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158 CHAPTER 5 
 
 
 
 
Now we look at the configuration of the chiral center in the bottom right corner of each drawing above (highlighted in 
gray). Notice that they have opposite configuration. This is the necessary feature that enables this compound to be 
accessible from either 1D or from 1L. Here is another example: 
 
 
 
Once again, this stereoisomer will be accessible from either 1D or from 1L. In contrast, the first six structures (in the 
answer to part c) do not have this feature. For example, consider the first structure: let’s draw it, rotate it 180 degrees, 
and then inspect the configuration in the bottom right corner of each drawing: 
 
 
 
Note that in this case, the configuration in the bottom right corner of each drawing of this structure is the same. 
Therefore, this stereoisomer can ONLY be made from 1D. It cannot be made from 1L. A similar analysis for the first 
six stereoisomers (in the answer to part c) shows that all six of these stereoisomers require a specific enantiomer for the 
starting material. Only the last four stereoisomers can be made from either 1D or from 1L. 
 
 
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