To a solution of tert-butyl 4-(1-aminopropyl)-2-nitrobenzoate (5.6 g) in 98% hydrous methanol (112 ml), D-tartaric acid (1.8 g) was added at room temperature, and the mixture was stirred at 60° C. for 1 hour and further stirred at room temperature for 5 hours. The precipitated crystals were collected by filtration and dried to obtain the title compound (3.4 g) as a colorless crystal. A small amount of obtained crystal salt was conveted to corresponding free form, and optical purity of obtained dree form was determined by high performance liquid chromatography (HPLC), whereupon it was found to be 62.2% ee. The obtained crystals (3.4 g, 62.2% ee) were suspended in 80% hydrous methanol (68 ml), and the resulting suspension was stirred under heating and refluxed for 1 hour, then further stirred at room temperature for 5 hours. The precipitated crystals were collected by filtration and dried to obtain the title compound (2.7 g, 94.4% ee) as a colorless crystal. To improve the optical purity, the obtained crystals (2.7 g, 94.4%ee) were suspended in 80% hydrous methanol (54 ml), and the resulting suspension was stirred under heating and refluxed for 1 hour, then further stirred at room temperature for 5 hours. The precipitated crystals were collected by filtration and dried to obtain the title compound (2.5 g, 99.1% ee) as a colorless crystal.
