Fischer convention

The diirality of a molecule is described by specifying its configuration. The system that is used is the Cahn Ingold-Prel6g convention, which uses the descriptors R and S. The Fischer convention, employing the descriptors D and L, is historically important and is still used with certain types of molecules. 

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. 

At the present time, use of the Fischer convention is almost entirely restricted to carbohydrates, amino acids, and biologically important molecules of closed related structural types. The problem with more general use is that there are no adequate rules for deciding whether a diiral atom is like D-glyceraldehyde or L-glyceraldehyde when the structures are not closely similar to the reference molecules. This relationship is clear for carbohydrates and amino acids. 

A 90° rotation breaks the Fischer convention by exchanging the groups that go into the plane and those that come out. In the following Fischer projections of (jR)-glyceraldehyde, the —H and -OH groups come out of the plane before rotation but go into the plane after a 90° rotation. As a result, the rotated projection represents (S)-glyceraldehyde. 

In the Fischer convention, all common proteinogenic amino acids belong to the L series, but w ith the CIP convention the S configuration does not universally apply, (/ )-cysteine being an exception. 

Ring forms of sugars are also often drawn according to the Fischer convention making use of elongated bent lines to represent ordinary simple bonds ... 

Note that this last step becomes particularly confusing in cases in which the tail projects up, whereas the Fischer convention features the carbons of the tail down (e.g., a-D-glucofuranose). Correct application of step 4 to these cases requires that you rotate the plane of the paper to place the tail down and indicate the Fischer convention OH assignments (Fig. Ill-10). However, there still is un-... 

The correct steric representation by the Fischer convention of two asym-... 

The Fischer convention is widely used in sugar chemistry and for a-amino acids. For sugars, which contain a number of asymmetric centers, and for diastereomers with only two centers, the Fischer convention defines a series as D or L according to whether the configuration at the highest numbered asymmetric center is analogous to D- or L-glyceralde-... 

Figure 7 The configurations of D-(+) and L-(-)-gIyceraIdehyde, D-erythrose, and D threose, according to the Fischer Convention.

The Fischer convention is often inexact and difficult to use, especially when complicated chemical transformations are required to convert the molecule under investigation into a molecule of known configuration. In addition, the assigned configuration, D or L, is often confused with the... 

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