Benzofuranyi-pyran-2-ones, -pyridazines, and -pyridones from naturally occurring furochromones (Visnagin and Khellin)

The novel and versatile enaminones 2a,b were synthesized by treatment of visnaginone methyl ether la or khellinone methyl ether 1b with N,N-dimethylformamide dimethylacetal. They were reacted with hippuric acid or N-acetylglycine to yield benzofuran-5-yl-2H-pyran-2-ones 3a-d. The reaction of 2a,b with cyanoacetamide and malononitrile dimer in sodium ethoxide gave benzofuran-5-yl-pyridones 4a,b and [benzofuran-5-yl-1H-pyridine-2-ylidene] malononitrile 5a, respectively. Refluxing 2a,b with hydrazine hydrate or with hydroxylamine afforded benzofuran-5-yl-1H-pyrazoles 6a,b and benzofuran-5-yl-isoxazoles 7a,b, respectively. Moreover, 2a,b coupled with aryl diazonium salt in the presence of sodium hydroxide to yield 3-(benzofuran-5-yl)-2-aryl-hydrazono-3-oxo-propanals 8a,b which were excellent precursors for the synthesis of pyridazines 9-12. (C) 2003 Wiley Periodicals, Inc.