Acetals and Ketals as Protecting Groups

In an organic synthesis, it sometimes happens that one of the reactants contains a functional group that is incompatible with the reaction conditions. Consider, for example, the conversion 

It looks as though all that is needed is to prepare the acetylenic anion CH3CCH2CH2C=C , then alkylate it with methyl iodide (Section 9.6). There is a complication, however. The carbonyl group in the starting alkyne will neither tolerate the strongly basic conditions required for anion formation nor survive in a solution containing carbanions. AcetyUde ions add to carbonyl groups (Section 14.6). Thus, the necessary anion is inaccessible. 

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