Peroxides reacting with enones

Isoxazolines 79, obtained from aromatic nitrile oxide cycloadditions to cyclohex-2-enone, reacted with nickel peroxide to give 3-aryl-6,7-dihydro[l] benzoisoxazol-4(5// )-ones 80. In contrast, the corresponding 2-bromocyclohex-2-enone underwent nitrile oxide cycloaddition, followed by dehydrobromination, to afford the regioisomeric 3-aryl-4,5-dihydro[l]benzoisoxazol-7(6//)-ones 81 (Scheme 1.23) (242). 

Cycloaddition of 5,6-dihydropyran-2-one with aromatic nitrile oxides leads to 3-aryl-3a,6,7,7a-tetrahydropyrano[3,4-d]isoxazol-4(47/)-ones 98. The latter react with nickel peroxide to give the corresponding dihydropyranoisoxazolones 99. Similar to 2-bromocyclohex-2-enone, 3-bromo-5,6-dihydropyran-2-one undergoes nitrile oxide cycloaddition, followed by dehydrobromination, to form regioi-someric 3-aryl-5,7-dihydropyrano 4,3-c/ isoxazol-7(4//)-ones 100 (Scheme 1.24) (242). 

Treatment of enones with basic tert-butyl hydroperoxide provides an alternative route for epoxidation of enones when the alkaline hydrogen peroxide procedure fails. For example, the enone shown below did not react with alkaline hydrogen peroxide but underwent chemo- and stereoselective oxidation with tert-h xty hydroperoxide in the presence of the base trimethylbenzylammonium hydroxide (Triton-B). To avoid a 1,3-diaxial interaction with the angular methoxymethyl substituent, the hydroperoxide anion attacked the enone from the a-face of the molecule. 

Reaction with enones and quinones. For an a,/3-unsaturated ketone to react with hydrogen peroxide a basic medium is required, the probable function being to... 

In contrast with metal-complex catalyzed transformations [52], enantioselective organocatalyzed intermolecular conjugate additions of O-nudeophiles seem to be limited to peroxides such as hydrogen peroxide or tert-butyl hydroperoxide. In these reactions the primary addition product, a / -peroxy enolate, reacts further to yield an epoxide (Scheme 4.31). Consequently, reactions of this type are covered in Section 10.2 Epoxidation of Enones and Enoates . 

Chemical reactions on - and Z-isomers usually give the same type of product, though often with different stereochemistry. The two geometrical isomers may also react at very different rates. For example, the reaction of these conjugated - and Z-enones with alkaline hydrogen peroxide gives in each case an epoxide, but with different stereochemistry and at very different rates. 

The two ylids react differently with 2,6,6-trimethylcyclo-2,4-heptadienone as shown. Write a mechanism for each process, and suggest a reason for the difference. (You may find it helpful to think about the mechanism of epoxidation of enones with the anion of hydrogen peroxide.)... 

---