Fischer projections manipulation

As you work with Fischer projections, you may notice that some routine strrrctural changes lead to pr edictable outcomes—outcomes that may r educe the number of manipulations you need to do to solve stereochemistry problems. Instead of listing these shortcuts, Problem 7.10 invites you to discover some of them for yourself. 

Follow the steps in the text. (1) Assign priorities to the four substituents on the chiral carbon. (2) Manipulate the Fischer projection to place the group of lowest priority at the top by carrying out one of the allowed motions. (3) Determine the direction 1 —> 2 — 3 of the remaining three groups. 

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. 

Keeping in mind the three-dimensional properties of molecules, Newman projections can be converted to wedged-dashed structures or Fischer projections as desired. It is important to develop facility for manipulating structures and... 

Manipulate the Fischer projection in part a in the same manner. In this case, the configuration is S. 

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) ... 

Manipulate the Fischer projection such that this hydroxyl is at the bottom after exchange with the terminal (C6) functional group. 

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. 

Note the a-glucosidase action will not allow us to specify the stereochemistry at the anomeric carbon of the pentose. Manipulation of the Fischer projection will allow us to draw the Haworth structure of the disaccharide ... 

The answer is C. The given representation is called Fischer projection. The Fischer projection is a 2-dimensional way of representing structures. To manipulate the Fischer projection, we should think in terms of 3-D. This can be done by interchanging two pairs of substituents at a time. If only... 

Having achieved this conversion, you can change one Fischer projection into another of the same molecule by using certain manipulations rotations and substituent switches. However, we shall see next that care has to be taken so you dou t inadvertently interconvert R and S configurations. 

Either by hand or by computer, it s reasonably easy to draw and manipulate real stereochemical representations of molecules with two chiral centers, sometimes even with three. However, many biological molecules have many more chiral centers than this, and Fischer devised a way of representing these in a flat form, in which it is easy to see stereochemical relationships and, particularly, to identify meso-compounds. Fischer projections consist of a cross motif, 7.89, and in this, groups b and d are pointing toward us, out of the paper, while groups a and c point away from us, behind the paper. 

Textbooks offer many rules for the manipulation of Fischer projections, but we already know the only one we need—any pairwise swap, at a single asymmetric carbon atom, reverses the chirality. Thus, two swaps will get us back to the same configuration we started with, and Figure 7.16 shows how we can prove that 7.93 and 7.95 are identical. While a few of you will be able to look at 7.93 and 7.95 and say it s obvious that they are the same, most find the mechanical process both more comfortable and more reliable. 

To determine the absolute configuration of a molecule from its Fischer projection, the projection must be manipulated (switches giving opposite enantiomers) so that the lowest priority group is at the top of the diagram. 

Fischer projections are a method of showing stereochemistry and particularly the relationship between chiral centers, with uncomplicated drawings. Thus, 16.46a is represented as 16.46b. For students with good spatial perception, some of the relationships described will be obvious, but this will not be true for everyone. If you can simply see what is happening, that s great—but for many people, a slow, mechanical, pedestrian approach, which will always work, is helpful. How can these projections be manipulated There are many rules in other textbooks, but we can make do with just one. Any pairwise swap reverses the absolute configuration at an asymmetric carbon atom. For those of you able to translate immediately between projection and three-dimensional structure, it may be obvious that 16.47a and 16.47c are identical, but the swapping system will let the rest of us catch up. 

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