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used, and especially, sulfonyl halide compounds (i.e. leaving group Lis a sulfonyloxy group) are preferably used. Most preferable combination of S 2 and leaving group L is that wherein S 2 is an ester residue and leaving group L is a sulfonyloxy group. 

[0051] The N-protected-3-aminopropionic acid deriva› tives (3) synthesized as described above may be isolated and/or purified by a conventional method such as extraction, crystallization, distillation, chromatography or the like. However, from the operational and/or economical viewpoint in the industrial production, the products may be used in the next reaction as they are without isolation and/or purification by the above conventional method. 

[0052] Next described is the second step wherein a steric configuration at 2-position carbon of N-protected-3-amino- 

propionic acid derivatives (3) is inverted to convert the derivatives into substituted-3-amino-2-hydroxypropionic acid derivatives (4). 

[0053] In this step, the N-protected-3-aminopropionic acid derivatives (3) obtained in the first step can be converted into substituted-3-amino-2-hydroxypropionic acid derivatives ( 4) having an inverted steric configuration at 2-position carbon, in a high steric inversion ratio. In this case, the N-protected-3-aminopropionic acid derivatives (3) synthe› sized in the above first step may be occasionally converted into substituted-3-amino-2-hydroxypropionic acid deriva› tives ( 4) having an inverted steric configuration at 2-position carbon under reaction conditions of the above first step. However, the process is included within the production process according to the present invention so far as the above N-protected-3-aminopropionic acid derivatives (3) are once formed as an intermediate. 

[0054] In this step, the inversion of a steric configuration at 2-position carbon can proceed with an essentially perfect inversion. The steric inversion ratio at 2-position carbon in the products can be expected to be at least 95% or more, usually 98% or more, preferably 100%. In this connection, the steric inversion ratio means, as a general concept, a ratio of enantiomer excess (% e.e.) or diastereomer excess (% d.e.) of a product having an inverted steric configuration at 2-position carbon [in this case, substituted-3-amino-2-hy- 

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droxypropionic acid derivatives ( 4)] to enantiomer excess (% e.e.) or diastereomer excess(% d.e.) of a starting material [in this case, N-protected-3-aninopropionic acid derivatives (3)]. 

[0055] In this step, oxazolidinone derivatives are usually obtained as the substituted-3-amino-2-hydroxypropionic acid derivatives ( 4) having an inverted steric configuration at 2-position carbon. However, as described below in more detail, for example, if water coexists at the beginning of this step for the purpose of producing 3-amino-2-hydroxypropi› onic acid derivatives in a high quality and a high yield, plural compounds can be formed as the substituted-3-amino-2- hydroxypropionic acid derivatives (4) having an inverted steric configuration at 2-position carbon. The typical examples are shown in the following scheme: 

[0056] wherein R1, R 2 , R 3 , R 4 and S 3 are as defined above; 

[0057] L represents a leaving group; and 

[0058] S 7 , S 8 and S 9 represent a hydrogen atom or an ester residue. 

[0059] The formation mechanism of these inversion prod› ucts is presumed as follows. 

[0060] Firstly, oxazolinium derivative (4a) is formed by substitution of leaving group L ofN-protected-3-aminopro› pionic acid derivative (3) through a nucleophilic substitution reaction by carbonyl oxygen of an alkoxycarbonyl group (-COOR 4 ) in the same molecule. The leaving group L eliminated in this substitution reaction can form a salt with oxazolinium derivative ( 4a) as a counter anion. If R 3 is a hydrogen atom, of course, the structure of the oxazolinium derivative (4a) maybe represented as oxazoline derivative ( 4d) formed by deprotonation. 

[0061] N-alkoxycarbonyl-3-amino-2-hydroxypropionic acid derivative ( 4b) can be formed by hydrolysis of the above oxazolinium derivative (4a), or by direct substitution of leaving group L of N-protected-3-aminopropionic acid derivative (3) with water. 

[0062] The oxazolidinone derivative (4c) can be formed by hydrolysis or the like of the above oxazolinium derivative 

