1,2-Pyrrolidinedicarboxylic acid,4-[4-[[2-[[4-[[4-[[[[(9H-fluoren-9-ylmethoxy)carbonyl]amino]acetyl]amino]phenyl]azo]phenyl]amino]-2-oxoethyl]amino]-4-oxobutyl]-5-oxo-,1-(1,1-dimethylethyl) 2-ethyl ester, (2S,4R)-