Kv. Gothelf et Kbg. Torssell, SYNTHESIS OF FLAVONES VIA APPLICATION OF THE NITRILE OXIDE AND THE STILLE REACTIONS, Acta chemica Scandinavica, 48(1), 1994, pp. 61-67
Hydroxylated and methoxylated benzaldehyde oximes have been chlorinate
d, dehydrohalogenated and cycloadded to tributylstannylacetylene to gi
ve 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-benzoqui
none with 3-aryl-5-tributylstannylisoxazoles gave 3-aryl-5-(2-hydroxya
ryl)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, h
ydrolysis and acid-catalyzed cyclisation gave flavones. The synthesis
of 2-iodo-3,5-dimethoxy-1,4-benzoquinone is described.