Fischer convention, projection formulas

In the Fischer convention, the ermfigurations of other molecules are described by the descriptors d and L, which are assigned comparison with the reference molecule glyceraldehyde. In ertqrloying the Fischer convention, it is convenient to use projection formulas. These are planar representations defined in such a w as to convey three-dimensional structural information. The molecule is oriented with the major carbon chain aligned vertically in such a marmer that the most oxidized terminal carbon is at the top. The vertical bonds at each carbon are directed back, away fiom the viewer, and the horizontal bonds are directed toward the viewer. The D and L forms of glyceraldehyde are shown below with the equivalent Fischer projection formulas. 

FIGURE 7-2 Three ways to represent the two stereoisomers of glyc-eraldehyde. The stereoisomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. By convention, in Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader vertical bonds project behind the plane of the paper, away from the reader. Recall (see Fig. 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away. 

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). 

The stereodescriptor sn indicates that the name results from the convention of stereospecific numbering of the atoms of glycerol. The structure must therefore be drawn as a Fischer projection formula with the l configuration. 

The stereogenic centers in sugars are often depicted following a different convention than is usually seen for other stereogenic centers. Instead of drawing a tetrahedron with two bonds in the plane, one in front of the plane, and one behind it, the tetrahedron is tipped so that horizontal bonds come forward (drawn on wedges) and vertical bonds go behind (on dashed lines). This structure is then abbreviated by a cross formula, also called a Fischer projection formula. In a Fischer projection formula ... 

Fischer projection formulas are also used for compounds like aldohexoses that contain several stereogenic centers. In this case, the molecule is drawn with a vertical carbon skeleton and the stereogenic centers are stacked one above another. Using this convention, all horizontal bonds project forward (on wedges). 

Fischer projection formulas. Formulas which, by convention, show bonds coming out of the plane drawn vertically, and bonds that project behind the plane drawn horizontally. 

In the Fischer Convention, the type of structural formula used is the Fischer projection. Thus we should convert the structural representation of the glyceraldehydes given in the problem into the Fischer projection. First, a cross is drawn. The location of the chiral (assymmetric) carbon is represented by the intersecting point of the two lines of the cross. The horizontal lines extending to the left and right of this point represent bonds extending forward, above the plane of the paper. The vertical lines represent bonds extending back, below the plane of the paper. 

This formula, as wdl as the later expressions CXXVII through CXXX (c/. Section III, 3) are drawn according to the Fischer convention (Ber., S4, 2683 (1891)). The horizontal lines project forward, and the vertical ones, backward. 

It is convenient in employing the Fischer convention to use projection formulas. These Fischer projections are obtained by orienting the molecule so that the most highly oxidized carbon atom of the main chain is at the top, with the vertical bonds from the asymmetric carbon atom directed back and the horizontal bonds directed forward. The d and l forms of glyceraldehyde then become as shown in the following three-dimensional representations and Fischer projection formulas ... 

Chemists, however, sometimes use formulas called Fischer projections to show three dimensions in chiral molecules such as acyclic carbohydrates. Fischer projection formulas are useful in cases where there are chirality centers at several adjacent carbon atoms, as is often the case in carbohydrates. Use of Fischer projection formulas requires rigjd adherence to certain conventions, however. Used carelessly, these projection formulas can easily lead to incorrect conclusions. 

Fischer projection (Sections 5.13 and 22.2C) A two-dimensional formula for representing the configuration of a chiral molecule. By convention, Fischer projection formulas are written with the main carbon chain extending from top to bottom with all groups eclipsed. Vertical lines represent bonds that project behind the plane of the page (or that lie in it). Horizontal lines represent bonds that project out of the plane of the page. 

The specification of configurations in diastereomeric species is quite simple, with each chiral center being designated R or S according to the sequence rules when the Cahn-Ingold-Prelog convention is used. An extension of the Fischer convention to systems with more than one asymmetric center that is based on carbohydrate structures and terminology is still used in relatively simple cases. This convention can be illustrated with the same stereoisomeric 2,3,4-trihydroxybutanals just discussed. The 2R,3R and 25,35 isomers are d- and L-erythrose, respectively. The 25,3/ and 2i ,35 isomers are d- and L-threose, respectively. The Fischer projection formulas are shown below ... 

Fig. 2. Planar zigzag projection of the backbone carbon atoms of a) isotactic and b) syndiotactic polypropylene, both in the saw-horse and in the Fischer notation. It should be noted that the latter notation is not strictly a Fischer projection formula, because, by the Fischer convention, the carbon chain is always written vertically. In addition, only the methyl group positions are...

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