Epoxidations using selenium dioxide

With the two methyl stereocenters successfully set, we next focused on installing the secondary C4-hydroxyl group. Using selenium dioxide, we were able to convert the allylic methyl ether in one step into an enal (174), which we then reduced with diisobutylaluminum hydride (175, Scheme 32). This allylic alcohol could be epoxidized with /m-CPBA, thus ensuring installation of the desired C4-hydroxyl group stereocenter under substrate-controlled conditions... 

It is well known that other non-metal oxides can react with hydrogen peroxide to form similar compounds which can be viewed as inorganic peracids. Such species include boron(III), arsenic(III) and selenium(IV). For example, selenium dioxide can be used as a catalyst for epoxidations or amine oxidations through perselenous acid (Figure 2.29).86... 

Transition metal catalysts not only increase the reaction rate but may also affect the outcome of the oxidation, especially the stereochemistry of the products. Whereas hydrogen peroxide alone in acetonitrile oxidizes alkenes to epoxides [729], osmic acid catalyzes syn hydroxylation [736], and tungstic acid catalyzes anti hydroxylation [737]. The most frequently used catalysts are titanium trichloride [732], vanadium pentoxide [733,134], sodium vanadate [735], selenium dioxide [725], chromium trioxide [134], ammonium molybdate [736], tungsten trioxide [737], tungstic acid [737],... 

Radical epoxidation. In the presence of this radical cation, iodosylbcnzcnc can effect epoxidation of stilbene, but only in low yield because of decomposition of the catalyst. Selenium dioxide (Se02) is a suitable oxidant for this catalyzed epoxidation, but suffers from low solubility in CH2CI2 at 0°. The most useful... 

Allylic oxidation. Selenium dioxide has been widely used for allylic oxidation of olefins, but this reaction is often troublesome because of formation of selenium-containing by-products and colloidal selenium, which are not easily eliminated. The combination of Se02 with H2O2 to reoxidize selenium species to Set) , has been used to convert olefins into diols and epoxides (2, 362), but this iiicIIuhI is not general for allylic oxidations. Umbreit and Sharpless have effected allylic oxidations with an excess of /-butyl hydroperoxide (90"i.) in combination... 

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