1,4,7,10-Tetraazacyclododecane-1,4,7-tricarboxylic acid, 10-[(2S)-4-methyl-1-oxo-2-[[2-[4,7,10-tris[(1,1-dimethylethoxy)carbonyl]- 1,4,7,10-tetraazacyclododec-1-yl]acetyl]amino]pentyl]-, 1,4,7-tris