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Five 3-(2-hydroxy-4-methoxyphenyl)coumarins (Ia–Ie) were oxidized with lead tetraacetate to the corresponding 6H-benzofuro[3,2-c][1]benzopyran-6-ones (IIa–IIe) and 3-(1-acetoxy-4-methoxy-2-oxo-3,5-cyclohexadienyl)coumarins (IIIa, IIIb, and IIId). Four 3-(3,4-dimethoxy-2-hydroxy-phenyl)coumarins (If–Ii) gave the corresponding 3-(1-acetoxy-3,4-dimethoxy-2-oxo-3,5-cyclohexadienyl)coumarins (IIIf–IIIi).
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Kurosawa et al. (1976) studied this question.
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