Hydroxy Ketone TMs The Dithiane Anion

Disconnection of an a-hydroxy ketone or aldehyde also results in an illogical synthon that requires a nucleophilic carbonyl carbon. 

One method that achieves the required umpoiung is using a 1,3-dithiane anion as a synthetic equivalent of the acyl anion (the Henry-Nef strategy using a nitroalkane anion offers an analogous approach, but it will not be discussed here). While the acyl anion is unstable and cannot be prepared, the 1,3-dithiane anion is stable due to the inductive withdrawal of electron density by the two sulfur atoms. The 1,3-dithiane anion can be used as a nucleophile and then converted to a carbonyl by hydrolysis. 

3-Dithiane Anion is the Synthetic Equivalent of an Acyl Anion 

To make a 1,3-dithiane anion, an aldehyde is first converted to a thioacetal by reaction with 1,3-propanedithiol and a Lewis add, such as BF3 (the formaldehyde derivative, called 1,3-dithiane, is commercially available). The resulting thioacetal is then deprotonated with a strong base, such as n-butyllithium. 

Reaction of a 1,3-dithiane anion nucleophile with a ketone or aldehyde electrophile, followed by a mercury-assisted hydrolysis, affords an a-hydroxy ketone. 

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