Quinones extended

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

This addition is general, extending to nitrogen, oxygen, carbon, and sulfur nucleophiles. This reactivity of the quinone methide (23) is appHed in the synthesis of a variety of stabili2ers for plastics. The presence of two tert-huty groups ortho to the hydroxyl group, is the stmctural feature responsible for the antioxidant activity that these molecules exhibit (see Antioxidants). 

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

The reactions of enamines with positively activated olefins have been extended to arylations with />-quinones (350,362-369) and quinone sulfoni-mides (365-368,370). Thus a new pathway for the facile formation of benzofurans and indoles became available. 

Br20 a dark-brown solid moderately stable at —60° (mp —17.5° with decomposition), prepared by reaction of Bt2 vapour on HgO (cf. CI2O p. 846) or better, by low-temperature vacuum decomposition of BrOa. The molecule has C2v symmetry in both the solid and vapour phase with Br-O 185 1pm and angle BrOBr 112 2° as determined by EXAFS (extended X-ray absorption fine structure). It oxidizes I2 to I2O5, benzene to 1,4-quinone, and yields OBr in alkaline solution. 

Mercaptoquinolines of this type exist in the zwitterion or the zwitterion-extended quinone form to a greater extent than do the analogous hydroxy compounds ° (see Table V and Section II,R), and the color of 8-mercaptoquinoline has been attributed to the zwitterion structure.The concentration of the zwitterion decreases as the dielectric constant of the solvent decreases in the order H2O > MeOH > EtOH > Bu OH as indicated by the change in the molecular extinction coefficient. ... 

Encouraged by the short synthesis of K vitamins, the chromium-mediated benzannulation was extended to the synthesis of vitamin E 68 [59]. The problem of imperfect regioselectivity of alkyne incorporation - which did not hamper the approach to vitamin K due to the final oxidation to the quinone - was tackled by demethylation of both regioisomeric hydroquinone monomethyl ethers 67 to give the unprotected hydroquinone. Subsequent ring closure yielded a-tocopherol (vitamin E) 68 (Scheme 39). 

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). 

These DFT data provide a consistent picture for the tautomerization equilibria involving the dimers 2-4, which highlight the extended quinone methides 2a, 3a, and 4a as the most stable tautomers for all biindolyl quinones investigated. 

Lemus, R. L. Skibo, E. B. Studies of extended quinone methides. Design of reductive alkylating agents based on the quinazoline ring system, j. Org. Chem. 1988, 53, 6099-6105. 

Repetitive Capture and Release of a Quinone Methide Extends Its Effective Lifetime... 

SCHEME 9.19 Reversible capture and release of quinone methides by dA extends their effective lifetime. 

The effects of changing Jt-conjugation at the 4-position on both the rate of isomerization of the initially formed o-quinones to QMs and the reactivity of the quinoids formed from 4-propylcatechol, 2,3-dihydroxy-5,6,7,8-tetrahydronaphtha-lene (2-THNC), hydroxychavicol, and 4-cinnamylcatechol were studied (Fig. 10.6).9 These catechols were selectively oxidized to the corresponding o-quinones or QMs and trapped with GSH. Microsomal incubations with the parent catechols produced only o-quinone-GSH conjugates. However, if GSH was added after an initial incubation period both o-quinone- and QM-GSH conjugates were observed. The results indicate that the extended Jt-conjugation at the para position enhances the rate... 

Quercetin is a naturally occurring flavonoid with both antioxidant and prooxidant activities (Scheme 10.12).90 It has been demonstrated in a variety of bacterial and mammalian mutagenicity experiments that quercetin has mutagenic properties that could be related to quinoid formation.91,92 Quercetin is initially oxidized to an o-quinone, which rapidly isomerizes to di-QMs that could also be called extended... 

SCHEME 10.12 Di-QMs or extended quinones formed from quercetin. 

Bolton, J. L. Comeau, E. Vukomanovic, V. The influence of 4-alkyl substituents on the formation and reactivity of 2-methoxy-quinone methides evidence that extended ji-conjugation dramatically stabilizes the quinone methide formed from eugenol. Chem.-Biol. Interact. 1995, 95, 279-290. 

Quinacridones are not the only industrially significant products. The list may be extended to include the derivative mentioned in Section 3.2.1.4, the linear trans-quinacridone quinone. There are two other synthetic pathways besides the hydroquinone method. The older method involves cyclization of the 2,5-bis-(2 -carboxyanilino-)-l, 4-benzoquinone 63 with concentrated sulfuric acid or polyphos-phoric acid at 150 to 200°C. The starting material 63 is obtained through condensation of 1,4-benzoquinone with anthranilic acid ... 

---