Omura-Sharma-Swern oxidation

Omura-Sharma-Swern Oxidation (TFAA-Mediated Moffatt Oxidation)... 

Alcohols react with compound 22 at low temperature in ca. 30 min, yielding an alkoxydimethylsulfonium salt 24 and one equivalent of trifluoroacetic acid. This mixture is normally stable at room temperature for several days. Nonetheless, alkoxydimethylsulfonium salts, derived from alcohols whose radicals are able to stabilize carbocations—particularly allylic and benzylic alcohols—suffer solvolyses by the action of trifluoroacetic acid from 0°C to room temperature, already in the absence of an amine, yielding the corresponding trifluoroacetates. This differential stability of alkoxydimethylsulfonium salts, derived from diverse alcohols, dictate different protocols in the Omura-Sharma Swern oxidation depending on the alcohol (vide infra). 

Most Omura-Sharma-Swern oxidations are performed in CH2CI2, although other apolar solvents, like toluene,125 can be equally effective. 

The nature of the amine, used for the decomposition of the alkoxydi-methylsulfonium salt, has a great influence in the yield of the aldehyde or ketone. Swern et al. proved1220 that best yields are obtained with hindered amines, like Hiinig s base (EtN/- Pr2). Nevertheless, most Omura-Sharma Swern oxidations are performed using Et3N instead of Hiinig s base, although... 

Interestingly, it is possible to perform an in situ addition of a Grignard reagent to a carbonyl compound, obtained by the Omura-Sharma-Swern oxidation. 

A 99% yield of ketone is obtained via an Omura-Sharma-Swern oxidation, while Dess-Martin periodinane delivers a 73% yield. 

As expected, acid sensitive functionalities, including THP,135 Tr,136 TBS137 and t-Bu138 ethers, orthoesters,139 acetals140 and glycosides,137a 141 as well as Boc-protected142 amines, are resistant to Omura-Sharma-Swern oxidations. 

After a substantial exploratory chemistry involving other oxidants, such as Swern, Ac20/DMS0, NaOCl, Al(Or-Bu)3/acetone, 5% TPAP/NMO and P205/DMS0, it was found that an Omura-Sharma-Swern oxidation was unique providing a 88% yield of the desired ketone, with no interference from the unprotected primary amine. 

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