Process for the production of DTPA-tetraesters of terminal carboxylic acids

Process for the production of diethylenetriaminepentacarboxylic acid tetraesters of general formula (I) ##STR15## in which R.sup.1 stands for a tert-butyl group or a benzyl group, and

• Z stands for hydrogen or for a saturated or unsaturated, branched or straight-chain C.sub.1 -C.sub.50 alkyl chain, in which the chain or parts of this chain optionally can form a cyclic C.sub.5 -C.sub.8 unit or a bicyclic C.sub.10 -C.sub.14 unit, which contains 0 to 10 oxygen and/or 0 to 2 sulfur atoms and/or 0 to 3 carbonyl, 0 to 1 thiocarbonyl, 0 to 2 imino, 0 to 2 phenylene, 0 to 13 indole, 0 to 1 methyl-imidazol-4-yl and/or 0 to 3 N--R.sup.3 groups and is substituted by 0 to 2 phenyl, 0 to 2 pyridyl, 0 to 5 R.sup.2 O, 0 to 1 HS, 0 to 4 R.sup.2 OOC, 0 to 4 R.sup.2 OOC--C.sub.1-4 alkyl and/or 0 to 1 R.sup.2 (H)N groups in which optionally present aromatic groups can be substituted zero to five times, independently of one another, by fluorine, chlorine, bromine, iodine, R.sup.2 O.sub.2 C, 4 R.sup.2 OOC--C.sub.1-4 alkyl, R.sup.2 (H)N, R.sup.2 NHOC, R.sup.2 CONH, O.sub.2 N, R.sup.2 O, R.sup.2 groups, and in which
• R.sup.2 stands for hydrogen or a branched or unbranched C.sub.1 -C.sub.4 alkyl radical, characterized in that a compound of general formula II ##STR16## which R.sup.1 has the above-indicated meaning, is reacted with a carboxylic acid ester of general formula III ##STR17## in which Z' has the meaning of an optionally protected group Z, in which Z has the above-indicated meaning and
• T.sup.1 stands for a straight-chain or branched C.sub.1 -C.sub.6 alkyl group, a benzyl, trimethylsilyl, triisopropylsilyl, 2,2,2-trifluoroethoxy, 2,2,2-trichloroethoxy group or a metal ion equivalent of an alkali or alkaline-earth element, in which T.sup.1 is always different from R.sup.1, and
• Nu stands for a nucleofuge
• and then, by cleavage of group T.sup.1 as well as of protective groups optionally contained in Z', the free acid of general formula I is produced.