Conformations and Cyclic Forms of Sugars

In reality, the cyclic hemiacetal forms of sugars and their derivatives may assume a wealth of different ring conformations. For purposes of nomenclature, however, linear Fischer Projections and Haworth Representations with their quasi conformation-averaging planar-cyclic habit are best suited to illustrate the manifold stereochemicd relationships significant for carbohydrates. 

Glycosides of 3-deoxy-3-nitroaldoses are readily available by the cyclization of sugar dialdehydes with nitromethane. Introduced in 1958 by Baer and Fischer,48 the method has been widely employed, and some aspects have been reviewed.49,50 The term sugar dialdehydes refers to products of glycol cleavage of glycosides.51 The formation, structure, conformation, and reaction of such dialdehydes have been discussed in detail in previous Chapters of these Volumes.52-54 The compounds exist in aqueous and alcoholic solutions as solvated, cyclic forms that are internal hemialdals or hemiacetals.54 However, under the conditions of the nitromethane cyclization, mobile equilibria are rapidly shifted from such cyclic products toward carbonyl species, and it is, therefore, permissible and customary in... 

Fischer projections choose the cyclic and planar, //-eclipsed conformation of the carbon skeletons of sugars as a basis, then pull the cycle straight to form a line and orient it so that the more highly oxidized end Cl is located at the top. This projection procedure remained alive for almost a century because inherent symmetry properties in carbohydrates become apparent here in the most simple way. Always remember Fischer projection formulas are the short notations of a/l-eclipsed cyclic conformations, although they look linear (Fig. 4.2.1). The terminal chiral center, C5 in hexoses, is usually R- or, in the Fischer notation, D-configured. 

The study of the conformations of cyclic sugars was developed more fully by Reeves in a series of papers, beginning in 1949, dealing with the formation of complexes of sugars and their derivatives in cuprammonia solution. Reeves was able to provide experimental evidence to indicate that some pyranoid sugars indeed adopt chair conformations in solution, and that, in such cases, one chair form usually appeared to preponderate over the other. Boat or skew conformations were considered possible if the nonbonded interactions between the substituents in the chair conformation became too large. " Reeves s investigations have formed the basis of much subsequent work in the conformational analysis of cyclic carbohydrate... 

Draw the most stable conformation of the most stable cyclic hemiacetal form of this sugar, and give the structure a complete systematic name. 

Recurring areas of concern in contemplating syntheses with sugars are the form of the molecule (/. e. linear or cyclic) and if cycUc, the size and conformation of the ring. Accordingly these areas will be examined briefly. 

Many of the stereoelectronic effects in the list above govern reactivity, but the next section will deal with how stereoelectronic effects affect structure—and in particular conformation. Some of the most important saturated oxygen heterocycles are the sugars. Glucose is a cyclic hemiacetal—a pentasubstituted tetrahydropyran if you like—whose major conformation in solution is shown below. About two-thirds of glucose in solution exists as this stereoisomer, but hemiacetal formation and cleavage is rapid, and this is in equilibrium with a further one-third that carries the hemiacetal hydroxyl group axial (<1% is in the open-chain form). 

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