Furanose rings five-member

Nucleic acids have two kinds of pentoses. The recurring deoxyribonucleotide units of DNA contain 2 -deoxy-D-ribose, and the ribonucleotide units of RNA contain D-ribose. In nucleotides, both types of pentoses are in their j3-furanose (closed five-membered ring) form. As Figure 8-3 shows, the pentose ring is not planar but occurs in one of a variety of conformations generally described as puckered. ... 

DFru—Dfructose in oligosaccharides the furanose or five-membered ring is indicated by/, as DFru/ is Dfructofuranosyl. 

Furanose form Five-membered ring arising via cyclic hemiacetal formation between the carbonyl group and a hydroxyl group of a carbohydrate. 

Very important furanose molecules (five-membered rings) are commonly foimd in nature. For example, D-ribose and D-deoxyribose are foimd as the building imits of nucleic acids, and fructose is a constituent of sucrose. These rings are not planar, either one (envelope form) or two (twist form) atoms are out of the plane containing the others. The different envelope and twist conformations... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Hemiacetal formation between the carbonyl group and the C 4 hydroxyl yields the five membered furanose ring form The anomenc carbon is a new chirality center its hydroxyl group can be either cis or trans to the other hydroxyl groups of the molecule... 

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

As viewed m the drawing a 120° counterclockwise rotation of C 4 places its hydroxyl group m the proper position At the same time this rotation moves the CH2OH group to a position such that it will become a substituent that is up on the five membered ring The hydrogen at C 4 then will be down m the furanose form... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

Most carbohydrates exist as cyclic hemiacetals Those with five membered rings are called furanose forms those with six membered rings are called pyranose forms... 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

Furanose (Section 25.5) The five-membered-ring form of a simple sugar. 

Cyclic hemiacetals or hemiketals of sugars with a five-membered (tetrahydrofuran) ring are called furanoses, those with a six-membered (tetrahydropyran) ring pyran-oses. For sugars with other ring sizes see 2-Carb-5. 

It should be noted that this reaction occurs with total regioselectivity only one five- membered ring thionocarbamate linked to the furanose moiety was formed. 

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both six-membered and five-membered rings, an aqueous solution consists almost completely of the six-membered hemiacetal form five-membered rings... 

The pentose ribose is also able to form six-membered pyranose and five-membered furanose rings. In solution, ribose exists mainly (76%) in the pyranose form interestingly, however, when we... 

Fructose is a ketose and, therefore, forms hemike-tal ring structures. Like ribose, it is usually found in combination as a five-membered furanose ring. 

Sucrose is composed of glucose and fructose, and again we have a six-membered pyranose ring coupled to a five-membered furanose ring. However, there is a significant difference when we compare its structure with that of lactulose in sucrose, the two sugars are both linked through their anomeric... 

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