(4R,5S,6S)-3-[(3R,5S)-5-((3R,4R)-3,4-Dihydroxy-pyrrolidine-1-carbonyl)-1-(4-nitro-benzyloxycarbonyl)-pyrrolidin-3-ylsulfanyl]-6-((R)-1-hydroxy-ethyl)-4-methyl-7-oxo-1-aza-bicyclo[3.2.0]hept-2-ene-2-carboxylic acid 4-nitro-benzyl ester