The preparation methods are as follows. (1) Using 3-methylpyridine as raw material, N-oxygen-3-methylpyridine is first formed by reaction with hydrogen peroxide, and then in the presence of acid binding agent diisopropylamine, phosphorus oxychloride is added to the solvent as chlorination agent for cyclic chlorination to obtain a mixture of 2-chloro-5-methylpyridine and 2-chloro-3-methylpyridine, and the separation technology combining crystallization and distillation method is adopted. 2-chloro-5-methylpyridine and 2-chloro-3-methylpyridine can be obtained, respectively. Side chain chlorination of 2-chloro-3-methylpyridine leads to 2-chloro-3-trichloromethyl pyridine, and further hydrolysis leads to 2-chloro-3-pyridine formyl chloride or 2-chloro-3-pyridine carboxylic acid (2-chloronicotinic acid). (2) Chlorine gas is injected into the mixture of phosphorus oxychloride and phosphorus trichloride, and the temperature is controlled at about 60 ° C until the remaining chlorine gas escapes, cool and add niacin n-oxide in batches, and heat the mixture. Reaction at 100 ~ 105ºC for 1 ~ 1.5h, after the reaction mixture is transparent, stir and react for 30min, and reduce pressure to remove phosphorus oxychloride, the resulting residue is cooled to room temperature, and water is added to obtain the finished product.
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