Pyridazinone dicarboxylic acids

Practically all pyridazine-carboxylic and -polycarboxylic acids undergo decarboxylation when heated above 200 °C. As the corresponding products are usually isolated in high yields, decarboxylation is frequently used as the best synthetic route for many pyridazine and pyridazinone derivatives. For example, pyridazine-3-carboxylic acid eliminates carbon dioxide when heated at reduced pressure to give pyridazine in almost quantitative yield, but pyridazine is obtained in poor yield from pyridazine-4-carboxylic acid. Decarboxylation is usually carried out in acid solution, or by heating dry silver salts, while organic bases such as aniline, dimethylaniline and quinoline are used as catalysts for monodecarboxylation of pyridazine-4,5-dicarboxylic acids. 

Saturated or unsaturated 1,4-dicarboxylic acids also serve as starting material for 3-hydroxy-6(l f/-pyridazinones. A detailed investigation of the reaction between succinic anhydride and phenylhydrazine revealed that, in glacial acetic acid at room temperature, five products are formed two of them are pyridazines 2 and 3 [84IJC(B/439]. Other hydrazines and... 

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