The invention relates to the method to prep. new compd. of (S)-5-(2-acetoxypropanamido)-2,4,6-triiodoisophthalic acid useful for the prodn. of triiodinated contrast agent, esp. iopamidol, with low content of acetyl and hydroxyacetyl analogs.  E.g., acylation of 5-amino-2,4,6-triiodoisophthalic acid with (S)-1-chloro-1-oxopropan-2-yl acetate gave (S)-5-(2-acetoxypropanamido)-2,4,6-triiodoisophthalic acid. The new compd. may then be converted to the resp. acid dichloride by reacting with a chlorinating reagent, followed by the amidation with 2-amino-1,3-propanediol and acetate hydrolysis to provide iopamidol.