Fischer projection rules

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. 

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). 

This is our Fischer projection. All of the configurations can be seen on this drawing, because we are able to picture in our minds what the 3D shape is. So the rule is that all horizontal lines are coming out at us, and all vertical lines are going away from us ... 

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)-... 

When naming (R) and (S) from Fischer projections with the hydrogen on a horizontal bond (toward you instead of away from you), just apply the normal rules backward. 

In applying the Cahn, Ingold, and Prelog RjS system the compound is oriented in a Fischer projection and the four groups or atoms bound to an asymmetric carbon are ranked by the following set of sequence rules (Fig. 8) ... 

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... 

Knowing the two rules provides a way to see if two projections reproi sent the same or different enantiomers, For example, three different Fischer projections of 2-butanol follow. Do all represent the same enantiomer, or is one different ... 

There are further rules for the use of Fischer projections (a) where relevant the vertical direction is that of the carbon chain (b) in the vast majority of cases the carbon in the highest oxidation state is located at the top of the diagram. 

Hudson lactone rule. The value of the rotation of aldonic acid lactones is decisively affected by the configuration of that carbon atom whose hydroxyl group is engaged in the cyclization. If, in the normal Fischer projection formula, the lactone ring is written on the right, the lactone is dextrorotatory if it is written on the left, the lactone is levorotatory. 

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