The diketopiperazine suggested to be the intermediate during the spont
aneous isomerization of beta-ODAP and alpha-ODAP was synthesized. Its
behaviour was studied at selected pH values and provided evidence that
its natural occurrence is unlikely. 2-Hydroxy-imidazolidine-2,4-dicar
boxylic acid (for the rearrangement beta-ODAP <---->alpha-ODAP) or 2-h
ydroxy-pyrimidine-2,4-dicarboxylic acid (for the rearrangement gamma-O
DAB <---->alpha-ODAB) are suggested to be the unstable intermediates.