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. 

## 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. 

## Crystallization example of threo-3-amino-2-hydroxy-4-phenylbutyric acid 

[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 

## Example 11 

## Example 12 

(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. 

## Crystallization example of threo-3-amino-2-hydroxy-4-phenylbutyric acid 

[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 13 

