SYNTHESIS OF FLAVONES VIA APPLICATION OF THE NITRILE OXIDE AND THE STILLE REACTIONS

Hydroxylated and methoxylated benzaldehyde oximes have been chlorinated, dehydrohalogenated and cycloadded to tributylstannylacetylene to give 3-aryl-5-tributylstannylisoxazoles in good to excellent yields. The palladium-catalyzed coupling reaction, the so-called Stille reaction, of hindered and electron-rich 2-iodophenols and a 2-iodo-1,4-benzoquinone with 3-aryl-5-tributylstannylisoxazoles gave 3-aryl-5-(2-hydroxyaryl)isoxazoles in moderate to excellent yields. The coupling reaction was studied under various conditions and with various Pd(II) and Pd(0) complexes. Reduction of the 3,5-diarylisoxazoles with H-2/Raney-Ni, hydrolysis and acid-catalyzed cyclisation gave flavones. The synthesis of 2-iodo-3,5-dimethoxy-1,4-benzoquinone is described.