MANUFACTURING OF PROTECTED DO3A.

Abstract

The invention relates to a process for the one-step preparation and isolation of a protected DO3A such as a tri-tert-butyl ester of DO3A as a solid salt.

Description

PROTECTED DO3A MANUFACTURING FIELD OF INVENTION The present invention relates to organic chemistry, particularly to the preparation of protected DO3A. More particularly, the invention relates to a one-step process for the preparation and isolation of protected DO3A, such as the tri-tert-butyl ester of DO3A (tri-tert-butyl ester of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) as a salt. BACKGROUND OF THE INVENTION Magnetic Resonance Imaging (MRI) is a renowned diagnostic imaging technique increasingly used in clinical diagnoses for a growing number of indications. The strong expansion of medical MRI has further benefited from the development of a class of compounds, MRI contrast agents, which act by causing a marked variation in the proton relaxation rates of nearby water in the tissues / organs / fluids where they are distributed, thereby adding physiologically relevant information to the impressive anatomical resolution commonly obtained in non-contrast MRI images. Examples of commercially available MRI contrast agents include Gd3+ ion complexes with linear chelating agents, such as the DTPA ligand, marketed as MAGNEVISTMR; the DTPA-BMA ligand Gd3+ complex, marketed as OMNISCANMR; the BOPTA Gd3+ complex, known as Gadobenate Dimeglumine and marketed as Mult¡HanceMR; and Gd3+ complexes with cyclic chelating agents, such as the DOTA ligand, marketed as DOTAREMMR; the hydroxylated tetraaza macrocyclic ligand Gd3+ complex known as HPDO3A, long marketed as ProHanceMR; and that of the corresponding butyl-triol derivative, known as Gadobutrol and marketed as GadavistMR. Key intermediate compounds in the preparation of many of these macrocyclic chelating ligands are DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) and protected derivatives thereof, such as the tri-tert-butyl ester of the formula ML / a / ZUZZ / UU / 1 uo Ηχ / --\ / —COOfBu \Ν\ fBuOOC— / \__ / —COOfBu DO3A tri-tert-butyl ester The preparation of the tri-tert-butyl ester of DO3A is commonly carried out by following two main synthetic routes that lead to the isolation of the product as a free base or, alternatively, as a salt with the anion of the alkylating agent. Isolating the product as a hydrobromide has proven preferable in large-scale preparations, as it allows the product to be collected in a stable form that can be conveniently stored for extended periods without degradation. When necessary, the hydrobromide salt can then be converted to the corresponding free base for use in subsequent reactions. Thus, many efforts have been made over time to optimize the synthesis and isolation of the tri-tert-butyl ester of DO3A as a hydrobromide salt. WO 93 / 02045 describes a process for the tris-alkylation of cyclen with tert-butyl bromoacetate in dimethylacetamide in the presence of NaOAc that requires 19 days of reaction time to give the desired hydrobromide with an overall yield of 56%. A procedure that allows reducing the reaction time to 60 minutes is described by Moore in Org. Synth. 2008, 85,10-14, involving the reaction of 1,4,7,10-tetraazacyclododecane and NaOAc with tert-butyl bromoacetate in DMAC and precipitation of the tri-tert-butyl ester hydrobromide salt by dilution of the crude reaction with diethyl ether and cooling. A complex and time-consuming fabrication procedure is then required, comprising multiple steps of dissolution, washing, and re-precipitation, leading to a final product with a yield of 65-80%. Jagadish et al. (Tetrahedron Lett., 2011, 52(17), 2058-2061) describe a procedure comprising tris-alkylating 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate in DMAC (dimethylacetamide) and in the presence of NaOAc for 24 h at room temperature, emptying the reaction mixture into water to give a clear solution, and precipitating the hydrobromide salt by the addition of KHCO3. The solid obtained is then collected, dissolved in CHCl3, washed with water, concentrated, and recrystallized by the addition of ether to give the desired product in approximately 80% yield. US8,138,332 describes a manufacturing process comprising reacting 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate in DMAC (dimethylacetamide) and ML / a / ZUZZ / UU / 1 uo in the presence of NaOAc at room temperature for 5 days, empty the reaction suspension into a large excess