A process for the preparation of the compounds of formula: wherein R 1 is independently selected from the group consisting of: carboxyl group (-COOH), carboxylic ester group (-COOR 3) and carboxamido group (-CONH 2, -CONHR 3 or -CONR4R5), R 2 is independently selected from the group consisting of: carboxylic ester group (-COOR 3) and carboxamido group (-CONH 2, -CONHR 3 or -CONR4R5), wherein R 3, R 4 and R 5 are, the same or different from each other, independently a straight or branched C 1 -C 4 alkyl group optionally substituted by one or more, preferably one or two hydroxyl groups, comprising: a) reacting an iodine oxyacid or the salt thereof with elementary iodine in a solvent under an acidic condition in the presence of an acid or a salt of a strong acid, to generate the iodinating reagent in situ, and in turn reacting the iodinating reagent with compounds of formula added to produce compounds of formula, wherein R 1 and R 2 are defined as in the formula, (b) during the iodination reaction, introducing excessive iodine in vapor form into the reaction system, and then adding a supplementary amount of iodine oxyacid or the salt thereof to the iodination reaction system after a period of reaction time, (c) subliming and collecting the excess free iodine after the iodination reaction is completed, (d) filtering and purifying the crude product of the iodination reaction to obtain the compounds of formula, and (e) optionally, recycling the excess free iodine collected after sublimation, the mother liquor of iodination reaction and/or the mother liquor of the purification step to the iodination reaction of step (a). R 1 is independently selected from the group consisting of: carboxyl group (-COOH), carboxylic ester group (-COOR 3) and carboxamido group (-CONH 2, -CONHR 3 or -CONR4R5), R 2 is independently selected from the group consisting of: carboxylic ester group (-COOR 3) and carboxamido group (-CONH 2, -CONHR 3 or -CONR4R5), wherein R 3, R 4 and R 5 are, the same or different from each other, independently a straight or branched C 1 -C 4 alkyl group optionally substituted by one or more, preferably one or two hydroxyl groups, comprising: a) reacting an iodine oxyacid or the salt thereof with elementary iodine in a solvent under an acidic condition in the presence of an acid or a salt of a strong acid, to generate the iodinating reagent in situ, and in turn reacting the iodinating reagent with compounds of formula added to produce compounds of formula, wherein R 1 and R 2 are defined as in the formula, (b) during the iodination reaction, introducing excessive iodine in vapor form into the reaction system, and then adding a supplementary amount of iodine oxyacid or the salt thereof to the iodination reaction system after a period of reaction time, (c) subliming and collecting the excess free iodine after the iodination reaction is completed, (d) filtering and purifying the crude product of the iodination reaction to obtain the compounds of formula, and (e) optionally, recycling the excess free iodine collected after sublimation, the mother liquor of iodination reaction and/or the mother liquor of the purification step to the iodination reaction of step (a). a) reacting an iodine oxyacid or the salt thereof with elementary iodine in a solvent under an acidic condition in the presence of an acid or a salt of a strong acid, to generate the iodinating reagent in situ, and in turn reacting the iodinating reagent with compounds of formula added to produce compounds of formula, wherein R 1 and R 2 are defined as in the formula, (b) during the iodination reaction, introducing excessive iodine in vapor form into the reaction system, and then adding a supplementary amount of iodine oxyacid or the salt thereof to the iodination reaction system after a period of reaction time, (c) subliming and collecting the excess free iodine after the iodination reaction is completed, (d) filtering and purifying the crude product of the iodination reaction to obtain the compounds of formula, and (e) optionally, recycling the excess free iodine collected after sublimation, the mother liquor of iodination reaction and/or the mother liquor of the purification step to the iodination reaction of step (a).