To 40 ml of tetrahydrofuran were added 1.54 ml (11 m mol) of diisopropylamine and 6.9 ml of 1.6M butyllithium-hexane solution, followed by stirring for 10 minutes under a stream of nitrogen. A solution of 2.43 g of methyl (S)-N-tert-butoxycarbonylpyroglutamate in 15 ml of tetrahydrofuran, was added and stirred for 10 minutes at -78° C. and further for 20 minutes at -40° C. To the reaction mixture which was again cooled to -78° C. was added dropwise a solution of 1.12 ml (11 m mol) of benzaldehyde in 4 ml of tetrahydrofuran. After stirring for 10 minutes, the reaction mixture was diluted with a saturated ammonium chloride solution, stirred until the temperature of the mixture reached room temperature and extracted with ethyl acetate. The combined organic layers were washed with water and then a saturated saline solution, dried over sodium sulfate and distilled to remove the solvent. The residue was subjected to a silica gel column (2.5×40 cm) chromatography (developed by ethyl acetate-hexane=1:3→1:2→1:1) to obtain 2.26 g (65%) of methyl (S)-N-tert-butoxycarbonyl-4-(α-hydroxybenzyl)pyroglutamate as colorless oil.
