The reasons for epimerization of 3-azido-2-hydroxysuccinates observed during the ring opening of epoxides or cyclic sulfites with sodium azide is now clarified. It is caused by the high acidity of the proton at the 3-position. This is proven by a proton deuterium exchange in assays with either D 2 O or DCl containing solvents. The anti-3-azido-2-hydroxysuccinates serve as intermediates for enantiomerically pure trans-aziridine-2,3-dicarboxylates for which an optimized synthetic pathway is presented. The first example of an enantiomerically pure mixed diester of the aziridine-2,3-dicarboxylic acid the synthesis of the allyl ethyl ester is reported herein.