Preparation and purification of iodixanol

1. A method of preparation of iodixanol, characterized by comprising the following steps:

a) dimerization of 5-acetamido-N,N′-bis (2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide (compound A) to prepare iodixanol, obtaining a solution of dimerization; wherein said dimerization step comprises dissolving Compound A in an excessive amount of alkali, neutralizing the excess alkali by a boron-containing acidic substance of salts thereof, and adjusting the pH to 10-13;

b) purification of the solution of dimerization by a macroporous adsorption resin chromatographic column, obtaining iodixanol crude product with a purity more than 97%; or

desalination of the solution of dimerization by anionic and cationic ion exchange resins or macroporous adsorption resin chromatographic column, obtaining desalted iodixanol crude product;

c) recrystallization of the iodixanol crude product with mixed solvents containing 2-methoxyethanol, obtaining iodixanol with a purity more than 99%.

2. A method according to claim 1 , wherein the iodixanol crude product obtained by desalination of the solution of dimerization contains 85-90% by weight of iodixanol, 3-7% by weight of iohexol, 2-5% by weight of compound A and other impurities.

3. A method according to claim 1 , wherein the dimerization agent is epichlorohydrin, 1,3-dichloro-2-hydroxypropane or 1,3-dibromo-2-hydroxypropane, and the dimerization reaction takes place in non-aqueous solvent, water, or mixture of water and one or more alcohols.

4. A method according to claim 3 , wherein the dimerization agent is epichlorohydrin, and the solvent is water, 2-methoxyethanol or methanol.

5. A method according to claim 3 or 4 , wherein the ratio of the dimerization agent/compound A is 0.45-0.60 mole dimerization agent per mole compound A.

6. A method according to claim 1 , wherein the alkali used is alkali metal hydroxides, and the ratio of compound A/alkali is 1.05-1.60 molar of alkali per mol of the compound A.

7. A method according to claim 6 , wherein the boron-containing acidic substance or salts thereof used to neutralize the excess alkali to form a buffer solution comprises a boron oxyacid, a mixed acid comprising a boron oxyacid, and borate,

wherein said boron oxyacid is boric acid, metaboric acid, pyroboric acid or tetraboric acid,

said mixed acid comprising boron oxyacid comprises another acid selected from a group consisting of acetic acid, hydrochloric acid, phosphoric acid, sulphuric acid and a mixture thereof,

said borate is selected from borax and metaborate, a mixed acid comprising boric acid and hydrochloric acid and a mixed acid comprising boric acid and phosphoric acid.

8. A method according to claim 7 , wherein the ratio of compound A/boron in the boron-containing acidic substance or salts thereof is 0.2-1.2 mole of boron per mole of compound A.

9. A method according to claim 1 , wherein the prepared iodixanol crude product is desalted by anionic and cationic ion exchange resins or a macroporous adsorption resin chromatographic column.

10. A method according to claim 9 , wherein the purification of iodixanol crude product is achieved using a macroporous adsorption resin chromatographic column to separate inorganic and organic impurities.

11. A method according to claim 1 , wherein the purified iodixanol by recrystallization has a global content of impurities content not higher than 0.2%.

12. A method according to claim 9 or 10 , wherein the macroporous adsorption resin of the chromatographic column is a polystyrene based resin, to pololyacrylic ester based resin, or a reticulated aliphatic polymer resin.

13. A method according to claim 9 or 10 , wherein the weight ratio of the macroporous adsorption resin and iodixanol crude product is 20:1-2:1.

14. A method according to claim 9 or 10 , wherein the weight ratio of the macroporous adsorption resin and iodixanol crude product is 2:1-0.5:1.

15. A method according to claim 10 , wherein compound A and iohexol are isolated by using water or an aqueous solution with less than 5% alcohol to elute.

16. A method according to claim 10 , wherein a larger quantity of water or an aqueous solution with less than 20% alcohol is used to elute, followed by nanofiltration and concentration of the filtrate, obtaining iodixanol crude product with a content of more than 97% iodixanol.

17. A method according to claim 10 wherein a aqueous solution with more than 30% alcohol is used to elute, followed by concentration to recover residues of iodixanol.

18. A method according to claim 15 , 16 or 17 , wherein the alcohol used to elute is C1 to C3 alkanol.

19. A method according to claim 15 , 16 or 17 , wherein the elution is carried out under normal pressure.

20. A method according to claim 15 , wherein the isolated compound A and iohexol are used to prepare iohexol, which is purified by the macroporous adsorption resin chromatographic column and the recrystallization with mixed solvents containing 2-methoxyethanol as defined in claim 1 , obtaining iohexol which meeting the USP requirements for iohexol.

