Pyrazines 3-Pyridazinone

Pyrimidines (237) react to give monoadducts but in the case of pyrazines (238) the monoadducts proved to be highly reactive to further cycloaddition (134). The 5-substituted pyridazinones (239) (X=EtS02, I, pyrrolidino. Cl) undergo cycloaddition to the C=C bond as shown followed by elimination of HX to give fused pyrazoles (135). 

B—1) Special products such as for instance thiadiazoles, isoxazo-les, pyridazinones, pyrazines, pyrazoles, etc. 

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