To a stirred solution of 5-acetamido-N1,N3-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide (7.5 g, 10 mmol) and boric acid (1.24 g, 20 mmol) in water (10 ml) and methanol (7 ml) is added potassium hydroxide (10 M) drop wise until a stable pH 12.5 is established. Allylbromide (860 ul, 10 mmol) is added and potassium hydroxide (10 M) is continuously added to maintain pH 12.5. for 3 h and the mixture is then left stirring over night. The following day the mixture is reduced in vacuo to remove methanol, the resulting aqueous phase is diluted with 50 ml water and treated with acidic and basic ion exchanger. Resins are removed and rinsed and the combined aqueous volume is reduced to dryness in vacuo to yield 5-(N-allylacetamido)-N1,N3-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophthalamide.
