As a key intermediate for the synthesis of sulfazecin derivatives, 3-amino-3-methoxy-2-azetidinone (24 or 26) was synthesized from penicillins, and various new compounds, including sulfazecin, were synthesized by acylation and sulfonation of 24 or 26. Some of these compounds (33, 36) showed higher antibacterial activity than the corresponding 3-demethoxy derivatives against a β-lactamase-producing strain of Escherichia coli.