2.6- Dimethyl-1,4-benzoquinone

Several products were also detected in base-degraded D-fructose solution acetoin (3-hydroxy-2-butanone 62), l-hydroxy-2-butanone, and 4-hydroxy-2-butanone. Three benzoquinones were found in the product mixture after sucrose had been heated at 110° in 5% NaOH these were 2-methylbenzoquinone, 2,3,5-trimethylbenzoquinone, and 2,5-dimethyl-benzoquinone (2,5-dimethyl-2,5-cyclohexadiene-l,4-dione 61). Compound 62 is of considerable interest, as 62 and butanedione (biacetyl 60) are involved in the formation of 61 and 2,5-dimethyl-l,4-benzenediol (63) by a reduction-oxidation pathway. This mechanism, shown in Scheme 10, will be discussed in a following section, as it has been proposed from results obtained from cellulose. 

Treatment of 5,6-dichloro-2,3-dimethyl- -benzoquinone with sodium sulfide followed by oxidation with nitric acid yields the thianthrene derivative (60). Sulfur extrusion from 60 with peracetic acid leads to 2,3,7,8-tetramethyl-l,4,6,9-dibenzothiophene tetrone (61) (overall yield 57%). ... 

O.NiCgHgCCH3)2 N.6 ( ) mp 250° diffc sol prepd by heating a soln of 1,4-dimethyl-benzoquinone-dioxime In NaOH with K ferric cyanide soln (Ref 1, p659 Ref 2)... 

PS II particles were prepared according to Berthold et al. (U) by triton-treatment of the thylaXoids (2 mg chlorophyll/ml) in a medium containing 5 mM NaCl, 20 mM hepes buffer pH 7.5, and 50 mg/ml triton X-100 for 30 min. at 4°C and centrifuged at 40.000 g. Activity was measured in a MDBQ (methylenedioxy-dimethyl-benzoquinone) system. 

ABBREVIATIONS, PS photosystem. DCMU 3-(3,4-dichlorophenyl)-1,1-dime thylurea. MBQ methyl- -benzoq iinone. phe-BQ phenyl- -benzoquinone. DMBQ dimethyl- -benzoquinone. BQ p-benzoquinone. 

Photosystem II (PSIl) membranes which are reasonably pure and stable can be isolated from chloroplasts and cyanobacteria (2,3). These PSIl membranes contain the antenna pigments, reaction centre chlorophylls, O2 evolution complex and the two polypeptides D2 and Di, which bind the primary quinone acceptors Qa and Qb respectively. They are, therefore, capable of light-induced O2 evolution in the presence of artificial PSIl acceptors such as dimethyl benzoquinone (DMBQ) or dichlorobenzoquinone (DCBQ). The PSIl membranes can be deposited on TI02 without appreciable loss of photosynthetic activity. 

Lagrange etal. (1994) found significant amounts of superoxide anion production (measured by spectrometric recording of reduction of acetylates ferricytochrome c at 550 nm) in homogenates of rats choroid plexus incubated with 2,5-dimethyl benzoquinone (109.3 nM/min/NADPH-cytochrome P450 reductase activity) or menadione (100 nM/min/NADPH-cytochrome P450 reductase activity). 

Both of the previous examples describe the use of achiral dendralenes to synthesize racemic products. In 2007, the Sherburn group illustrated that chiral 3-substituted [3]dendralene 34 allowed for the preparation of enantiomerically pure cycloadducts (Scheme 12.6) [7]. DA addition of maleic anhydride, followed by in situ lactonization, yielded carboxylic acid 36. This was esterified with diazomethane to improve solubility, and then subjected to a high-pressure double DA sequence using 2,6-dimethyl-/ -benzoquinone, forming the C2 symmetric heptacycle 38 in 85% yield. 

Scheme 1-158. Sequential 1,2- and 1,4-addition of methyllithium and vinylmagnesium bromide onto 2,6-dimethyl-/ -benzoquinone.

Most examples of Diels-Alder reactions reported for both 2-vinyl and 3-vinylindoles involve typical electrophilic dienophiles such as benzoquinone, A"-phenylmaleimide and dimethyl acetylenedicarboxylate (see Table 16.1). T hese symmetrical dienophiles raise no issues of rcgiosclectivity. While there arc fewer examples of use of mono-substituted dienophiles, they appear to react... 

