In boiling acetic anhydride o-formylphenylacetic acid is partly dehydrated to benzopyran-3-one (λmax. 441 nm.), which forms appropriate adducts with dienophiles. In sulphuric acid o-formylphenylacetic acid is completely converted into the conjugate acid of benzopyran-3-one. Experiments leading to 1-methyl-benzopyran-3-one, the more stable 1-phenyl derivative, and their conjugate acids are described.
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Holland et al. (1970) studied this question.