N,N-dicyclohexylcarbodiimide (10.83 g, 52.5 mmol), N-hydroxybenzotriazole (2.03 g, 15 mmol) and triethylamine (7.7 ml, 55.2 mmol) were added to a solution (130 ml) of N-(tert-butoxycarbonyl)-3-(thiazol-4-yl)-L-alanine (1) (13.62 g, 50 mmol) obtained by the method described in literatures (J. Am. Chem. Soc. 73, 2935 (1951) and Chem. Pharm. Bull. 38, 103 (1950)) and 2(R)-2-methylpyrrolidine p-toluenesulfonic acid (2) (12.79 g, 50 mmol) obtained by the method described in a literature (Helv. Chim. Acta, 34, 2202 (1951)) in tetrahydrofuran. The mixture was stirred for 20 hours at room temperature. After the precipitates are filtered off, the obtained filtrate was concentrated under reduced pressure. Thus-obtained residue was dissolved in ethyl acetate (200 ml) and the solution were washed with an aqueous solution of sodium hydrogencarbonate and water, successively. The organic layers were dried over magnesium sulfate and concentrated under reduced pressure to give a title compound (3) (16.45 g, 100%) as oil.
