(-)-3β-((1S)-1-hydroxyethyl)-4α-<3-<<(p-nitrobenzyl)oxy>carbonyl>-2-oxopropyl>azetidin-2-one

4-[(R)-3-((R)-1-Formyloxy-ethyl)-4-oxo-azetidin-2-yl]-3-oxo-butyric acid 4-nitro-benzyl ester