Dimethyl-l,4-benzoquinone [527-61-7] M 136.1, m 72" (sealed tube). Crystd from water/EtOH (8 1). 

Arylation of enamines with p-benzoquinones takes a somewhat different course (70). The enamine (16) reacts exothermally with p-benzoquinone in benzene solution to give 2-(dimethylamino)-2,3-dihydro-3,3-dimethyl-5-benzofuranol (92). The reaction of enamines with quinone dibenzenesul-fonimide proceeds similarly (68). The product from the enamine (28) is the tetrahydrocarbazole derivative (93). 

The benzoquinone adduct is a 2 1 molecular complex of 2,3-dimethylquinoxaline and quinol, and is readily prepared by crystallizing quinol from toluene in the presence of excess of 2,3-dimethyl-quinoxaline. 2,3-Dimethylquinoxaline and A -phenylmaleimide undergo Michael addition to form the quinoxalinylmethyl-A -phenylsuccinimide... 

Benzoquinone Fivalic [Propanoic acid, 2,2-dimethyl-] 2-tert-Butyl [2,5-Cyclohexadiene-1,4-dione, 2- (1,1 -di methy lethy 1) - ] 67" 2... 

Benzaldehyde, 43, 73, 123 Benzaldehyde diethyl acetal, 108, 110,122—3 Benzaldehyde dimethyl acetal, 34, 111 Benzenesulphonylhydrazonc, 13 Benzophenone, 137 Benzophenone cyanohydrin, 137 p-Benzoquinone, 137... 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

A = Acrylonitrile B = Benzoquinone C = Naphthoquinone D = 5,8-Quinolinequinone E = Methyl vinyl ketone F = Dimethyl acetylene dicarboxylate G = Methyl acrylate ... 

Dimethyl-1,4-benzoquinone (328) and ethylenediamine (329) gave 7,8-dimethyl-6(4//)-quinoxalinone (330) (EtOH-CH2Cl2, 20°C, light exclusion 35% the required oxidation was probably provided by an excess of the quinone). ... 

Similarly, 1-vinylcyclohexane can be trapped with dimethyl acetylenedicarboxylate in refluxing xylene to afford 195 in 78% yield (equation 126) . Benzo[ ]anthracene can be obtained by the reaction of 196 and 1,2-dihydronaphthalene (equation 127) and oxidation of 197 with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone . 

Dimethyl 2-(17/-indol-2-yl)but-2-enedioate, easily available by reaction of 4,7-dihydroindole with dimethyl acet-ylenedicarboxylate followed by oxidation with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), treated with PTAD provided compound 703, which was thermaly isomerized to 704 (Scheme 111) <2006JOC7793>. 

Figure 2.106 OTTLE cell cyclic voltammogram of 0.01 M 1,4-benzoquinone in dimethyl-suiphoxide solution containing 0.5M tetraethylammonium perchlorate. Scan rate 0.5mVs. ...

Hong and coworkers have investigated the cycloaddition chemistry of fulvenes with a wide variety of alkenes and alkynes in great detail [191]. As one example, the reaction of 6,6-dimethylfulvene with benzoquinone is shown in Scheme 6.92. Under microwave conditions in dimethyl sulfoxide (DMSO) at 120 °C, an unusual hetero-[2+3] adduct was formed in 60% yield, the structure of which was determined by X-ray crystallography. The adduct is a structural analogue of the natural products aplysin and pannellin and differs completely from the reported thermal (benzene, 80 °C) Diels-Alder cycloaddition product of the fulvene and benzoquinone (Scheme 6.92) [191]. 

Whereas for the hexamethyl compound 150 only products formed by the linear route have been detected with a sizeable number of dienophiles (X=X inter alia TCNE, maleic anhydride, benzoquinone, 1,4-naphthoquinone, acrolein, methyl acrylate102), the parent system 4 undergoes threefold Diels-Alder addition in a star-shaped manner leading to 164 with dimethyl acetylenedicarboxylate and to 165 with fumaroyl chloride followed by methanolysis (equation 20)92. 

The generation and trapping of 5,6-bis(trimethylsilyl)benzo[c]furan 126 was reported by Wong utilizing Warrener s s-tetrazine methodology. The trapping of the silylated isobenzofuran with A-phenylmaleimide is illustrated below. A number of other dienophiles such as dimethyl acetylenedicarboxylate, benzoquinone, naphthoquinone and anthra-l,4-quinone have also been used <00TL5957>. 

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