Isopaque,Winthrop,France,1973
3,5-Diacetamido-2,4,6-triiodobenzoic acid (diatrizoic acid) (see Diatrizoate entry for synthesis) (10 g) is suspended in water (10 ml), 5 N potassium
hydroxide (4.3 equivalent) is added and the mixture cooled to about 15°C.
Dimethyl sulfate (0.5 equivalent) dissolved in an equal volume of acetone is
added drop by drop while stirring. After the reaction mixture has been stirred
for about 1 hour hydrochloric acid (1:1) is added, with stirring to pH about
0.5. The precipitate is filtered, washed and suspended moist in 4 parts of
water, concentrated ammonia is added to pH about 7 and the ammonium salt
solution is isomerized at 90°C to 100°C for about one-half hour whereafter
additional ammonia is added to pH about 9 followed by solid ammonium
chloride (about 10% weight/volume) and the solution stirred overnight and
the excess of 3,5-diacetamide-2,4,6-triiodobenzoic acid recovered as
ammonium salt on the filter. The filtrate is precipitated by means of
hydrochloric acid (1:1 ) at pH about 0.5 and the N-methyl-3,5-diacetamido-
2,4,6-triiodobenzoic acid collected on a filter, washed and dried.
