( )-(Pivaloyloxymethyl(4R,5S,6S)-6-((R)-1-hydroxyethyl)-4-methyl-7-oxo-3-(((R)-5-oxopyrrolidin-3-yl)thio)- 1-azabicyclo(3.2.0)hept-2-ene-2-carboxylate 157542-49-9 CS-834