Diastereomers Fischer projections

Relative to each other both hydroxyl groups are on the same side m Fischer pro jections of the erythrose enantiomers The remaining two stereoisomers have hydroxyl groups on opposite sides m their Fischer projections They are diastereomers of d and L erythrose and are called d and l threose The d and l prefixes again specify the con figuration of the highest numbered chirality center d Threose and l threose are enan tiomers of each other... 

Organic chemists use an informal nomenclature 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 molecule is described as the erythro diastereomer. When like substituents are on opposite sides of the Fischer projection, the molecule is described as the threo diastereomer. Thus, as seen in the... 

Aldopentoses have three chirality centers. The eight stereoisomers are divided into a set of four D-aldopentoses and an enantiomeric set of four L-aldopentoses. The aldopentoses are named ribose, arabinose, xylose, and lyxose. Fischer projections of the d stereoisomers of the aldopentoses are given in Figure 25.2. Notice that all these diastereomers have the same configuration at C-4 and that this configuration is analogous to that of D-(+)-glyceraldehyde. 

Given Fischer projections of two isomers, tell their relationship (for example, same structure, enantiomers, diastereomers). 

Use the Fischer projection of meso-tartaric acid and carry out even and odd exchanges of the groups follow these exchanges with a model. Does an odd exchange lead to an enantiomer, a diastereomer, or to a system identical to the meso form (15a) Does an even exchange lead to an enantiomer, a diastereomer, or to a system identical to the meso form (15b) ... 

Aldohexose 14, written with its primary alcohol group at the top of the chain and its aldehyde group at the bottom, also produces diacid 13 on oxidation with nitric acid. A rotation of 180° in the plane of the page (recall that such a rotation of a Fischer projection does not change the configuration of the compound) puts 14 in the more common form with the aldehyde group at the top of the chain. When drawn this way, it is apparent that 14 is an L-aldohexose and is a diastereomer of 7. 

Summary Fischer Projections andTheir Use 201 Diastereomers 201 Summary Types of Isomers 203 5-12 Stereochemistry of Molecules withTwo or More Asymmetric Carbons 204 5-13 Meso Compounds 205 5-14 Absolute and Relative Configuration 207 5-15 Physical Properties of Diastereomers 208 5-16 Resolution of Enantiomers 209 EssentialTerms 213 Study Problems 215... 

Erythrose is the aldotetrose with the OH groups of its two asymmetric carbons situated on the same side of the Fischer projection, and threose is the diastereomer with the OH groups on opposite sides of the Fischer projection. These names have evolved into a shorthand way of naming diastereomers with two adjacent asymmetric carbon atoms. A diastereomer is called erythro if its Fischer projection shows similar groups on the same side of the molecule. It is called threo if similar groups are on opposite sides of the Fischer projection. 

Diastereomers having similar groups on the same side (erythro) or on opposite sides (threo) of the Fischer projection. This terminology was adapted from the names of the aldotetroses... 

Problem 26.4 Draw the Fischer projection of a threonine diastereomer, and label its chirality centers as iJ or S (Problem 26.3). 

Two common prefixes used to distinguish diastereomers are threo and erythro. When drawn in the Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sites. 

Notice that the synthesis leads to a pair of C-2 epimers because the first step of the reaction converts the carbonyl carbon in the starting material to an asymmetric carbon. Therefore, the OH bonded to C-2 in the product can be on the right or on the left in the Fischer projection. The two epimers are not obtained in equal amounts, however, because the first step of the reaction produces a pair of diastereomers and diastereomers are generally formed in unequal amounts (Section 5.19). 

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