The present invention relates a process for preparation of crystalline sodium or potassium (2S,3S)-3-[[(lS)-l-isobutoxymethyl-3-methylbutyl]carbamoyl]oxirane-2-carboxylate, comprising the following steps (i) to (iv): (i) causing an ester of (2S,3S)-3-[[(lS)-l-isobutoxymethyl-3-methylbutyl] carbamoyl]oxirane-2-carboxylic acid react with a basic sodium or potassium (2S,3S)- 3-[[(lS)-l-isobutoxymethyl-3-methylbutyl]carbamoyl]oxirane-2-carboxylate, (ii) crystallizing the sodium or potassium salt obtained in the preceding step from a mixed solvent of water and acetone, to obtain crystalline sodium or potassium salt, (iii) recrystallizing the sodium or potassium salt obtained in the preceding step using an aliphatic alcohol, and (iv) drying the product recrystallized in the preceding step under reduced pressure.
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