A N-(2-methoxy-2-oxoethyl)-N-(phenylsulfonyl)glycine monomethyl ester of the respec-tive dicarboxylic acid was involved in a reaction with imines promoted by acetic anhydride at an elevated temperature. Instead of the initially expected δ-lactam products of the Castagnoli–Cush-man-type reaction, medicinally important 3-amino-2-azetidinones were obtained as the result of cyclization, involving a methylene group adjacent to an acid moiety. In contrast, replacing alcohol residue with hexafluoroisopropyl in the same substrate made another methylene group (adjacent to the ester moiety) more reactive to furnishing the desired δ-lactam in the Castagnoli–Cushman fashion.