Quinones with chromium trioxide

Method 1. Cool a solution of 33 g (0.33 mol) of hydroquinone in 150 ml of 60 per cent acetic acid contained in a 600-ml beaker to below 5 °C in an ice bath. Dissolve 42 g (0.42 mol) of chromium trioxide in 70 ml of water, and add 30 ml of glacial acetic acid. By means of a separatory funnel with bent stem and supported over the beaker, add the chromium trioxide solution to the mechanically stirred hydroquinone solution at such a rate that the temperature does not rise above 10 °C the addition takes about 2 hours. Filter the mixture at once and wash the quinone several times with 10 ml portions of ice-cold water. Spread the material upon filter paper until dry, but no longer or the quinone will be lost through sublimation. The yield of quinone (a bright yellow crystalline solid), m.p. 115 °C, is 21 g (66%) it darkens when exposed to light. 

One final interesting application of CAN which exemplifies one of the many possible reactions of qui-nones comes from Hassall and coworkers in their synthesis of 4-demethoxydaunomycinone (47). Thus, oxidation of the boronate (42) with CAN gave the crude quinone (43) which was reacted with trans-1,2-bis(acetoxy)-l,2-dihydrobenzocyclobutene (44) to give the tetracyclic quinone (45) in an impressive 79% overtdl yield. Deacetalization and reductive acetylation to the naphthacene (46), followed by oxidation with anhydrous chromium trioxide and deprotection with boron trichloride afforded the target compound (47 Scheme 10). 

Tribromo-m-cresol is oxidized by chromium trioxide in 70% acetic acid at 70-75 °C within 10 min to dibromo-m-toluquinone (3,5-dibromo-2-methyl-p-benzophenone) in 77% yield [559]. 2-BVomo-4-hy-droxy-5-methoxybenzyl alcohol treated with Fremy salt at pH 6 at room temperature for 1 h gives an 84% yield of 2-bromo-5-methoxy -benzo-quinone [487]. 4-Benzyl-2,6-dibromophenol, on oxidation with bromine in a sealed tube, yields 2,6-dibromophenylchinomethide [732] (equation 312). The oxidation of 2,5-di-tert-butyl-4-methoxyphenol with mercuric oxide or... 

Boil the hydrocarbon (0 5 g) with glacial acetic acid (10 ml) until it has dissolved. Add chromium trioxide (1 g) in water (2 ml). Reflux gently for 15-60 minutes, allow to cool, add water if necessary to precipitate the quinone, filter, and recrystallize the derivative from glacial acetic acid. 

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