Synthesis of Pyridines via Rh-Catalyzed Ring Expansion of Isoxazoles

Significance

Reported is the one-pot synthesis of highly functionalized pyridines via the ring expansion of isoxazoles using Rh catalysis. Optimal conditions were found to be slow addition of the diazo compound 1 to a toluene solution of the catalyst and isoxazole 2 at 60 ˚C to ensure complete formation of the Rh carbenoid. Intermediate 3 may be isolated. Increasing the temperature to reflux effects cyclization to the dihydropyridines 4, which were oxidized with DDQ to the aromatic derivatives 5 in moderate to excellent yield. The substrate scope was widely studied, with the lower yields observed for the more electron-rich hetero­aromatic vinyldiazoacetates.