Silyl enol ethers quinones

Silylated cyanohydrins have also been prepared via silylation of cyanohydrins themselves and by the addition of hydrogen cyanide to silyl enol ethers. Silylated cyanohydrins have proved to be quite useful in a variety of synthetic transformations, including the regiospecific protection of p-quinones, as intermediates in an efficient synthesis of a-aminomethyl alcohols, and for the preparation of ketone cyanohydrins themselves.The silylated cyanohydrins of heteroaromatic aldehydes have found extensive use as... 

Addition of silyl enol ethers to o-quinones.1 This reaction requires mild conditions because of the sensitivity of these quinones, but can be achieved with a... 

Lead tetraacetate was employed by Stoodley and coworkers for an oxidative isomerization in their synthesis of 4-demethoxydaunomycinone (47). The diene (48) reacted with the oxirane dienophile (49) via the least hindered endo transition state to give the cycloadduct (50) in 86% yield. Hydrolysis of the silyl enol ether followed by reduction of the oxirane and introduction of the acetylene moiety gave the compound (51), which was oxidatively isomerized with LTA in acetic acid to give the quinone (52). All that remained now to complete the synthesis was conversion of the acetylene to a methyl ketone and dealkylation of the ether, llie last two steps were accomplished in an over l yield of 38%, the low yield attributable to problems in formation of the hydroxy group from the ether (Scheme 11). Bulman-Page and Ley employed LTA for a similar transformation in their synthesis of demethoxydaunomycinone and related anthracyclinones. 

The oxidation of ketones to enones via the reaction of their silyl enol ethers with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) has been suggested originally to proceed via allylic hydride abstraction [195-198]. A recent reinvestigation, however, [199] has established the intermediate formation of a substrate-quinone adduct 96 which was presumably formed from a geminate radical ion pair after electron transfer. Decomposition of the adduct then finally afforded the observed enone product 97. Recently, the critical role of solvent polarity in the formation of 97 from the PET reaction of 93 and chloranil has been identified by time-resolved spectroscopy [200]. 

Silyl enol ethers are converted to a-thioalkyl and a-thioaryl ketones via a sulfe-nylation method, driven by aromatization of an added quinone mono-0,5 -acetal in the presence of Me3SiOTf. ... 

Radical coupling followed by nucleophilic attack of hydroxyl on a quinone methide intermediate is postulated as the mechanism of the key step in the syntheses of silybin and eusiderin (eq 10). 1,4-Diketones are produced in the reaction of silyl enol ethers with Ag20 in DMSO (eq 11). ... 

Quinazoline-2,4-dione Quinazolinone Dihydro Quinoline Quinolone Quinone Selenide Selenoxide Silane Tetraalkyl Siloxy Ether Silyl Chloride Silyl Enol Ether Silyl Ether... 

Bockman, T.M., Perrier, S., and Kochi, J.K., Dehydrosilylation versus a-couphng in the electron-transfer of silyl enol ethers to quinones. Strong effect on photogenerated ion pairs, /. Chem. Soc., Perkin Trans. 2, 595,1993. 

Di-endoperoxides are also formed from the enol ethers and silyl enol ethers of phenylpyruvic add, phenylacetone, and phenylacetaldehyde however, carefully controlled photooxygenation gives the mono-endoperoxides, which can be reduced to the o-hydroxyphenyl derivatives or used as the equivalent of o-quinone methides, as in the formation of compounds 10 ... 

Making the Diels-Alder disconnection by drawing the mechanism of the reverse reaction 31a gives a new enol ether 32 and a quinone 33. The enol ether 32 is a derivative of the simple enone 34 and can be made by trapping the kinetic enolate with a suitable silyl group. 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

Dehydrosilylation versus oxidative addition of enol silyl ethers to quinones... 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

Michael additions to quinones. In the presence of TrC104, enol silyl ethers undergo 1,4-addition to benzoquinone to give adducts that cyclize to benzofurans.1 A similar reaction with diimidoquinones produces indole derivatives. 

Arisawa, M., Ramesh, N.G., Nakajima, M., Tohma, H., and Kita, Y, Hypervalent iodine(lll)-induced intramolecular cychzation of a-(aryl)alkyl-P-dicarbonyl compounds a convenient synthesis of benzarmulated and spirobenzannulated compounds, /. Org. Chem., 66, 59, 2001. Eberson, L., Reaction between organic and metal ion species, in Electron Transfer Reactions in Organic Chemistry, Reactivity and Structure, Vol. 25, Hafner, K., Lehn, J.-M., Rees, C.W., von Rague-Schleyer, R, Trost, B.M., and Zahradnik, R, Eds., Springer-Verlag, Berhn, 1987, chap. 7. Bockman, T.M., ShuMa, D., and Kochi, J.K., Photoinduced electron transfer from enol silyl ethers to quinone. Part 1. Pronounced effects of solvent polarity and added salt on the formation of a-enones, J. Chem. Soc., Perkin Trans. 2, 1623, 1996. 

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