1-Azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid,3-[[1-[4-[(3-hydroxy-1-azetidinyl)carbonyl]-2-oxazolyl]-3-azetidinyl]thio]-6-[(1R)-1-hydroxyethyl]-4-methyl-7-oxo-, monosodium salt, (4R,5S,6S)-