The base-catalysed condensation of 3-carbamoyl-2-iminochromen with aromatic aldehydes yields 2-aryl-4-oxo-2,3-dihydrobenzopyrano[2,3-d]pyrimidines, which isomerise to stable 2-aryl-4-hydroxy-5H-benzopyrano[2,3-d]pyrimidines. The synthesis of 3-oxo-2H-benzopyrano[2,3-c]pyrazole is also described.
C. N. O'Callaghan, J. Chem. Soc., Perkin Trans. 1, 1980, 1335 DOI: 10.1039/P19800001335
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