Jun.22,2006  17  US 2006/0135784 Al  analysis (area percent method) confirmed that the conver› sion ratio of erythro-3-( ethoxycarbonyl)amino-2-methane› sulfonyloxy-4-phenylbutyric acid methyl ester was 98% and  the remaining ratio of threo-3-( ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric acid methyl ester was  99%.  [0175]  The reaction solution thus obtained was concen› trated under reduced pressure, and water was added to the  resultant residue (329 mg) to obtain an aqueous solution  (1.51 g). To the solution, a 30% aqueous solution of sodium  hydroxide (672 mg) was added, and the mixture was reacted  at 60°  C. for 12 hours. Determination by HPLC revealed that  the amount of threo-3-amino-2-hydroxy-4-phenylbutyric  acid in the resultant reaction solution was 74 mg. Also, the  HPLC area ratio of threo-isomer/erythro-isomer was 93/7,  and the diastereomer excess was 87% d.e.  Example 11  Conversion of 3-( ethoxycarbony l)amino-2-methane› sulfony loxy-4-pheny lbutyric acid methyl ester  (diastereomer mixture) into threo-3-amino-2-hy- droxy-4-phenylbutyric acid  [0176]  The ethyl acetate solution (831 mg) of the diaste› reomer mixture of 3-( ethoxycarbonyl)amino-2-methane› sulfonyloxy-4-phenylbutyric acid methyl ester obtained in  Example 7 [ corresponding to 3-( ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric acid methyl ester (151  mg), calculated as an erythro-isomer; HPLC area ratio of  erythro-isomer/threo-isomer=83/17] was solvent-replaced  with toluene to obtain a toluene solution (901 mg). Pyridine  (1.69 g) was added to the solution, and the mixture was  reacted at 100°  C. for 18 hours. HPLC analysis (area percent  method) confirmed that the conversion ratio of erythro-3- ( ethoxycarbony l)amino-2-methanesulfony loxy-4-pheny lbu› tyric acid methyl ester was 98% and the remaining ratio of  threo-3-( ethoxycarbonyl )amino-2-methanesulfony loxy-4- pheny lbutyric acid methyl ester was 99%.  [0177]  The reaction solution thus obtained was concen› trated under reduced pressure, and water was added to the  resultant residue (327 mg) to obtain an aqueous solution  (1.52 g). To the solution, a 30% aqueous solution of sodium  hydroxide (665 mg) was added, and the mixture was reacted  at 60°  C. for 12 hours. Determination by HPLC revealed that  the amount of threo-3-amino-2-hydroxy-4-phenylbutyric  acid in the resultant reaction solution was 74 mg. Also, the  HPLC area ratio of threo-isomer/erythro-isomer was 93/7,  and the diastereomer excess was 87% d.e.  [0178]  The ethyl acetate solution (8.29 g) of the diastere› omer mixture of 3-( ethoxycarbonyl)amino-2-methanesulfo› nyloxy-4-phenylbutyric acid methyl ester obtained in  Example 7 [ corresponding to 3-( ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric  acid methyl  ester  (1.50 g), calculated as an erythro-isomer; HPLC area ratio of  erythro-isomer/threo-isomer=83/17] was treated in the same  manner as that in Example 5. The resultant 4-benzyl-2- oxazolidinone-5-carboxylic acid methyl ester (1.00 g) was  mixed with water (10 ml), lithium hydroxide monohydrate  (757 mg) was added to the mixture, and the mixture was  reacted at 60°  C. for 20 hours. The resultant reaction solution  was filtrated with a filter to remove insoluble matters, and  the filter was then washed with water (1 ml) to obtain a  filtrate (12.49 g) free from the insoluble matters. The amount  of 3-amino-2-hydroxy-4-phenylbutyric acid in the resultant  filtrate was 695 mg calculated as a three-isomer. Also, the  HPLC area ratio ofthreo-isomer/erythro-isomer was 83/17,  and the diastereomer excess was 66% d.e.  [0179]  To the filtrate thus obtained, cone. hydrochloric  acid was added slowly, and the pH was adjusted to pH 4.6  to precipitate crystals. Acetone (12 ml) was added to the  solution over 1 hour, the mixture was stirred for further 3  hours under ice-cooling to mature the precipitation of the  crystals well, and the precipitated crystals were then fil› trated. The resultant wet crystals were washed with acetone  (3 ml) and then dried in vacuo to obtain white crystals (564  mg). The purity of threo-3-amino-2-hydroxy-4-phenylbu› tyric acid was 96% by weight, and the water content was 2%  byweight (crystallization yield 94%). Also, the HPLC area  ratio ofthreo-isomer/erythro-isomer was 99/1, and the dias› tereomer excess was 98% d.e.  [0180]  The ethyl acetate solution (8.35 g) of the diastere› omer mixture of 3-( ethoxycarbonyl)amino-2-methanesulfo› nyloxy-4-phenylbutyric acid methyl ester obtained in  Example 7 [ corresponding to 3-( ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric  acid  methyl  ester  (1.51 g), calculated as an erythro-isomer; HPLC area ratio of  erythro-isomer/threo-isomer=83/17] was solvent-replaced  with toluene to obtain a toluene solution (8.95 g). N,N› Dimethyl formamide (10 ml) was added to the solution, and  the mixture was reacted at 80° C. for 20 hours. HPLC  analysis (area percent method) confirmed that the conver› sion ratio of erythro-3-( ethoxycarbonyl)amino-2-methane› sulfonyloxy-4-phenylbutyric acid methyl ester was 99% and  the remaining ratio of threo-3-(ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric acid methyl ester was  99%.  [0181]  The reaction solution thus obtained was mixed with  water (10 ml), and the mixture was concentrated under  reduced pressure. Lithium hydroxide monohydrate (761 mg)  was added to the resultant residue (1.03 g), and the mixture  was reacted at 60°  C. for 20 hours. The amount of3-amino- 2-hydroxy-4-phenylbutyric acid in the resultant reaction  solution was 772 mg calculated as a threo-isomer. Also, the  HPLC area ratio of threo-isomer/erythro-isomer was 94/6,  and the diastereomer excess was 88% d.e.  [0182]  To the reaction solution thus obtained, cone. hydro› chloric acid was added slowly, and the pH was adjusted to  pH 4.5 to precipitate crystals. Cone. hydrochloric acid was  further added slowly to the mixture, and the pH was adjusted  to pH 2 to dissolve most of the precipitated crystals. The  solution was filtrated with a filter to remove insoluble  matters, and the filter was further washed with water (1 ml)  to obtain a light brown filtrate (12.49 g) free from the  insoluble matters. The resultant filtrate was then washed  with toluene (10 ml), whereby it was possible to transfer  Example 12  Crystallization example of  threo-3-amino-2-hydroxy-4-phenylbutyric acid  Example 13  Crystallization example of  threo-3-amino-2-hydroxy-4-phenylbutyric acid 