Process for Preparing Crystalline 3,6,9-triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic Acid and Use for Production of Primovist®

2) Method for preparing crystalline 3,6,9-triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic acid of formula I

• characterized in that 3,6,9-triaza-3,6,9-tris(tert-butoxycarbonylmethyl)-4-(4-ethoxybenzyl)undecanedioic acid di-tert-butyl ester of formula II
• is dissolved in a lower alcohol, hydrolysed with an aqueous alkali metal hydroxide solution, a resultant reaction mixture is concentrated, a residue dissolved in water and a resultant solution acidified by slow addition of an aqueous inorganic acid and filtered from a precipitate.

4) Use of 3,6,9-triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic acid of formula I

• for producing a gadolinium complex of 3,6,9-triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic acid (Gd-EOB-DTPA).

5) Use of crystalline 3,6,9-triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic acid of formula I

• for producing a galenical formulation of the gadolinium complex of 3,6,9-triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic acid (Gd-EOB-DTPA).

6) 3,6,9-Triaza-3,6,9-tris(carboxymethyl)-4-(4-ethoxybenzyl)undecanedioic acid in a purity of greater than 99%.