Reaction with 1,4-benzoquinones

Nan ya et al. (97) also reported the synthesis of isoindolediones by the reaction of miinchnones with 1,4-benzoquinones. Reactions with an unsymmetrical mtinchnone were not regioselective. Several groups have examined the reactions of miinchnones with unsaturated nitriles, including 2-chloroacrylonitrile (98), cinna-monitrile (78,99) and fumaronitrile (78) to give unexpected products in several cases. Eguchi and co-workers (100) smdied the cycloaddition of several mtinch-nones with electron-deficient trifluoromethylated olehns. Thus, miinchnones 176... 

Benzoquinone, reaction with 6-amino-l,3-dimethyluracil, 55, 200 aryl isocyanides, 57, 5... 

Reaction with an amino-propyl derivative of glass or hornblende with glutaraldehyde cross-linking reaction with titanium(iv) oxide coated with polymerized, diazotized 1,3-diaminobenzene Reaction with agarose activated with 4-benzoquinone Reaction with agarose cyclic imidocarbonate... 

At least two pathways have been proposed for the Nenitzescu reaction. The mechanism outlined below is generally accepted." Illustrated here is the indolization of the 1,4-benzoquinone (4) with ethyl 3-aminocrotonate (5). The mechanism consists of four stages (I) Michael addition of the carbon terminal of the enamine 5 to quinone 4 (II) Oxidation of the resulting hydroquinone 10 to the quinone 11 either by the starting quinone 4 or the quinonimmonium intermediate 13, which is generated at a later stage (HI) Cyclization of the quinone adduct 11, if in the cw-configuration, to the carbinolamine 12 or quinonimmonium intermediate 13 (IV) Reduction of the intermediates 12 or 13 to the 5-hydroxyindole 6 by the initial hydroquinone adduct 7 (or 8, 9,10). 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

Dimethylquinoxaline undergoes reaction with typical dienophiles such as maleic anhydride, p-benzoquinone, and AT-phenylmaleimide. The products were formulated as Diels-Alder adducts primarily since analogous products were not isolated from reactions with other quin-... 

Benzoquinone is an excellent dienophile in the Diels-Alder reaction. What product would you expect from reaction of benzoquinone with 1 equivalent of 1,3-butadiene Prom reaction with 2 equivalents of 1,3-butadiene ... 

The next major obstacle is the successful deprotection of the fully protected palytoxin carboxylic acid. With 42 protected functional groups and eight different protecting devices, this task is by no means trivial. After much experimentation, the following sequence and conditions proved successful in liberating palytoxin carboxylic acid 32 from its progenitor 31 (see Scheme 10) (a) treatment with excess 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) in ie/t-butanol/methylene chloride/phosphate buffer pH 7.0 (1 8 1) under sonication conditions, followed by peracetylation (for convenience of isolation) (b) exposure to perchloric acid in aqueous tetrahydrofuran for eight days (c) reaction with dilute lithium hydroxide in H20-MeOH-THF (1 2 8) (d) treatment with tetra-n-butylammonium fluoride (TBAF) in tetrahydrofuran first, and then in THF-DMF and (e) exposure to dilute acetic acid in water (1 350) at 22 °C. The overall yield for the deprotection sequence (31 —>32) is ca. 35 %. 

Ethyl H-, 2-diazepine-l-carboxylate functions as a 2 -component in the Diels-Alder reaction with tetrachloro-l,2-benzoquinone to give a mixture of the regioisomers 20 and 21.100... 

The HQ reaction with oxygen proceeds through several steps, and the precise reaction path is pressure-dependent. The first step is the formation of benzoquinone is shown in the following section. Further reactions result in the formation of low MW alcohols, ketones, andother compounds. 

A large number of other sensitizers has been investigated for use in photolytic de-diazoniation. The excited states of these compounds (S ) react either by direct electron transfer (Scheme 10-97), as for pyrene, or by reaction with an electron donor with formation of a sensitizer anion radical which then attacks the diazonium ion (Scheme 10-98). An example of the second mechanism is the sensitization of arenedi-azonium ions by semiquinone, formed photolytically from 1,4-benzoquinone (Jir-kovsky et al., 1981). 

A proof for the formation of alkyl radicals was found by their addition to the aci-nitromethane anion (CH2=N02 ) and by their reaction with p-benzoquinone to give the optically active nitroalkane radical-anion and the semiquinone radicals, respectively. In the case of di-r-butyl sulfoxide the f-butyl radical was observed directly by its absorption spectra. 

Scheme 2.7). The phenols were formed during isolation (chromatography on silica gel) from the corresponding cycloadducts. In the reaction with p-benzoquinone, a product was unexpectedly obtained from a hetero-T>ie s-Alder reaction with the quinone acting as a carbonyl dienophile. 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

An interesting example of accelerating a reaction when high pressure is applied is the synthesis of a series of highly functionalized 4a,5,8,8a-tetrahy-dro-l,4-naphthalenediones 10 by cycloaddition of p-benzoquinone (8) with a variety of electron-poor dienic esters 9 at room temperature (Equation 5.2) reported by Dauben and Baker [6]. Using conventional methods, these heat-sensitive cycloadducts are difficult to synthesize free of the isomeric hydroquin-ones. When the reactions were carried out under thermal conditions, the primary cycloadducts were mostly converted into the corresponding hydroqui-nones. 

Benzoxazoles are produced in high yield from a-acylphenol oximes by a Beckmann rearrangement using zeolite catalysts <95SC3315>. The reaction of the o-benzoquinone 40 with aromatic aldehyde oximes produces the benzoxazoles 41 <95ZOR1060>. The fused oxazolium salts 43 (R = Me, Et, Pr , or Ph R2 = Me or Pr ) are formed from tropone and nitrilium hexachloroantimonates 42 <96JPR598>. 

Similarly, 2,3,5-trimethyl-1,4-hydroquinone (TMHQ), a key intermediate in the synthesis of vitamin E, is produced via oxidation of 2,3,6-trimethylphenol to the corresponding benzoquinone. Originally this was performed by reaction with chlorine followed by hydrolysis, but this process has now been superseded by oxidation with O2 in the presence of a Cu2Cl2/LiCl catalyst (see Fig. 2.20) (Mercier and Chabardes, 1994). Alternatively, this oxidation can also be cataly.sed by a heteropolyanion (Kozhevnikov, 1995). 

Coles, B., Wilson, I. and Wardman, P. (1988). The spontaneous and enzymatic reaction of N-acetyl-/>-benzoquinone imine with glutathione, a stopped flow kinetic study. Arch. Biochem. Biophys. 264, 253-260. 

Issa et al. [34] used 2,3-dichloro-5,6-dicyano-p-benzoquinone for the spectropho-tometric determination of primaquine and other antimalarials. The drugs were determined in tablets by a spectrophotometric method based on the reaction with 2.3-d ich loro-5.6-dicyano-p-benzoquinone and measurement of the absorbance at 460 nm. The reaction occurred fastest in methanol and acetonitrile to yield a radical anion, which was detected by electron spin resonance. The color attained its maximum intensity after 5 min and remained stable for at least 1 h. The absorbance versus concentration curve obeyed Beer s law in the concentration range 1-4 mg per 100 mL. The recovery was 99.9-102.6%. 

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