(R)-2-[(2R,3R)-2-(Benzothiazol-2-yldisulfanyl)-3-(1,3-dioxo-1,3-dihydro-isoindol-2-yl)-4-oxo-azetidin-1-yl]-3-ethylcarbamoyloxymethyl-but-3-enoic acid 4-nitro-benzyl ester