1-Azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid,3-[[1-(cyclopropylthioxomethyl)-3-azetidinyl]thio]-6-[(1R)-1-hydroxyethyl]-4-methyl-7-oxo-, (4-nitrophenyl)methyl ester, (4R,5S,6S)-