Monosaccharides chair form

Fig. 2.8 Preferred conformations adopted by cyclic monosaccharides chair form for six-membered rings (e.g. glucose) and envelope form for five-membered rings (e.g. fructose). Each carbon atom in the cyclic system possesses one axial and one equatorial substituent the axial bonds are represented by vertical lines.All OH groups and the CH2OH group are equatorial in 3-D-glucose.

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. 

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. 

A final consideration in describing the structure of a monosaccharide is its conformation. Most aldopyra-noses adopt a chair conformation, the most stable form being that in which as many as possible of the bulkier OH and CH2OH groups are equatorial, as shown for glucose in Fig. 2.8. A corresponding envelope confor-... 

The pyranoid forms of monosaccharides present an interesting problem in conformation because of the two different chair-conformations which, ignoring the non-bonded interactions in substituents, are energetically equivalent. The actual conformation of the free molecule is believed to be determined by the non-bonded interactions of substituent groups, as discussed principally by Hassel and Ottar, Reeves, and Barker and... 

There are two possible types of strainless six-member pyranose rings, the boat (B) and the chair (C) forms and of these, the chair is usually preferred as there are fewer interactions across the ring between snbstituents. The rings can be defined as " 1 if C4 is above the plane (described by C2, C3, C5 and 05) and Cl is below it, or as C4 for the alternative ring. P-D-Glncopyranose has all substituents (OH) that are equatorial in the " 01 conformation, a possible reason why D-Glc is a common monosaccharide in nature. 

Convert each chair conformation to an open-chain form and then to a Fischer projection. Name the monosaccharide you have drawn. 

When drawing either a Haworth projection or a chair conformation, the convention is to place the oxygen atom in the upper, rear-right position, as highlighted in red. Notice that in a cycfic form of D-glucose all substituents can occupy equatorial positions, which renders the compound particularly stable and explains why D-glucose is the most common naturally occurring monosaccharide. 

Fucose (Fuc, shown in Fig. 2) is a deoxy-monosaccharide, formed by replacing the hydroxymethyl group of Gal with a methyl group it can be termed, 6-deoxy-galactose. Xylose (Xyl) is a pentose, without the flexible hydroxymethyl group in its pyranoside chair configuration, the OH2,3,4 groups have the same orientation (all equatorial) as Glc. 

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