24.2: Conformations and Cyclic Forms of Sugars (2023)

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Simple sugar anomers: glucose mutarotation Summary exercises Answer Credits and Attributions
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    Goals

    After completing this section, you should be able to

    1. determine whether a particular monosaccharide exists as piranosa or furanosa.
    2. Draw the cyclic pyranose form of a monosaccharide according to its Fischer projection.
    3. Draw the Fischer projection of a monosaccharide based on its cyclic pyranose form.
    4. draw from memory the cyclic pyranose form of D-glucose.
    5. determining whether a given pyranose cyclic form represents the D or L form of the monosaccharide in question.
    6. describe the phenomenon known as mutarotation.
    7. Use chemical equations to explain what exactly happens at the molecular level during the mutarotation process.
    key terms

    Make sure you can define the key terms below and use them in context.

    • alpha-anomer
    • anomer
    • anomeric center
    • Beta-anomer
    • furanosa
    • mutarotation
    • Pyranos
    study notes

    Before attempting to study this section, consider looking at the formation of hemiacetals discussed in Section 19.10.

    As mentioned above, the preferred structural form of many monosaccharides may be that of a cyclic hemiacetal. Five and six member rings are preferable to other ring sizes due to their low angle and obscured elongation. Cyclic structures of this type are called furanose (five-membered) or pyranose (six-membered), reflecting the relationship of ring size to the common heterocyclic furan and pyran compounds shown to the right. Ribose, an important aldopentose, generally adopts a furanose structure as shown in the figure below. Following the D-family convention, the five-membered furanose ring is drawn in an edge projection with the ring oxygen positioned away from the viewer. The anomeric carbon atom (here colored red) is placed on the right. The upper bond to this carbon is defined as beta, the lower bond is then alpha.

    24.2: Conformations and Cyclic Forms of Sugars (1)

    24.2: Conformations and Cyclic Forms of Sugars (2)

    The cyclic pyranose forms of various monosaccharides are often represented in a planar projection known as the Haworth formula, after the British chemist Norman Haworth. As with the furanose ring, the anomeric carbon is placed on the right with the oxygen ring at the back in vertical view. In the D family, the alpha and beta bonds have the same orientation defined for the furanose ring (beta up and alpha down). These Haworth formulas are useful for representing stereochemical relationships, but they do not represent the true shape of the molecules. We know that these molecules are actually coiled up in a way that we call a chair conformation. Examples of four typical pyranose structures are shown below, both as Haworth projections and more representative chair conformers. Anomeric carbons are colored red.

    24.2: Conformations and Cyclic Forms of Sugars (3)

    24.2: Conformations and Cyclic Forms of Sugars (4)

    The size of the cyclic hemiacetal ring that a given sugar assumes is not constant, but can vary with substituents and other structural features. Aldolhexoses normally form pyranose rings, and their pentose counterparts tend to prefer the furanose form, but there are many counterexamples. The formation of acetal derivatives illustrates how subtle changes can alter this selectivity. A pyranose structure for D-glucose is drawn in the pink box on the left. Acetal derivatives have been prepared by acid-catalyzed reactions with benzaldehyde and acetone. Benzaldehyde usually forms six-membered cyclic acetals, whereas acetone preferentially forms five-membered acetals. The top equation shows the formation and some reactions of 4,6-O-benzylidene acetal, a commonly used protecting group. A methyl glycoside derivative of this compound (see below) leaves the C-2 and C-3 hydroxyl groups exposed to reactions such as periodic acid cleavage shown as the final step. The formation of an isopropylidene acetal at C-1 and C-2, intermediate structure, leaves the C-3 hydroxyl as the only unprotected function. A selective oxidation to a ketone is then possible. Finally, direct derivatization of glucose with di-O-isopropylidene by reaction with excess acetone results in a change to a furanose structure in which the C-3 hydroxyl is again unprotected. However, the same reaction with D-galactose, shown in the blue shaded box, produces a pyranose product in which the C-6 hydroxyl is deprotected. Both derivatives do not react with Tollens' reagent. This difference in behavior is attributed to the cis orientation of the C-3 and C-4 hydroxyl groups in galactose, allowing the formation of a less tense five-membered cyclic acetal compared to the trans C-3 hydroxyl groups. and C-4. in glucose. Shunts of this type allow selective reactions at different sites in these highly functionalized molecules.

