Compounds with More Than One Stereogenic Center Diastereomers

Note that if the lowest-priority group is on a horizontal arm, you can still determine the absolute configuration of the stereocenter. This can be done by rotating three of the four groups so that the lowest-priority group is located on a vertical arm and then proceeding as above  

It can also be done by remembering that a substituent on a horizontal arm is pointing out at the viewer. If the sequence of the three priority groups 1— -2— 3 is counterclockwise, as in this case, the absolute configuration is R. (If the direction were clockwise, the absolute configuration would be S.) 

PROBLEM 5.16 Determine the absolute configuration of the following enantiomer of 2-butanol from its Fischer projection  

Fischer projections are used extensively in biochemistry and in carbohydrate chemistry, where compounds frequently contain more than one stereocenter. In the next section, you will see how useful Fischer projections are in dealing with compounds containing more than one stereogenic center. 

If Compounds with More Than One Stereogenic Center Diastereomers 

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The prefix 5a- or 5 - indicates the stereochemistry (trans or cis) of A-B ring fusion 5a-androstane and 5 -androstane, respectively. Coprostanol (5b-cholestan-3b-ol) and cholestanol (5a-cholestan-3b-ol), for example, are epimers that differ from each other at the C5 stereogenic center (Figure 1.8). Compounds with more than one stereogenic center (diastereomers) are discussed in more detail in section 1.3.3. 

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