Benzoquinone carbonyl

The adducts obtained from acyclic dienes and cyclic dienophiles are frequently formed in accordance with the endo rule. A classic example is found in the Woodward synthesis of reserpine, which started with the Diels-Alder reaction of i -pentadienoic acid and benzoquinone (3.67). In this cycloaddition reaction, the endo adduct 78, in which the carboxylic acid and the benzoquinone carbonyl groups become cLs to one another, is obtained as the exclusive product. 

The major alternative mechanism of the Nenitzescn indole synthesis is that proposed by Steck and colleagues [7], which involves the initial obvious condensation of the aminoaoto-nate nitrogen with the benzoquinone carbonyl group to give 26. This mechanism is shown in abbreviated form in Scheme 6. The proposed steps from 26 to 27 to 28 (not shown) make no chemical sense. In any event, Allen and colleagues have shown that componnds of type 26 are not intermediates in the formation of indoles [16, 17]. Thus, whereas the reaction between qninone 29 and enaminone 30 gave amine-carbonyl condensate 31 and cycUzed carbinol amine 32, this latter compound did not fnmish indole 33 under conditions of... 

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. 

With trisubstituted benzoquinones and use of the cationic oxazaborolidinium catalyst B, 2-[tra-(isopropyl)silyloxy]-l,3-butadiene reacts at the monosubstituted quinone double bond. The reactions exhibit high regioselectivity and more than 95% e.e. With 2-mono- and 2,3-disubstimted quinones, reaction occurs at the unsubstituted double bond. The regiochemistry is directed by coordination to the catalyst at the more basic carbonyl oxygen. 

The 13C NMR spectral data for 9, 16, and 17 are shown in Table 2. The chemical shifts of carbonyl carbons of anthraquinones are characteristically observed at about 180 ppm.9 The chemical shifts for carbonyl carbons of 1,4-naphthoquinone and 1,4-benzoquinone appear at about 185 ppm, while those of carbonyl carbons adjacent to a methylene or methyl carbon are at about 200 ppm. The chemical shifts of the Cl and C4 of 9 are observed at 200.8 ppm and assigned to the 1,4-diketo form 9a. In the 13C spectrum of 17, the chemical shifts of carbonyl carbons are at 199.9 and 172.2 ppm. The former value corresponds to a carbonyl carbon adjacent to the methylene carbon, and the latter corresponds to the carbonyl carbon in the 9-position. The methylene carbons of 17 show two signals at 34.5 and 23.8ppm. From these results, 17 is considered to exist exclusively as an unsymmetrical 4,9-diketo form, 17a. Thus, these NMR spectral data suggest... 

A cyclocondensation involving the carbonyl of a benzoquinone was used for the synthesis of a cinnoline 15 of interest as a potential anti-fungal agent <06BMCL1850>. 

Benzoquinones can also serve as carbonyl compounds in this reaction. The deriving spirooxetanes can be converted to phenols (4.80)494). 

The carbonyl group of p-benzoquinone is capable of adding to dienes on irradiation t yield the spiro-compounds 36 (equation 21)23. 

The tropone ring of cyclohepta[c]thiophen-6-one reacts preferentially at the C=C bond, instead of at the carbonyl group, with both dichloro- and dibromocarbene to give mono- and bis-adducts in relatively low yields (5-40%) [60]. Benzoquinones produce anfi-bis-insertion adducts in their reaction with chloroform (95%), or bromoform (57%), under basic conditions [29]. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

It oxidizes thiophenol to diphenyl disulphide, hydroquinone to p-benzoquinone, benzoin to benzyl, benzylic alcohols to the corresponding carbonyl compounds and cleaves hydrobenzoin to benzaldehyde. 

An interesting entry to functionalized dihydropyrans has been intensively studied by Tietze in the 1990s using a three-component domino-Knoevenagel Hetero-Diels-Alder sequence. The overall transformation involves the transient formation of an activated heterodienophile by condensation of simple aldehydes with 1,3-dicarbonyls such as barbituric acids [127], Meldrum s acid [128], or activated carbonyls. In situ cycloaddition with electron-rich alkenes furnished the expected functionalized dihydropyrans. Two recent examples concern the reactivity of 1,4-benzoquinones and pyrazolones as 1,3-dicarbonyl equivalents under microwave irradiation. In the first case, a new three-component catalyst-free efficient one-pot transformation was proposed for the synthesis of pyrano-1,4-benzoquinone scaffolds [129]. In this synthetic method, 2,5-dihydroxy-3-undecyl-1,4-benzoquinone, paraformaldehyde, and alkenes were suspended in ethanol and placed under microwave irradiations to lead regioselectively the corresponding pyrano-l,4-benzoquinone derivatives (Scheme 38). The total regioselectivity was... 

The electronic and vibrational spectra of benzoquinones are very diagnostic. Of the two possible isomers remaining, the 2,5-substituted quinone (5) would be expected to give two different electronic transitions of equal intensity around 280 nm, and only one carbonyl stretching band. The Fourier transform infrared spectrum (CH2CI2) of the haustorial Inducer showed strong absorptions at 1698 (vC=0), 1646 (vC=0) and 1597 cm (vC=C). These spectroscopic data established the haustorial inducer as 2,6-diraethoxy-2"benzoquinone (2,6-DMBQ, 4). 

The synthesis of acrylic acid or its ester (228) from ethylene has been investigated in AcOH from the standpoint of its practical production 12]. The carbonylation of styrene is a promising commercial process for cinnamate (229) production[207,213,214]. Asymmetric carbonylation of styrene with Pd(acac)2 and benzoquinone in the presence of TsOH using 2,2 -dimethoxy-6, 6 -bis(diphenylphosphino)biphenyl (231) as a chiral ligand gave dimethyl phenylsuccinate 230 in 93% ee, although the yield was not satisfactory, showing that phosphine coordination influences the stereochemical course of the oxidative carbonylation with Pd(II) salt[215]. 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

Recently from a comparison of the infra-red absorption spectrum of this compound with that of o-benzoquinone Glowiak [10] came to the conclusion that dinitro-benzenediazo-oxide has a quinonoid structure. Both substances show the presence of the strong absorption band of the carbonyl group 1666 cm-1 for dinitrobenzene-diazo-oxide and 1680 cm-1 for o-benzoquinone. In addition dinitrobenzenediazo-oxide gives a band with a frequency of 2190 cm-1, characteristic of a double bond between nitrogen atoms. (Some derivatives of this compound may also have the diazo structure (Ha), which is discussed later on.)... 

Other organic mediators act as hydride atom-abstracting agents. This is true, for example, with 2,2-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and the oxoammonium ion which is anodically accessible from 2,2,6,6-tetramethylpiperidyl oxide (TEMPO). DDQ has been electrochemically regenerated either externally or internally The in situ electrochemical oxidation, of TEMPO to the active oxoammonium ion is performed in lutidine-containing acetonitrile. Thus, primary alcohols can be oxidized to the aldehydes, while secondary ones are stable Primary amines are transformed to nitriles. If water is present, the amines are cleaved via the Schiff bases to the corresponding carbonyl compounds... 

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