Regiodivergent Synthesis of Butenolide-Based α- and β-Amino Acid Derivatives via Base-Controlled Azirine Ring Expansion.

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 SN2' substitution of the bromine in the azirine by the carboxylate ion.