Process for the preparation of a dicarboxylic acid di-chloride

Process for the preparation of S-(-)-5-››2-(acetyloxy)-1-oxopropyl!amino!-2,4,6-triiodo-1,3-benzenedicarb oxylic acid dichloride comprising the following steps:

• a) catalytic hydrogenation of 5-nitro-1,3-benzenedicarboxylic acid in neutral or basic environment, which gives an aqueous solution of 5-amino-1,3-benzenedicarboxylic acid sodium salt;
• b) direct iodination of the 5-amino-1,3-benzenedicarboxylic acid sodium salt solution deriving from step a), without further purification, with a solution of ICl in HCl, being the 5-amino-1,3-benzenedicarboxylic acid sodium salt solution previously added with HCl and H.sub.2 SO.sub.4 to give 5-amino-2,4,6-triiodo-1,3-benzenedicarboxylic acid;
• c) chlorination of the 5-amino-2,4,6-triiodo-1,3-benzenedicarboxylic acid dichloride, in heterogeneous phase between 5-amino-2,4,6-triiodo-1,3-benzenedicarboxylic acid and thionyl chloride, in a solvent selected from:
• (C.sub.7 -C.sub.16) linear or branched hydrocarbons, (C.sub.7 -C.sub.8) aromatic hydrocarbons, 1,1,1-trichloroethane, n-butylacetate, diglyme (diethylenglycoledimethylether), and in the presence of a catalytic amount of a tertiary amine, to give 5-amino-2,4,6-triiodo-1,3-benzenedicarboxylic acid dichloride;
• d) reaction between S-(-)-›2-(acetyloxy)!propionic acid chloride and 5-amino-2,4,6-triiodo-1,3-benzenedicarboxylic acid dichloride, in an aprotic dipolar solvent and in presence of a halogenhydric acid.