4-[3-(p-nitrobenzyloxycarbonyl)-3-diazo-2-oxopropyl]-azetidin-2-one

(4-Oxo-azetidin-2-yl)-acetic acid benzhydryl ester

4-[1-(2,4-Dimethoxy-benzyl)-4-oxo-azetidin-2-yl]-3-oxo-butyric acid 4-nitro-benzyl ester

2-Diazo-4-[1-(2,4-dimethoxy-benzyl)-4-oxo-azetidin-2-yl]-3-oxo-butyric acid 4-nitro-benzyl ester