of water (3.9:1 (w / w) over DMAC) to give a clear aqueous solution, adjust the pH of the solution to 9 with solid NaHCO3 and precipitate the hydrobromide salt of the tri-tert-butyl ester of DO3A by adding a salt such as KBr to the solution. The hydrobromide salt is collected by filtration with a yield of approximately 73%. The above procedures commonly require reaction times of several days, followed by costly and slow purifications of the raw products, and are therefore not suitable for large-scale production. The growing interest in macrocyclic Gd-based contrast agents such as ProHance and Gadobutrol, and more generally, in DO3A derivatives, makes it highly desirable to have optimized manufacturing procedures that overcome the disadvantages mentioned above and allow convenient preparation of this important starting material on a larger scale, for example. BRIEF DESCRIPTION OF THE INVENTION The Applicant has now unexpectedly found that dilution with water of the raw material obtained by reacting 1,4,7,10-tetraazacyclododecane (or Cyclen, as used interchangeably herein) with an activated acetic ester, such as tert-butyl bromoacetate in an organic solvent and in the presence of a base, makes it possible to isolate the respective DO3A triester as a solid salt directly from the crude reaction. The present invention relates in general to an improved process for the manufacture of protected DO3A, such as tri-tert-butyl ester of DO3A, wherein the compound is collected as a solid salt directly from a crude organic reaction diluted with water. More particularly, the invention relates to an optimized process for the preparation of DO3A having ester-protected carboxyl groups comprising essentially diluting with water the crude mixture obtained by reacting 1,4,7,10-tetraazacyclododecane with an activated acetic ester, such as a tert-butyl ester, in an organic solvent and in the presence of an auxiliary base, and then collecting the triester product as a solid salt, directly from the diluted mixture. Optionally, the process may involve the addition of supplemental water to the organic solvent in which the reagents are mixed. One aspect of the invention relates to a one-step process for manufacturing a DO3A salt protected by formula (I) ML / a / ZUZZ / UU / I uo * (HX)y(I) where X is a chlorine, iodine or, preferably, a bromide anion; and is an integer from 1 to 3, for example, 1, 2 or 3, and R is a Ci-Ce alkyl or aryl group; The process essentially comprises: 1) reacting the Cyclone with an activated acetic ester of the formula XCH2OOR, in an organic solvent and in the presence of an auxiliary base, to give a crude reaction mixture; 2) adding water to the crude mixture of step 1), to obtain a suspension comprising the protected DO3A in the form of a solid salt of formula (I); and 3) collect and wash the DO3A protected salt. The organic solvents for use in the reaction of step 1) preferably include dipolar aprotic solvents such as DMF, DMSO, MeCN, and DMAC. Most preferably, the organic solvent is DMAC. The auxiliary base is preferably selected from weak bases such as NaOAc, NaHCO3, Na2CO3, KHCO3, K2CO3, DIPEA, and triethylamine; most preferably, it is NaOAc. Preferably, in the above formula (I), X is a bromine, and ey is 1 or 2; more preferably y is 1. In one embodiment of the above formula (I), R is a benzyl, more preferably R is a Ci-Ce alkyl. The Ci-Ce alkyls according to the invention include a linear or branched chain comprising 1 to 6 carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, and the like. Much more preferably, in formula (I) R is tert-butyl. In one embodiment, the reaction mixture in step 1 also includes water. In a preferred embodiment, step 1) of the process comprises reacting 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate, to manufacture DO3A protected as a tri-tert-butyl ester, such as the tri-tert-butyl ester monohydrobromide salt of DO3A. Advantageously, the process of the invention allows the collection of tri-tert-butyl ester of DO3A as a hydrobromide salt directly from the crude reaction mixture with good yield and optimum purity, which can thus be used as such (or conveniently stored) without the need for additional complicated and time-consuming purifications or recrystallizations. ML / a / ZUZZ / UU 11 uo DETAILED DESCRIPTION OF THE INVENTION In one embodiment, the present invention relates to a one-step process for the manufacture of a DO3A-tric-tert-butyl ester salt of the formula (IA) \ / \z—COOíBu *(HBr)v N Ny (IA) N tBuOOC—7\__ / X—COOíBu where y is an integer from 1 to 3, preferably from 1 to 2 and, most preferably, is 1, comprising: 1) reacting the Cyclone with tert-butyl bromoacetate in an organic solvent and in the presence of an auxiliary base to give a mixture; 2) adding water to the mixture of step 1), to obtain a suspension comprising the tri-tert-butyl ester of DO3A as a solid salt of formula (IA); and 3) collect and wash the DO3A-tric-tert-butyl ester salt. Step 1 Step 1) of the process involves reacting Cyclone with tert-butyl bromoacetate, essentially as outlined in the following synthetic scheme 1 Scheme 1 Cyclen DO3A tri-tert-butyl-(HBr)y ester This step generally involves adding tert-butyl bromoacetate to Cyclen in an organic solvent and in the presence of an auxiliary base to give a mixture comprising the tri-tert-butyl ester of DO3A as a solid salt. For example, Cyclen and the auxiliary base are first mixed in an organic solvent, to give a mixture that is then loaded with tert-butyl bromoacetate. In one embodiment, tert-butyl bromoacetate is added to the mixture as is, without any prior dilution of it. In a preferred embodiment, step 1) of the process of the invention comprises i) obtaining a solution of tert-butyl bromoacetate in an organic solvent, and i) adding the obtained solution to a mixture of Cyclen and an auxiliary base in the same organic solvent. Suitable organic solvents include dipolar aprotic solvents as stated above; in a preferred embodiment, the organic solvent for use in the process of the invention is DMAC. For example, tert-butyl bromoacetate is first diluted with DMAC, for example, at room temperature, to give a tert-butyl bromoacetate solution with a concentration preferably of 3 to 7 mol / L (where L refers to DMAC), more preferably of 3 to 5, and much more preferably of about 4-5 mol / L. The resulting solution is then added to a mixture of Cyclen and an auxiliary base in DMAC. Preferably, the auxiliary base in the mixture is a weak base; in a preferred embodiment, it is NaOAc. More typically, the mixture is a suspension. In some forms, the mixture of Cyclen and the auxiliary base in DMAC from step 1) may also include a certain amount of water (or dilution water). When present, the amount of dilution water is (by weight) from 0.1 to 2, more preferably from 0.5 to 1.5 times the amount of Cyclen in the mixture. More preferably, it is substantially equal to the amount by weight of Cyclen. The process preferably comprises preparing a suspension of Cyclen and NaOAc in DMAC (and optionally water) in which the final concentration of Cyclen is 0.5 to 1.0, preferably 0.6 to 1.0, and much more preferably 0.6 to 0.85 mol / L. An amount of tert-butyl bromoacetate is then loaded into the suspension that is sufficient to provide the desired trialkylation of Cyclen while avoiding further unwanted alkylation. In one embodiment, the alkylation reaction of step 1) is carried out using a substantially stoichiometric Cyclon:tert-butyl bromoacetate ratio of 1:3 (mol / mol). In an alternative embodiment, an excess of tert-butyl bromoacetate, for example, of approximately 1–30%, preferably 1–10%, corresponding to a Cyclon:tert-butyl bromoacetate ratio of 1:3 to 1:4 (mol / mol), may be used. Similarly, in one embodiment, a substantially stoichiometric ratio of 1:3 between the Cyclon and the weak base NaOAc is used, whereas in the alternative embodiments, an excess of the base, ranging from 1 to 30% and preferably from 1 to 10% above the stoichiometric amount, may be used. The appropriate amount of tert-butyl bromoacetate solution is preferably loaded into the suspension obtained from Cyclen and NaOAc kept under stirring and at a temperature of 0 to 25 °C. The addition is conveniently carried out in a time of 1-4 hours, preferably 2-3 hours. After the addition, the reaction mixture is kept under stirring until the completion of the alkylation reaction, for example, for a time of approximately 16MA / a / ZUZZ / UU / I uo hours. In one embodiment, the addition of tert-butyl bromoacetate and the completion of the reaction are carried out at the same temperature, for example, ranging from 0 to 252C, preferably from 5 to 15SC, much more preferably from 10 to 15SC, for example, from about 12SC; then the resulting reaction mixture is raised (or maintained) at 25aC and stirred at that temperature for about 2-4 hours. In an alternative embodiment, the addition of tert-butyl bromoacetate is carried out at a lower temperature, preferably 0 to 