Optical resolution of racemic 5-oxo-1-phenyl-pyrazolidine-3-carboxylic acid 2 with L-amino acid methyl ester via the diastereomers formation was investigated. Treatment of racemic 5-oxo-1-phenylpyrazolidine-3-carboxylic acid 2 with L-valine methyl ester gave diastereomers with a total yield of 86%. The diastereomeric dipeptides can be easily separated by flash column chromatography. Acidic cleavage of the derived diastereomers gave both the optically pure (+)-(R)- and (-)-(S)-5-oxo-1-phenyl-pyrazolidine-3-carboxylic acid ((+)-(R)-2 and (-)-(S)-2) with a total yield of 94% and 95%, respectively.
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