Quinol ether

In equation 7, ttimer radical (4) is produced when (3) dissociates. Whenever (4) couples with the other product of equation 7, ie, the 2,6-dimethylphenoxy radical, the tetramer is produced as described. These redistribution reactions of oligomers that proceed by ketal formation and subsequent dissociation ultimately generate terminal quinol ethers which enolize to the more stable terminal phenol (eq. 8). 

The hydrogenation of a quinol ether imine derivative on Pt/C led to the hydrogenolysis of the C-O bond (Scheme 4.39).175... 

Other methods which have proved of value include the formation of substituted methides by the action of silver oxide on phenols (70JOC3666). It is postulated that upon oxidation of the phenol a phenoxy radical is formed which dimerizes to the quinol ether. Disproportionation to the methide and the original phenol follows. 

The other mechanism proposed also involved coupling of oligomeric radicals to give quinol ethers as intermediates and is illustrated as follows for dimer. 

In 1987, Kita and co-workers first developed a general and high yielding (59% quant.) route to p-benzoquinone monoacetals (2a) and spirolactones from para-substituted phenols (la) with PIFA in MeCN in the presence of alcohols (R"OH) [27]. Similar methods for preparing quinone monoacetals and quinol ethers have been developed independently by Lewis et al. (PIDA/CH2C12-R"OH (11-65% yields)) [28] and Pelter et al. (PIDA/R"OH (65-99% yields)) [29] [Eq.(l)]. 

Addition to quinone monoketals and quinol ethers.2 Complexation of quinone monoketals or quinol ethers with MAD permits 1,4-addition of organo-lithium and Grignard reagents. Highest yields obtain with aryl, vinyl, and acetylenic organometallics. 

Electron transfer from oxygen to copper gives a phe-noxyl radical, which couples with another copper-bound radical to form the C—O—C dimer and Cu(I). The reaction behaves as a step reaction rather than a chain reaction. A quinol ether rearrangement occurs to equilibrate polymer and monomer. High-molecular-weight polymer is formed only in the late stages of reaction. Indeed, other phenols are incorporated into the polymer if they are added at the end of the reaction because of the quinol ether rearrangement. 

The proximity effect of the functional groups in 47 is instructive. Thus, A-aryl aldimines of the type, ArCH=NAr, are reported to give azoarenes [Ar N=NAr ] and aldehydes [ArCH=0] with DAIB (77IJC(B)376), while phenols are oxidized to quinones, quinol ethers, or quinone acetals, depending on the nature of the reaction medium (92MI2, 01OR327). 

DAIB and BTIB oxidations of phenols proceed through aryloxyiodane 129 and/or aryloxenium ion 130 intermediates and are quite useful for the preparation of quinones, quinol ethers, and quinone acetals (e.g., Scheme 39) (88TL677, 92MI2, 93JCS(P1)1891, 01OR327). When phenols bearing nucleophilic side chains are used as substrates, such oxidations provide fertile ground for the assembly of heterocyclic structures. This can be accomplished by oxidative-cyclization reactions of different types. 

The combination of MAD with some complex aluminum hydride reagents enables the conjugate reduction of a -unsaturated ketones [142]. Although selectivity is profoundly affected by the structure of substrates, the 1,4 addition of hydride to quinone monoketals and quinol ethers is successfully mediated by MAD to give reduction products in good yield (Sch. 104) [143]. 

The products obtained via phenol oxidations involving phenoxy radieals range from simple ortho- or p ra-benzoquinones and quinol ethers to biphenyls arising via C-C coupling processes. Suitably substituted phenoxy radieals with sterieally bulky ortho- uh-stituents R and nonbulky par<7-substituents R (Me, Et, Bu) have been found to exist in equilibrium with their eorresponding parn-quinol ether derivatives (VII), as in Eq. (2). The results are more complex when R is bulkier [9,16,17]. 

The constant current electrolysis of methyl eugenol serves to illustrate some of the effects of ring substitution and electrolyte on the course of such reactions. When carried out in methanol containing 2% NaOH or KOH (with or without added NaC104), an approximately 1 1 mixture of or// o-quinone bisketal (LXVI) and para-quinol ether (LXVII) was obtained. In contrast, when the oxidation was carried out in methanol-NaC104 in the absence of base, only side-chain substitution product (LXVIII) was obtained [Eq. (33)] [77]. This work clarified an earlier mechanistic interpretation of the reaction [78]. 

Replacement of the methoxy group with a hydroxyethyl ether function was found to largely overcome this limitation of the chemistry, affording the corresponding cyclic quinol ether ketals (LXXXVII) in good yield [Eq. (44)]. Mild acidic hydrolysis of the cyclic ketals to the quinol ether derivatives was nearly quantitative except when R = t-Bu [Eq. (44)], wherein loss of /-butyl led to reduction during the hydrolysis [98,99]. 

