Fischer projection, definition

The contrast between formulas 20 and 21, both pertaining to isotactic polyethylidene, should be noted This contrast occurs because the polymer repeating unit has only one carbon atom in the chain and thus there is no correspondence between such periodicity and that of the zigzag representation. The classical definition of an isotactic polymer (as one in which all substituents are on the same side of the chain) holds true, in general terms, only if the polymer is represented in the Fischer projection. Analogous considerations pertain to syndiotactic polyethylidene 22 and 23. 

Fischer projections are however, unsatisfactory when considering the physical properties and chemical reactivity of monosaccharides for which definitive spatial formulations are necessary. These are given below for D-glyceraldehyde, D-erythrose and D-threose, for which the (/ ,S configuration may be readily assigned at the appropriate chiral carbons. 

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). 

Establish the orientation of the asymmetric carbon created by hemiacetal or hemiketal formation. By definition, an a-anomeric OF is on the same side of the Fischer projection as the... 

Fischer projections are still essential for showing genetic relations between carbohydrates, but are definitely unsuitable for precise stereochemical deductions and reliance on them may have been responsible for some... 

Chiral, definition of, 260 Chiral axis. See Stereogenic axis Chiral center. See Stereogenic center Chiral drugs, 273 Chiral molecules, 259—263, 290 absolute configuration, 267, 292 Fischer projection formulas, 271-272, 278, 280, 292-293... 

Unfortunately, there is more than one definition for threo- and erythro. Winstein and co-workers gave the following definition of these diastereomers In a compound with two asymmetric carbons that has two common ligands and a third that differs, the isomers that would be meso if the third ligand were identical are erythro diastereomers.An alternative definition is If two asymmetric carbons have only one ligand in common, then the other four ligands are paired in the same commonsense way and isomers that would have equal pairs eclipsed in any conformation are erythro. Eliel, Mislow and their co-workers defined erythro and threo in terms of Fischer projections. The aldol products (see sec. 9.4.A) 68 and 69, and the ester-aldehyde condensation products 70 and 71.30 shown with the erythro and threo notation. 

An alternative definition specifies that the more highly oxidized terminal carbon atom be at the top of the Fischer projection. For carbohydrates and amino acids, there is ordinarily no conflict between these two definitions. For historical notes and an elaboration of the d and l nomenclature system, see Slocum, D. W. Sugarman, D. Tucker, S. P. /. Chem. Educ. 1971,48,597 reference 140a, pp. 88-92. 

A major 1981 lUPAC document (9) addresses polymer stereochemistry in depth. Key chapters of this document discuss basic definitions (configurational unit, configurational base and repeating units, stereorepeating imit, different types of tacticity, tactic block polymers, and stereoblock polymers) sequences conformations and supplementary definitions. Most of the illustrations use the rotated Fischer projections, but some three-dimensional representations are included. 

What happens when we rotate a Fischer projection in the plane of the page by 90° Does the result depict the spatial atraugement of the original molecule The definition of a Fischer projection—horizontal bonds are pointed above, vertical ones below the plane of the page—tells us that the answer is clearly no, because this rotation has switched the relative spatial disposition of the two sets The result is a picture of the enantiomer. On the other hand, rotation by 180° is fine, because horizontal and vertical lines have not been interchanged The resulting drawing represents the same enantiomer. 

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