Ammonium Chloride → (3-Bromo-6-methoxy-pyrazolo[5,1-b]thiazol-7-yl)-carbamic acid tert-butyl ester

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To a mixture of tert-butyl (6-methoxypyrazolo[5,1-b][1,3]thiazol-7-yl)carbamate (16.5 g, 61.4 mmol) and THF (410 mL) was added n-butyllithium (2.77 M solution in n-hexane: 62.1 mL, 172 mmol) while stirring at −78° C. After further stirring the mixture at −78° C. for 40 minutes, 1,2-dibromotetrafluoroethane (10.2 mL, 86 mmol) was added and stirring was continued for 2 hours while heating to room temperature. A saturated aqueous solution of ammonium chloride and ethyl acetate were added to the mixture, and then acetic acid was added to a weakly acidic pH of the mixture. After thoroughly shaking the mixture, the organic layer was separated and the organic layer was washed with brine and dried over anhydrous magnesium sulfate. The mixture was filtered, and then the solvent in the filtrate was distilled off under reduced pressure. The residue was purified by silica gel column chromatography (mixture of n-heptane and ethyl acetate: n-heptane/ethyl acetate=1/0 then 4/1) to obtain the title compound (14.3 g, 41.1 mmol).

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