Problems and exercisesAcetal Formation

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1. An alkyne with a ketone

How would you plan the synthesis of the alkyne in the figure (5-heptyn-2-one)?

Structure of 5-heptyn-2-one: CH₃–C≡C–CH₂–CH₂–CO–CH₃

Hint: use the protection and deprotection of the C=O group.

2. Hydrolysis of acetals

What happens to the following acetals when they are treated with a large excess of water in basic medium? And in acid medium?

Three acetals: the diethyl acetal of 4-ethoxybenzaldehyde, the cyclic ethylene glycol acetal of cyclohexanone and cis-2-methoxy-5-methyltetrahydrofuran

3. Grignard and a protected aldehyde

How would you synthesize 5-hydroxy-5-phenylpentanal using Grignard or organolithium reagents?

4. Protecting alcohols with DHP

DHP or dihydropyran (oxacyclohex-2-ene) reacts with alcohols in acid to give an acetal. This is a method for protecting alcohols. Can you propose mechanisms for the formation of the acetal (protection of the alcohol) and for its cleavage in aqueous acid (deprotection of the alcohol)?

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