15°C, while the completion of the alkylation reaction is carried out at a higher temperature, preferably 20 to 35°C. For example, in one embodiment, step 1) of the process comprises adding a solution of tert-butyl bromoacetate in DMAC to a suspension of Cyclone and NaOAc in the same solvent (and optionally water) at a temperature of approximately 10°C, which is carried out for approximately 2.5 h, during which the temperature of the reaction mixture is maintained at 10–15°C. The resulting reaction mixture is then kept under stirring at this temperature until the completion of the alkylation, for example, for 20–48 h, preferably for 20–30 h, and more preferably for 20–25 h.After completion, the temperature of the reaction mixture is preferably raised to room temperature (e.g., about 25°C) and the mixture is kept under stirring at this temperature for an additional 2 to 4 hours. In an alternative embodiment, after the addition of the tert-butyl bromoacetate solution, the temperature of the resulting reaction mixture is raised, for example, to 23–35°C, and the mixture is kept under stirring at that temperature until the alkylation is complete. Preferably, the reaction mixture is left under stirring at a temperature of 25–30°C for 20–30 h, preferably for 20–25 h. Under the above conditions, selective trialkylation of Cyclen is obtained, with the formation of tri-tert-butyl ester of DO3A that remains in suspension as a solid salt, for example as a mixture of hydrobroms, more particularly as a monohydrobromide. This yields a crude mixture, usually a suspension or thick slurry, comprising DO3A-tric-tert-butyl salts (e.g., as hydrobromide), by-product salts (e.g., NaBr) and optional minor amounts of unreacted base and / or organic impurities in the solid suspended phase. Step 2 Step 2) is conveniently carried out by adding water (also identified as “process water”) to the raw suspension obtained in step 1). The applicant has actually unexpectedly found that a suitable addition of water to the suspension collected from step 1) of the process allows the solubilization of by-products and IVIA / a / ¿U¿¿ / UU / I Uo unreacted components while leaving the tri-tert-butyl ester salt (hydrobromide) of DO3A as essentially the only remaining solid in the resulting crude suspension, which can then be collected. The amount of water (by weight) to be added to the raw mixture collected from step 1) can be adequately determined, for example, with respect to the amount of Cyclen (by weight) subjected to the alkylation reaction. The appropriate amounts of process water (by weight) can be, for example, ten times or less than the amount of starting Cyclen undergoing the alkylation reaction. This amount of process water is generally independent of the presence of optional amounts of water in the initial reaction mixture comprising Cyclen and a DMAC auxiliary base (which optional amounts, however, are more than twice the amount of Cyclen). In a preferred embodiment, step 2) of the process is carried out by diluting the crude suspension obtained from step 1) with an amount of process water that is preferably more than twice (for example, approximately 2.5 times) the amount of Cyclen (w / w) undergoing the alkylation reaction. Preferably, the amount of water is, by weight, from 3 to 10 times, more preferably from 3 to 8, much more preferably from 3 to 6, and particularly preferably from 4 to 6, such as approximately 4, 5, or 6 times the amount of Cyclen (w / w) undergoing the reaction. In fact, as confirmed by experimental results, diluting the crude suspension obtained from step 1) with an amount of water more than 2 times, for example, corresponding to 2.5 - 8 times, preferably 3 - 8 times the amount (by weight) of the respective starting Cyclon (subjected to trialkylation), allows solubilizing most of the undesirable salts / impurities in suspension, while avoiding the loss of the desired product, thus leading to achieving the desired product with good yield and purity. Dilution is usually carried out with purified water, such as Milli-Q or purified water, for example, by reverse osmosis which has a temperature of 15-20sC. Preferably the dilution is carried out at a temperature of 20-25sC, more preferably at approximately 20sC, and in a time of approximately 0.5-1 hours, more preferably at approximately 0.5 h. In a particularly preferred embodiment, step 2) of the process comprises diluting the collected suspension from step 1) with an amount of purified water as above, which