Para-quinone

The synthetic procedure described is based on that reported earlier for the synthesis on a smaller scale of anthracene, benz[a]anthracene, chrysene, dibenz[a,c]anthracene, and phenanthrene in excellent yields from the corresponding quinones. Although reduction of quinones with HI and phosphorus was described in the older literature, relatively drastic conditions were employed and mixtures of polyhydrogenated derivatives were the principal products. The relatively milder experimental procedure employed herein appears generally applicable to the reduction of both ortho- and para-quinones directly to the fully aromatic polycyclic arenes. The method is apparently inapplicable to quinones having an olefinic bond, such as o-naphthoquinone, since an analogous reaction of the latter provides a product of undetermined structure (unpublished result). As shown previously, phenols and hydro-quinones, implicated as intermediates in the reduction of quinones by HI, can also be smoothly deoxygenated to fully aromatic polycyclic arenes under conditions similar to those described herein. 

The mechanisms for model condensation reactions of para-hydroxymethyl-substituted phenol (and therefore para-quinone methide) with reactive ortho positions are described in Fig. 7.29. The phenolate derivatives react with para-quinone... 

Figure 7.28 Preferential formation of para-quinone methides.

Para-quinone methide intermediates, 403 Para-quinone methides, 404 para-Trishydroxybenzylamine, reaction with 2,4-xylenol, 397 Partial aromatic polyamides, 136, 143, 180-184... 

When the ortho-para directing group is one with an unshared pair (this of course applies to most of them), there is another effect that increases the amount of para product at the expense of the ortho. A comparison of the intermediates involved (p. 683) shows that C is a canonical form with an ortho-quinoid structure, while D has a para-quinoid structure. Since we know that para-quinones are more stable than the ortho isomers, it seems reasonable to assume that D is more stable than C, and therefore contributes more to the hybrid and increases its stability compared to the ortho intermediate. 

Much attention has been devoted to the development of methods to generate quinone methides photochemically,1,19-20 since this provides temporal and spatial control over their formation (and subsequent reaction). In addition, the ability to photogenerate quinone methides enables their study using time-resolved absorption techniques (such as nanosecond laser flash photolysis (LFP)).21 This chapter covers the most important methods for the photogeneration of ortho-, meta-, and para-quinone methides. In addition, spectral and reactivity data are discussed for quinone methides that are characterized by LFP. 

Rabin, O. Vigalok, A. Milstein, D. Metal-mediated generation, stabilization, and controlled release of a biologically relevant, simple para-quinone methide BHT-QM. 

The oxidation behavior of 3-oxa-chromanols was mainly studied by means of the 2,4-dimethyl-substituted compound 2,4,5,7,8-pentamethylM /-benzo[ 1,3]dioxin-6-ol (59) applied as mixture of isomers 27a it showed an extreme dependence on the amount of coreacting water present. In aqueous media, 59 was oxidized by one oxidation equivalent to 2,5-dihydroxy-3,4,6-trimethyl-acetophenone (61) via 2-(l-hydroxyethyl)-3,5,6-trimethylbenzo-l,4-quinone (60) that could be isolated at low temperatures (Fig. 6.41). This detour explained why the seemingly quite inert benzyl ether position was oxidized while the labile hydroquinone structure remained intact. Two oxidation equivalents gave directly the corresponding para-quinone 62. Upon oxidation, C-2 of the 3-oxa-chroman system carrying the methyl substituent was always lost in the form of acetaldehyde. 

FIGURE 6.41 Oxidation of 3-oxa-chromanol 59 in aqueous media (excess water present), leading to acetophenone 61 with an equimolar amount of oxidant, and further to para-quinone 62 in the presence of excess oxidant. 

Syntheses of the 1,2- and 7,8-dihydrodiols of 5-MC have been described (60,87,103). The 1,2-dihydrodiol (30a) is most efficiently prepared from l-hydroxy-5-MC (29a) by Method IV (87), with the difference that an a-phenol is employed rather than a 8-phenol as in previous examples. Oxidation of 29a with (KSO NO furnished a single isomeric quinone identified as 5-MC 1,2-aione (Figure 18). Formation of an ortho rather than a para quinone in the oxidation of an a-phenol with Fremy s reagent is unusual. Apparently the bay region methyl group serves to sterically block oxidative attack in the adjacent bay region site. Reduction of 5-MC 1,2-dione with NaBH in ethanol gave 5-MC 1,2-dihydrodiol. 

Quinone and some of its derivatives may be used in the non-sulphur vulcanisation of natural rubber. The best-known derivative is para-quinone dioxime used as a curing agent for butyl rubbers. 

L.F. Fieser A.E. Oxford, JACS 64, 2060-65(1942)(PrePn of diacetyl peroxide and the alkylation of para quinones)... 

Quercetin (see also Bracken fern) para-Quinone (see also 1,4-Benzoquinone) Quintozene... 

Further digging in the literature led us to focus on the simplest, ethylenediamine-derived Co(salen), 18, which used catalytically in the presence of oxygen appeared to offer great promise in that phenols could be oxidized all the way to para-quinones with little, if any, accompanying ort/to-oxidation (Scheme 10).27 Initial studies on model system 19 indicated that while 10% Co(salen) in THF gave a low 19% yield of... 

Using small amounts of water in these reaction allows the efficient synthesis of para-quinones 38 as shown in Scheme 18, starting either from phenols 37 (R = OH) [88-91] or from the corresponding anilines 37 (R = NH2) [92]. 

Similar para-quinone derivatives can be obtained by oxidative demethyla-tions of phenol ethers which can be performed in water. Either [bis(trifluo-roacetoxy)iodo]benzene 4 or the polymer-supported reagent 19 can be used for the generation of para-quinones 38 from 1,4-dimethoxybenzene derivatives of type 39, Scheme 19 [93]. 

The Dess-Martin periodinane 8 is also able to oxidize aromatic compounds to the corresponding quinones. The presence of water is important and, starting from anilides 42 substituted in the 2-position, the rare class of ortho-imido-quinones 43 is accessible, Scheme 21. It has been shown that compounds of type 43 are interesting building blocks and can lead to polycyclic molecules of diverse molecular architecture [95,96]. They can undergo subsequent Diels-Alder reactions and intramolecular versions have been used for a rapid access to natural products and for synthesis of scaffolds for further manipulation.para-Quinones 45 are also easily accessible, however, only in modest yields by reacting 4-sub-stituted anilines 44 under the same reaction conditions, Scheme 21 [97]. 

Scheme 21. Synthesis of orf/zo-imidoquinones and para-quinones by oxidation with Dess-Martin periodinane...

The formation of the products 43 and 45 has also been studied from a mechanistic point of view. Labeling studies with H2180 revealed that two molecules of acetyl-2-iodoxybenzoic acid (formed by the reaction between Dess-Martin Peri-odinane 8 and water) are involved in para-quinone formation. It is suspected that the substituent in 2-position in 42 blocks another molecule of acetyl-2-iodoxybenzoic acid attacking the initially formed product leading to the formation of ortho-imidoquinones. Anilides substituted in the 3-position does lead to complex mixtures in the oxidation reaction. 

---