Thermolysis of 5-azido-4-arylpyridazin-3(2h)-ones : An efficient and versatile synthesis of pyridazino[4,5-b]indoles

5-Azido-4-arylpyridazin-3(2H)-ones, which are easily available from 4,5-dihalopyridazin-3(2H)-ones in few steps, were found to undergo in high yields a thermally induced cyclization into pyridazino[4,5-b]indole derivatives ("aza-carbolinones") by formation of the N-5/C-5a bond via a nitrene insertion process. This new method is complementary to a previously reported pathway in which the C-4a/N-5 bond of the ring system is formed.