Benzofuranyl‐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 1a 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 hydroxyla‐ mine 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.