    Simple sugar anomers: glucose mutarotation

    24.2: Conformations and Cyclic Forms of Sugars (5)

    Illustration 1:D-glucose cyclization. D-glucose can be represented with a Fischer projection (a) or three-dimensionally (b). The cyclic monosaccharide (c) is formed by reacting the OH group of the fifth carbon atom with the aldehyde group.

    When a straight-chain monosaccharide, such as one of the structures shown in Figure 1, forms a cyclic structure, the carbonyl oxygen atom can be shifted up or down, resulting in two stereoisomers as shown in Figure 2. The structure shown on the left of Figure 2, with the OH group on the first carbon atom projected downwards, represents what is known asAlpha form (α). The structures on the right with the OH group on the first carbon atom facing up are theForma beta (b). These two stereoisomers of a cyclic monosaccharide are known as anomers; They differ in structure around the anomeric carbon atom, that is, the carbon atom that in straight-chain form was the carbonyl carbon.

    24.2: Conformations and Cyclic Forms of Sugars (6)

    Figure 2:monosaccharides. In an aqueous solution, monosaccharides exist as an equilibrium mixture of three forms. The transformation between the forms is denoted asmutarotation, shown for D-glucose (a) and D-fructose (b).

    It is possible to obtain a crystalline glucose sample in which all the molecules have the α structure or all have the β structure. The α form melts at 146°C and has a specific rotation of +112°, while the β form melts at 150°C and has a specific rotation of +18.7°. However, when the sample is dissolved in water, a mixture containing both anomers and the straight-chain form in dynamic equilibrium is soon formed (part (a) of Figure 2). You can start with a sample of pure crystalline glucose made up entirely of either anomer, but once the molecules dissolve in water, they open up to form the carbonyl group and then close up again to form the α or β anomer. The opening and closing are continuously repeated in a continuous interconversion between anomeric forms and is called mutarotation (lat.change, which means "change"). At equilibrium, the mixture consists of approximately 36% α-D-glucose, 64% β-D-glucose, and less than 0.02% of the open-chain aldehyde form. The observed rotation of this solution is +52.7°.

    Although only a small percentage of the molecules are in the open-chain aldehyde form at any given time, the solution still exhibits the reactions characteristic of an aldehyde. As the small amount of free aldehyde is consumed in a reaction, the equilibrium changes to give more aldehyde. Therefore,inthe molecules can eventually react even though there is very little free aldehyde present at any one time.

    Typically (for example, in Figures 1 and 2), the cyclic forms of sugars are represented using a convention first proposed by Walter N. Haworth, an English chemist. The molecules are drawn as flat hexagons, with a dark border representing the side facing the viewer. The structure is simplified to show only the functional groups attached to the carbon atoms. Any group written to the right on a Fischer projection appears below the ring plane on a Haworth projection, and any group written to the left on a Fischer projection appears on an above-plane Haworth projection.

    The difference between the α and β sugar forms may seem trivial, but such structural differences are often critical to biochemical reactions. This explains why we can get energy from the starch in potatoes and other plants, but not from cellulose, even though both starch and cellulose are polysaccharides made up of glucose molecules linked together.

    Summary

    Monosaccharides containing five or more carbon atoms form cyclic structures in aqueous solution. Two cyclic stereoisomers can be formed from any straight chain monosaccharide; these are known as anomers. In aqueous solution, an equilibrium mixture is formed between the two anomers and the straight-chain structure of a monosaccharide in a process known as mutarotation.

    exercises

    1. Draw the cyclic structure of β-D-glucose. Identify the anomeric carbon.

    2. Since aldohexose D-mannose differs from D-glucose only in the configuration of the second carbon atom, draw the cyclic structure of α-D-mannose.

    Answer

    1. 24.2: Conformations and Cyclic Forms of Sugars (7)

    2.24.2: Conformations and Cyclic Forms of Sugars (8)

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