preferably ranges from 3 to 8, more preferably from 3 to 6, and much more preferably from 4 to 6 times the amount (by weight) of the starting Cyclen, for a time of about half an hour and at a temperature of 20-25sC, and then keeping the crude mixture under agitation at about 20sC for a time of 0.5-3 hours, preferably about 2 hours. ML / a / ZUZZ / UU Z 1 ÜO This results in a suspension where the solid phase consists essentially of the hydrobromide salt of the tri-tert-butyl ester of DO3A and, optionally, smaller residual amounts of unwanted reaction salts. The total amount of water in the suspension (i.e., including water added in step 2) and the optional water from step 1) is at least 2.5 times, more preferably at least 3 times, and even more preferably at least 4 times the amount of Cyclen added in step 1) (w / w), up to 10 times, preferably up to 8 times the amount of Cyclen. Step 3 Step 3) of the process involves collecting and then washing the solid product present in the suspension resulting from step 2). The solid can be conveniently collected using procedures known to the skilled professional. In one modality, the solid is collected by filtration of the crude suspension. In an alternative modality, which is particularly preferred when working on an industrial scale, the suspension obtained in step 2) of the process is subjected to centrifugation, allowing the removal, in the liquid phase, of residual amounts of DMAC, water, solubilized reaction salts and optional liquid impurities, to obtain a wet solid comprising the raw product as a solid salt which is then collected. Centrifugation is usually carried out at high speed (e.g., between 1800 and 2500 rpm). The cake, which comprises the raw product, is then washed. Preferably, the raw product collected is washed with water (or “wash water”). In a preferred embodiment, the collected wet solid comprising crude product is washed with an amount of water that, by weight, is 4 to 20 times the weight of the Cyclen subjected to the reaction. At this level, the appropriate amount of water that allows dissolving and thus removing optional residual salts / impurities is preferably determined with reference to the amount of water added to the crude suspension in step 2) of the process. In reality, it is clear to a skilled professional that smaller dilutions of the raw mix in step 2) are preferentially associated with washing the raw product in step 3) with larger quantities of water. Conversely, dilutions that use larger quantities of water from the raw mix allow for the use of smaller quantities of wash water. For example, as confirmed by experimental results, when the crude suspension obtained from step 1) of the process is diluted with an amount of process water approximately 8 times (w / w) the amount (by weight) of the starting Cyclen, the crude product collected in step 3) can be appropriately washed twice with a IVIA / a / 4ÍU24Í / UU / 1 Uo amount of wash water that corresponds to each one, by weight, from 2 to 8 times or, more preferably, from 2 to 4 times the amount by weight of the Cyclen, still obtaining a product with optimal purity. In contrast, when the crude suspension is diluted with smaller quantities of process water, for example, approximately 3-4 times the weight of the Cyclen, the collected crude product is preferably washed twice with a larger quantity of water, for example, each time ranging from 4 to 10 or, more preferably, from 4 and 8 times the weight of the starting Cyclen. In a preferred embodiment, the crude suspension obtained from step 1) of the process is diluted with an amount of water that is approximately 4 times the amount (by weight) of Cyclen, to give a crude product that is then washed twice, each time with an amount of water approximately 4 times the amount (by weight) of Cyclen, which corresponds, in this case, to an organic solvent:total water ratio of approximately 1:1 (w / w). After washing, the product is collected, for example, by filtration or centrifugation of the solid, and the wet product is then dried, for example, at temperatures above ambient temperature (rt) and / or under reduced pressure. In a preferred embodiment, the product is dried at a temperature of 35-40sC and reduced pressure (e.g., 5 to 25 mbar) for a time of 20-25 h, to give DO3A tritert-butyl ester as a mono-hydrobromide salt with a yield of 73-83%. Although the above description refers specifically to the preparation of the tri-tert-butyl ester hydrobromide salt of DO3A, i.e., a compound of