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tion, under reaction conditions of the above third step followed: 

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[0141] wherein R 1 , R 2 , R 3 , S1, S 3 , S 4 , S 5 , S 6 and Lare as defined above; and 

[0142] S $^{10 }$represents a hydrogen atom or an ester resi- due. 

[0143] Thus, if a mixture of erythro-isomer (3) and threo› isomer (3) is used, it is possible to convert erythro-isomer (3) selectively into threo-isomer ( 4) having an inverted steric configuration at 2-position carbon, and at the same time, to retain threo-isomer (3) as it is, in the second step, as shown in the above scheme. Subsequently, in the third step, leaving group L ofretained threo-isomer (3) may be converted into a hydroxyl group to obtain threo-isomer ( 4), and then, substituents may he removed to obtain 3-amino-2-hydrox› ypropionic acid derivative (1) having a threo configuration [hereinafter, also referred to as threo-isomer (1)]. This process can greatly increase the diastereomer excess of the products. 

[0144] In the above process, R$^{1 }$is a substituted or unsub› stituted alkyl group having 1 to 30 carbon atoms, a substi› tuted or unsubstituted aryl group having 6 to 30 carbon atoms, or a substituted or unsubstituted aralkyl group having 7 to 30 carbon atoms, and R $^{2 }$is a hydrogen atom. However, R$^{1 }$is preferably a substituted or unsubstituted aralkyl group having 7 to 30 carbon atoms, more preferably a substituted or unsubstituted benzyl group, and most preferably an unsubstituted benzyl group. Also, leaving group L is pref› erably the above sulfonyloxy group. 

[0145] In the above process, in order to convert erythro› isomer (3) into three-isomer ( 4) having an inverted steric configuration at 2-position carbon, and at the same time, to retain threo-isomer (3) as it is, it is preferable to carry out the reaction in the above second step under mild conditions, for example, by using an acidic substance or a basic substance in a small amount, by retaining a low reaction temperature or by combining these conditions. As the conditions become mild, it is possible to retain threo-isomer (3) efficiently. 

[0146] The above acidic substance is not limited to a particular one and may be selected from acidic substances used in the above second step. The amount of the acidic substances used varies depending on the kind of acids, and they are usually used in a molar amount below 5-fold, preferably in a molar amount below 2-fold, and more preferably in a molar amount below I-fold, relative to the total amount of erythro-isomer (3) and threo-isomer (3). 

[0147] The above basic substance is not limited to a particular one and weakly basic substances used in the above second step are preferably used. Among others, N,N-dim› ethyl formamide, pyridine or the like may be used prefer› ably. The amount of the above basic substances used varies depending on the kind of bases, and they are usually used in a molar amount below 50-fold, preferably in a molar amount below 20-fold, and more preferably in a molar amount below 10-fold, relative to the total amount of erythro-isomer (3) and threo-isomer (3). 

[0148] Also, the reaction temperature varies, for example, depending on the kind of the above acidic substances or basic substances, the amount of them used and the like, and it may be usually below 120° C., preferably below 100° C., and more preferably below 80° C. 

[0149] Reaction solvents used in this reaction are not limited to particular ones and, various organic solvents used in the above second step may be used. Examples of the reaction solvents include, for example, aliphatic hydrocar› bons such as hexane, heptane or methylcyclohexane; aro› matic hydrocarbons such as toluene or xylene; halogenated hydrocarbons such as methylene chloride or chloroform; ethers such as tetrahydrofuran, 1,4-dioxane or t-butyl methyl ether; aliphatic acid esters such as methyl acetate, ethyl acetate or isopropyl acetate; nitriles such as acetonitrile; alcohols such as methanol, ethanol, isopropanol or benzyl alcohol. Toluene is a particularly preferable aromatic hydro› carbon, 1,4-dioxane is a particularly preferable ether, and ethyl acetate is a particularly preferable aliphatic acid ester. Among others, aromatic hydrocarbons are preferably used. These organic solvents may be used alone or in a mixture of two or more solvents. Besides, if the basic substances used in the above second step are liquid at the above reaction temperature, the above basic substances may be used so as to also serve as a reaction solvent. 

[0150] By the above process, it is possible to obtain 3-amino-2-hydroxy-4-phenylbutyric acid wherein the dias› tereomer excess of the threo-isomer is 87% d.e. from N-ethoxycarbony 1-3-amino-2-methanesulfony loxy-4-phe› nylbutyric acid methyl ester [in the above general formula (2), R$^{1}$=benzyl group, R$^{2}$=hydrogen atom, P$^{1}$=ethoxycarbo› nyl group, and R $^{4}$=methyl group] wherein the diastereomer excess of the erythro-isomer is 66% d.e., for example, as shown in Example 9 below. Thus, the process can greatly increase the diastereomer excess. 

[0151] As described above, according to the present invention, itis possibleto prepare conveniently and efficiently, and with industrial advantages, 3-amino-2-hydroxypropi› onic acid derivatives (1) having a particular steric configu› ration from N-protected-3-amino-2-hydroxypropionic acid derivatives (2) having a reverse steric configuration at 2-position carbon. 

