To a solution of L-glutamic acid 1,5-dibenzyl ester p-toluenesulfonate (10.0 g), Boc-Ser (4.1 g) and TEA (2.0 g) in DMF (200 ml) are added with stirring WSC (5.0 g) and HOBt (4.0 g) under ice-cooling. The mixture is stirred at room temperature for 20 hours, and thereto is added ethyl acetate (200 ml). The mixture is washed successively with 1N hydrochloric acid, a saturated aqueous sodium hydrogen carbonate solution, and a saturated aqueous sodium chloride solution, dried, and concentrated under reduced pressure. The precipitated solid is collected by filtration, and washed with ether to give N-tert-butoxycarbonyl-L-seryl-L-glutamic acid 1,5-dibenzyl ester (9.7 g) as colorless crystals.
