## Conversion of (2S,3S)-3-( ethoxycarbonyl)amino-2- hydroxy-4-phenylbutyric acid into (2S,3S)-3- ( ethoxycarbony l)-amino-2-methanesulfonyloxy-4- pheny lbutyric acid methyl ester 

[0159] Thionyl chloride (596 mg) was added to methanol (1.39 g) over 10 minutes under ice-cooling, and the mixture was reacted at the same temperature for 1 hour. The resultant solution was added to a solution of (2S,3S)-3-(ethoxycar› bonyl)amino-2-hydroxy-4-phenylbutyric acid (HPLC area ratio of erythro-isomer/threo-isomer=99/1) (1.04 g) and toluene (10 ml) over 10 minutes under ice-cooling. The mixture was reacted at the same temperature for 2 hours and then warmed to room temperature. From the resultant reac› tion solution, methanol and thionyl chloride as well as components secondarily produced from these reagents and having a low boiling point such as sulfurous acids hydrogen chloride or the like were distilled off under reduced pressure. Tetrahydrofuran was added to the resultant residue to obtain a tetrahydrofuran solution (10.12 g) of (2S,3S)-3-(ethoxy› carbonyl)amino-2-hydroxy-4-phenylbutyric acid methyl ester. The HPLC area ratio of erythro-isomer/threo-isomer was 99/1, and the diastereomer excess was 99% d.e. 

[0160] To the tetrahydrofuran solution (5.04 g) of (2S,3S)- 3-( ethoxycarbony l)amino-2-hydroxy-4-pheny lbutyric acid methyl ester thus obtained, triethylamine ( 421 mg) was added under ice-cooling. Then, methanesulfony chloride (1376 mg) was added to the mixture over 10 minutes, and the mixture was reacted at the same temperature for 2 hours. Toluene (5 ml) and water (5 ml) were added to the resultant reaction solution, and the aqueous phase was separated. The resultant organic phase was concentrated under reduced pressure to obtain (2S,3S)-3-( ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric acid methyl ester (erythro-isomer) (685 mg) as a white solid. The HPLC area ratio of erythro-isomer/threo-isomer was 99/1, and the dias› tereomer excess was 99% d.e. 

[0161] $^{1}$H-NMR Spectra (CDC1$_{3}$ , TMS internal standard) of the resultant (2S,3S)-3-( ethoxycarbonyl)amino-2-meth› anesulfonyloxy-4-phenylbutyric acid methyl ester were: Ii 1.19 (3H, t, 1=7.2 Hz), 2.79-2.86 (2H, m), 3.20 (3H, s), 3.64 (3H, s), 4.08 (2H, q, 1=7.2 Hz), 4.55-4.63 (lH, m), 4.92 (lH, d, 1=8.8 Hz), 5.31 (lH, brs), 7.17-7.33 (5H, m). 

[0162] Also, $^{13}$C NMR spectra (CDC1$_{3}$) were: Ii 14.4, 35.6, 39.2, 52.8, 53.0, 61.3, 78.61 127.1, 128.6, 129.3, 135.9, 155.8, 167.4. 

## Conversion of (2S,3S)-3-( ethoxycarbonyl)amino-2- methanesulfonyloxy-4-phenylbutyric acid methyl ester into (2R,3S)-3-amino-2-hydroxy-4-phenylbu- tyric acid 

[ 0163] (2S,3S )-3-Ethoxycarbony l)amino-2-methanesulfo› ny loxy-4-pheny lbutyric acid methyl ester (HPLC area ratio of erythro-isomer/threo-isomer=99/1) (148 mg) obtained in Example 4 was dissolved in N,N-dimethyl formamide (3 ml), the temperature of the mixture was gradually raised from 60° C., and the mixture was reacted finally at 100° C. for 8 hours. The resultant reaction solution was concentrated under reduced pressure, water (2 ml) and a 30% aqueous 

## Example 4 

## Example 5 

solution of sodium hydroxide (551 mg) were added to the resultant residue, and the mixture was reacted at 60° C. for 15 hours. Determination by HPLC revealed that the amount of (2R,3S)-3-amino-2-hydroxy-4-phenylbutyric acid (threo› isomer) in the resultant reaction solution was 71 mg. Also, (2S,3S)-3-amino-2-hydroxy-4-phenylbutyric acid ( erythro› isomer) was not detected. 

## Conversion of (2S,3S)-3-( ethoxycarbonyl)amino-2- hydroxy-4-phenylbutyric acid through 3-( ethoxycar› bonyl)amino-2-chloro-4-phenylbutyric acid into (2R,3S)-3-amino-2-hydroxy-4-phenylbutyric acid 

[0164] Thionyl chloride (694 mg) was added to a solution of (2S,3S)-3-( ethoxycarbonyl)amino-2-hydroxy-4-phenyl› butyric acid (HPLC area ratio of erythro-isomer/threo-iso› mer=99/1) (277 mg) and tetrahydrofuran (2 ml) over 2 hours under ice-cooling, and the temperature of the mixture was then raised to 60° C. stepwise. The mixture was reacted at 60° C. for 24 hours. From the resultant reaction solution, tetrahydrofuran and thionyl chloride as well as components secondarily produced from thionyl chloride and having a low boiling point such as sulfurous acid, hydrogen chloride or the like were distiled off under reduced pressure. The resultant residue was treated with a silica gel colunm (silica gel: WAKOGEL C-200, developing solvent: ethyl acetate/ hexane=30/70) to obtain a concentrate (100 mg) of 3-( ethoxycarbonyl)amino-2-chloro-4-phenylbutyric acid as a solid. 

[0165] LC/MS Spectra (ESI) of the resultant 3-(ethoxy› carbonyl)amino-2-chloro-4-phenylbutyric acid were: m/e 286.1 [100%, M(Cl$^{35}$)+1], 288.2 [36%, M(Cl$^{37}$)+1]. 

[0166] The concentrate (73 mg) thus obtained was dis› solved in N,N-dimethyl formamide (1 ml), and the mixture was reacted at 100° C. for 2 days. The resultant reaction solution was concentrated under reduced pressure, water (0.4 ml) and a 30% aqueous solution of sodium hydroxide (0.4 ml) were added to the resultant residue, and the mixture was reacted at 60° C. for 18 hours. Determination by HPLC revealed that the amount of the (2R,3S)-3-amino-2-hydroxy- 4-phenylbutyric acid in the resultant reaction solution was 46 mg. Also, the HPLC area ratio of threo-isomer/erythro› isomer was 97/3, and the diastereomer excess was 94% d.e. 

## Conversion of 3-( ethoxycarbonyl)amino-2-hydroxy- 4-phenylbutyric acid (diastereomer mixture) into 3-( ethoxycarbonyl )-amino-2-methanesulfony loxy-4- phenylbutyric acid methyl ester 

[0167] Thionyl chloride (3.46 g) was added to methanol (2.64 g) over 30 minutes under ice-cooling, and the mixture was reacted at the same temperature for 1 hour. The resultant solution was added to a toluene solution (54.51 g) containing the diastereomer mixture ( 4.05 g) of 3-( ethoxycarbony› l)amino-2-hydroxy-4-phenylbutyric acid obtained in Example 2 (HPLC area ratio of erythro-isomer/threo-iso› mer=83/17, diastereomer excess=66% d.e.) over 30 minutes under ice-cooling. The mixture was reacted at the same temperature for 2 hours, and the temperature of the mixture was then raised to room temperature. From the resultant reaction solution, methanol and thionyl chloride as well as 

## Example 6 

## Example 7 