formula (I) where X is a bromine anion and R is tert-butyl, a person skilled in this field will nevertheless know that the described process can be similarly implemented for the preparation of compounds of formula (I) Ηχ / \ / —C00fí* (HX)V N N. ] EITHER ROOC— / \__ / —COOfí where X is a chlorine, fluorine or iodine anion; and is 2 or 3, and R is a benzyl or, preferably, a Ci-Ce alkyl other than tert-butyl. The use of the process of the invention advantageously allows a significant reduction in both reaction and process times. For example, the invention process allows the desired product to be achieved with a yield of approximately 83%, or even higher when working on an industrial scale, in ML / a / ZUZZ / UU / I uo a reaction time of approximately 24 hours, which is well below the reaction times required by known procedures. More specifically, the proposed process allows obtaining tri-tert-butyl ester of DO3A as a hydrobromide salt with the above yield and a purity of at least 95%, preferably at least 97%, more preferably at least 98%, and much more preferably at least 99%, for example, up to 99.5% directly from the crude reaction mixture (obtained by reacting 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate in an organic solvent such as DMAC and in the presence of a weak base, such as NaOAc), by simply diluting the crude product with water, collecting and washing the solid hydrobromide salt with water. Therefore, the collected product can be used advantageously as is, without requiring any additional costly and / or time-consuming purification or re-precipitation or recrystallization step, where this makes the process particularly advantageous for large-scale productions, e.g., industrial productions. The purity of the collected hydrobromide can be determined by different analytical methods, for example, including NMR vs a standard, which confirms the structure and assay of the collected compound, HPLC to determine purity, and by titration, to determine the optional presence of residual NaBr. In another aspect, the invention relates to a new process for the manufacture of a macrocyclic chelating ligand of formula (II) HOOC^ \__ / COOH ινΐΛ / a / zuzz / uu / 1 uo wherein Ri and R2 are both -CH2OH or R1 is H and R2 is -CH3, or a chelating complex thereof with a paramagnetic metal ion selected from the group consisting of Fe2+, Fe3+, Cu2+, Cr+, Gd3+, Eu3+, Dy3+, La3+, Yb3+ or Mn2+, or an alkaline earth metal ion, or a salt thereof, wherein the process comprises: a) prepare a hydrobromide salt of the tri-tert-butyl ester of DO3A according to the synthetic process described above; b) convert the obtained hydrobromide salt into the desired ligand of formula (II); and, c) where applicable, form the ligand obtained into a complex with a metal ion and isolate the complex. Preferably, the paramagnetic metal ion is Gd3+ and the alkaline earth metal ion is Ca2+ and the prepared chelating complex is Gadobutrol, Gadoteridol or Calteridol. More preferably, the metal ion is Gd3+ and the prepared chelating complex is Gadoteridol or Gadobutrol of the formula Gadoteridol Gadobutrol More specifically, in an additional aspect, the invention relates to a new process for the synthetic preparation of Gadoteridol or Gadobutrol comprising: a) preparing tri-tert-butyl ester of DO3A as a hydrobromide salt according to the process of the present invention; b) convert the obtained hydrobromide salt into the desired ligand of formula (II); and c) form the ligand obtained into a complex with a Gd3+ metal ion and isolate the respective Gadoteridol or Gadobutrol complex. In the above process, step a) comprising the preparation of the tri-tert-butyl ester of DO3A as a hydrobromide salt is carried out by the manufacturing process of the present invention, as comprehensively reported above, while steps b) and c) in their entirety, including experimental conditions and optional variants thereof, are carried out in accordance with the procedure known in the art. For example, step b) may comprise i) neutralizing the collected hydrobromide salt and deprotecting it to give the DO3A ligand where the carboxylic groups are in the acidic, deprotected form; and i) alkylating the DO3A to obtain the desired ligand in the acidic form. Neutralization and removal of the protecting groups from the hydrobromide salt collected in step a) can be carried out according to known techniques, for example, by hydrolysis in the presence of a base, or treatment with trifluoroacetic acid. Alkylation of