21. A method according to claim 1 , wherein the iodixanol crude product used for the recrystallization is the solution of dimerization or the desalted aqueous solution thereof or a solid thereof obtained by spray-drying.

22. A method according to claim 1 , wherein the recrystallization time is 1-4 days.

23. A method according to claim 1 , wherein the temperature of recrystallization is above 60° C.

24. A method according to claim 1 , wherein the mixed solvent for the recrystallization contains 2-methoxyethanol, water and co-solvent, and the co-solvent is a C1 to C4 alkanol or a mixture thereof.

25. A method according to claim 24 , wherein the co-solvent is ethanol, iso-propanol, n-butanol, sec-butanol, t-butyl alcohol or their mixtures.

26. A method according to claim 1 , wherein the mixed solvent for the recrystallization include a mixed solvent of 2-methoxyethanol and iso-propyl alcohol, or 2-methoxyethanol and n-butanol.

27. A method according to claim 1 , wherein the ratio of the mixed solvent for recrystallization/iodixanol crude product is about 1 to 30 grams of mixed solvent per gram of iodixanol crude product.

28. A method according to claim 24 , wherein the composition of the mixed solvent for the crystallization is 0.2-50 grams of co-solvent per gram of 2-methoxyethanol.

29. A method according to claim 24 , wherein the ratio of the mixed solvents for recrystallization/water is 1-100 grams of mixed solvent per gram of water.

30. A method according to claim 1 , wherein the recovered product which contains more than 20% iodixanol is purified by the macroporous adsorption resin chromatographic column, obtaining iodixanol crude product with a content of more than 97% iodixanol, wherein the recovered product with a iodixanol content of more than 20% is the residue in the macroporous adsorption resin chromatographic column, or is the refined mother liquor obtained after the recrystallization.

31. A method according to claim 3 , wherein the dimerization agent is epichlorohydrin, and the solvent is water.

32. A method according to claim 1 , wherein the alkali used is potassium hydroxide.

33. A method according to claim 1 , wherein the alkali used is sodium hydroxide.

34. The method of claim 7 , wherein said borate comprises sodium metaborate.

35. The method of claim 7 , wherein said borate comprises potassium metaborate.

36. The method of claim 7 , wherein said boron-containing acidic substance or salts thereof is boric acid.

37. The method to claim 7 , wherein the ratio of compound A/boron in the boron-containing acidic substance or salts thereof is 0.2-0.8 of boron per mole of compound A.

38. A method according to claim 9 , wherein the macroporous adsorption resin of the chromatographic column is a styrene-divinylbenzene copolymer based resin.

39. A method according to claim 10 , wherein the macroporous adsorption resin of the chromatographic column is a styrene-divinylbenzene copolymer based resin.

40. A method according to claim 15 , wherein the alcohol used to elute is methanol.

41. A method according to claim 16 , wherein the alcohol used to elute is methanol.

42. A method according to claim 17 , wherein the alcohol used to elute is methanol.

43. A method according to claim 15 , wherein the elution is carried out under pressure of 1.5 MPa.

44. A method according to claim 16 , wherein the elution is carried out under pressure of 1.5 MPa.

45. A method according to claim 17 , wherein the elution is carried out under pressure of 1.5 MPa.

46. A method according to claim 1 , wherein the recrystallization time is 1-3 days.

47. A method according to claim 1 , wherein the recrystallization time is 1-2 days.

48. A method according to claim 1 , wherein the temperature of recrystallization is about 90-120° C.

49. A method according to claim 1 , wherein the temperature of recrystallization is about 102-115° C.

50. A method according to claim 24 , wherein the co-solvent is n-butanol.

51. A method according to claim 24 , wherein the co-solvent is iso-propanol.

52. A method according to claim 1 , wherein the ratio of the mixed solvent for recrystallization/iodixanol crude product is about 2 to 10 grams of mixed solvent per gram of iodixanol crude product.

53. A method according to claim 24 , wherein the composition of the mixed solvent for the crystallization is 0.8-8 grams of co-solvent per gram of 2-methoxyethanol.

54. A method according to claim 25 , wherein the ratio of the mixed solvents for recrystallization/water is 15-50 grams of mixed solvent per gram of water.