The synthetic pathway is shown in Scheme 19316401a: Treatment of (S)-5-[(2-acetyloxy-1-oxopropyl)amino]-2,4,6-triiodo-1,3-benzenedicarbonyldichloride (I) with 2-amino-1,3-propanediol in diglyme yields (S)-3-[(2-acetyloxy-1-oxopropyl)amino]-5-[[[2-hydroxy-1-(hydroxymethyl)ethyl]amino]carbonyl]-2,4,6-triiodobenzenecarbonylchloride (II). Compound (II) is treated with 1,3-diamino-2-propanol to give the dimeric compound [(S)-(R*,R*)]-N,N''-(2-hydroxy-1,3-propanediyl)bis[N'-[2-hydroxy-1-(hydroxymethyl)ethyl]-5-[(2-acetyloxy-1-oxopropyl)amino]-2,4,6-triiodo-1,3-benzenedicarboxamide] (III). Hydrolysis of compound (III) and desalting of its aqueous solution gives a crude [S-(R*,R*)]-N,N''-(2-hydroxy-1,3-propanediyl)bis[N'-[2-hydroxy-1-(hydroxymethyl)ethyl]-5-[(2-hydroxy-1-oxopropyl)amino]-2,4,6-triiodo-1,3-benzenedicarboxamide, which is subsequently purified through a bed of adsorbent resin.