Five-membered furanoses

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... 

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... 

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... 

Only acetal C —OMe is hydrolyzed the other ether linkages are stable.) The unmethylated OH s of (A) are those involved in ring formation. They are C and C" if the ring is five-membered (furanose) or C and C for a six-membered ring (pyranose). See Fig. 22-13. (Wavy lines denote cleavages.)... 

RGURE 8-18 Structural variation in DNA. (a) The conformation of a nucleotide in DNA is affected by rotation about seven different bonds. Sx of the bonds rotate freely. The limited rotation about bond 4 gives rise to ring pucker, in which one of the atoms in the five-membered furanose ring is out of the plane described by the other four. This conformation is endo or exo, depending on whether the atom is displaced to the same side of the plane as C-5 or to the opposite side (see Fig. 8-3b). (b) For purine bases in nucleotides, only two conformations with respect to the attached ribose units are sterically permitted, anti or syn. Fyrimidines generally occur in the anti conformation. 

Monosaccharides tend to form ring structures known as intramolecular hemiacetals, especially when the product is a five-member (furanose) ring or a six-member (pyranose) ring. Depending on which way the ring forms about the reactive carbon, the structure is called an a- or /3-hemiacetal. 

In this case, a hemiacetal is formed,that contains a five-membered furanose ring. Once again, there are two isomers, depending on how the OH group attacks the C=0 group. 

In general, a six-membered pyranose form is preferred over a five-membered furanose form because of the lower ring strain, and these cyclic forms are very much favoured over the acyclic aldehyde or ketofte forms. As can be seen in Table 1.3, at equilibrium, the anomeric ratios of pyranoses differ considerably between aldoses. These observations are a direct consequence of differences in anomeric and steric effects between monosaccharides. The amount of the pyranose and furanose present in aqueous solution varies considerably for the different monosaccharides. Some sugars, such as D-glucose, have undetectable amounts of furanose according H-NMR spectroscopic measurements whereas others, such as D-altrose, have 30% furanose content under identical conditions. 

With D-ribose, the OH group used to form the five-membered furanose ring is located on C4. Cyclization yields two anomers at the new stereogenic center, which are called a-D-ribofuranose and p-D-ribofuranose. 

Figure 11.6. Ring Structures of Fructose. Fructose can form both five-membered furanose and six-membered pyranose rings. In each case, both a and P anomers are possible.

The approaches available for predicting the relative stabilities and hence equilibrium populations of alternative conformations have been thoroughly reviewed (65MI1 75MI5), and for free sugars this can extend to comparison of the relative stabilities of alternative isomers, such as five-membered (furanose) rings or open chains. They begin from two complementary, but quite different approaches. 

The five-membered furanose ring can adopt two main conformations the envelope (E), in which the reference plane is formed by four adjacent coplanar atoms with only one atom outside it (above or below the plane) and the twist (T), where the reference plane is formed by three adjacent coplanar atoms with the remaining two atoms placed on the opposite sides of it (Figure 2.26). Accordingly, there are ten different envelope conformations Ce, 1, % E2, E, 3, % 4, and 0) and ten different twist conformations (°Ti, Tq, T2, Ti, T2, Tq, and which... 

There is an anomeric carbon, resulting from the equilibrium between the acychc form and the four cyclic forms of a sugar monomer in solution. These cychc forms are comprised of a five-membered furanose ring and the generally more abundant six-membered pyranose ring. Mutorotation around the anomeric centre converts the a-form into the P-form. 

In some cases, the pyranose ring formation can be either obstructed or blocked, and a five-membered furanose ring dominates for the given sugar. The molecular... 

D-Fructose, a ketohexose, can potentially form either a five-membered (furanose) or a six-membered (pyranose) ring involving formation of an internal hemiketal linkage between C2 (the anomeric carbon atom) and the C5 or C(, hydroxyl group, respectively. The hemiketal linkage introduces a new asymmetrical center at the C2 position. Thus, two anomeric forms of each of the fructo-furanose and fructopyranose ring structures are possible (Figure 9-10). In aqueous solution at equilibrium, fructose is present predominantly in the )3-fructopyranose form. 

---