2,8,10,16,18,24-Hexaoxa-5,13,21-triazapentacosanedioic acid,5,21-bis[2-[[(1,1-dimethylethoxy)carbonyl]oxy]ethyl]-13-(2-hydroxypropyl)-7,19-dimethyl-9,17-dioxo-, bis(1,1-dimethylethyl) ester