acetic acid 3S-{5R-{3'-[4S-acetoxy-1-(2S-tert-butoxycarbonylaminobutyryl)pyrrolidin-2R-ylmethyl]-6,6'-difluoro-1H,1'H-[2,2']biindol-3-ylmethyl}-1-(2S-tert-butoxycarbonylaminobutyryl)pyrrolidin}-3-yl ester