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In tetrahydrofuran; sodium hydrogencarbonate; ethyl acetate;
(a) (R)-4-Amino-2-[[(benzyloxy)carbonyl]amino]-4-oxobutanoic acid A solution of benzyl chloroformate (58.32 ml., 0.41 mole) in tetrahydrofuran (50 ml.) is added dropwise to a solution of D(-)asparagine hydrate (50 g., 0.33 mole) dissolved in 1 l. of saturated sodium bicarbonate. The solution is allowed to stir at room temperature for 6 hours and then extracted with ethyl acetate (3*200 ml.). The aqueous layer is concentrated in vacuo to remove traces of ethyl acetate, then acidified to pH of 3. A white precipitate is collected and subsequently washed with water and then dried over phosphorus pentoxide to give 74 g. of (R)-4-amino-2-[[(benzyloxy)carbonyl]amino]-4-oxobutanoic acid, m.p. 206-210.