(4R,5S,6S)-6-((R)-1-Hydroxy-ethyl)-3-[(3S,5S)-5-(5-hydroxymethyl-isoxazol-3-yl)-1-(4-nitro-benzyloxycarbonyl)-pyrrolidin-3-ylsulfanyl]-4-methyl-7-oxo-1-aza-bicyclo[3.2.0]hept-2-ene-2-carboxylic acid 4-nitro-benzyl ester