A solution of ((1S)-1-azidomethyl-2methylpropyl)-carbamic acid t-butyl ester (3.26 g, 13.3 mmol) (obtained as described in Reference Example 61(2)) in methanol (160 ml) was subjected to catalytic hydrogenation in the presence of 10% palladium on charcoal (3.26 g) at room temperature for 1 hour. After checking the completion of the reaction, the reaction mixture was filtered in order to remove the catalyst and the filtrate concentrated under reduced pressure. To a solution of the obtained crude product in methylene chloride (160 ml) were added chloroformic acid p-nitrobenzyl ester (4.31 g, 20.0 mmol) and triethylamine (2.80 ml, 20.0 mmol) in an ice bath, and then the mixture was stirred at room temperature overnight. After checking the completion of the reaction, the reaction mixture was partitioned between ethyl acetate and saturated aqueous sodium hydrogencarbonate solution. The obtained organic layer was washed with saturated aqueous sodium chloride solution, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The residue was purified by chromatography on a silica gel column using n-hexane:ethyl acetate (8:1→1:1) as the eluant to afford ((2S)-2-t-butoxycarbonylamino-3-methylbutyl)carbamic acid p-nitrobenzyl ester (3.13 mg, yield 62%) as pale yellow crystals.
