Quinone methides isomerization

Phenolic quinone methides isomeric with LVII have been considered as the product of oxidative transformations of another antioxidant, 2,2 -methylenebis(4-methyl-6-tert-butylphenol) XXXVII84. Rubber films, prepared from high-pH lattices stabilized with this antioxidant, turn pink during the thermal oxidation. The discolouration is ascribed84 to the formation of ion LXII. However, the oxidation of XXXVII seems to proceed in a more complicated way. Scheme 6 shows the origin of oxidation coupling products prepared under conditions which simulated the inhibition process49 85. The increased relative content of radicals tert-BuO 2 in the... 

Wan s group showed that the observed photodehydration of hydroxybenzyl alcohols can be extended to several other chromophores as well, giving rise to many new types of quinone methides. For example, he has shown that a variety of biphenyl quinone methides can be photogenerated from the appropriate biaryl hydroxybenzyl alcohols.32,33 Isomeric biaryls 27-29 each have the benzylic moiety on the ring that does not contain the phenol, yet all were found to efficiently give rise to the corresponding quinone methides (30-32) (Eqs. [1.4—1.6]). Quinone methides 31 and 32 were detected via LFP and showed absorption maxima of 570 and 525 nm, respectively (in 100% water, Table 1.2). Quinone methide 30 was too short lived to be detected by LFP, but was implicated by formation of product 33 that would arise from electrocyclic ring closure of 30 (Eq. 1.4). 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Quinone Methide Regeneration is Required for Isomerization between Its N1 and 6-Amino Adducts of dA... 

Iverson, S. L. Hu, L. Q. Vukomanovic, V. Bolton, J. L. The influence of the para-alkyl substituent on the isomerization of o-quinones to p-quinone methides potential hioac-tivation mechanism for catechols. Chem. Res. Toxicol. 1995, 8, 537-544. 

Bolton, J. L. Pisha, E. Shen, L. Krol, E. S. Iverson, S. L. Huang, Z. van Breemen, R. B. Pezzuto, J. M. The reactivity of o-quinones which do not isomerize to quinone methides correlates with alkylcatechol-induced toxicity in human melanoma cells. Chem.-Biol. Interact. 1997, 106, 133-148. 

Ralph, J. Adams, B. R. Determination of the conformation and isomeric composition of lignin model quinone methides by NMR. J. Wood Chem. Technol. 1983, 3, 183-194. 

The anion derived from 2,3-dimethyl-l,4-naphthoquinone behaves as a quinone methide and undergoes a [l,4]-cycloaddition with the benzoquinone (193). The product is the xanthene derivative (194) (70JCS(C)722). There is no indication of the formation of the isomeric xanthene. A [l,3]-cycloaddition occurs simultaneously which leads to the fluorene derivative (195). 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

The Chemistry of Ring B.—The isolation of taxodione (49), a diterpenoid tumour inhibitor, has stimulated interest in the introduction of oxygen functions at C-6. Taxodione itself has been synthesized from podocarpic acid. The latter was converted to ferruginol benzoate (46). The C-11 hydroxy-group was introduced via the diazo-compound and the product acetoxylated at C-7 to afford (47) this was converted into the A -olefin (48), which was epoxidized and isomerized to the C-6 ketone. The product was then oxidized to give the quinone-methide of taxodione (49). 

The hydroformylation of the orf/zo-prop-2-enylphenol 81 which contains no benzylic hydroxy group gives a mixture of the open-chain aldehyde 82 and the seven-membered cyclic hemiacetal 83 in a 82 83 ratio of approximately 40 60 (equation 34) °. The ben-zofuran epoxide 85 and its valence-isomeric quinone methide 86, both readily obtainable from benzofuran 84, rearrange thermally above —20°C to form the allylic alcohol 87 and the tautomeric phenol 88 (equation 35) . ... 

J Ralph, BR Adams. Determination of the Conformation and Isomeric Composition of Lignin Model Quinone Methides by NMR. J Wood Chem Technol 3 183, 1983. 

In the oxidation of Metilox , the formation of monomeric quinone methide LIV was also detected by spectra73. However, it is unstable and isomerizes to LV. Also the combined quinone methide and cyclohexadienonyl derivative LVI was mentioned among reaction products. 

The effect of benzoquinones on thermal oxidation of hydrocarbons was investigated in detail on a liquid substrate. It was found182 that 4-tert-alkyl-l,2-benzoquinones are at 60 °C more powerful retarders in the initiated oxidation of tetra-hydronaphthalene than 2-tert-alkyl-l,4-benzoquinones. The effect of both isomeric hydroxy-tert-alkylbenzoquinones of types CXLin and CLI on this oxidation is entirely negligible. Unsubstituted 1,4-benzoquinone retards the oxidation of ethylbenzene213. However, the retardation effect of 2-tert-butyl- and 2,6-di-tert-butyl-1,4-benzoquinones CXXXVIII and XXII and, in particular, of 3,5,3 -tetra-tert-butyl-4,4 -diphenoquinone XX is much weaker than that of derivatives of quinone methide, e. g., XXIX Moreover, benzoquinones resume their retardation... 

QMs exist in three isomeric forms, namely, o-, m-, and p-quinone methides (also known as o-, m-, and p-QMs) (Figure 7.5). The importance of o- and p-QMs in organic synthesis and their role in biochemistry have been studied in detail. Urdike benzoquinones, o- and p-QM derivatives are highly polarized compounds, usually observed with difficulty or postulated as reactive intermediates because of facile reactions driven by the formation of aromatized phenol derivatives. 

N- -alanylnorepinephrine. Further studies of purified H. sexta laccase are needed to determine whether it is a bifunctional enzyme which promotes not only o-diphenol oxidation, but also isomerization of the o-quinone to a p-quinone methide. Unlike the laccase preparation, tyrosinase from M. sexta has little effect on the rate of yS-hydroxylation. The functional role of laccase... 

Yoshida et al. have reported the coupling of an aromatic aldehyde and 2equiv. ofaryne, affording 9-arylxanthenes (Scheme 12.53) [101]. The reaction presumably starts with a nucleophilic attack of the carbonyl oxygen of 188 on the aryne, leading to zwitterion 189, which subsequently undergoes cyclizahon to afford benzoxete 190. Subsequent isomerization of 190 leads to o-quinone methide 191 which then undergoes [4+2] cycloaddition with a second molecule of aryne to afford the xanthene 192. 

Exists in 3 isomeric forms in soln. (E -form quinone methide Z-form). Schiff s base from fruits of Crinum asiaticum (Amaryllidaceae). Considered as a direct precursor to Amaryllidaceae alkaloids. Orange-yellow needles (MeOH). Mp 220°. 

---