1,4,7,10-Tetraazacyclododecane-1,4,7-triacetic acid,10-[2-[[[4-[2-[2-[(2R)-2-[[(1,1-dimethylethoxy)carbonyl]amino]-4-methyl-1-oxopentyl]hydrazinyl]-2-oxoethyl]phenyl]methyl]amino]-2-oxoethyl]-,1,4,7-tris(1,1-dimethylethyl) ester