Acylation at nucleophilic carbon other than enols and enolates

Acylation at nucleophilic carbon (other than enols and enolates) 

We should not leave the subject of acylation at carbon without considering a problem that affects all such reactions to some degree. It can be understood most easily if we imagine some functional group Z that is able to stabilize a carbanion, and the acylation of that car-banion with an acid chloride—something like this. 

All looks well until we consider what might happen to the product under the reaction conditions. It too can form an anion, and a very stable one at that, because, not only is it stabilized by Z, but it is also an enolate. 

Since this anion is more stable (less basic) than the original anion, if there is an equilibrium between the two carbanions in the reaction mixture, the original carbanion will be sufficiently basic to act as the base that removes the proton from the product. 

instead of being acylated, the starting anion is protonated. This side-reaction could reduce the maximum possible yield in the acylation reaction to 50% half the starting material forms the product by acylation, while the other half simply deprotonates the product. How is this to be avoided  

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