<I>t</I>-ブチルフェノール類の位置選択的酸素酸化とその合成化学的応用
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概要
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In the oxygenation of 4-alkyl-2, 6-di-<I>tert</I>-butylphenols catalyzed with <I>t</I>-BuOK in aprotic solvents such as DMF, DMSO, and HMPT, O<SUB>2</SUB> is incorporated into the para position of the phenols exclusively to give the corresponding epoxy-<I>p</I>-quinols produced by an intramolecular decomposition of <I>p</I>-per-oxide anion intermediates. 4-Aryl-2, 6-di-<I>tert</I>-butylphenols are not susceptible to the oxygenation under these conditions. In the <I>t</I>-BuOK-catalyzed oxygenation of 2, 4, 6-tri-<I>tert</I>-butylphenols and 4-aryl-2, 6-di- <I>tert</I>-butylphenols in <I>t</I>-BuOH, on the other hand, O<SUB>2</SUB> is incorporated into the <I>o</I>-position of these phenols exclusively to give epoxy-<I>o</I>-quinols. With a large excess of <I>t</I>-BuOK in DMF the methyl group of <I>p</I>-cresols is selectively oxygenated to give the corresponding 4-hydroxybenzaldehydes. When 4-alkyl-2, 6-di-<I>tert</I>-butylphenols are oxygenated in HNEt<SUB>2</SUB> containing N<SUB>a</SUB>NH<SUB>2</SUB>, the corresponding <I>p</I>-quinols are obtained in nearly quantitative yield. The <I>p</I>-quinols are converted quantitatively to hydroquinones with migration of the 4-alkyl group to the adjacent carbon on the ring. The base-catalyzed reaction of acetates of the <I>p</I>-quinols results in the selective formation of <I>m</I>-hydroxyphenylacetic acids. It has been newly found that the generation of carbanion adjacent to the carbonyl group in esters of hydroperoxides selectively formed from the <I>t</I>-butylated phenols extremely weakens the peroxy bond to give unique products even at -70°C. Epoxy-<I>o</I>-quinols are quantitatively converted to the corresponding cyclopentadienones by base catalysis at an elevated temperature. Cyclopenten-ones are also obtained in good yield from epoxyquinols. A non-radical mechanism for the incorporation of O<SUB>2</SUB> into the phenolates is argued on the basis of the regioselectivity in the oxygenation and of results obtained in the reaction of phenoxy radicals with O<SUB>2</SUB><SUP>-</SUP>.
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