3.5- Di-f-butyl-l,2-benzoquinone

Reaction of 4,6-disubstituted-3-methoxy-l,2-benzoquinones with dialkyl azodicarboxylates and triphenylphosphine gives dihydro-1,2,3-benzoxadiazoles 99 in good yield (64—94%) (Scheme 25). A multistep mechanism leading to the zwitterionic intermediates 98, which then cyclise to the products 99, has been proposed (05OL5139). An N,N-diphenyl-2,3-dihydro-l,2,3-benzoxadiazole has been obtained (40% yield) by reaction of 3,5-di-f-butyl-l,2-benzoquinone with W-phenyliminophosphorane (PhN - PPhg) (see also Section 3.1.3.2) (02SC2779). 

Reaction of 3,5-di-f-butyl-l,2-benzoquinone with pyridine cupric methoxychloride and anhydrous ammonia in pyridine gives the imide 159 (-50%) (81JA5795). The lactone dimer 160 is formed in 51% )deld from 3,6-di-f-butyl-l,2-benzoquinone and 3,3-dimethylbut-l-ynyl-lithium (99MI350). 

A number of studies of the oxidation of 3,5-di-f-butyl-l,2-benzoquinone with hydrogen peroxide (81JCS(P2)1176, 82JOC3766, 83JA5035,... 

Choice of a quinone reagent should be made by considering its solubility, stability, and reduction potential in a test solution. The use of 1,4-benzoquinone or 1,2- benzoqui-none is not favorable, because both of them are rather unstable in aqueous and alcoholic solutions. Instead, 2-methyl-l,4-naphthoquinone (so-called vitamin Kj, abbreviated as VK3) and 3,5-di-f-butyl-l,2-benzoquinone (DBBQ) are preferred, since they facilitate acid determination due to their stability and solubility in solvents. Each of them gives a well-defined reduction peak on its voltammogram (see Figure 36.1, curve a) and is available commercially at a reasonable cost. 

F. tert-Butyhyanoketene. Tj-pically the ketene is prepared by dissolving 1 g. of 2,5-diazido-3,6-di-starting material as well as the intermediate cyclopentenedione is followed by thin layer chromatography v(Note 17). When the cyclopentenedione is no longer detectable, after approximately 90 minutes, the heating is stopped. The solution contains ferf-butylcyanoketene in amounts equivalent to at least a 95% yield (Note 18). 

Diazido-3,6-di-tert-butyl-l,4-benzoquinone refluxed 45 min. in anhydrous benzene tert-butylcyanoketene. Y almost 100%. - The product is a surprisingly stable yet very reactive ketene. F. e. and reactions of the product s. H. W. Moore and W. Weyler, Jr., Am. Soc. 92, 4132 (1970). 

Anhydrous HCl rapidly bubbled through a soln. of 3-bromo-2,5-di-/ r/-butyl-1,4-benzoquinone in glacial acetic acid during 1 hr., and the intermediate quinol oxidized with N2O4 in chloroform -> 2-diloro-3-bromo-5-tert-butyl-l,4-benzo-quinone. Y 75%. F. e. s. H. W. Moore et al., J. Org. Chem. 57, 1984 (1972). 

The base-induced rearrangement of the 1,3-dipolar cycloadducts of 2,5-di-f-butyl-/ -benzoquinone with 2,6-dichlorobenzonitrile oxide has been shown to afford isoxazole-fused / -quinol derivatives (250) by way of the pathway outlined in Scheme 64. Reduction of 2,4-dihydro-l//-pyrazino[2,l-fc]quinazoline-3,6-diones (251) with sodium borohydride has been reported to yield 11,1 la-dihydro derivatives... 

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