Pyranoses, monosaccharides

Table 1 Naturally Occurring Monosaccharides (pyranose carbon numbering system)... 

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%). 

Some monosaccharides also exist in a five-mem be red cyclic hemiacetal form called a furanose form. D-Fructose, for instance, exists in water solution as 70% /Tpvranose, 2% a-pyranose, 0.7% open-chain, 23% /3-furanose, and 5% a-furanose. The pyranose form results from addition of the -OH at C6 to the carbonyl group, while the furanose form results from addition of the —OH at C5 to the carbonyl group (Figure 25.5). 

Deoxy sugars, as we saw in Section 25.7, have an oxygen atom "missing." That is, an —OH group is replaced by an -H. The most common deoxy sugar is 2-deoxyribose, a monosaccharide found in DNA (deoxyribonucleic acid). Note that 2-deoxyribose exists in water solution as a complex equilibrium mixture of both furanose and pyranose forms. 

The compounds usually known as monosaccharide anhydrides or glycose anhydrides (earlier glycosans ), formation of which involves the anomeric hydroxy group, are named by the same procedure. In these cases the order of preference of ring size designators is pyranose > furanose > septanose. However, three- or four-membered rings should normally be cited as anhydro if there is a choice. 

This is the form employed by the carbohydrate databank CarbBank, and is preferred for most purposes. Each symbol for a monosaccharide unit is preceded by the anomeric descriptor and the configuration symbol. The ring size is indicated by an italic / for furanose or p for pyranose, etc. The locants of the linkage are given in parentheses between the symbols a double-headed arrow indicates a linkage between two anomeric positions. In CarbBank, omission of a/p, D/L, or ftp means that this structural detail is not known. 

The structures of the a-D-aldo-pento and -hexo-pyranose monosaccharides are shown in Pig. 1. In all cases, these sugars will be studied as the six-membered-ring tautomer, as shown. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

Bhaskar and Loganathan96 described O-peracetylation of monosaccharides, disaccharides, and methyl glycosides (94) with acetic anhydride under catalysis by acid zeolites. From the panel of zeolites tested (HY, HEMT, HZSM-5, HZSM-12, HZSM-22, and H-beta), the large-pore zeolite H-beta provided the best yields of the fully acetylated sugars, most of them being over 85% and up to 99%, with the pyranose forms 95 accounting for 66-100% of the reaction products (Scheme 22). 

In many monosaccharides the proportion of the a-anomer in aqueous solution is higher than the expected ratio only based on steric inteactions. In particular, while the predicted ratio in aqueous solution at 25 °C based on the A value for axial vs. equatorial cyclohexanol is 11/89, the observed ratio for D-glucose is 36/64 for a-pyranose/p-pyranose.10 Based on... 

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... 

It has also been observed281 that, for a number of fluorinated monosaccharides, geminal 19F- H coupling-constants for the pyranose series are larger when the fluorine substituent is axially rather than equatorially disposed. Exceptions to this generalization were subsequently found, as in the virtually identical value of/F 2, H-2 for peracetylated 2-deoxy-2-fluoro-a-D-gluco- and -manno-pyranosyl fluorides.71... 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

With monosaccharides, the structures of the relatively rigid pyranoses and methylpyranosides in the crystalline state can be accurately pr icted if the hydroxyl groups are oriented as in the crystal, in the directions appropriate to form... 

Acetal and ketal linkages are widely foimd in nat-inal sugars and polysaccharides. The structure of sucrose is a splendid example. Sucrose is a disaccharide, composed of two linked monosaccharide imits, glncose in pyranose ring form and frnctose in fmanose ring form. As we have seen above,... 

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