Rotations Fischer projection

Throughout this document, stereochemical formulae for polymer chains are shown as Fischer projections rotated through 90°, i.e., displayed horizontally rather than vertically,... 

FIGURE 7 9 Repre sentations of (2/ 3R) dihy droxybutanoic acid (a) The staggered conformation is the most stable but is not properly arranged to show stereochemistry as a Fischer projection (b) Rotation about the C 2-C 3 bond gives the eclipsed conforma tion and projection of the eclipsed conformation onto the page gives (c) a correct Fischer projection... 

To verify that the Fischer projection has the R configuration at its chirality center, rotate the three-dimensional r epresentation so that the lowest-ranked atom (H) points away from you. Be careful to maintain the proper stereochemical relationships during the operation. 

A 90° rotation breaks the Fischer convention by exchanging the groups that go into the plane and those that come out. In the following Fischer projections of (jR)-glyceraldehyde, the —H and -OH groups come out of the plane before rotation but go into the plane after a 90° rotation. As a result, the rotated projection represents (S)-glyceraldehyde. 

B A Fischer projection can have one group held steady while the other three rotate in either a clockwise or a counterclockwise direction. The effect is simply to rotate around a single bond, which does not change the stereochemistry. 

Fischer projection, 975-978 carbohydrates and, 977-978 D sugars, 980 i., sugars, 980-981 rotation of, 976 R.S configuration of, 977 conventions for, 975-976 Fishhook arrow, radical reactions and, 139, 240... 

Thus the trans relationship between the hydroxymethyl group and the C-l hydroxy group in a-D-glucopyranose, and the cis relationship between the methyl group and the C-l hydroxy group in P-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5. 

Fischer projection of acyclic form, 56-57 glycosides, 132-135 C-glycosyl compounds, 139-140 N-glycosyl derivatives, 137-139 glycosyl halides, 136-137 glycosyl residues, 125 isotopic substitution and isotopic labelling, 91 me so forms, 59 optical rotation, 59 parent structure choice, 53... 

The Fischer projections are two-dimensional representations of three dimensional objects. Further a Fischer projection may be rotated in the plane of paper by 180°, but not by 90° as illustrated in the following examples—... 

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)-... 

Enantiomers have very similar chemical properties, but they rotate polarized light in opposite directions (optical activity, see pp. 36,58). The same applies to the enantiomers of lactic acid. The dextrorotatory L-lactic acid occurs in animal muscle and blood, while the D form produced by microorganisms is found in milk products, for example (see p.l48). The Fischer projection is often used to represent the formulas for chiral centers (cf.p. 58). 

In this article the use of formula 3 will always refer to rotated Fischer projections. In any case, the formulas must represent a section of chain long enough to illustrate the structural features excluding, unless explicitly required, the terminal groups. This representation corresponds to the use of a macromolecular model with an infinite chain length. 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

The use of rotated Fischer projections corresponds to the practice of using horizontal lines to denote polymer backbone bonds, but it is most important to note that this does not give an immediately visual impression of the zigzag chain. In the projections as used in this document, at each individual backbone carbon atom the horizontal lines represent bonds directed below the plane of the paper from the carbon atom while the vertical lines project above the plane of the paper from the carbon atom. Thus, the rotated Fiseher projection... 

The use of rotated Fischer projections has been retained in the present edition in order to provide a link with, and an explanation of, the bulk of existing published polymer literature, although the present common practice [4] is to depict main-chain bonds in planar, extended zigzag (all-trans) conformations, together with a stereochemical representation of side-groups at tetrahedrally-bonded atoms. 

Usually in Fischer projections the earbon atoms of the main chain are omitted [4]. In general, in the area of maeromolecular chemistry, element symbols are not omitted in the backbone and are usually shown in the rotated Fischer projection [3]. 

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