Quinone masked

The solution may have a brown tint, partially masking the yellow color of the quinone. The dark color is probably due to reaction of the diethylamine with the 2,5-dichloro-3,6-di-fcr -butyl-l,4-benzoquinone. 

Molecular electrostatic potentials have been used to explain the regioselectivity exhibited in the Diels-Alder cycloaddition reactions between 1-trimethylsilyloxy-butadiene and the quinones 5-formyl-8-methyl-1,4-naphthoquinone, 5-methoxy-7-methyl-1,4-phenanthrenequinone, and 5,6,7-trimethyl-1,4-phenanthrenequinone.128 The intramolecular Diels-Alder reaction of masked o-benzoquinones (123) with a variety of dienes provides adducts (124) which rearrange to functionalized ris-decal ins (125) with complete stereocontrol of up to five stereocentres. This methodology ... 

Anodic treatment of 1,2- or 1,4-dihydroxy-substituted benzenes to form the corresponding quinones or masked congeners is well known, since they represent valuable synthetic intermediates [64]. Benzoquinone ketals of electron rich arenes like 18 can be challenging since the oxidative aryl-aryl coupling reaction usually competes. When using BDD anodes the benzoquinone ketal 19 is obtained in an almost quantitative manner, demonstrating the superior properties of this electrode material. Despite the basic conditions, no deblocking of the silyl-protected phenol moiety is observed [65] (Scheme 9). 

In chapter 7, special emphasis has been placed on the synthesis of representative polycyclic quinones and their photochromic behavior, including the spectral, kinetic, and fatigue characteristics of such systems. Potential applications are focused on recording and multiplication of images, optical memories, and gradation masking. 

Thiophenes can act as dienophiles in Diels-Alder reactions with electron-poor dienes such as hexachlorocyclopen-tadiene, tetrazines, or o-quinone monoimines. The masked o-benzoquinone 64 can undergo inverse electron demand cycloadditions with thiophene itself or simple derivatives such as 2-methyl-, 2-methoxy-, and 2,4-dimethylthiophene (Scheme 5) <2001TL7851>. Depending on the substitution pattern on the thiophene skeleton, different cycloadducts can be observed. The basic thiophene skeleton gives rise to a bis-adduct 65. By blocking the second double bond with a methyl or methoxy group, a 1 1 adduct 66 or 67, respectively, is obtainable in moderate yield. 

A novel approach to the problem of effecting regiospecific quinone-isoprene coupling has been reported (Scheme 4). Addition of the allylic bromide (142) to the masked quinone (143) gave the epimeric masked quinol (144). Cope rearrangement of the latter, followed by oxidation gave 2-isopentenyl-p-benzoquinone (145). 

The method could be applied to the synthesis of many natural benzo- and naphthoquinones. The masked quinone (146), which may serve as a general precursor to the menaquinones, was prepared and isoprenylated by a similar series of reactions. 

Birch reduction of aromatic ethers is well known to afford alicyclic compounds such as cyclohexadienes and cyclohexenones, from which a number of natural products have been synthesized. Oxidation of phenols also affords alicyclic cyclohexadienones and masked quinones in addition to C—C and/or C—O coupled products. All of them are regarded as promising synthetic intermediates for a variety of bioactive compounds including natural products. However, in contrast to Birch reduction, systematic reviews on phenolic oxidation have not hitherto appeared from the viewpoint of synthetic organic chemistry, particularly natural products synthesis. In the case of phenolic oxidation, difficulties involving radical polymerization should be overcome. This chapter demonstrates that phenolic oxidation is satisfactorily used as a key step for the synthesis of bioactive compounds and their building blocks. 

Kna 7 has used the oxyanion-accelerated rDA reaction to advantage in the synthesis of conduritol A. The masked dienol used was 9-[(benzyloxy)methoxy]anthracene (81), which upon reaction with benzo-quinone (82) gave the protected adduct (83) (equation 38). Further transformations yielded (84), which upon treatment with potassium hydride caused rDA fragmentation to occur at 35 °C. Two subsequent steps produced conduritol A (86a), a naturally occurring cyclitol, in 39% overall yield from (82). The room-temperature DA activity of anthranol reported by Rickbom, coupled with the observed low-temperature rDA reactivity of its adducts, makes anthranol a highly useful alkene protecting group via a DA/iDA sequence. 

The FTIR difference spectra obtained from steady state illumination of preparations containing only or both Qa and Qb surprisingly show no large changes for the transitions PIQ>i —> P+IQ>i and PIQaQb —> P IQaQb One explanation could be that the vibrations arising from Q i" and occur at approximately the same frequencies another possible explanation is that the absorption bands are very small and therefore are masked by stronger absorbance bands. Further studies with isotopically labelled quinones and with amino acid labelled RC s should provide reliable assignments of bands and thereby more detailed information on the molecular events involved in the primary processes of photosynthesis. 

Aromatic amines and phenols are among the few classes of compounds in which a large proportion of them exhibit useful fluorescence. Parker and Barnes [21] found that in solvent extracts of rubbers the strong absorption by pine tar and other constituents masks the absorption spectra of phenylnaphthylamines, whereas the fluorescence spectra of these amines are sufficiently unaffected for them to be determined directly in the unmodified extract by the fluorescence method. In a later paper Parker [22] discussed the possibility of using phosphorescence techniques for determining phenylnaphthylamines. Drushel and Sommers [7] have discussed the determination of Age Rite D (polymeric dihydroxy quinone) and phenyl-2-naphthylamine in polymer films by fluorescence methods and Santonox R and phenyl-2-naphthylamine by phosphorescence methods. 

Finally, the enzyme was incubated with the possible electrophilic hydrolysis products of compound 1. Lack of inhibition indicated that the electrophilic quinone methide was not formed until compound 1, masked to resemble the natural substrate, localized in the active site. Thus, compound 1 satisfied the usual tests for suicide-like inhibition. 

Simple quinone monoketals, such as dimethyl ketal variants of t5q)e 9/10 (Fig. 7), constitute the second most often used group of simple quinonoids in s5mthesis. They are usually described as monoprotected or masked forms of their quinone counterparts, with which they share some of the basic reactivity features related to the... 

The asymmetric construction of the benzoxepinone core (138) has been reported twice in the course of two similar works. Both methods rely on an enantioselective NHC-catalysed formal [4-1-3] annulation of enals and o-quinone methides but, while o-quinone methides might been used as such,122 a method for the in situ generation of more reactive o-quinone methide from its masked precursor (139) has been developed using fluorides.12 ... 

Actually, a redox reaction takes place. It is due to the reducing properties of para and ortho diphenols, stmctures that were masked in a-tocopherol. Hence, the driving force of this particular reaction is the formation of a quinone. It is sufficiently strong to induce the break of the ether-oxide bond of pyran. This is remarkable. The identification reaction of a-tocopherol is called Emmeri-Engel s reaction. 

The photooxygenation of masked o-benzoquinones, such as 173, in methanol may generate a cationic intermediate, like 175, that gives functionalized cyclopentenones, such as 177, through a ring contraction (Scheme 18.41). On the other hand, when the reaction is performed in chloroform, endoperoxides are isolated. The required quinones 173 are readily prepared from 2-methoxyphenols 172 using the iodine(III) reagent PhI(OAc)2. ... 

Evans, D.A. and Wong, R.Y. (1977) Synthesis of anti-bacterial p-quinols from marine sponges. Synthetic applications of masked quinones. J. Org. Chem., 42, 350-351. 

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