tert-Butyl 1-((2R)-1-(8-amino-2-oxo-1-(thiophen-3-ylmethyl)-1,2,3,4-tetrahydroquinolin-3-ylamino)-4-methyl-1-oxopentan-2-ylamino)-2-methyl-1-oxopropan-2-ylcarbamate (100 mg) and succinic anhydride (21 mg) were added to toluene (1 mL), and the mixture was refluxed for two hours while being stirred. The reaction mixture was concentrated under reduced pressure, and the thus-recovered residue was purified through silica gel column chromatography (ethyl acetate, chloroform:methanol (5:1)), whereby the Compound A (114 mg) was yielded. The Compound A (110 mg), sodium acetate (75 mg), and acetic anhydride (1 mL) were added to toluene (1 mL), and the mixture was stirred at 80° C. for 20 minutes. The mixture was cooled to room temperature, and water was added thereto. The mixture was extracted with ethyl acetate twice, and the organic layers were combined. The combined organic layer was dried over sodium sulfate, and the solvent was evaporated under reduced pressure, whereby the title compound (100 mg) was yielded.
