Cyclic Structures of Monosaccharides Anomers

Thomson Click Organic Interactive to learn to draw cyclic forms of simple monosaccharides. 

Wc said in Section 19.10 that aldehydes and ketones undergo a rapid and reversible nucleopliilic addition reaction with alcohols to form hemiacctals. 

Mutarotalion occurs a reversible ring-opening of each anomer to the open-chain aldehyde, followed by reclosiire. Although equilibration is slow at neutral pH, it is catalyzed by both acid and base. 

Draw jS-L-glucopyranose in its more stable chair conformation. 

Problem 25.11 Ribose exists largely in a furanose form, produced by addition of the C4 -OH group 

Both anomers of n-glucopyranose can be crystallized and purified. Pure fv-n-glucopyranose has a melting point of 146 °C and a specific rotation, (all), of -112.2 pure jS-D-glucopyranose has a melting point of 148 to 155 °C and a specific rotation of +18.7. When a sample of either pure anomer is dissolved in water, however, the optical rotation slowly changes and ultimately reaches a constant value of -52.6. That is, the specific rotation of the -anomer solution decreases from +112.2 to +52.6, and the specific rotation of the j3-anomer solution increases from +18.7 to - -52.6. Called mutarotation, this change in optical rotation is due to the slow conversion of the pure anomers into a 37 63 equilibrium mixture. 

FIGURE 21.4 Glucose in its cyclic pyranose forms. As explained in t)ie text, two anomers are formed by cyclization of glucose. Tlie molecule wliose newly formed -OH group at Ci is cis to tlie oxygen atom on the lowest chirality center (C5) in a Fischer projection is the a anomer. The molecule whose newly formed -OH group is trans to the oxygen atom on the lowest chirality center in a Fischer projection is the (3 anomer. 

When an open-chain monosaccharide cyclizes to a pyranose form, a new chirality center is generated at the former carbonyl carbon and two diastereo-mers, called anomers, are prodnced. The hemiacetal carbon atom is referred 

Pyranose and furanose forms of fructose in aqueous solution. The two pyranose anomers result from addition of the C6 -OH group to the C2 carbonyl, while the two furanose anomers result from addition ofthe C5 -OH group to the C2 carbonyl. Co to this book s student companion site at www.cengage.com/ chemistry/mcmurry to explore an interactive version of this figure. 

WORKED EXAM PLE 21.3 Drawing the Chair Conformation of an Aldohexose 

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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. 

D-(+)-galactose (15) is an example of the consecutive numbering of the carbon ring atoms in a monosaccharide (disaccharide see p. 253). Carbohydrates can exist in a cyclic and an acyclic structure. For this reason there is a special position in the structure of a monosaccharide, the carbon atom C-l and so called anomeric center. You can see that there is an equilibrium between er-anomer a-15 and / -anomer /3-15 of D-(+)-glucopyranose over the acyclic aldehyde structure 16. Both are cyclic hemi-acetals. The /Tanomer is the preferred conformation, but there are a few effects, like sterical or stereoelectronical effects (anomeric effect, inverse anomeric effect), which have influence on the a /i rate. 

The carbonyl and alcohol groups within the same monosaccharide may react together if the carbon chain is long enough. The result is a cyclic hemiacetal. A new chiral center is formed at the carbon which was previously the carbonyl. The two optical isomers that can result are called anomers. Five- and six-membered cyclic structures predominate with the alcohol oxygen as the last member of the ring. These are referred to as furanoses and pyranoses, respectively. Cyclic structures exist in equilibrium with the open-chain form. 

In the crystalline state, reducing monosaccharides exist exclusively in cyclic structures (a- or P-anomers). In solutions, a balance between the a- and P-anomers and acyclic forms is established as the individual forms interconvert over time. This process, schematically shown in Figure 4.2, is called mutarotation. The mechanism of mutarotation assumes cleavage of the saccharide cyclic forms... 

Ans. The cyclic hemiacetal and hemiketal structures have no carbonyl function but are in equilibrium with the open-chain carbonyl structure. As the small amount of the open-chain carbonyl structure reacts, some of the a- and /3-anomers undergo ring-opening to replenish the equilibrium concentration of the open-chain structure. The newly formed open-chain structure undergoes reaction. More a- and -anomers open up. There is a continuous shifting of a- and /3-anomers to the open-chain carbonyl structure until all of the monosaccharide initially present has reacted. 

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