{(2R,3S)-3-[(R)-1-(tert-Butyl-dimethyl-silanyloxy)-ethyl]-2-[4-(4-nitro-benzyloxycarbonylamino)-butylsulfanylthiocarbonylsulfanyl]-4-oxo-azetidin-1-yl}-(triphenyl-λ5-phosphanylidene)-acetic acid 4-nitro-benzyl ester