Chirality centers naming compounds with

C HOW TO Name Compounds with More Than One Chirality Center... 

How To Test for Chirality Planes of Symmetry 201 5.7A How To Assign (R) and (S) Configurations 202 5.12A How To Draw Stereoisomers for Molecules Having More Than One Chirality Center 218 5.12C How To Name Compounds with More Than One Chirality Center 222... 

Given the importance of stereochemistry in reactions between biomolecules (see below), biochemists must name and represent the structure of each biomolecule so that its stereochemistry is unambiguous. For compounds with more than one chiral center, the most useful system of nomenclature is the RS system. In this system, each group attached to a chiral carbon is assigned a priority. The priorities of some common substituents are... 

Diastereoisomers. Whereas compounds with one chiral center exist as an enantiomorphic pair, molecules with two or more chiral centers also exist as diastereoisomers (diastereomers). These are pairs of isomers with an opposite configuration at one or more of the chiral centers, but which are not complete mirror images of each other. An example is L-threonine which has the 2S, 3R configuration. The diastereoisomer with the 2S, 3S configuration is known as i-a//o-threonine. L-isoleucine, whose side chain is -CH(CH3) CH2CH3, has the 2S, 3R configuration. It can be called 2(S)-amino-3(R)-methyl-valeric acid but the simpler name L-isoleucine implies the correct configuration at both chiral centers. 

The name of the compound is (/ )-2-bromopentane. Reaction of (5)-2-pentanol with PBr3 to form (R)-2-bromopentane occurs with a change in stereochemistry because the configuration at the chirality center changes from S to R. 

The trivial names of the aldoses can be used as prefixes to determine the stereochemistry for other chiral compounds. However, in this case, the -ose suffix is omitted and the prefix is written in italics (Figure 2.8). In this respect, it should be mentioned that the chiral centers can be separated by one or more nonchiral centers. If the number of chiral centers exceeds four (e.g., in heptose, octose, nonose, etc. derivatives), a multiple configurational prefix is added to the stem name (Figure 2.9). In this case, the first four chiral centers are selected, followed by the remaining ones. However, the name of the compound starts with the prefix of the chiral center with the highest location. In the case of ketoses, if the carbonyl group is not at C-2, then its location should also be given in the name. 

In some cases, particularly natural products with many chiral centers, it is convenient to name the mirror image of a substance having a trivial or common name by simply indicating that the structure is the enantiomer (en t-) of the natural compound. The enantiomer of cholesterol (67) is therefore... 

Stereoisomers (a) and (b) are nonsuperposable mirror images and are, therefore, a pair of enantiomers. Stereoisomers (c) and (d) are also nonsuperposable mirror images and are a second pair of enantiomers. One way to describe the four stereoisomers of 2,3,4-trihydroxybutanal is to say that they consist of two pairs of enantiomers. Enantiomers (a) and (b) of 2,3,4-trihydroxybutanal are given the names (2R,3R)-erythrose and (2S,3S)-erythrose enantiomers (c) and (d) are given the names (2R,3S)-threose and (2S,3R)-threose. Note that all of the chiral centers in a molecule are reversed in its enantiomer. The molecule with the 2R,3S configuration is the enantiomer of the molecule with 2S,3R, and the molecule with 2S,3S is the enantiomer of the molecule with 2R,3R. Erythrose and threose belong to the class of compounds called carbohydrates, which we discuss in Chapter 25. Erythrose is found in erythrocytes (red blood cells), hence the derivation of its name. 

As with all compounds, when a chirality center is present, the configuration is indicated at the beginning of the name for example ... 

Draw the structures of eight different carboxylic acids with molecular formula C6H12O2. Then, provide a systematic name for each compound, and identify which three isomers exhibit chirality centers. 

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