Dioxiranes Rubottom oxidation

The highly potent antithrombotic (+)-rishirilide B was synthesized in the laboratory of S.J. Danishefsky. One of the tertiary alcohol functionalities was introduced via the Rubottom oxidation of a six-membered silyl dienol ether with dimethyl dioxirane (DMDO). The oxidation was completely stereoselective, and it was guided by the proximal secondary methyl group. Subsequently, the enone was converted to the enedione, which was used as a dienophile in the key intermoiecuiar Dieis-Aider cycioaddition step. 

Many variations of the Rubottom oxidation employ oxidants other than m-CPBA in order to execute the transformation under mild conditions or to allow for enantioselective synthesis. Use of dimethyl dioxirane (DMDO) for the oxidation of enolsilanes has become a popular alternative to traditional conditions for Rubottom oxidations. This mild oxidant has been used to facilitate the isolation of 2-silyloxyoxiranes, which are stable under the essentially neutral reaction conditions." For example, treatment of 26 with DMDO at -40 °C afforded 27 in 99% yield.1 Ib These compounds can subsequently be converted to 2-hydroxyketones, as described above, or can be used in other transformations.12 Chiral dioxiranes generated in situ from chiral ketones and oxone have also been employed in enantioselective Rubottom oxidations developed independently by Shil3a and Adam.13b As shown above, enolsilane 28 was transformed to a-hydroxyketone 29 in 80% yield and 90% ee.l3a... 

In addition to dioxiranes, a wide variety of other oxidants have also been employed in Rubottom oxidations including /-butyl hydroperoxide, oxone, and benzoyl peroxide.2 However, use of these other oxidants is much less common than use of tw-CPBA or DMDO. Chiral Mn(salen)complexesI4a and the Sharpless asymmetric dihydroxylation system14b have also been used in asymmetric versions of the Rubottom oxidation. 

Silyl enol ethers are a class of electron-rich, nonaromatic compounds that easily form reactive radical cations on one electron oxidation. The silyl enol ether functional group is closely related to the carbonyl function and consequently, syntheses of silyl enol ethers generally make use of enolates. In addition, silyl enol ethers can be described as masked enols or enolates since their reactions often yield ketones. A number of oxidation reactions of silyl enol ethers making use of oxygen or oxygen-containing reagents such as peroxides, peracids (known as Rubottom oxidation), dioxirane, osmium tetraoxide, or triphenyl phosphite ozonide have been described in the literature. In all cases either a-hydroxy-ketones or the silyl enol ether epoxides are formed. 

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