In a flask under argon was placed (S)-2-benzyloxycarbonylamino-4-hydroxy-butyric acid methyl ester (535 mg, 2.0 mmol) in tetrahydrofuran (5 mL). To this mixture was then added imidazole (330 mg, 4.8 mmol) and t-butyl dimethyl silyl chloride (360 mg, 2.4 mmol) and within a minute solids started to appear and N,N-dimethylformamide (1 mL) was added to solubilize the material. The mixture was stirred for 16 h at 25° C. and then diluted with water (50 mL) and the aqueous layer extracted with diethyl ether (2×50 mL) dried over sodium sulfate, filtered and concentrated in vacuo with silica gel (2 g). Purification by Biotage flash chromatography (Aspire 40 g column, 16% ethyl acetate/hexanes) afforded (S)-2-benzyloxycarbonylamino-4-(t-butyl-dimethyl-silanyloxy)-butyric acid methyl ester (478 mg, 63%) as a colorless oil.
