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CHAPTER 7 223
 
7.86. In this target molecule, the only bond we know how to make (based on reactions covered thus far) is the one 
next to the cyano group. 
 
 
Cyanide is a good nucleophile, and a primary substrate is ideal for an SN2 process, so these starting materials would 
lead to a good synthesis of 4-fluorophenylacetonitrile. The forward synthesis is shown here. 
 
 
 
7.87. When comparing the starting compound to the target molecule, we note that the ester group is new, and the 
configuration of the adjacent chiral center has been inverted. The latter observation indicates that an SN2 process must 
be involved. So our retrosynthesis should focus on disconnection of the following bond: 
 
 
 
We need a substrate with the opposite configuration (the leaving group should be on a wedge, rather than a dash), as 
shown here: 
 
 
 
This substrate must be made from the starting alcohol: 
 
 
 
The best way to convert the OH group into a leaving group and retain its stereochemistry is with the use of tosyl 
chloride to make the tosylate. This tosylate can then function as the substrate in an SN2 reaction. The forward 
synthesis is shown here: 
 
 
The use of a secondary substrate (for the SN2 process) is reasonable in this case, since the nucleophile involved is not a 
strong base (the resonance-stabilized carboxylate anion is the conjugate base of a relatively acidic carboxylic acid), so 
there is little competition with E2. Backside attack of the nucleophile results in inversion of stereochemistry to give the 
desired target molecule. 
 
7.88. Disconnection of bond a is a logical retrosynthesis that leads to a familiar nucleophile (an alkoxide ion) and a 
methyl substrate (ideal for SN2): 
 
 
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