A method for the preparation of 5-aminobutenolides from 2-bromo-2H-azirine-2-carboxylic esters/amides with arylacetic acids has been developed. The reaction regioselectivity can be switched by a change of the basic catalyst, making it possible to prepare both butenolide-based α- and β-amino acid derivatives. The change in the regioselectivity is interpreted in terms of the stability and reactivity of the enolates formed during the S_N2′ substitution of the bromine in the azirine by the carboxylate ion.