The synthesis of optically active 2H-azirine-2-carboxylicesters was achieved by Swern oxidation of the corresponding aziridine-carboxylic esters. For both trans and cis aziridine esters this oxidation gives a regioselective introduction of the double bond which is not in conjugation with the ester function. The methyl ester of ent-Azirinomycin was prepared in this manner.