Rules of the Fischer Projection

Introduction of a chiral center will result in stereoisomers the number of stereoisomers is 2 where n corresponds to the number of the chiral atoms. Before their real structure was known. 

FIGURE 2.1 Formation of polyhydroxy-aldehydes and ketones from formaldehyde. 

FIGURE 2.2 Carbohydrate derivatives with different oxidation states. 

If more than one stereogenic center is present in the molecule, then the reference atom should have the highest number of stereogenic centers that is, it should be the most distant from the carbonyl group. 

FIGURE 2.5 Different drawings of D- and L-glucose in the Fischer projection = stereogenic reference center. 

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Rules of the Fischer Projection Trivial and Systematic Names Absolute and Relative Configuration... 

Rules of the Fischer Projection Mutarotation The Haworth Projection The Mills Projection The Reeves Projection Conformations of the Six-Membered Rings Conformations of the Five-Membered Rings Conformations of the Seven-Membered Rings Conformations of Fused Rings Steric Factors... 

The (I ,S)-nomenclature still reminds the user of the right and left handed helical pattern arising from Fresnel s 29> interpretation of optical activity. These patterns are characterized by the combination of a translational and a rotational direction. The Ta skeletal symmetry of tetracoordinate systems submits itself to the pictorial models not applicable to other configurational types. The CIP rules may as well be used to define a configurational nomenclature on the basis of the Fischer projection. If one specified that in such a projection of an (R)-... 

A consequence of the Fischer projection rules is that such formulae must not be rotated 90°, because this would mean an inversion of the configuration of the stereogenic center, but they can be rotated 180° without altering the configuration (double inversion = retention) (see Figure 2.4). 

As one of the empirical rules applicable to determination of the configuration of the asymmetric carbon linked with four different atoms, B, C, andZ), Brewster [11] proposed that a tetrahedral system of the Fischer projection has a positive optical rotation when the polarizability of the four atoms decreases in the order A> B> C> D. 

To assign Ihe configuration of each chirality center, we can use the rule of thumb that was pointed out in the solution to Problem 24.3. Specifically, an OH on the right side of the Fischer projection indicates the R configuration, while an OH on the left side of the Fischer projection indicates the S configuration ... 

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 answer is D. The given structure is an ethane derivative, not a methane derivative. This rules out Choices A and B. In the Fischer projection, the horizontal lines represent the bonds extending out of the plane of the paper toward you. The vertical lines represent the bonds pointing into the plane of the paper (away from you). With this in mind, we can start the switching of groups as follows ... 

Since the Cahn-Ingold-Prelog and the Fischer conventions are based on different principles, there is no direct correlation between them. That d-glyceraldehyde (Fischer convention) corresponds to 2 -glyceraldehyde (sequence rule) is only coincidence. One need only examine the case of cysteine to be convinced that there is no direct correspondence. The Fischer projection of L-cysteine is shown ... 

CH2—CHR-, and - CH2—CHR—O-. Without going any further into individual types of conformations at this stage, it need only be mentioned that the Fischer projection corresponds to an eclipsed, and the Natta projection to a staggered conformation. Newman projections can represent both eclipsed and staggered conformations. As a rule, trans-staggered and as-eclipsed conformations are used as the basis of such projections. (For the significance of these expressions see Section 4.5.)... 

The specification of configurations in diastereomeric species is quite simple, with each chiral center being designated R or S according to the sequence rules when the Cahn-Ingold-Prelog convention is used. An extension of the Fischer convention to systems with more than one asymmetric center that is based on carbohydrate structures and terminology is still used in relatively simple cases. This convention can be illustrated with the same stereoisomeric 2,3,4-trihydroxybutanals just discussed. The 2R,3R and 25,35 isomers are d- and L-erythrose, respectively. The 25,3/ and 2i ,35 isomers are d- and L-threose, respectively. The Fischer projection formulas are shown below ... 

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