2.6- Dichloro-p-benzoquinone

The resin-bound 1,3-oxazinium salt 116, obtained by oxidation of 4//-l,3-oxazines 115 with 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ), behaved as /3-diketone equivalents and formed pyrazoles 117 through a functionalizing release process on treatment with hydrazines (Scheme 18). When the hydrazines were substituted (R = Me, Ph), the oxazinium salts reacted selectively to afford one regioisomer 117 <2004JC0846>. 

Figure 3. pH dependence of for the electron transfer from cw-[Et2Co(bpy)2] to p-benzoqui-none derivatives in H20-EtOH (5 1 v/v) at 298 K 2,6-dichloro-p-benzoquinone (O) 2,5-dichloro-p-benzoquinone ( ) chloro-p-benzoquinone (A) p-benzoquinone (A) methyl-p-benzoquinone ( ) and 2,6-dimethyl-p-benzoquinone ( ). 

Transient electronic spectra of the 1 1 and 1 2 complexes are also observed in the electron transfer reduction of 2,5-dichloro-p-benzoquinone and 2,5-dimethyl-p-benzoquinone [132]. Although the formation constant K for the 1 1 complex is too large to be determined, the formation constant Kj for the 1 2 complex can be determined as 4.5 M and the Kj value decreases with a decrease in the electron-donating ability of X-substituted semiquinone radical anion (X = 2,5-Mc2 > H > 2,5-Cl2) [132]. Thus, Mg + acts as a Lewis acid which can bind with the radical anion base, although the radical anion-Mg + complexes are unstable owing to the facile disproportionation [132]. The formation of such complexes is also confirmed... 

Ruthenium tetroxide (RuOa) is also utilized for phenolic oxidation. Sodium 2,6-dichlorophenoxide (750) was oxidized with RuOa in H2O to afford 2,6-dichloro-p-benzoquinone (751) (60%), while the use of acetone as a solvent provided the corresponding biphenol 752 as the only isolatable product (20%) (Scheme 148). 

An intriguing reaction occurs between dichloro-p-benzoquinones 71 and 2-[(arylamino)methylene]cyclohexanones 72 to form dibenzofurans 73, Scheme 22 (83CB152). The reaction is assumed to go via oxidation of 72 to 2-[(arylamino)methylene]-3-cyclohexenones followed by cycloaddition of 71. 

On spraying with 2,6-dichloro-p-benzoquinone-4-chloroimine, ceric sulfate reagent (Seher, 1960, 1961) and especially with antimony penta-chloride (BoUiger, 1962), the tocopherols give different colors which vary also with the adsorbant, as shown in Table IX and Figs. 7 and 8. The reaction with antimony pentachloride (20% in chloroform) depends on the number and position of the methyl gi-oup on the benzene ring and on the nature of the side chain. These color complexes are not stable, however. 

In 1985, O Malley et al. published the total syntheses of rac-averufin (103) and rac-nidurufin (104) (65). These are both early precursors of the aflatoxins in their biosynthesis. Nidurufin (104) is the direct successor of averufin (103) and the direct precursor of versiconal hemiacetal acetate (12, see Scheme 2.1). Nidurufin (104) and averufin (103) are accessible by the same synthesis route only the two last steps differ firom each other (see Scheme 2.17). The first reaction was a double Diels-Alder reaction with dichloro-p-benzoquinone (97) and two equivalents of diene 98. Then, three of the four alcohol functions were selectively MOM-protected (—> 99). The remaining alcohol was converted into the allyl ether and then subjected to a reductive Claisen rearrangement, followed by MOM-protection of the redundant alcohol ( 100). By addition/elimination of PhSeCl, 101 was formed. Deprotonation of t-butyl 3-oxobutanoate, followed by reaction with 101 yielded the pivotal intermediate 102. This could be converted into rac-averufin (103) by deprotection of the alcohols and decarboxylation at the side chain. The last step was a p-TsOH-catalyzed cyclization to give 103. By treating 102 with /m-CPBA, the double bond is epoxidized. rac-Nidurufin (104) was then formed by cyclization of this epoxide under acidic conditions. 

Fig.3. Amplitudes of the stable absorbance changes at 295 nm in BBY membranes with and without artificial electron acceptor (2,5-dichloro-p-benzoquinone), normalized as indicated. The first flash is off scale.

Photosystem II preparations were isolated using the method of Berthold et al. (3). Salt-washed PSII membranes were obtained by exposing the PSII preparations to 2 M NaCl for 30 min. (dark) at 1.5 mg Chl/ml and washed once in MES buffer (pH 6.0). For reductant treatment, intact and salt-washed PSII preparations were incubated with a reductant in the dark for different periods of time (30 min. for intact and 3 min. for salt-washed) at 2 mg Chl/ml. Oxygen evolution was assayed in a Clark-type O2 electrode in 10 mM CaCl2 and 50 mM MES using 2,6-dichloro-p-benzoquinone as accepter. 

In thylakoids the pattern of proton release into the lumen can be measured with neutral red as indicator and bovine serum albumin as membrane impermeable buffer of the bulk phase (2). With reaction centre complexes both the donor and the acceptor side of PS II are exposed to the dye. Therefore, selective buffering is impossible and it makes no difference whether one uses membrane adsorbed dyes, like neutral red, or hydrophilic ones, like bromo-cresol purple. In fact, both dyes showed the same extent and also the same kinetics of proton release. We prefered to use bromo-cresol purple which pK (6.2) is in the range of the pH-optimum for O2 evolution by reaction centre complexes. Whether we used ferricyanide alone or in combination with 2,5-Dichloro-p-benzoquinone as terminal electron acceptor, had only little effect on the signal. That ferricyanide here acted as acceptor for the PS II reaction centre complexes is demonstrated by fig. 2.. 

Antioxidants Not stated Light petroleum -ethyl acetate (9 1) (a) Ethanolic 2,6-dichloro-p-benzoquinone-4-chloramine + 2% aq. Na2B40y (b) Diazotised p-nitroaniline [11]... 

The elution solvents used were benzene-ethylacetate-acetone (100 5 2) for antioxidants and chloroform-hexane (2 1) for UV absorbers and butanol-glacial acetic acid (97 3) for organotin compounds. An ethanolic solution of 2,6-dichloro-p-benzoquinone-4-chlorimine was used as spray reagent for antioxidants and UV stabilisers. 

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