3,6,10,13,16,20,23-Heptaazapentacosanedioic acid,8,18-bis[bis[2-(1,1-dimethylethoxy)-2-oxoethyl]amino]-13-(carboxymethyl)-3,6,20,23-tetrakis[2-(1,1-dimethylethoxy)-2-oxoethyl]-9,17-dioxo-,1,25-bis(1,1-dimethylethyl) ester, (8R,18R)-