Electrophilic epoxidizing agent

The electrophilic epoxidizing agents such as m-CPBA, which you met in Chapter 20, are less good with electron-deficient alkenes we need a nucleophilic epoxidizing agent instead. There is another sig-... 

Posner and coworkers proposed that the highly electrophilic epoxide could not be isolated due to its inherent instability but was a potent alkylating agent responsible for parasite death. However, Wu and coworkers have isolated a small quantity (1-2% yield) of the epoxide 16 in their iron(II) degradations of artemisinin using iron(II) sulphate in aqueous acetonitrile s. These authors conclude that it is not the active killing species since any external nucleophiles would have to compete with the in-built nucleophile (the OH moiety). Moreover, Avery and coworkers also concluded that the epoxide could not... 

While enol acetates from saturated ketones were useful a-oxygenadon substrates, the corresponding dienol acetates are not. The relatively electron-deficient alkene bearing the acetoxy group is less attractive to electrophilic oxygenating agents than the unsubstituted double bond. Thus, for example, peracid treatment leads to epoxidation of die unfunctionalized alkene. However, it would seem likely that re-... 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

---