Photochemical reactions. Part 21. Photochemical isomerization of pyridazinium and triazinium betaines

Abstract

Irradiation of 5-oxidopyridazinium betaines (1) results in the smooth isomerization leading to pyrimidin-4(3H)-ones (2). When 1 -oxidophthalazinium betaine (6) and 3-oxidopyridazinium betaine (11) are photolysed in water, the corresponding phthalazin-1 (2H)-one (8) and pyridazin-3(2H)-one (13) are isolated in high and low yields, respectively. Upon employment of acetonitrile instead of water as solvent, irradiation of the betaines (6) and (11) gives the stable fused diaziridines (7) and (12) which are converted easily into the phthalazin-1 (2H)-one (8) and pyridazin-3(2H)-one (13) by the action of water. Photolysis of 4-oxido-1,2,3-benzotriazinium betaines (15) causes photoisomerization to give benzotriazin-4(3H)-ones (16) in high yields.