1.45 g (3.9 mmol) of the above N-t-butoxycarbonyl-4-t-butyldimethylsilyloxy-D-pyro-glutamic acid methyl ester were dissolved in 15 ml of anhydrous THF (tetrahydrofuran) in an atmosphere of argon gas. The resulting solution was cooled to −78° C., 2.8 ml (4.8 mmol; 1.7 mM/ml) of LHMDS (lithium hexamethyldisilazane) were added to it, and the mixture was stirred for an hour. A solution obtained by dissolving 1.30 g (4.2 mmol) of N-t-butoxycarbonyl-3-bromomethylindole in 4 ml of THF was added dropwise into the reacted solution and it was stirred at −78° C. for 25 minutes and then at room temperature for two hours. An aqueous solution of ammonium chloride was added to the reacted solution, and extraction was performed twice with 50 ml of ethyl acetate. The organic layer was washed with 50 ml of water and 50 ml of a saturated saline solution and was dried with anhydrous magnesium sulfate. The magnesium sulfate was removed by filtration, and the filtrate was concentrated under reduced pressure. The residue was refined by PTLC (preparative thin-layer chromatography) to yield 1.21 g (2.01 mmol) of N-t-butoxycarbonyl-(4R)-4-t-butyldimethylsilyloxy-4-(N-t-butoxycarbonyl-3-indolylmethyl)-D-pyroglutamic acid methyl ester. Its 1H-NMR spectrum showed a peak marked by a very small amount (several percent) of impurity beside the isomers of the compound.
