Oxygen reaction with silyl dienol ethers

While the reaction of singlet oxygen with silyl enol ethers was governed by competing prototropic and silatropic ene processes (see Section 2.3.2.1.3.ii), the interaction with dienol ethers displays a different mode of reactivity. Singlet oxygen generated from triphenyl phosphite ozonide at low temperature... 

This enolate is more stable than the isomeric 1,3-enolate in which the O substituent is in the 2-position three resonance forms can be written for the former enolate while only two can be written for the latter. Me3SiCl then reacts with the dienolate A at the oxygen atom. The dienol silyl ether C is obtained in this way. Silyl ether C reacts with MeLi—in analogy to the reaction B — A shown in Figure 13.19—via the silicate complex B to give the desired enolate. Note that the product enolate is not accessible by treatment of cyclohexanone with LDA (Figure 13.14). 

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