Pairs of optically pure enantiomers of substituted 1,1 -diamines have been prepared from the azides of L-amino-acids via the Curtius rearrangement.This synthesis is based on the interchange of two groups attached to the asymmetrically substituted tetrahedral α-carbon atom of the parent amino acid. Such an interchange occurs without the intermediate formation of a racemate. In addition, some side reactions of isocyanates of N-benzyloxycarbonyl-L-amino acids in aqueous acidic solution have been elucidated.