Stereoisomerism Fischer projections

Fischer projections of the stereoisomeric 2 3 dihydroxybutanoic acids compounds I and II are erythro stereoisomers and III and IV are threo... 

Draw Fischer projections or make molecular models of the four stereoisomeric 3 ammo 2 butanols and label each erythro or threo as appropriate... 

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

Of the eight stereoisomeric aldopentoses Figure 25 2 shows the Fischer projections of the d enantiomers (d ribose d arabinose d xylose and D lyxose) Likewise Figure 25 2 gives the Fischer projections of the eight D aldohexoses... 

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. 

Fischer Projections Stereoisomerism in Cyclic Compounds Methods of Determining Absolute Configuration Asymmetric Synthesis... 

Organic chemists nse an informal nomenclatnre system based on Fischer projections to distinguish between diastereomers. When the carbon chain is vertical and like substituents are on the same side of the Fischer projection, the molecnle is described as the erythro diastereomer. When like substitnents are on opposite sides of the Fischer projection, the molecule is described as the threo diastereomer. Thus, as seen in the Fischer projections of the stereoisomeric 2,3-dihydroxybntanoic acids, compounds I and II are erythro stereoisomers and III and IV are threo. 

PROBLEM 7.19 There are two other stereoisomeric tartaric acids. Write their Fischer projections, and specify the configuration at their stereogenic centers. 

Fischer projection formulas can be used to represent molecules with several stereoisomeric centers, and they are commonly used for carbohydrate molecules. For other types of structures, a common practice is to draw the molecule in an extended conformation with the main chain horizontal. In this arrangement, each tetrahedral carbon has two additional substituents, one facing out and one in. The orientation is specified with solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

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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