P-Quinonemethides

The cleavage process occurs by p-quinonemethide formation after acetate hydrol-ysis. 

The same chromophore is present in the p-quinonemethide precursors of DL-pinoresinol (twice) and dehydrodiconiferyl alcohol (once). Measurements in a fast-recording ultraviolet spectrometer (Cary apparatus) indicated a half-life of the quinonemethides of about 1 hour (dilute solution in 70% aqueous dioxane at 20°C. and pH 5.5) (5, 18). They may also become stabilized to a small extent by polymerization (see XLIV). Since the quinonemethide (XXI) has no opportunity to become stabilized by intramolecular prototropy, it adds on external electrolytes, particularly hydroxyl compounds and preferably water 31). 

The mesomeric quinonemethides and 0-quinonemethides described above are somewhat more stable than the simple p-quinonemethides whose properties are already well known even from classical studies. The o-quinonemethides XX and XVII do not add on water even in solution in aqueous organic solvents their solution in dioxane/water is stable for months. They do not add on methanol or higher alcohols and react only slowly with phenols and organic acids. The addition of water is not catalyzed by mild alkalies the red color of the phenoxide ion (XVIII) prevails for weeks in soda solution. Addition of water occurs more rapidly in strongly alkaline solution. The addition of mineral acids and reduction by sodium borohydride are instantaneous. The addition of HC1 is rapid even at pH 4.0, the conditions used for determining the carbonyl content of lignin by the hydroxylamine hydrochloride reaction 13). 

Several efficient syntheses of ( )-cherylline (595) have been reported, and the common strategic feature of these entries is the cyclization of p-quinonemethides or functional equivalents thereof (223-225). For example, reaction of 609, which was readily accessible from 4-benzyloxystyrene by sequential reaction with bromine and methanol, with 3-benzyloxy-4-methoxy-(V-methylbenzylamine gave the tertiary amine 611 (223). In related work, 611 was obtained via ami-nolysis of the mesylate 610 that was prepared in two straightforward steps from 4-benzyloxystyrene oxide (224). When 611 was heated in strong acid, cycliza-... 

A general approach to precursors of p-quinonemethides has been developed that involved the olefinations of p-quinoneketals as 615 with suitable nucleophiles. For example, the reaction of 615 with the anion derived from 613 led directly to the p-quinonemethide ketal 616 via a Peterson reaction (Scheme 57) (225). Alternatively, 616 was prepared by the initial reaction of 615 with the anion of 614 followed by mild dehydration of the intermediate tertiary alcohol... 

Jakobsons, J., and S. M. Shevchenko. 1990. Theoretical conformational analysis of p-quinonemethides as lignin models. Latv. PSR Zinat. Akad. Vestis, Kim. Ser. 1990(2) 186-191. 

The p-quinonemethide route, introduced in the late 1980s,starts with the reaction of a p-H-calixarene with HCHO and a dialkylamine to produce a Mannich base, followed by methylation of the Mannich base to give the quaternary salt, and treatment with two equivalents of a nucleophile to produce a p-CH2Nu-calixarene, as illustrated by the conversion of 194 to 196. The Mannich reaction appears to occur with greater facility with the fully hydroxylated calixarenes than with their partial ethers, perhaps a consequence of the greater acidity of the former. For example, 4 reacts smoothly at room temperature (24 h) to afford the tetrakis(dimethylaminomethyl)calix[4]arene, whereas its A,C-dimethyl ether fails to react under these conditions and requires 66 h reaction time in refluxing dioxane. The A,C-dibenzoate and the A,C-diallyl ether of 4 both fail to undergo the Mannich reaction even at 140°C. ... 

To the list of nucleophiles used in the p-quinonemethide route (see ref. 1, p. 142) has been added CH(C02Et)3, from which calix[4]- and -[8]arenes 196b... 

N-Acetyldopamine A -[2-(3,4-dihydroxyphenyl)ethyl]-acetamide and N-P-alanyldopamine (3-amino-A -[2-(3,4-dihydroxyphenyl)ethyl]propanamide) occur in insects where they serve to form the sclerotized cuticle required for the exoskeleton. They are transformed by enzymatic oxidation to the corresponding o-quinones and p-quinonemethides which cross-link structural proteins with each other and with chitin. The biosynthesis of N-acetyldopamine starts from dopamine and acetyl-CoA by way of arylamine-A acyltransferase (EC 2.3.1.5). In contrast to the biosynthesis of (/f)- noradrenaline, the p-quinonemethides, generated non-selectively by enzymatic oxidation from N-ace-tyldopamine or )V-)3-alanyldopamine, undergo addition of water to furnish the racemic side-chain hydrox-ylated derivatives N-acetylnoradrenaline ()V-[2-(3,4-dihydroxyphenyl)-2-hydroxyethyl]acetamide) or, respectively N- alanylnoradrenaline (3-amino-)V-[2-(3,4-dihydroxyphenyl)-2-hydroxyethyl]propan-amide). 

The HPLC analysis of quercetin solution treated with MPO for 30 minutes (Figure 1(d)) revealed that concentration of quercetin (retention time 10 minutes) was negligible. Also, formation of major oxidation product (retention time 5.26 minutes) which was more polar than quercetin was detected (Figure 1(d)). From literature data and the obtained spectrophotometric and HPLC results in this work, we assume that the oxidation product detectable at 336—342 nm results from H20 addition on the p-quinonemethide formed by H-atom abstraction at 3-OH and 4 -OH of quercetin and subsequent rearrangement of the central ring [16]. The complete sequence of quercetin autoxidation induced by oxidants and catalyzed by metal ions is described earlier [15, 16] (Scheme 1). 

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