Furanose and pyranose

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... 

Sfrucfurally O glycosides are mixed acefals fhaf involve fhe anomeric posifion of furanose and pyranose forms of carbohydrates Recall fhe sequence of mfermediafes m acefal formalion (Secfion 17 8)... 

A particular- carbohydrate can interconvert between furanose and pyranose for-rns and between the a and p configuration of each for-m. The change from one for-m to an equilibrium mixture of all the possible hemiacetals causes a change in optical rotation called mutarotation. 

Haworth formulas (Section 25.6) Planar representations of furanose and pyranose forms of carbohydrates. 

The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

It is evident from the above discussion that more-detailed studies of the P.M.R. spectra of carbohydrate derivatives will reveal an ever-increasing number of long-range couplings. Studies in this laboratory are directed towards the investigation of the occurrence of these couplings in the spectra of both furanose and pyranose derivatives. 

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. 

In the 1920s, Haworth and his school proposed the terms furanose and pyranose for the two forms. Haworth also introduced the Haworth depiction for writing structural formulae, a convention that was soon widely followed. 

The variants are distinguished by the locants of those ring atoms that lie outside a reference plane (defined below) and are listed for some examples in Table 1. The locants of ring atoms that lie on the side of the reference plane from which numbering appears clockwise (i.e. the upper side in the normal Haworth representation of furanoses and pyranoses) are written as superscripts and precede the letter those that lie on the other side are written as subscripts and follow the letter. Heteroatoms (e.g. O, S) are indicated by their subscript or superscript atomic symbols. Table 1 gives the notations and Chart III some examples. 

Reactions of furanose and pyranose with -benzyl hydroxyl amine at 110°C... 

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... 

The direct 0-alkylation of the anomeric center (Scheme 1, path C) by treatment of furanoses and pyranoses with base and then with simple alkylating agents, for instance an excess of methyl iodide or dimethyl sulfate, has long been known (1,3). Surprisingly, no studies employing this simple method for syntheses of more-complex glycosides and saccharides have been reported prior to our work (1,37,38). 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

Monomeric carbohydrates in their cyclic form (furanoses and pyranoses) are hemiacetals, which, to become acetals, form 0-glycosyl conjugates. The C-atom C(l) that bears two O-atoms is the reactive, electrophilic center targeted by glycosidases. Nonenzymatic hydrolysis is also possible, although, as a rule, under physiological conditions of pH and temperature, the reaction is of limited significance. 

These compounds come from methylenation reactions of the corresponding carbonyl derivative by means of an ylide. Several experimental conditions have been described. In most cases, CF2Br2 and HMPT (hexamethyl phosphorotriamide) are employed. The reaction occurs with aldehydes as well as with ketones in the furanose and pyranose series. The reaction can also be performed with lactones the fluoromethyl group is then introduced in the anomeric position. With these substrates, the Julia olefmation, which uses difluoromethyl sulfone, has also been reported to be an efficient method. Some examples of these reactions are shown in Figure 6.24. 

The remarkable stability of the thiazole ring allowed synthetic manipulations of the thiazolyl ketol acetates, which extended considerably the scope of the above C-formylation method of furanoses and pyranoses. Instead of the reductive removal of the acetoxy group, the /V-glycosidation of either a- or p-anomer 73 with TMSN3 afforded stereoselectively the azido galactopyranoside 75 in 88% isolated yield (Scheme 22) [77]. The cleavage of the... 

Although the characteristic differences discussed above permit ready distinction between furanose and pyranose derivatives, their fragmentation patterns have much in common. The fragments of the A, C, and J series are formed in a similar manner from both of the two types of derivatives, although there is a difference in the contribution of some of the isomeric ions.1 87... 

For this reason, the names furanose and pyranose have been coined to denote five- and six-membered rings in cyclic sugars. The two forms of glucose are appropriately identified by the names a-D-glucopyranose and /3-D-glucopyra-nose. Likewise, L-arabinose, D-xylose, D-galactose, and D-mannose occur naturally as pyranoses, but D-ribose (in combined form) and D-fructose occur as furanoses (see Figures 20-1 and 20-2). 

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