A mixture of pyrmetazOle (0.108 mole) and potassium bicarbonate (0.13 moles) in 115 mL of water and 180 mL of methylene chloride was cooled to 0° C. to 4° C. while being stirred. A solution of MCPBA (0.108 moles, 72% pure) in 51 mL of methylene chloride and 13.3 mL of ethanol was added dropwise over 3 hours while the temperature was maintained between 2° to 4° C. After the addition was complete the reaction mixture was stirred an additional 0.5 hour and analyzed by HPLC to ascertain the extent of conversion. The reaction mixture was then diluted with 115 mL of cold water, and the pH was adjusted to 13-13.5 with approximately 15 mL of 50% aqueous sodium hydroxide (NaOH) while the reaction temperature was maintained between 2° to 5° C. The reaction mixture was then stirred an additional 15 minutes and the layers were separated. The organic layer was extracted with pH 13.5 NaOH in water. The basic aqueous phases were combined and treated with approximately 12 mL of methyl formate which was added over a 4 hour period. When the reaction mixture pH reached approximately 10.8, the reaction mixture was seeded with omeprazole. When the methylformate addition was complete the product was collected by filtration and rinsed with water and methanol. Drying provided 32.2 g of crude omeprazole (86% yield).
