12.14 : 乙缩醛和硫缩醛作为醛和酮的保护基团

Acetals can selectively protect aldehydes and ketones against different reaction conditions. They are inert towards bases, oxidizing and reducing agents, and nucleophiles.

Since the aldehyde more rapidly transforms to an acetal than the ketone, in a compound containing both these groups, if reduction of the ketone is desired, the more reactive aldehyde is first selectively masked, and then the ketone is reduced.

After that, the aldehyde can be deprotected under mildly acidic conditions to obtain the selectively reduced product.

Analogously, thioacetal protection and deprotection can also be explored for selective reduction of functional groups.

Thioacetals are similar to acetals, except they are also stable under acidic conditions and can protect the carbonyl group in acidic and basic environments.

Mechanistically, thioacetals follow the same three steps: selective protection, desired reduction, and finally, deprotection. 

In contrast to an acetal, which is hydrolyzed under mildly acidic conditions, a thioacetal, being inert to acids, is removed in the presence of mercuric chloride in aqueous acetonitrile.