4-Pentylbicyclo[2.2.2]octane-1-carboxylic acid (I) was converted to the corresponding acid chloride with SOCl2 and then coupled to L-aspartic acid beta-tert-butyl ester (II) to afford amide (III). Subsequent coupling of (III) with further L-aspartic acid beta-tert-butyl ester (II) in the presence of DCC and NHS produced diamide (IV). This was then condensed with (aminobutyl)diethylenetriamino-pentaacetic acid penta-tert-butyl ester (V) to provide (VI).
After deprotection of the tert-butyl ester groups of (VI) with HCl in dioxan, the resulting heptacarboxylic acid (VII) was treated with gadolinium (III) oxide at a controlled pH value to furnish the target gadolinium chelate.