Stereochemistry Fischer projections

Monosaccharides have chiral centers and thus exhibit optical activity. Let s explore the structure of glyceraldehyde. In the three-dimensional representation, the dotted wedges indicate the bonds that extent backwards from the chiral carbon (away from you or into the plane of the page) and the solid wedges indicate the bonds that are projected toward you (out of the plane of the page). In stereochemistry, Fischer projection is an important way to represent the spatial orientation of molecules. In Fischer projection representation, the bonds that are pointed backward (away from you or into the page) are indicated by vertical lines, and the bonds that extend toward you (out of the page) are represented by horizontal lines. For a more detailed approach, refer to Chapter 19 (Stereochemistry). 

As you work with Fischer projections you may notice that some routine structural changes lead to predictable outcomes—outcomes that may reduce the number of manip ulations you need to do to solve stereochemistry problems Instead of listing these short cuts Problem 7 10 invites you to discover some of them for yourself... 

FIGURE 7 9 Repre sentations of (2/ 3R) dihy droxybutanoic acid (a) The staggered conformation is the most stable but is not properly arranged to show stereochemistry as a Fischer projection (b) Rotation about the C 2-C 3 bond gives the eclipsed conforma tion and projection of the eclipsed conformation onto the page gives (c) a correct Fischer projection... 

Fischer projections and d-l notation have proved to be so helpful m representing carbohydrate stereochemistry that the chemical and biochemical literature is replete with their use To read that literature you need to be acquainted with these devices as well as the more modern Cahn-Ingold-Prelog R S system... 

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open chain form is present at equilibrium To understand their structures and chemical reac tions we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms Consider first cyclic hemiacetal formation m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

Fischer projections and d-l notation are commonly used to describe car bohydrate stereochemistry The standards are the enantiomers of glycer aldehyde... 

Make a molecular model corresponding to the stereochemistry of the Fischer projection of 2-phenyl-2-butanol shown in the equation and verify that it has the R configuration. 

L-Amino acid (Section 27.2) A description of the stereochemistry at the a-carbon atom of a chiral amino acid. The Fischer projection of an a-amino acid has the amino group on the left when the carbon chain is vertical with the carboxyl group at the top. 

Because carbohydrates usually have numerous chirality centers, it was recognized long ago that a quick method for representing carbohydrate stereochemistry is needed. In 1891, Emil Fischer suggested a method based on the projection of a tetrahedral carbon atom onto a flat surface. These Fischer projections were soon adopted and are now a standard means of representing stereochemistry at chirality centers, particularly in carbohydrate chemistry. 

B A Fischer projection can have one group held steady while the other three rotate in either a clockwise or a counterclockwise direction. The effect is simply to rotate around a single bond, which does not change the stereochemistry. 

Problem 25.2 Convert the following Fischer projections into tetrahedral representations, and, assign R or S stereochemistry to each ... 

For a thorough understanding of stereochemistry, it is useful to examine molecular models (like those depicted in Fig. 4.1). However, this is not feasible when writing on paper or a blackboard. In 1891, Emil Fischer greatly served the interests of chemistry by inventing the Fischer projection, a method of representing tetrahedral carbon atoms on paper. By this convention, the model is held so that the two bonds in front of the paper are horizontal and those behind the paper are vertical. 

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... 

The structures above show some of the fundamental features of carbohydrates. Initially, we have drawn these compounds in the form of Fischer projections, a depiction developed for these compounds to indicate conveniently the stereochemistry at each chiral centre (see Section 3.4.10). The Fischer projection is drawn as a vertical carbon chain with the group of highest oxidation state, i.e. the carbonyl group, closest to the top, and numbering takes place from the topmost carbon. 

The stereochemistries of the various substituents may be deduced by considering the implications of the Fischer projection (see Box 3.16). 

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