Pyranose and Furanose Structures

Show that the percentages of a- and /S-D-glucose in aqueous solution at equilibrium can be calculated from the specific rotations of the pure a and )8 forms and the specific rotation of the solution at equilibrium. 

Solution The equilibrium rotation is +52°, and the rotations of pure a and /3 forms are -1-112 and +19 , respectively. Assuming that no other forms are present, we can express these values graphically as follows  

The six-membered cyclic form of most monosaccharides is the preferred structure. These structures are called pyranose forms after the six-membered oxygen heterocycle pyran. The formula at the extreme left of eq. 16.3 is more completely named a-D-glucopyranose, with the last part of the name showing the ring size. 

Cyclic monosaccharides with six-membered and five-membered rings are called pyranoses and furanoses, respectively. 

Pyranoses are formed by reaction of the hydroxyl group at C-5, with the carbonyl group. With some sugars, however, the hydroxyl group at C-4 reacts instead. In these cases, the cyclic hemiacetal that is formed has a five-membered ring. This type of cyclic monosaccharide is called a furanose, after the parent five-membered oxygen heterocycle furan. 

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N. Asano, K. Oseki, H. Kizu, and K. Matsui, Nitrogen-in-the-ring pyranoses and furanoses Structural basis of inhibition of mammalian glycosidases, J. Med. Chem., 37 (1994) 3701-3706. 

It is interesting to note that the formation of the dioxane ring stabilizes the anhydrides as to their pyranose or furanose structures, difructose anhydride I as 2,1 l,2 -di-D-fructofuranose and dihetero-levulosan as 2,1 l,2 -di-D-fructopyranose. 

Gas-liquid chromatography may be the method of choice when it is necessary to identify or to quantitate one or more carbohydrates especially when they are present in small amounts. Although this technique is often used because it is possible to resolve carbohydrates with very similar structures, the fact that a and /3 anomers and pyranose and furanose forms of the same carbohydrate all give separate peaks is sometimes a disadvantage. 

It follows that, when we dissolve a sugar such as glucose or ribose in water, we create a mixture of various equilibrating structures. The relative proportions of pyranose and furanose forms, and of their respective anomers for the eight aldohexoses, are shown in Table 12.1. In each case, the proportion of non-cyclic form is very small (<1%). 

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... 

Although much work has been done in an attempt to elucidate the structure of iV-substituted glycosylamines, rigid proof of their structures has not yet been achieved.43 Spectroscopic evidence is inconclusive,69 whilst chemical methods are of limited value because equilibrium between open-chain, pyranose, and furanose forms is possible. However, all such derivatives as ethers and esters so far prepared directly from A-glycosylamines have been shown to be pyranoid, and periodate oxidation of some N-glycosylamines has confirmed this conclusion.70 71... 

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.

The ring-chain equilibrium 46A 46B was revealed by H- and 13C-NMR spectroscopy (93ZOR278) in solutions of D-glucose, D-mannose, d-galactose, and D-arabinose thiosemicarbazones. The equilibrium state depends rather unpredictably on the structures of the initial monosaccharide and of the substituents R2 and R3 in the thiosemicarbazone moiety. In solutions of D-fructose thiosemicarbazones, pyranose and furanose tautomers participate in the equilibrium. 

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

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. 

The substitution of pentose by hexose sugars has received much attention because oligonucleotides containing hexose sugars have many useful properties. The first hybridisation system between pyranose and furanose nucleic acids has been reported. a-Pyranose oligonucleotides (a-homo-DNA) forms stable duplexes with RNA in a parallel orientation. NMR studies show that the a-homo-DNA bases (in a-homo-DNA RNA duplexes) are equatorially arranged, whilst in the RNA strand they are pseudoaxial. The helical structure does not conform to either A- or B-forms. 

The anomeric carbon atom, C-l, in pyranose and furanose sugars is unique in that it is the only carbon atom in these molecules which is bonded to two atoms which are more electronegative. These more-electronegative atoms necessarily have lone-pair electrons. It is the electronic structure which arises from the electronegativity differences and the presence of these lone-pair electrons that gives rise to the special reactivity and structural properties of the anomeric carbon center. 

The ketoses are classified as 2-ketoses, 3-ketoses, etc., following the carbonyl position on the chain. The 2 of 2-ketoses, a common natural structure, can be removed. The suffix ose is replaced by ulose in the parent name. Likewise, fructose is a 2-hexulose, or more simply, a hexulose. For the complete name, it is preceded by the configuration descriptor. The systematic name of fructose 4.37 would be D-araZ>Z o-2-hexulose. In the case of 2-ketoses, there is no possible ambiguity for the configuration of pyranoses and furanoses. The nomenclature is copied from that of aldoses as, for example, /3-D-fructopyranose 4.38. Methyl glycoside 4.39 of sialic acid is called methyl 5-acetamido-3,5-dideoxy-D-gZycera-a -D-gaZacfo-2-nonulopyranosidonic acid. 

Mass spectrometry has proved to be a useful complement to other methods for elucidating the structure of cyclic acetals. The reader is referred to several excellent reviews on the mass spectrometry of carbohydrates that have appeared within the time span covered by this article.65a 65<1,66 Mass spectrometry is a valuable aid for distinguishing pyranose from furanose structures, and monoacetals from diacetals. It may also facilitate the assignment of ring location. However, it is generally insensitive to configurational differences. 

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