehyde. Open-chain and cyclic forms are shown. The oxacyclohexane (pyranose) form is more stable than the oxacyclopentane (furanose) form for the free sugar. The oxacyclopentane structure is shown for ribose because this is the form in which it occurs in many important substances, such as the nucleic acids. Only the a anomers are shown (see Section 20-2B).

Most, if not all, of the stable forms of crystalline aldose and ketose monosaccharides exist in the pyranose structure. Each in solution, as with D-glucose, exists as an equilibrium mixture of open chain and of a- and / -anomers of the cyclic forms. The cyclic five- and six-membered structures formulated below are an illustrative selection of monosaccharides. 

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. 

Cellulose is formed by elimination of water from adjacent j8-anomers of the two cyclic forms of glucose (Eq. 15.2), hence it is a dehydro polymer. 

Anomers Diastereomers of glycosides, hemiacetals, or related cyclic forms of sugars, differing in configuration only at C-1 of an aldose, C-2 of a 2-ketose, etc. 

Most sugars exist in cyclic forms with five- or six-membered rings. The cyclization process involves the carbonyl group and gives rise to another chiral center in addition to the ones already present in the sugar molecule. The two possible cyclic isomers, called anomers, are designated a and p. 

Arts. The p- does not refer to the linkage between glucose units. The convention is that the type of 1,4-linkage in a carbohydrate does not appear in its name, it being understood whether the linkage in a particular carbohydrate is a-1,4- or /3-l,4-. The p- in the name -o-maltose refers to the fact that the structure drawn is that of the -anomer of o-maltose. C-1 of the far right glucose unit of maltose is a hemiacetal carbon, which means there are three forms of maltose—an open-chain, the a-anomer of the cyclic form, and the /3-anomer of the cyclic form. 

The aldohexose D-galactose exists predominantly in cyclic forms. Given the structure below, draw the Haworth structure for the anomer. Label the new compound as a or p. 

Hemiacetal formation between the carbonyl group and the C-4 hydroxyl yields the five-membered furanose ring form. The anomeric carbon is a new chirality center its hydroxyl group can be either cis or trans to the other hydroxyl groups. The two cyclic forms are diastereomers and are referred to as anomers because they have different configurations at the anomeric carbon. 

In solution, the sugars are found predominantly in the cyclic form, more specifically, the hexoses in the pyranose form and the pentoses in the furanose form. The formation of rings creates a new asymmetric center. The two new isomers are distinguished from one another as a and p anomers, because they are epimeric with respect to C-1. In the a anomers, the anomeric hydroxyl group is axially oriented (while the anomeric hydrogen atom is equatorial), whereas in the P anomers it is equatorial. In D-glucose (d-glucopyranose) it follows that... 

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