Benzoquinone ketal

Cationic [5 + 2]cycloaddition involving a styrene and p-benzoquinone ketals in the presence of a Lewis acid is the key reaction to a number of neolignans fe nin [187]. The oxygen atoms of the dioxolane function in the cycloaddend rrinforce the a character of their neighboring carbons, and this character is transmitted to the termini which participate in the C-C bond formations. 

Scheme 9 Electroorganic formation of benzoquinone ketal 0 r S ° TBDPS BDD anode "O O " (fV ° TBDPS...

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). 

Bamford-Stevens decomposition of tosylhy-drazones, 351 p-Benzoquinone, 308 Benzyl ether hydrogenolysis, 139 Benzyl thioenol ethers, 87 Birch reduction, 11, 49, 50 Birch reduction of estrone methyl ether diethyl ketal, 51... 

Hoechst113-114) took a different approach to the synthesis of / -benzoquinone. By oxidation of benzene in methanol solution in the presence of tetraalkylammonium fluorides as conductive salts p-benzoquinone tetramethyl ketal is formed. This can be converted to p-benzoquinone in a simple procedure while methanol can be recycled. 

Kinetic data are available for the hydrolysis of benzophenone diethyl ketal in various dioxane—water mixtures [164]. General catalysis cannot be detected with certainty [165, 188,189]. The activation parameters for the H30+ catalyzed reaction are AH = 13.9 kcal. mole-1 and AS = +1.4 eu as expected for an A1 reaction. Another interesting example is the acid catalyzed hydrolysis of the tetramethyl ketal of p-benzoquinone which takes place in two stages [190]... 

It is postulated that the ionic magnesium complex (4) holds the open-chain progenitor (3) in the proper conformation for intramolecular Michael condensation. Trials with other basic catalysts were unsuccessful. The product of condensation of the ketal (2) with dimethyl oxalate similarly could be cyclized to give 2,5-dihydroxy-l,4-benzoquinone (7). 

Further examples of synthetic application of the para-alkoxylation reaction include the preparation of dimethoxy ketal 231, which is an essential precursor in the enantioselective synthesis of the potent antifungal agent (-)-jesterone [297], the synthesis of various dimethoxy ketals of para- and ort/to-benzoquinones [298] and the methoxylation of various phenolic substrates, such as 232, using (diacetoxyiodo)benzene in methanol (Scheme 3.95) [299-301]. 

Likewise, the oxidative dearomatization ofpara-methoxy substituted N-protected anilines 288 using (diace-toxyiodo)benzene in the presence of methanol gives p-quinone monoimide ketals 289 (Scheme 3.120) [358]. If the oxidation of aniline derivatives is performed in the presence of water, the final isolated products are the respective / -benzoquinones or p-benzoquinone monoketals resulting from the hydrolysis of initially formed monoimide ketals 289 [358,359]. 

The Lewis acid-catalysed orientation reversal in the reaction between substituted cyclohexa-1,3-dienes and 2,6-dimethyl-l,4-benzoquinone ° has been employed in an interesting synthesis of quassin (218). ° Thus, reaction at room temperature of the diene (215) with the above quinone in the presence of an equivalent quantity of Bp3,OEt2 gave the adduct (216) which was converted by several subsequent steps into (218). In the absence of the catalyst the alternative adduct (217) was obtained. Periodic acid oxidation of substituted o-cresols ° and of 2-methoxyphenols in methanol solution affords intermediate o-quinol methyl ethers or o-quinone dimethyl ketals which dimerize to give dienediones with structures related to those of (216) and (217). Another report concerns the formation of a Diels-Alder dimer upon hypochlorite oxidation of 2,2 -methylenebis(4-methyl-6-t-butyl)phenol. ... 

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