Formulas Fischer projection

When this formula is rotated so that the group of lowest priority attached to C2 is directed away from the viewer, it resembles the following  

The order of progression from the group of highest priority to that of next highest priority (from —Br, to —CHBrCHs, to —CH3) is clockwise. So C2 has the (R) configuration. When we repeat this procedure with C3, we find that C3 also has the (R) configuration  

Give names that include (R) and (S) designations for compounds B and C in Section 5.12A. 

Chloramphenicol (at right) is a potent antibiotic, isolated from Streptomyces venezuelae, that is particularly effective against typhoid fever. It was the first naturally occurring substance shown to contain a nitro (—NO2) group attached to an aromatic ring. Both chirality centers in chloramphenicol are known to have the R) configuration. Identify the two chirahty centers and write a three-dimensional formula for chloramphenicol. 

There is no obvious relationship between configuration R or S) and sign of rotation (+ or —). For example, (R)-lactic acid is levorotatory. When (i )-lactic acid is converted to its methyl ester (eq. 5.1), the configuration is unchanged because none of the bonds to the stereogenic carbon is involved in the reaction. Yet the sign of rotation of the product, a physical property, changes from — to +. 

Enantiomers often behave differently in a biological setting because these properties usually involve a reaction with another chiral molecule. For example, the enzyme lactic acid dehydrogenase will oxidize ( + )-lactic acid to pyruvic acid, but it will not oxidize ( —)-lactic acid (eq. 5.2). 

Why The enzyme itself is chiral and can distinguish between right- and left-handed lactic acid molecules, just as a right hand distinguishes between left-handed and right-handed gloves. 

Enantiomers differ in many types of biological activity. One enantiomer maybe a drug, whereas its enantiomer maybe ineffective. For example, only (—)-adrenaline is a cardiac stimulant (-l-)-adrenaline is ineffective. One enantiomer maybe toxic, another harmless. One may be an antibiotic, the other useless. One may be an insect sex attractant, the other without effect or perhaps a repellant. Chirality is of paramount importance in the biological world. 

Instead of using dashed and solid wedges to show the three-dimensional arrangements of groups in a chiral molecule, it is sometimes convenient to have a two-dimensional way of doing so. A useful way to do this was devised many years ago by Emil Fischer the formulas are called Fischer projections. 

Arrange the carbon chain vertically with the most oxidized group (—CHO in glyceraldehyde) at the top.  

Place the carbon atom at the chiral center in the plane of the paper. It is C-2 in glyceraldehyde. 

Because C-2 is bonded to four groups, the CHO group and the CH OH group extend behind the plane of the page, and the hydrogen atom and the hydroxyl group extend up and out of the plane. 

Project these four groups onto a plane. The carbon atom at the chiral center is usually not shown in this convention. It is located at the point where the bond lines cross. The vertical lines project away from the viewer. The horizontal lines project toward the viewer. 

Fischer projection formulas can be drawn for any pair of enantiomers. These formulas imply that we know the configuration at the chiral carbon atom. However, the true configuration could not be determined by early chemists because there was no way to determine the arrangement of the atoms in space. Therefore, Fischer arbitrarily assigned a configuration to one member of the enantiomeric pair of 

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Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

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. 

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. 

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

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). 

Without consulting chapter figures, draw Fischer projection formulas for glycine, aspartate, leucine, isoleucine, methionine, and threonine. 

Scheme 9. The threo 2,3-diol problem. The erythro/threo notation is based on Fischer projection formulas. For example, if threo 2,3-diol 28, shown here in a staggered zigzag conformation, was depicted in an eclipsed Fischer projection, the adjacent hydroxyls attached to carbons 2 and 3 would reside on opposite sides of the carbon chain. An alternative, perhaps less ambiguous, descriptor is the syn/anti notation.63...

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. 

Fig. 3.—Formulas Representing KDO (1). [a, Fischer projection-formula (acyclic form) b, Haworth formula (ketopyranose) c, conformational drawing (ketopyranose).]...

In the simplest and oldest system the formula of the compound is compared with a standard substance taken as the key compound. In the series of a amino or a -hydroxy acid, the key is the top part of the projection formulas (Fischer projection formulas). 

Problem 5.10 Structures of CHClBrF are written below in seven Fischer projection formulas. Relate structures (b) through (g) to structure (a). 

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). 

Fischer further proposed that the amino acid in this orientation could be projected onto the paper and drawn with ordinary lines for all the bonds. This gives the previously shown Fischer projection formula of... 

Filamin 370 Filaria worms 24 Fimbriae 6. See also Pili Fingerprinting. See also Peptide mapping of DNA 259 of proteins 118, 360 First Law of Thermodynamics 282 First order reactions 457 Fischer, Edmond H. 84 Fischer, Emil H. 42, 83 Fischer projection formula 42 of monosaccharides 163 FK506 488 Flagella... 

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