Quinone acetals

The procedure described is essentially that of Belleau and Weinberg and represents the only known way of obtaining the title compound. One other quinone acetal, 1,4,9,12-t6traoxadispiro[4.2.4.2]tetradeea-6,13-diene, has been synthesized by a conventional method (reaction of 1,4-cyclohexanedione with ethylene glycol followed by bromination and dehydrobromination ) as well as by an electrochemical method (anodic oxidation of 2,2-(l,4-phenylenedioxy)diethanol ). Quinone acetals have been used as intermediates in the synthesis of 4,4-dimethoxy-2,5-cyclohexadienone,. syw-bishomoquinone, - and compounds related to natural products. ... 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

Quinolines, 0-halo, 56, 34 Quinone acetals, 57, 94 o-Quinones, 58, 125 Quinones, alkylation of, 56, 68... 

The cycloaddition reactions of 2,5-bis(trimethylsilyloxy)furans have been discussed (Section 3.11.2.7.1). The reactions of these compounds with aldehydes and ketones and with quinone acetals have also been investigated (80TL3431). 

The N-benzylcinchonidinium catalyst 70 was successfully employed by Barrett et al. in the synthesis of (—)-preussomerin G [112]. As a key step the epoxide 72 was obtained from the quinone acetal 71 in 81% yield and with 95% ee in the presence of 10 mol% of the ammonium salt 70 (Scheme 10.15). Adam et al. recently reported the highly enantioselective epoxidation of isoflavones [113]. The best results, i.e. ee up to 98% with essentially quantitative yields, were achieved when... 

Both o- and p-quinone acetals, also prepared in situ, reacted with 3-cyanophthalide affording hydroxy-anthraquinones [38],... 

The proximity effect of the functional groups in 47 is instructive. Thus, A-aryl aldimines of the type, ArCH=NAr, are reported to give azoarenes [Ar N=NAr ] and aldehydes [ArCH=0] with DAIB (77IJC(B)376), while phenols are oxidized to quinones, quinol ethers, or quinone acetals, depending on the nature of the reaction medium (92MI2, 01OR327). 

DAIB and BTIB oxidations of phenols proceed through aryloxyiodane 129 and/or aryloxenium ion 130 intermediates and are quite useful for the preparation of quinones, quinol ethers, and quinone acetals (e.g., Scheme 39) (88TL677, 92MI2, 93JCS(P1)1891, 01OR327). When phenols bearing nucleophilic side chains are used as substrates, such oxidations provide fertile ground for the assembly of heterocyclic structures. This can be accomplished by oxidative-cyclization reactions of different types. 

Intramolecular isotope effects on loss of ketene have also been reported for phenyl and quinone acetate anions. Again, the methyl group in the acetate was present as CH2D. With the o-acetate phenoxide ions, the isotope effects were in the range 2.0—2.5 [46] and with quinone acetate ions, in the range 1.5—2.5 [920]. These isotope effects were much the same for source reactions and metastable ions. 

T=80-1500C) Hydrogenation of p-carboxy-benzaldehyde to p-toluenic acid (T=254 C) Hydrogenation of furanto tetrahydrofuran (T=1400C) Hydrogenation of quinone. acetic anhydride to 2-methyl-1,4 ... 

Hydrogenation of quinone acetic anhydride to vitamin K (T=132oC) Pd-Ni y=95% (vs. 80% for traditional process) Gryaznov, 1986... 

Isocorypal1ine (4, R =H, R =Me) has been oxidised by lead (IV) -acetate to the quinone acetal (5) which suffers Thiele acetylation to a product that may be hydrolysed and O-methylated to tehuanine (H. Hara et al.. Heterocycles, 1982, ]] (Special Issue), 293. Corypalline (4, R =Me, R =H) with lead tetra-... 

Cycloaddition of diphenylisobenzofurans to (101), to sulphenes, to 2-phenyl-3-spirocyclopropylazirines, and to o-benzoquinonedi-imines have been reported. The preparation and trapping of 1-methoxy- and 1-benzyl-isobenzofuran have been described and a high degree of regio-and stereo-specificity is observed in the reactions of a range of 1-substituted isobenzofurans with quinone acetals, for example in the formation of... 

While the Engler indole synthesis has not been pursued by others, Kita and coworkers developed a related intramolecular amine cyclization onto benzoquinones leading to 5-oxygenated indoles (Scheme 4) [7]. The yields are excellent, and several examples are shown (yields are overall from the starting quinones or quinone acetal). 

Kita has described an excellent synthesis of 5-oxygenated indoles viathephenyliodine (III) bis(trifluoroacetate) (PIFA) oxidation of 2-aminoethyl oxygenated phenols (Scheme 2, equations 1 and 2) [5]. This method was extended to the preparation of 5-hydroxyindole, 5-methoxyindole, and 7-bromo-5-hydroxyindole in 65% to 100% yield from the requisite quinone or quinone acetal. 

Whereas the chalcones represent model substrates for the evaluation of new oxidations protocols, many authors have applied these exact or similar conditions to the epoxidation of naphthoquinones and quinone derivatives. Thus, Taylor s group epoxidized quinone-acetal 97 (Scheme 12.23) in good ee (89%) and moderate yield (32%). Epoxide 98 was used in the synthesis of (+)-Manumycin A [154]. Other naphthoquinones have been oxidized in good yields using similar conditions and slight modifications on the cinchona catalyst [155]. 

Wessely and co-workers [90] have applied TLC on a preparative scale instead of fractional distillation in order to isolate the hydro-quinone acetate formed during treatment of hromo phenols with lead tetraacetate. They worked with benzene on silica gel G layers and visualised by spraying with diazotised sulphanilic acid or a I % solution of p-dimethylaminobenzaldehyde (Rgt. No. 72). 

Substituted 2,5-dimethoxyacetophenones can be oxidized directly to quinone acetic acid esters (156) by using a modification of Taylor and McKillop s original procedure for the oxidation of acetophenones to phenyl acetates by thallium(ili) nitrate. The product is usually contaminated with small amounts of the a-methoxy-derivative cf. 3, 75). 

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