Novel optically active or racemic azethidine carboxylic acid derivatives (azethidine-2-carboxylic acid N-carboxyamino acid anhydrides) which are useful as drug intermediates and an industrially advantageous process for producing the same are provided. Namely, a process for obtaining azethidine-2-carboxylic acid N-carboxyamino acid anhydrides starting with azethidine-2-carboxylic acids is provided. Via the azethidine-2-carboxylic acid N-carboxyamino acid anhydrides according to the invention, azethidine derivatives having peptide bond, which are useful as drugs, can be obtained by an industrial and economic process without resorting deblocking reaction.
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