Fischer’s convention

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

The merit of Fischer s convention is that it enables the systematic stereochemical presentation of a large number of natural products, and this convention is still useful for carbohydrates or amino acids today. Its limitations, however, become obvious with compounds that do not resemble the model reference compound glyceraldehyde. For example, it is very difficult to correlate the terpene compounds with glyceraldehyde. Furthermore, selection of the correct orientation of the main chain may also be ambiguous. Sometimes different configurations may even be assigned to the same compound when the main chain is arranged in a different way. 

Following these rules, D-glyceraldehyde 6 in Fischer s convention can be assigned an ( -configuration. 

D-Glyceraldehyde in Fischer s convention (f )-glyceraldehyde in the Cahn-Ingold-Prelog convention... 

L-amino acids are levorotatory, and the convention shown in Figure 3-4 was needed to avoid potential ambiguities about absolute configuration. By Fischer s convention, l and d refer only to the absolute configuration of the four substituents around the chiral carbon, not to optical properties of the molecule. 

The preceding formulas VII and XI to XVI have been written purposely without the use of d and I symbols for distinguishing the members of a pair of enantiomorphs in order to emphasize that the mechanical models and Fischer s conventional formulas for representing them are quite independent of any plan of nomenclature. Recognizing this independence, let us consider next the historical origin and development of systematic nomenclature for such models and stereo-formulas in the carbohydrate group. 

It is worthwhile to read Bijvoet s original paper, because it reinforces some ideas that are carried through this text. Bijvoet pointed out that "Fischer s convention. .. appears to answer to reality" (emphasis in the original paper), and he explicitly referred to the assignment of stereochemistry as a model. 

Absolute configuration (Section 9.5) The exact three-dimensional structure of a chiral molecule. Absolute configurations are specified verbally by the Cahn-Ingold-Prelog R,S convention and are represented on paper by Fischer projections. 

Hudson, C. S., Historical Aspects of Emil Fischer s Fundamental Conventions for Writing Stereo-formulas in a Plane, III, 1-22... 

In the formula for D-mannitol (XIV), by Emil Fischer s second convention,84 the hydroxyl on carbon atom 5 is placed on the right. Since D-arabitol (XV) is optically active, the hydroxyl on carbon atom 3 must then be on the left, for regardless of the configuration at carbon atom 4, arabitol would otherwise be an optically inactive meso form. Finally, by reason of the equivalent symmetry of D-mannitol, the... 

With conventional methods, the formation of indole derivatives from anilines proceeds at the expense of both unsubstituted ortho positions in the phenyl ring. This leads to undesirable by-products. Particularly, the formation of by-products takes place during Fischer s synthesis of benzohet-erocycles. In the previously described ion-radical variant of the synthesis, only one indole isomer is formed—the isomer that corresponds strictly to the structure of the starting haloaniline. 

The historical aspects of Emil Fischer s fundamental conventions for writing stereoformulas in a plane have been thoroughly delineated.2 It may be mentioned that the perspective formulas generally attributed to W. N. Haworth were actually a revival of J. Bdeseken s cyclic formulas. ... 

Know the meaning of stereogenic carbon atom, stereogenic center, R-S convention, priority order, E-Z convention, Fischer projection. 

In historical accuracy, it should be remembered that Fischer s original second convention was applied to the enantiomorphous saccharic acids as the reference substances or standards his formula for natural glucose (XV) followed as a sequel. It has now become customary to select the enantiomorphous forms of glyceraldehyde as the reference substances or "standards, from the stereo-formulas of which there follow... 

Ordinary (that is, dextrorotatory) tartaric acid (XX) corresponds to XXI and not to XXII, as is often stated erroneously in textbooks. Formula XXII applies to the enantiomorph of natural tartaric acid. Formulas XXI and XXIII are drawn in conformity with Fischer s first convention but formula XXII does not follow it. 

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