Diisopropylethylamine → N-[[1-[(2-Benzothienylcarbonyl)amino]cyclohexyl]carbonyl]-L-methioninal

Under an argon gas atmosphere, 245 mg (1.9 mmol) of N,N-diisopropylethylamine was added dropwise to a solution of 297 mg (1.9 mmol) of sulfur trioxide-pyridine complex in 10 ml of anhydrous dimethyl sulfoxide and 5 ml of anhydrous methylene chloride under ice-cooling, and the mixture was stirred for 15 minutes. Further, under ice-cooling, 100 mg (0.23 mmol) of N-[[1-[(2-benzothienylcarbonyl)amino]cyclohexyl]carbonyl]-L-methioninol was added to the reaction solution, and the mixture was stirred at the same temperature for 2 hours. The reaction solution was poured to ice-water and extracted with ethyl acetate twice. The organic layer was washed with a 10% aqueous citric acid solution, a saturated aqueous sodium hydrogencarbonate solution and then saturated brine, and it was dried with anhydrous magnesium sulfate, the solvent was distilled off under reduced pressure. 20 ml of diisopropyl ether was added to the residue, and the mixture was stirred at room temperature for 18 hours. The obtained crystal was collected by filtration to quantitatively obtain the title compound.