Structural formulas Fischer projections

If you have access to a set of molecular models, converting between perspective formulas, Fischer projections, and skeletal structures is rather straightforward. If, however, you are interconverting these three-dimensional structures on a two-dimensional piece of paper, it is easy to make a mistake, particularly if you are not good at visualizing structures in three dimensions. Fortunately, there is a relatively foolproof method for these interconversions. All you need to know is how to determine whether an asymmetric center has the / or the 5 configuration (Sections 4.7 and 4.14). Look at the following examples to learn how easy it is to interconvert the various stractural representations. 

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. 

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). 

Refer to the Fischer projection of D-(+)-xylose in Figure 25.2 (Section 25.4) and give structural formulas for... 

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. 

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 Cahn-Ingold-Prelog rules (1956) are used to designate the configuration of each chiral C in a molecule in terms of the symbols R and S. These symbols come from the Latin, R from rectus (right) and S from sinister (left). Once told that the configuration of a chiral C is R or S, a chemist can write the correct projection or Fischer structural formulas. In our statement of the three rules the numerals 1, 2, 3, and 4 are used some chemists use letters a, b, c. and d in their place. 

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

Figure 2.2 Structural formulae of a- and /3-lactose, (a) Fischer projection, (b) Haworth projection and (c) conformational formula.

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). 

Exercise 20-3 Determine for each of the following sets of structures whether they correspond to the same stereoisomer, The left structure in each example is a Fischer projection formula. Models will be helpful ... 

Fischer projection formulas show three-dimensional structures in two dimensions. In such formulas, horizontal groups project toward the viewer, and vertical groups project away from the viewer. 

Using Fischer projection formulas, we can write the structures of D-glucitol (eq 16.9) and D-mannitol (see the answer to Problem 16.9). 

What products are obtained from the bromination of cinnamic acid [(E)-3-phenylpropenoic acid] Draw the structural formulae of the products as Fischer projection formulae. What relationship do the products have to one another ... 

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. 

In the course of investigations of aspartyl dipeptide esters, we had to draw their chemical structures in a unified formula. In an attempt to find a convenient method for predicting the sweettasting property of new peptides and, in particular, to elucidate more definite structure-taste relationships for aspartyl dipeptide esters, we previously applied the Fischer projection technique in drawing sweet molecules in a unified formula 04). 

The Fischer projection is a convenient way of showing the configurations of the linear forms of monosaccharides. This convention depicts the concepts of stereochemistry established by Jacobus Henricus van t Hoff and Joseph Achille Le Bel in a simplified form. While these abbreviated structural formulas are simple to write and easy to visualize, there are some guidelines that should be taken into account when converting a three-dimensional structure into a Fischer projection and in its manipulation (Fig. 1.2) ... 

Consequently, as a result of this information, it is possible to depict the structure of sphingosine from natural sources, by a Fischer projection formula (Figure 1-8). 

These structures, when written as shown below (left), are called Fischer projection formulas, which are attempts to represent three-dimensional molecules in two dimensions. For example, the two molecules would appear in three-dimensional space as shown below (right)... 

If the structures of the two tetroses are written alongside those of D-erythrose and D-threose using Fischer projection formulas, it is seen that two pairs of mirror images are given. That is, the four aldotetroses constitute two pairs of enantiomers ... 

The Fischer projection formula for D-glucose (Example 2.3) is also known as the open- or straight-chain structure. This structure occurs only in solution. There are two crystalline forms of D-glucose, known as a and /3, which also have different optical activities when dissolved. X-ray diffraction studies have confirmed chemical evidence that a- and /3-D-glucose are structures containing a ring of five carbon atoms and one oxygen atom ... 

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 ... 

Do not rotate a Fischer projection formula in the plane of the page, because you might inadvertently convert a compound into its enantiomer. When using Fischer projections it is usually best to convert them to structures with wedges and dashes, and then manipulate them. Although a Fischer projection formula can be used for the stereogenic center in any compound, it is most commonly used for monosaccharides. 

Figure 6 Structures of the four stereoisomers of sphingosine. Sphingosine has two chiral carbon atoms (C-2 and C-3). The Fischer projection formula of each structure is also shown, with C-1 at the top, to illustrate the D/L and erythro/threo stereochemical nomenclature. C-3 has an erythro orthreo configuration as it relates to C-2, depending on whether the similar groups (amino and hydroxy) are on the same or opposite side of the Fischer projection. D versus L refers to the configuration at C-2 relative to the configuration of D-glyceraldehyde versus L-glyceraldehyde.

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