Ethyl 2-[bis(tert-butoxycarbonyl)amino]-4-formyl-1,3-thiazole-5-carboxylate obtained in Example (62d) (915.5 mg, 2.29 mmol) was dissolved in diethyl ether (30 mL), followed by cooling to −78° C. Then, tetrabutylammonium bromide (890 mg, 2.76 mmol) and ethylmagnesium bromide (1 M solution in THF, 2.8 mL) were added, and the mixture was stirred for one hour. Diethyl ether was added to the reaction solution, and the organic layer was washed with brine and dried over anhydrous sodium sulfate. Following concentration under reduced pressure, the residue was purified by silica gel column chromatography (elution solvent: hexane/ethyl acetate=91/9, 85/15) to obtain 327.2 mg of the title compound (33%).
