Fischer projection sugars

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

L-(-F)-Arabinose is a naturally occurring L sugar. It is obtained by hydrolysis of the polysaccharide present in mesquite gum. Write a Fischer projection for L-(-F)-arabinose. 

Figure 25.2 Some naturally occurring D sugars. The -OF group at the chirality center farthest from the carbonyl group has the same configuration as

Only the o sugars are shown in Figure 25.3. Draw Fischer projections for the following i. sugars ... 

Problem 25.10 > The following model is that of an aldopentose. Draw a Fischer projection of the sugar, name it, and identify it as a n sugar or an ). sugar. 

Draw Fischer projections of the following molecules, placing the carbonyl group at the top in the usual way. Identify each as a n or t. sugar. 

Fischer projection, 975-978 carbohydrates and, 977-978 D sugars, 980 i., sugars, 980-981 rotation of, 976 R.S configuration of, 977 conventions for, 975-976 Fishhook arrow, radical reactions and, 139, 240... 

If a cyclic form of a sugar is to be represented in the Fischer projection, a long bond can be drawn between the oxygen involved in ring formation and the (anomeric)... 

The ring system in these dianhydro hexitols is of interest and worthy of some discussion. The formula of isomannide (LXX) based on the Fischer projection formula for sugars does not convey the real character of the molecule and the author has chosen to write these substances as two fused tetrahydrofuran rings. Scale models show this to be a more exact representation. Thus isomannide is written as LXXI, isosorbide as LXXII, and L-isoidide as LXXIII. 

Fischer projections provide a fnrther approach to the two-dimensional representations of three-dimensional formulae. They become particularly useful for molecules that contain several chiral centres, and are most frequently encountered in discussions of sugars (see... 

However, as we shall see shortly, Fischer-projection-derived eclipsed conformers are particularly useful in deducing the stereochemistry in cyclic forms of sugars (see Box 3.16). 

Stereochemistry in hemiacetal forms of sugars from Fischer projections... 

Fig. 1. Higher-order sugars L-g/yceroD-manno-Heptose (1, open-chain and 2, a-pyranose) and A -acetylneurami ilic acid (3, a-pyranose form). Structures 4-8 depict D-glucose (4) as its open-chain Fischer projection, and as the a-oxetose (5), a-furanose (6), a-pyranose (7), and a-septanose (8) ring forms.

For molecules with two stereogenic centers, the traditional descriptors are erythro and threo to which, henceforth, a c (short for carbohydrate) is added for a reason that will become apparent (Table 10). They are unambiguously defined for simple sugars. The backbone is given by the carbon chain the reference conformation is the conformation with all carbon atoms eclipsed. This statement is identical with the requirement of a Fischer projection. If two identical substituents X are located on the same side/opposite sides of the plane traced by the backbone, then the c-erythrojc-threo configuration is assigned. 

By convention, one of these two forms is designated the d isomer, the other the l isomer. As for other biomolecules with chiral centers, the absolute configurations of sugars are known from x-ray crystallography. To represent three-dimensional sugar structures on paper, we often use Fischer projection formulas (Fig. 7 2). 

Monosaccharides generally contain several chiral carbons and therefore exist in a variety of stereochemical forms, which may be represented on paper as Fischer projections. Epimers are sugars that differ in configuration at only one carbon atom. 

---