the resulting DO3A can then be carried out, for example, by using a suitable epoxide as described, for example, in EP0988294, to obtain the desired 10-(2-hydroxypropyl)1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (HPDO3A) or the [10[2,3-dihydroxy-1-(hydroxymethyl)propyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid ligand (DO3A butyltriol). Alternatively, the ligand of formula (II) can be prepared by alkylation in the presence of a base of the hydrobromide salt collected from step (a), or of the tri-tert-butyl ester of DO3A collected after neutralization of the salt, carried out in a separate step. The alkylation reaction can be carried out according to a conventional procedure, for example, including the reaction of the ester with an optionally protected alkylation group, or with an epoxide, for example, as outlined in Scheme 1 below. Scheme 1 MA / a / 4ÍU2¿ / UU ri ÜO ch2oh The alkylated ester obtained is then deprotected according to conventional techniques, for example, by using trifluoroacetic acid, to obtain the respective deprotected ligand. Step c) of the process comprising forming the ligand obtained from step b) into a complex with gadolinium can be carried out, for example, by stoichiometric addition of a suitable Gd(lll) derivative, particularly a Gd(111) salt or oxide, to a solution of the ligand, for example, working in accordance with well-known experimental methods, for example as reported in EP 230893. In a further embodiment, the invention relates to a new process where the hydrobromide salt of the tri-tert-butyl ester of DO3A obtained according to the method of the invention is used as a starting material for the preparation of the compounds described in WO 2017 / 098038, WO2017 / 098044 or WO2018 / 108780. All starting materials, including solvents, and auxiliary reagents, such as NaOAc or other bases used in the process of the invention, are commercially available. Further details in the process of the invention are then reported in the following Experimental Section, which constitutes a general reference to the operating conditions employed in a process according to the invention. EXPERIMENTAL PART Abbreviations and Definition of Terms DO3A: 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid tri-tert-butyl ester of DO3A 1,4,7-tris(1,1-dimethylethyl) ester of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid tri-tert-butyl ester of DO3A-(HBr) and hydrobromide salt of 1,4,7-tris(1,1-dimethylethyl) ester of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid. HPDO3A: 10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid. NMR: Nuclear Magnetic Resonance. MRI: Magnetic Resonance Imaging. NaOAc: Sodium acetate. DMAC: N,N-dimethylacetamide. DMF: N,N-dimethylformamide. DMSO: Dimethyl sulfoxide. MeCN: Acetonitrile. CHCh: Chloroform. NaHCO3: Sodium bicarbonate. Na2CO3: Sodium carbonate. KHCO3: Potassium bicarbonate. K2CO3: Potassium bicarbonate. Potassium carbonate DIPEA N,N-diisopropylethylamine rpm Revolutions per minute HPLC determination of the DO3A-tric-fer-butyl-HBr assay General procedure HPLC characterization of the collected tri-tert-butyl ester hydrobromide salt of DO3A The HPLC characterization of the tri-tert-butyl ester hydrobromide salt of DO3A obtained with the process of the invention was carried out using the Agilent 1100 liquid chromatography system. The experimental setup of the HPLC measurements is summarized below. Analytical conditions HPLC system equipped with a solvent delivery system, autosampler, column thermostat, degasser, and diode array detector or variable wavelength detector (or equivalent). Stationary phase: Zorbax Eclipse XDB-C8, 5 pm, 150 x 4.6 mm. Temperature: 45 °C Mobile phase column: A: 0.01 µM K2HPO4, 0.017 µM H3PO4 B: Acetonitrile Elution: Gradient Time (min) %B 0 5 30 80 35 80 38 5 45 5 Flow rate 1 mL / min Temperature 45 °C Detection UV, 210 nm, Bw=8 nm; Reference 360 ​​nm, Bw=100 nm Injection volume 10 pL Detection time 35 min Reference peak DO3A 3tBu Retention time DO3A 3tBu = 14.5 min. Determination of the Bromide assay in DOSA-tric-te / '-butyl-HBr. General procedure The determination of the bromide ion content in the collected tri-tert-butyl HBr DO3A is carried out by potentiometric titration using 0.1 N silver nitrate solution with a combined Ag / AgCl electrode, according to known procedures. Example 1: Synthesis of the tri-tert-butyl ester monohydrobromide salt of DO3A: 4x of process water v 2(4x) of wash water (n / p) on Cvclen of starting The synthesis