Ethyl 1‐aryl‐5‐cyano‐1,6‐dihydro‐4‐methyl‐6‐oxo‐3‐pyridazinecarboxylates 1 react with elemental sulphur in ethanolic basic solutions to yield thieno[3.4‐d]pyridazines 2. Compounds 2 readily react with electron‐deficient olefins to yield phthalazines.
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Elnagdi et al. (1989) studied this question.
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