Acetals and thioacetals are used for the protection of hydroxy groups and thiol groups, the same as for carbonyl groups of aldehydes and ketones. The protecting groups are introduced by the reaction of alcohols or thiols with carbonyl compounds under acidic conditions. Acetals are stable under basic conditions and reductive conditions, and are inert against nucleophiles such as organometallic reagents. The deprotections are carried out by hydrolysis under acidic conditions. Thioacetals are usually stable in both acidic and basic aqueous solutions, and deprotected by hydrolysis with the assist of mercury salts.
In addition, it is known that dithiotosylates can be used for converting active methylene compounds into cyclic dithioacetals and the given cyclic dithioacetals are further transformed into carbonyl groups by hydrolysis. Also, the starting methylene groups are given by reductive desulfurization.1,2)
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