The product (6.0 g) obtained in the above (3) is dissolved in acetonitrile (10 ml), and thereto is added dropwise a solution of methanesulfonic acid (8.181 g) in acetonitrile (15 ml) at a temperature below 20° C., and the mixture is stirred at room temperature for 30 minutes. At a temperature below 20° C., to the mixture are added dropwise DMF (20 ml), triethylamine (8.608 g) successively, and the mixture is stirred for 10 minutes. To the mixture are added N-Boc-β-alanine (3.547 g) and HOBT.H2O (3.13 g), and then further added thereto WSC.HCl (3.928 g) at a temperature of from 5° to 10° C., and the mixture is stirred for 30 minutes. The mixture is further stirred at room temperature for 12 hours, and poured into water, and extracted three time with ethyl acetate. The organic layer is washed successively with 1N HCl (twice), a saturated aqueous sodium hydrogen carbonate solution (twice) and a saturated brine (twice), and dried over anhydrous magnesium sulfate. The desiccant is removed by filtration, and the filtrate is concentrated under reduced pressure. The residue is purified by silica gel column chromatography (silica gel; 400 g, solvent; chloroform/acetone=3:1 to 2:1). The fractions containing the title compound are concentrated under reduced pressure to give the title compound (6.974 g).
