An improved process of synthesis of 5-methoxy-2-[(4-methoxy-3,5-dimethyl-2-pyridyl)methyl]sulfinyl-1H-benzimidazole (omeprazole) by oxidation of 5-methoxy-2-[[(4-methoxy-3,5-dimethyl-2-pyridyl)methyl]thio]benzimidazole with 3-chloroperoxy-benzoic acid in ethyl acetate wherein omeprazole is poorly soluble, at a temperature between -10° C. and 5° C. is disclosed. The second step is a purification of the crude product by dissolution and reprecipitation of the final product.
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