The condensation of 3-[2-(acetoxymethyl)-5-(benzyloxycarbonyl)-4-methyl-1H-pyrrol-3-yl]propionic acid methyl ester (VII) with 3,4-diethyl-1H-pyrrole (VIII) by means of Ts-OH in ethanol gives the bis adduct (IX). The reduction of both methoxycarbonyl groups of (IX) with borane/THF complex yields bis-propanol derivative (X), which is debenzylated by hydrogenation with H2 and TEA over Pd/C in THF to afford the dicarboxylic acid (XI). The reduction of (XI) with TFA, triethyl orthoformate and LiOH in methanol/water provides the dialdehyde (XII), which is cyclized with the intermediate phenylenediamine (VI) by means of HCl in methanol to give the pentacyclic compound (XIII).
The intermediate phenylenediamine derivative (VI) has been obtained as follows: The reaction of 2-[2-(2-methoxyethoxy)ethoxy]ethanol (I) with Ts-Cl and NaOH in THF/water gives the tosylate (II), which is condensed with pyrocatechol (III) by means of K2CO3 in methanol, yielding the aryl ether (IV). The nitration of (IV) with HNO3 in acetic acid affords the dinitro derivative (V), which is finally reduced to the target phenylenediamine intermediate (VI) with hydrazine in ethanol catalyzed by Pd/C.
Finally, compound (XIII) is treated with gadolinium triacetate and TEA in methanol and dehydrogenated with air to furnish the target gadolinium complex.