To a 60% mineral oil dispersion of sodium hydride (1.9 g) in a tetrahydrofuran (50 ml) solution, methyl iodide (2.7 ml) was added under ice cooling, then the (2S)-3-[(tert-butoxycarbonyl)amino]-2-(5-chloro-2-methoxybenzyl)propanoic acid (5 g), described in WO 06-059801A, in a tetrahydrofuran (20 ml) solution was slowly added, then the mixture was stirred at room temperature for 72 hours. To the reaction solution, water (50 ml) and ethyl acetate (20 ml) were added and the layers were separated. The organic layer was extracted with water (20 ml) and was combined with the aqueous layer previously obtained, and acidified (pH2) with 6M hydrochloric acid. This aqueous mixture was extracted with ethyl acetate, then the combined extract was washed with water, saturated aqueous sodium thiosulfate solution, and brine. The organic layer was dried over sodium sulfate, then concentrated to obtain (2S)-3-[tert-butoxycarbonyl(methyl)amino]-2-(5-chloro-2-methoxybenzyl)propanoic acid (5.28 g) (compound S17a).