The migration of oxygen from a quaternary center in a cyclohexadienone may be preferred to a carbon shift, when present as an ether or ester function rather than free hydroxy. Thus the p-quinol acetate (117) yields the orcinol monoacetate (118 79%) on treatment at room temperature with trifluoroacetic anhydride, and the p-quinol ether (119) forms the resorcinol diethyl ether (120 71%) in ethanolic sulfuric acid. In the second case, hemiketalization must intervene also some methyl shift (12%) is observed. With the quinol (121), treatment with acetic anhydride-sulfuric acid leads to the lactone (122) acetylation or lactonization probably precedes oxygen shift. A number of related examples can be found in the steroid area. - Thermal 1,3-shifts of p-quinol acetates can also be induced acetate (117) yields catechol acetate (123 50-60%, 45 °C) by way of isomerization of the first-formed acetate (124). In the o-quinol acetate series, 1,2-acetoxy shift is seen in (125) (126 92%) and in (127) (128 90%), both in... 

Becker used Attenburrow Mn02 for the conversion of hindered phenols into quinol ethers. For example, 2,4,6-tri-f-butylphenol is oxidized in benzene to give OH O O... 

Such a phenol keto-tautomer equivalent strategy was used for conjugate reduction of cyclic enones (equation 5). The quinone monoketals 9 and para-quinol ethers 10 were used as precursors to keto-tautomer equivalents of substituted phenols, namely enones 11, which were prepared by action of bis(2,6-di-fert-butyM-methylphenoxy)methylaluminium (MAD), followed by addition of lithium tri-iec-butyl borohydride (L-Selectride). The enones 11 obtained are reasonably stable at a freezer temperature without aromatization. ... 

The quinone monoketals 9 and para-quinol ethers 10 mentioned above (Section n.A.l) can be obtained by anodic oxidation of the corresponding O-protected phenols 103 ° (equation 41) or upon oxidation of substituted phenols 104 with one equivalent of phenylrodonium diacetate (PIDA) at an ambient temperature (equation 42). 

Co(salen)-catalyzed oxidation of phenols with fert-butyl hydroperoxide in CH2CI2 at room temperature provides predominantly tert-butylperoxylated products . On catalytic oxidation of 2,6-di(tert-butyl)-4-acetylphenol oxime O-methyl ether (261), both o- and p-(ferf-butylperoxy)quinol ethers (262 and 263) were obtained in 8.1 and 87.3% yields, respectively. On the other hand, catalytic oxidation of 264 provided the corresponding o- and p-quinol ethers (265 and 266) in 91.1 and 6.8% yields, respectively (Scheme 56). The remarkable difference of the o/p ratio in the reactions of 261 and 264 reflects clearly a combination of both steric and electronic factors. 

In the case of 4-alkenyl-2,6-di(ferf-butyl)phenols having three potential reaction sites for attack by f-BuOO , three possible ferf-butylperoxylated compounds will be produced depending on substituents on the olefinic side chain. Co(salen)-catalyzed oxidation of 267 with f-BuOOH provided quinomethane 268 and p-(ferf-butylperoxy)quinol ether 269 (81.5 and 9.5%, respectively). In the case of the substrate 270 bearing a fully substituted olefin, the corresponding o-substituted quinol ether 271 was obtained as a sole product (73%) (Scheme 56). Detailed studies on Co(salen)-catalytic oxidation of 2-alkenyl-4,6-and 4-alkynyl-2,6-di(terr-butyl)phenols with f-BuOOH have also been conducted . ... 

Complexation of quinone monoketals and quinol ethers with MAD, followed by addition of organolithium or Grignard reagents gives products from 1,4-addition (Scheme 6.96) [120]. The success of these 1,4-additions is in marked contrast to re-... 

Chemical interaction between transformation products of both the amine and phenol is involved. Derivatives of cyclohexadienone like 77, deeply coloured compounds with structure of indophenols or BQMI 57 are expected to be formed as intermediates [251], The compound 77 is formally analogous to quinol ethers of 1,4-benzoquinone dioxime 163 (R = tert-butyl, methoxy, phenyl) thermally releasing the corresponding phenoxyl and therefore imparting the antioxidant effect [253]. 

Acid-catalysed rearrangement of the quinol ether (91), itself obtained by phenolic oxidative coupling, unexpectedly gave an aporphine which on O-methylation afforded 1,9,10-trimethoxyaporphine (92). Ferricyanide oxidation of reticuline supplied isoboldine and the morphinandienone pallidine. ... 

In equation 6, the dimer (2) is formed by enolization of the quinol ether (1). Oxidation of the dimer produces the dimer radical, which can couple with other radicals that are present. If it couples with a 2,6-dimethylphenoxy radical... 

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