Rearrangement, dienone-phenol

The driving force in the overall reaction (the dienone-phenol rearrangement) is of course creation of an aromatic system.136 It may be noted that 50 and 51 are arenium ions (p. 502), the same as those generated by attack of an electrophile on a phenol.137 Sometimes, in the reaction of a phenol with an electrophile, a kind of reverse rearrangement (called the phenol-dienone rearrangement) takes place, though without an actual migration.I3B An example is... 

Similarly, many A-norsteroids 244 have been subjected to phenol-dienone rearrangement in HF—SbF5 medium854 [Eq. (5.312)]. 1H NMR spectroscopic studies at low temperature confirms the formation of 0-protonated intermediates (Scheme 5.87), which subsequently rearrange to diprotonated precursors of the dienones 245. 

Phenol-dienone rearrangement The reverse of the dienone-phenol rearrangement. 

Zincke-Suhl reaction. Phenol-dienone rearrangement of p-cresols by addition of carbon tetrachloride in the presence of aluminum chloride, with formation of 4-methyl-4-trichloromethylcyclo-hexa-2,5-dienone. 

It is generally known that the processes of reversible oxidation of phenols, i.e. the conversions of phenolic systems into quinone structures and vice versa, are of great importance in biochemical reactions. The reaction partners mentioned above can serve as donors and acceptors of electrons and protons, i.e. as antioxidant systems. The conversions of phenols into cyclohexadienones are accompanied by the loss of aromaticity and in essence are not rearrangements, although the term phenol-dienone rearrangement is found in the literature. A review which summarizes in detail the oxidation reactions of phenols under conditions of halogenation, nitration and alkylation as well as radical reactions appeared . The various transformations of phenols upon oxidation with nickel peroxide were also reviewed . Therefore, only recent reports concerning the phenols-to-quinones conversions are described in this section. 

Phenol-azaaromate complexes 377 Phenol-benzonitrile complex 177, 178 Phenol-dienone rearrangements 651-655 Phenol-formaldehyde ratio 1457 Phenol-formaldehyde resins 606, 608, 626-628, 1457 functionahty in 1457-1460 Phenolic acids 965-967... 

Finally, it should be noted that there are various reactions which can be called phenol-dienone rearrangements. They proceed upon halogenation, nitration and alkylation of phenols as well as in the course of radical reactions of phenols . 

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