Fischer projections representations

Conventional representation of a carbon atom (e.g. C-2 of D-glucose) in the Fischer projection. Representation (e) will be used in general in the present document. 

Fischer projection Representation of a configuration with horizontal lines for bonds pointed toward the viewer, vertical lines for bonds pointed aw,ny. 

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). 

FIGURE 25 1 Three dimensional representations and Fischer projections of the enantiomers of glycer aldehyde... 

A simpler representation of molecules containing asymmetric carbon atoms is the Fischer projection, which is shown here for the same lactic acid configurations. A Fischer projection involves... 

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. 

Since the main chain in this representation is in an entirely staggered conformation, whereas in the Fischer projection formulas the conformation represented is completefy eclipsed, an anti relationsh between two adjacent substituents in an extended structure corresponds to being on the same side in a Fischer projection formula (erythro) whereas a syn relationship corresponds to being on opposite sides in die Fischer projection (three). 

Next, translate the Fischer projection of i-serine to a three-dimensional representation, and orient it so that the lowest ranked substituent at the chirality center is directed away from you. 

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

It is rather difficult to represent a tetrahedron which is three-dimensional with a drawing of a formula that is two-dimensional. Two types of representations are used, perspective and projection formulae. Fischer projections are widely used because of their simplicity. 

Note. The Fischer projection is not intended to be a true representation of conformation. 2-Carb-4. Configurational symbols and prefixes 2-Carb-4.1. Use of d and l... 

Thus the trans relationship between the hydroxymethyl group and the C-l hydroxy group in a-D-glucopyranose, and the cis relationship between the methyl group and the C-l hydroxy group in P-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5. 

The following schematic representation of pyranose ring closure in D-glucose shows the reorientation at C-5 necessary to allow ring formation this process corresponds to the change from Fischer to modified Fischer projection. 

The orientation of the model described above results in a clockwise numbering of the ring atoms. Groups that appear to the right of the modified Fischer projection appear below the plane of the ring those on the left appear above. In the common Haworth representation of the pyranose form of D-aldohexoses, C-6 is above the plane. 

For the depiction of structural formulas of hexofuranoses, a combination of a three-dimensional, Haworth-perspective tetrahydrofuran ring with a Fischer projection of the C-5-C-6 side-chain is commonly used, as exemplified by formulas 3 and 6. With the formal closure of the second ring and formation of a 2,6-dioxabicyclo[3.3.0]octane system, however, the depiction of the C-6-C-3 ring, as in formula 7, also assumes three-dimensional geometry, and this does not correspond to the Fischer projection rule.11 Consequently, structural representations of such bicyclic molecules should be as close as possible to the actual steric situation, as shown by structures 4 and 8. 

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. 

The Fischer projections are two-dimensional representations of three dimensional objects. Further a Fischer projection may be rotated in the plane of paper by 180°, but not by 90° as illustrated in the following examples—... 

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... 

Manipulations we can do to a Fischer projection may at first glance appear confusing, but by reference to a model of a tetrahedral array, or even a sketch of the representation, they should soon become quite understandable, perhaps even obvious. The molecular manipulations shown are given to convince you of the reality of the following statements. 

In this article the use of formula 3 will always refer to rotated Fischer projections. In any case, the formulas must represent a section of chain long enough to illustrate the structural features excluding, unless explicitly required, the terminal groups. This representation corresponds to the use of a macromolecular model with an infinite chain length. 

The contrast between formulas 20 and 21, both pertaining to isotactic polyethylidene, should be noted This contrast occurs because the polymer repeating unit has only one carbon atom in the chain and thus there is no correspondence between such periodicity and that of the zigzag representation. The classical definition of an isotactic polymer (as one in which all substituents are on the same side of the chain) holds true, in general terms, only if the polymer is represented in the Fischer projection. Analogous considerations pertain to syndiotactic polyethylidene 22 and 23. 

The use of rotated Fischer projections has been retained in the present edition in order to provide a link with, and an explanation of, the bulk of existing published polymer literature, although the present common practice [4] is to depict main-chain bonds in planar, extended zigzag (all-trans) conformations, together with a stereochemical representation of side-groups at tetrahedrally-bonded atoms. 

To verify that the Fischer projection has the R configuration at its chirality center, rotate the three-dimensional representation so that the lowest-ranked atom (H) points away from you. Be careful to maintain the proper stereochemical relationships during the operation. 

The sawhorse or Newman representations of 2-butanol, 5a and 5b and 6a and 6b, are excellent for showing the arrangements of the atoms in conformations, but are needlessly complex for representing the stereochemical configuration. Fischer projection formulas are widely used to show configurations and are quite straightforward, once one gets the idea of what they represent. 

The Newman representation 25a or 26a of meso-tartaric acid does not have a mirror plane. Why is it different from the Fischer projections in this respect The reason is that the projection formulas represent a particular eclipsed conformation 27 of tartaric acid that does have a mirror plane ... 

Redraw the perspective drawings a, b, and c as Fischer projection formulas, leaving the configuration at the chiral centers unchanged. Similarly, redraw d and e in perspective, using a staggered sawhorse representation for e. 

In order of decreasing rank, the substituents attached to the stereogenic center in lactic acid are —OH, —C02H, —CH3, and —H. The Fischer projection given for (-F)-lactic acid (a) corresponds to the three-dimensional representation (b), which can be reoriented as in (c). When (c) is viewed from the side opposite the lowest ranked substituent (H), the order of decreasing precedence is anticlockwise, as shown in (d). (+)-Lactic acid has the 5 configuration. 

The structure of L-glycerol 3-phosphate is shown in a Fischer projection. Translate the Fischer projection to a three-dimensional representation. 

If you view the second formula from the top, you will see that it is just a three-dimensional representation of the Fischer projection. Horizontal groups at each stereogenic center come up toward you, and vertical groups recede away from you. The second formula represents an eclipsed conformation of D-threose. The third and fourth formulas represent sawhorse and Newman projections, respectively, of a staggered conformation of D-threose. 

To enable representation of these isomers on the flat surface of a page, Fischer developed a notation system (Fischer projections). Fischer projections denoted right- and left-handed isomers. These were called D (dextrorotatory) and L (levorotatory), respectively, and the compound gfyc-eraldehyde (which exists in two forms) was used as a reference. 

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