Lewis-acid-catalysed reaction of oxygen with 1, 3-diphenylisobenzofuran, tetraphenylfuran and tetraphenylcyclopentadiene

1, 3-Diphenylisobenzofuran is converted into o-dibenzoylbenzene (40-70%) by oxygen, either in dichloromethane at -78° in the presence of 1-2 equiv. of a variety of Lewis acids, or on silica gel at 20°, in the dark. Tetraphenylfuran is converted into (Z)-dibenzoylstilbene (55-65 %) under the former conditions in the presence of 0 05 equiv. of I₂ or SnI₄ under irradiation from a tungsten lamp. The conversions, which correspond to a mono-oxygenation process, do not involve as intermediates the furan endoperoxides, which were independently generated from the furans by use of singlet oxygen. 2, 5-Dimethylfuran and p-menthofuran do not react with oxygen in the presence of Lewis acids under the foregoing conditions. Tetraphenylcyclopentadiene is converted by oxygen into the corresponding endoperoxide (70-85%) in the presence of 0.05 equiv, of I₂ and SnI₄ or 0.1 equiv. of Ph₃C⁺BF₄- in dichloromethane at -78° under irradiation.