The invention discloses a preparation method of
cimetidine. The method comprises the following steps: (1) carrying out
condensation reaction on 2-(methylimidazole-4-yl) methyl thioethylamine and N-
cyanamide dialkyl
carbonate in hot
alcohol dehydratum, distilling out
alcohol at ordinary pressure, cooling to a
room temperature, crystallizing and filtering to obtain N-cyanogroup-N'-[2-(5-methylimidazole-4-yl) sulphur methyl ethyl]-O-R isourea; (2) carrying out
amination reaction on the N-cyanogroup-N'-[2-(5-methylimidazole-4-yl) sulphur methyl ethyl]-O-R isourea and
methylamine in a reaction medium, after reaction is ended, distilling
methylamine at ordinary pressure, cooling to
room temperature and crystallizing to obtain a
cimetidine crude product; and (3) decoloring the
cimetidine crude product in absolute ethyl
alcohol, cooling, crystallizing,
drying in vacuum, and crushing to obtain a cimetidine refined product. According to the preparation method, the environment
pollution caused by by-product methyl mercaptan /
sodium methyl mercaptide in an existing cimetidine production process can be solved; the production process is simplified; the cost is reduced; and the product quality is improved.