A new compound, (S)-5-(2-acetoxypropanamido)-2,4,6-triiodoisophthalic acid, of formula II (S)-5-(2-acetoxypropanamido)-2,4,6-triiodoisophthalic acid. Said new compound is of use for the production of triiodinated contrast agent, especially lopamidol, with low content of acetyl and hydroxyacetyl analogs. The new compound may be formed from 5-amino-2,4,6-triiodoisophtalic acid by acylating with (S)-1-chloro-1-oxopropan-2-yl acetate. The new compound may then be converted to the respective acid dichloride by reacting with a chlorinating reagent, which is a further object of the present invention, followed by the amidation with 2-amino-1,3-propanediol and acetate hydrolysis.