Pyranose and Furanose Rings

Chemisorption by these forces, e.g., of sucrose to saccharase, in such a manner that the mean planes of the pyranose and furanose rings are... 

While the pyranose and furanose ring forms of the sugars dominate the carbohydrate literature, the uncommon septanose ring forms have long intrigued sugar chemists, with particularly notable contributions by Stevens in Australia. The chapter in this volume from Saha and Peczuh (Storrs, Connecticut) provides a comprehensive overview of the subject from both the synthetic and structural viewpoints and presents a detailed analysis of the conformational behavior of these ring forms. It may be noted... 

Figure 4-1 Natural abundance 13C-NMR spectrum of honey showing content of a and (3 pyranose ring forms of glucose and both pyranose and furanose ring forms of fructose. Open chain fructose, with a peak at 214 ppm, was present in a trace amount. From Prince et al.6...

The pyranose and furanose ring carbon resonances are observed between 40 and 100 ppm. 

The second difficulty peculiar to carbohydrates is the allowance for conformational flexibility in the pyranose and furanose rings, which is not easily accounted for by the comparatively simple computer program described above. 

The many different isomers that can be formed because of the multiple reactive sites of the nucleophile, the pyranose and furanose ring forms, and the different degrees of substitution often make analysis of the obtained product mixtures troublesome. 

Figure 14.1. Construction of query saccharide structure for energy minimization with SWEET. Three-letter codes (IUBMB) for monosaccharides and amino acids are employed to construct oligosaccharide or oligopeptide chains. For monosaccharides, a- and p-anomers are prefixed with a and b, respectively. Pyranose and furanose rings are denoted as p and f, respectively.

Esters, ethers (most frequently pyranose and furanose ring oxygen atoms)... 

The full importance of the precise shape of molecules in determining stability and chemical reactivity has only recently been recognized. Calculations of the shape of various rings had previously been made, and pyranose and furanose rings had been examined theoretically and by x-ray analysis, but the significance of the calculated shapes, and methods for selecting the more probable shapes when alternatives were possible, could not be properly assessed until the importance of interactions between non-bonded atoms was realized. 

The same nomenclature applies to the furanose ring form of fructose, except that a and p refer to the hydroxyl groups attached to C-2, the anomeric carbon atom (see Figure 11.5). Fructose forms both pyranose and furanose rings. The pyranose form predominates in fructose free in solution, and the furanose form predominates in many fructose derivatives (Figure 11.6). Pentoses such as d-ribose and 2-deoxy-d-ribose form furanose rings, as we have seen in the structure of these units in RNA and DNA. 

Pyranose and Furanose Rings Can Assume Different Conformations... 

Identifying glycosidic linkages and numbering pyranose and furanose rings... 

Fig. 5.13. Pyranose and furanose rings formed from glucose and fructose. The anomeric carbons are highlighted.

In both pyranose and furanose rings there are several conformational isomers available for each sugar. In the pyranoses these fall into the three categories of boat , chair and skew forms and there are a number of each. The range of isomers in the furanose series is smaller. 

The reactions of carbohydrates, with few exceptions, are the reactions of functional groups that we have studied in earlier chapters, especially those of aldehydes, ketones, and alcohols. The most central reactions of carbohydrates are those of hemiacetal and acetal formation and hydrolysis. Hemiacetal groups form the pyranose and furanose rings in carbohydrates, and acetal groups form glycoside derivatives and join monosaccharides together to form di-, tri-, oligo-, and polysaccharides. 

Monosaccharides are generally stable in dilute mineral acids, but in dilute alkali mutarota-tion may occur through open-chain forms. If glucose is exposed to dilute alkali, for example, after a few days the mixture also contains fructose and mannose (10.9) implying equilibria between pyranose and furanose rings (10.10). Equilibria of this kind may be enzyme catalysed (Chapter 11.4). 

The -OH groups in pyranose and furanose rings may be easily replaced by -OMe, O CO Me, OSO3H, OPO3H2 and so forth, the phosphate esters being of special importance in biochanistry. 

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