Phenolic ethers, phenol-dienone rearrangement

Claisen rearrangement Thermal conversion of an allyl phenyl ether to an o-allyl phenol. The rearrangement proceeds via a cyelohexa-dienone intermediate. 

Cross-conjugated dienones are quite inert to nucleophilic reactions at C-3, and the susceptibility of these systems to dienone-phenol rearrangement precludes the use of strong acid conditions. In spite of previous statements, A " -3-ketones do not form ketals, thioketals or enamines, and therefore no convenient protecting groups are available for this chromophore. Enol ethers are not formed by the orthoformate procedure, but preparation of A -trienol ethers from A -3-ketones has been claimed. Another route to A -trien-3-ol ethers involves conjugate addition of alcohol, enol etherification and then alcohol removal from la-alkoxy compounds. 

The reaction is undoubtedly intramolecular. Evidence for this comes from the absence of crossed-over products when two different ethers were rearranged together121. Further, optically active ortho- 1,3-dimethylallyl phenol is formed from optically active 1,3-dimethylallyl phenyl ether122 and the presence of dienone intermediates has been demonstrated. Claisen and Tietze123 first proposed intermediate dienones such as LXXIX and LXXX. This was established124 when... 

Photoaddhion of electron donor olefins such as vinyl ethers and stilbene to variously methyl and halogeno-substituted 1,4-benzoquinones resulted in the formation of dihydrobenzofurans via a dienone-phenol rearrangement of the primary product spirooxetanes <96H(43)619>. High-temperature water seems to be an alternative to use of acid catalysts or organic solvents by the cyclization of allyl phenyl ethers to dihydrobenzofurans <96JOC7355>. 

Diazo ketone cyclizathn. Some years ago Mander and his group1 demonstrated that the protonated diazomethylcarbonyl group can initiate cyclizations in unsaturatcd systems. In the case of phenolic diazo ketones, formation of spirodienones can predominate over competing side reactions (dienone-phenol rearrangement). Tetra-fluoroboric acid or boron trifluoride etherate can be used, but trifluoroacetic acid is usually the acid of choice. 

Rearrangement of phenols to dienones occurs readily in superacids. Some simple bicyclic phenols and their ethers have been investigated as model compounds by use of HF-SbFs [73]. This method is also applicable to natural products [74]. For example, treatment of estrone derivatives in HF-SbFs, then aqueous bicarbonate work-up led to estra-4,9-diene-3,17-dione (Eq. 30). The opposite rearrangement, i.e. that of dienones to phenols, can also be achieved in an HF-SbFs medium (Eq. 31) [75]. 

Electron-rich olefins such as vinyl ethers and stilbenes will photocycloadd to various 1,4-benzoquinones to give a spirooxetane, which following dienone-phenol rearrangement, gives a dihydrobenzofuran. Irradiation of o-benzoqui-... 

Discovery of the dienone-phenol rearrangement of quinol acetates has made possible the synthesis of dihydric phenols that were difficult of access by other routes. The starting materials are obtained from phenols and lead tetraacetate, and with acetic anhydride and sulfuric acid (Thiele acetylation) or with boron trifluoride in ether they give, respectively, di- and mono-acetyl derivatives of resorcinol or hydroquinone.309 When treated with lN-sodium hydroxide, 0-quinol acetates of type (1) undergo nucleophilic addition of an OH" ion, giving resorcinol derivatives (2).310 Occurrence of the reaction is considered... 

In the aromatic Claisen rearrangement (65), the ether-phenol transformation occurs at about 2(X)° S-C allyl shifts are more difficult to achieve (66). As the formation of the dienone intermediates is rate-determining, the analysis of perturbance around the heteroatoms (X) leading to the intermediates should provide clues to the relative rates, for changes are the same elsewhere. Thus, the crux of the problem involves only the net change of a -X to a Qpi -X bond. Such a change is favored with X = O rather than that with X = S because trigonal carbon is harder. 

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