Quinones diene additions

Ketenes participate as dienes as well as dienophiles in [4+2] cycloaddition reactions. For example, several ketenes with a suitable substituent in the a-positions readily participate in [4+2] cycloaddition reactions. The substituents include unsaturated groups, a-oxo, a-thio or a-imino groups. In their role as dienophiles, ketenes participate in the reaction with dienes, azadienes, diimines and o-quinones by addition across the C=C bonds, and sometimes the C=0 bonds. The [2+2+2] adducts obtained in the reaction of ketenes with C=N double-bonded substrates (see Section 4.1.4.2) are another example of [4+2] cycloaddition reactions of ketenes. 

A-l,3-dimethylbutyl-A -phenyl quinone diimine (6QD1) has been introduced as a multifunctional additive for diene rubbers and provides an advantage in mixing characteristics (functions as peptizer and improves scorch safety) as well as improved performance (better antioxidant activity than paraphenylenediamine antidegradants) of the end products [36]. 

Anodic addition converts enolacetates into a-acetoxyketones or enones, depending on the reaction conditions [135], conjugated dienes into 1,2- or 1,4-dimethoxy alkenes [65], and hydroquinone dimethyl ethers into quinone bisketals [136, 137]. Anodic addition also affords products, some of which are of industrial interest, such as propylene oxide [138a] or 1,4-dimethoxydihydrofuran [138b]. To... 

In contrast to Nicolaou s model system, the IMDA reaction of quinone 86 to adduct 85 proceeded through an exo transition state geometry. This difference is due to the two additional substitutes attached to the diene side chain of quinone 86. A more detailed analysis (molecular modeling) of the four theoretically possible transition states (Fig. 10) reveals that an enophilic endolexo... 

Besides alkenes and alkynes, other multiple bonds can be used as acceptors in addition reactions of carbon radicals provided the usual requirements of reactivity and selectivity are met. Other types of carbon-carbon multiple bonds that have been used as acceptors include dienes,162 allenes,61 enolates (and ni-tronates, see below) and quinones.223 Even highly strained cr-bonds have served as acceptors on occasion.224... 

The reaction has been shown to involve a stereospecific syn-addition with respect to the dienophile. For example, the reaction of 2,3-dimethylbuta-1,3-diene with maleic anhydride gives cis-l,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride (Expt 7.20). An example of the use of a quinone as the dienophile is provided by the synthesis of cis-1,4,4a,9a-tetrahydro-2,3-dimethyl-9,l0-anthra-quinone which upon dehydrogenation (most simply by the action of oxygen upon its solution in alcoholic potassium hydroxide) yields 2,3-dimethylanthra-quinone (Expt 7.21). Tetraphenylcyclopentadienone (tetracyclone) readily undergoes the addition of dienophiles, such as maleic anhydride, to give an adduct, which then extrudes a molecule of carbon monoxide on heating, as in the preparation of 3,4,5,6-tetraphenyldihydrophthalic anhydride (Expt 7.22). 

Initial addition of 2,5-dimethyl-p-benzoquinone (160) (1 equiv.) to ATPH (2.2 equiv.) in CH2CI2 at -20 °C and subsequent treatment with cyclohexadiene (1.1 equiv.) resulted in only trace amounts of the endo cycloadduct 161, because of virtually complete protection against diene approach by the efficient encapsulation of the quinone substrate (route A in Sch. 126). In marked contrast, however, mixing of cyclohexadiene (1.1 equiv.) and ATPH (2.2 equiv.) in CH2CI2 at -20 °C and subsequent addition of 2,5-dimethyl-p-benzoquinone (160) (1 equiv.) under similar reaction conditions afforded the endo cycloadduct 161 in 65 % yield (route B in Sch. 126). It should be noted that this implies the effective inclusion of the diene component by electronic interaction of the diene portion with the concave aromatic interior of the cavity of ATPH (162 in Sch. 126), ensuring smooth cycloaddition on complexation and capsule formation with quinone substrate 160. 

Conjugated dienes can be epoxidized (1,2-addition), although the reaction is slower than for corresponding alkenes, but a,p-unsaturated ketones do not generally give epoxides when treated with peroxyacids.The epoxidation of a,p-unsaturated ketones with hydrogen peroxide under basic conditions is known as the Waits-Schejfer epoxidation, discovered in 1921. This fundamental reaction has been extended to a,()-unsaturated ketones (including quinones), aldehydes, and... 

An investigation of nitrosobenzene in alkaline solution is complicated by the reaction of nitrosobenzene with hydroxyl ions [163,164]. There is a relatively fast, reversible addition of hydroxyl ion to the nitroso group. Besides that, there is in aqueous solutions at pH > 13 in the absence of dioxygen a relatively slow reaction giving mainly a mixture of azoxybenzene and 4-hydroxynitrosobenzene (p-quinone monoxime) in a reaction suggested as involving a hydride ion transfer from the initially formed 4-hydroxycyclohexa-diene oxime anion to nitrosobenzene hydride ion transfer to nitrosobenzene probably also occurs in alcoholic solutions when an alkoxide is present. In the presence of dioxygen, some nitrobenzene is formed. 

The initial attack in the anodic oxidation of papaverine [75] probably involves a similar attack further oxidation and dimerization leads to the isolated product, 12,12 -bis-(2,3,9,10-tetramethoxyindolo[2,l-fl]isoquinolyl). An analogous reaction is the electrooxidation of a tetramethoxy-substituted 2-methyl-l-phenethyl-l,2,3,4-tetrahydroisoquinoline to a dibenzoquinolizinium derivative [76] and the oxidation of A,A -triphenyl-( -phenyle-nediamine to 9,10-diphenylphenazine [77]. Intramolecular Michel addition of nitrogen in a tetrahydroquinoline derivative to an o-quinone moity have resulted in the formation of a 5,6-dihydrodibenz[6,d]indolizine derivative [78]. A similar ring closure occurs during the oxidation of various catecholamines [79] and similar compounds [79] to indoles. Cyclic a-carbonylazo compounds, generated by anodic oxidation of the hydrazines, may be trapped by reaction with dienes to the expected heterocycles [80]. 

Other carbohydrate-derived dienes in the open-chain form have also been investigated. Dienes 5, upon addition to the quinone 6, generated in situ from hydroquinone, reportedly give single adducts 7, which are epimerized on basic alumina to compounds 82. 

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