Cyclohexa-2,4-dienones, 6,6-disubstituted

Scheme 2.13 Cu-catalysed 1,4-addition of AlMcj to 4,4-disubstituted cyclohexa-2,5-dienone with thioether ligands.

Irradiation of 4,4-disubstituted cyclohexa-2,5-dienones yields bicydo[3.1.0]hex-3-en-2-ones (4.87) in a process that resembles the first type of rearrangement just described for cydohexenones. The... 

Disubstituted cyclohexa-2,5-dienones from BTI oxidation of phenols... 

Recently, copper-catalyzed enantioselective addition reactions in the presence of phosphorus amidite 258 have also been applied to different Michael acceptors. Thus, both symmetrical and unsymmetrical 4,4-disubstituted cyclohexa-2,5-dienones 261 were found to add diethylzinc with moderate to high enantioselectivities and good... 

Zimmerman 103) applied the well known rearrangement of 4,4-disubstituted cyclohexa-2,5-dienones (e.g. the 4,4-diphenyl derivative tives the 3,4-disubstituted phenols l)). Using acetic anhydride and a small amount of concentrated sulfuric acid, 97.5 % of 3,4-diphenylphenolacetate is formed, which in turn is hydrolyzed to give 3,4-diphenylphenol. This is one of the easiest synthesis of this phenol. 2-Bromo-4,4-diphenylcyclohexa-2,5-dienone rearranges by a similar route to give a 49 % yield of 2-bromo-3,4-diphenylphenolacetate and 47 %2-bromo-4,5-diphenylphenolacetate U4). A [1.2]-phenyl shift of 4-methoxy-2,3,4,6-tetraphenyl-2,5-dienone in acetic anhydride and zinc chloride gives rise to 4-methoxy-2,3,5,6-tetraphenylacetate (m.p. 284 °C, 85% yield)115 . 

The mechanism is illustrated by the rearrangement of 4,4-disubstituted cyclohexa-dienones to 3,4-disubstituted phenols, as displayed here. 

Disubstituted phenols such as 350 undergo PhI(OAc)2-mediated oxidation in the presence of MeOH as a nucleophile resulting in the formation of two possible cyclohexa-dienones (351 and 352) (Scheme 73). The initially formed intermediate 353 is converted to the cyclohexadienones by two plausible routes. In route A, heterolytic dissociation generates a solvated phenoxonium ion 354, which further reacts with MeOH to afford 351 and/or 352. In route B, both 351 and 352 are produced by direct attack of MeOH on the intermediate (353). In the latter case, the reaction will be strongly influenced by steric factors and a homochiral environment using chiral solvents and chiral oxidants to induce some asymmetric induction, particularly in the formation of 352. 

Oxidative dearomatization of 2-substituted phenols 225 with [bis(acyloxy)iodo]arenes in the presence of external or internal nucleophile provides a convenient approach to 6,6-disubstituted cyclohexa-2,4-dienones 226 according to general Scheme 3.92. Specific examples of this reaction are provided below in Schemes 3.112-3.114. 

The oxidative dearomatization of ort/io-substituted phenols 225 leads to 6,6-disubstituted cyclohexa-2,4-dienones 226 (Scheme 3.92), which can be conveniently utilized in situ as dienes in the Diels-Alder cycloaddition reaction. When the oxidation of phenols is performed in the absence of an external dienophile, a dimerization via [4-f2] cycloaddition often occurs spontaneously at ambient temperature to afford the corresponding dimers with an extraordinary level of regio-, site- and stereoselectivity [348-350]. A detailed experimental and theoretical investigation of such hypervalent iodine induced Diels-Alder cyclodimerizations... 

The study of phenol alkylation has a long history. Claisen and co-workers (178) showed that metal phenoxides react with active alkyl halides (e.g., allyl and benzyl bromides) in nonpolar solvents to give o-alkylphenols. Cyclohexa-dienones may be prepared in this way starting from 2,6-disubstituted phenols... 

When this reaction is carried out on a p,p disubstituted a,p-unsaturated complex, cyclohexa-2,4-dienones are obtained, evidently because tautomerization to a phenol is... 

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