N1,N3-bis(2,3-dihydroxypropyl)-5-(N-(2,3-dihydroxypropyl)acetamido)-2,4,6-triiodomethylbenzene dicarboxamide (Compound A, 1120.5 kg, 1.50 kmol) and 1-chloro-2,3-propylene oxide were synthesized as 5-(acetamido)-N,N'-bis(2,3-dihydroxypropyl)-5-(N-(2,3-dihydroxypropyl)acetamido)-2,4,6- Triiodo-m-toluene dicarboxamide and 5,5'-((2-hydroxypropyl-1,3-diyl)bis(acetylazanediyl))bis(N1,N3-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-m-toluene dicarboxamide) in the following general steps:
1. dissolve compound A (1120.5 kg, 1.50 kmol) in a solution of water (1232.6 kg) containing KOH (140.0 kg, 2.25 kmol), controlling the temperature below 20 °C.
2. boric acid (64.9 kg, 1.05 kmol) and hydrochloric acid (30.4 kg, 0.30 kmol) were added in batches.
3. 1-chloro-2,3-epoxypropane (83.3 kg, 0.90 kmol) was added slowly and dropwise, keeping the pH of the reaction system between 10-11.
4. The progress of the reaction was monitored by HPLC during the reaction.
5. When the content of compound A was below 5%, the reaction was quenched by addition of water (1232.6 kg) and the pH was adjusted to 5-6 with 18% hydrochloric acid.
6. The reaction mixture was decolorized by activated carbon and filtered.
7. HPLC analysis showed that the filtrate contained 86.3% iodixanol, 7.5% iodohexanol, 2.9% compound A and 2.7% O-alkylation by-products and other impurities.