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温度

0 °C

Intermediate 4 (1.56 g, 5.46 mmol) was dissolved in a mixture of DCM (25 mL) and pyridine (10 mL) and cooled using an ice bath. A solution of 4-nitrophenyl chloroformate (705 mg, 3.50 mmol) in DCM (25 mL) was added over 30 minutes. The reaction mixture was stirred for 1 hour at 0° C. and then the solvent was removed in vacuo. The resulting residue was purified by silica gel chromatography eluting with a gradient of 0-20% EtOAc in cyclohexane to gave the title compound as a yellow solid. The product thus obtained was used in the subsequent step without further analysis (2.0 g).

1. temperaturecooled
2. customthe solvent was removed in vacuo
3. customThe resulting residue was purified by silica gel chromatography
4. washeluting with a gradient of 0-20% EtOAc in cyclohexane