is carried out according to the following synthetic scheme 'n ní [ I / -x DMAC + Br COOtBu ——► |\T NaOAc HZ \___ / XH l\T íBuOOC— / \__ / —COO / Bu Cyclen DO3A 3tBu-HBr ML / a / zuzz / uu / I Uo A suspension of commercially available 1,4,7,10-tetraazacyclododecane (15.1 kg; 87.65 mol; conc. = 0.83 mol / L DMAC) and sodium acetate (22.65 kg; 276.11 mol) in DMAC (99 kg; 105.3 L), and a solution of tert-butyl bromoacetate (53.87 kg; 276.11 mol; conc. = 4.88 mol / L DMAC) in DMAC (53.23 kg; 56.63 L) were added at 10 °C for 2.5 h. The temperature was then raised to 25 °C and the mixture was stirred for 24 h. Water (60.0 kg; 4:1 w / w on starting Cyclone) was then added over 0.5 h. After 2 h the mixture was centrifuged, and the collected cake was washed with water (2 x 60.0 kg = 2 x 4:1 w / w on starting Cyclen). The wet solid was dried under vacuum, yielding 38.43 kg of the desired hydrobromide salt as a white powder. Performance: 73.5% performance. HPLC of the title: (against the standard) is 100% Title NMR: (vs. standard) 99.86% Bromide test: 100.7% Example 2: Synthesis of the tri-tert-butyl ester hydrobromide salt of DO3A 4x process water and 5.3x + 4x wash water (w / w) on starting Cyclen Cyclon (9 kg; 52.24 mol; 0.83 mol / L DMAC) and sodium acetate (13.5 kg; 164.57 mol) in DMAC (59.3 kg; 63.1 L) were loaded into a reactor. The resulting slurry was stirred for 30 min at 25 °C. The slurry was then cooled to 10 °C and a solution of tert-butyl bromoacetate (32.1 kg; 164.57 mol; 4.85 mol / L DMAC) in DMAC (31.9 kg; 33.9 L) was added for 3 h. The tubing was washed with DMAC (2.15 kg; 2.29 L), which was then added to the reaction mixture. The temperature was then raised to 25 °C, and the mixture was stirred for 24 h. Water (36.1 kg; 4:1 w / w on starting Cyclen) was then added at 0.5 h and after 2 h the mixture was centrifuged, and the cake was washed with water (47.4 kg + 36.5 kg). The wet solid was dried under vacuum, yielding 22.70 kg of the hydrobromide salt. Yield: 72.8%. HPLC of the title: (against the standard) is 100% Title NMR: (vs. standard) 99.86% Bromide test: 100.6% Example 3: Synthesis of the tri-tert-butyl ester hydrobromide salt of DO3A 4x process water and 2(4x) wash water (w / w) on the starting Cyclen Cyclon (9 kg; 52.24 mol; 0.83 mol / L DMAC) and sodium acetate (13.5 kg; 164.57 mol) in DMAC (59.15 kg; 62.93 L) were loaded into a reactor. The resulting suspension was stirred for 30 min at 25 °C. The suspension was then cooled to 10 °C and a solution of tert-butyl bromoacetate (32.14 kg; 164.74 mol; 4.86 mol / L DMAC) in DMAC (31.85 kg; 33.88 L) was added for 3 h. The tubing was washed with DMAC (2.15 kg; 2.29 L), which was then added to the reaction mixture. The temperature was then raised to 25 °C, and the mixture was stirred for 24 h. Water (36 kg) was then added at 0.5 h and after 2 h the mixture was centrifuged, and the cake was washed with water (36.7 kg + 36.8 kg). The wet solid was dried under vacuum, yielding 24.16 kg of the hydrobromide salt. Yield: 76.7%. HPLC of the title: (against the standard) is 100% Title NMR: (vs. standard) 98.84% Bromide assay: 99.9% Example 4: Synthesis of the tri-tert-butyl ester bromohydrol salt of DO3A 4x process water (w / w) and 2(4x) wash water (w / w) on starting Cyclen Cyclon (10.0 g; 58.05 mmol) and sodium acetate (17.9 g; 217.68 mmol) in DMAC (72.5 mL) were loaded into a reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. The suspension was then cooled to 12 °C and a tert-butyl bromoacetate solution (42.5 g; 217.68 mmol) in DMAC (37.5 mL) was added at 12 °C for 1.5 h. The mixture was stirred for 24 h at this temperature, then heated to 25 °C and stirred for 2 h at this temperature. After cooling to approximately 18-20 °C, water (40 mL) was added at 0.25 h and after 1 h the mixture was filtered through a porous septum P3 and washed with water (40 mL + 40 mL). The wet solid was dried under vacuum. Yield: 82.9% HPLC Area %: 99.2% Title NMR (vs. standard): 97.6% Bromide test: 102.% Example 5: Synthesis of the tri-tert-butyl ester hydrobromide salt of DO3A 1x dilution water, 8x process water and 2(2x) wash water (w / w) on starting Cyclen Cyclon (10.0 g; 58.05 mmol) and sodium acetate (17.9 g; 217.68 mmol) in DMAC (72.5 mL) were loaded into a reactor. The resulting suspension was stirred (250 rpm) for 30 min at 25 °C. The suspension was then cooled to 12 °C and diluted with water (10 mL). Maintaining the temperature at 12 °C, a solution of tert-butyl bromoacetate (42.5 g; 217.68 mmol) in DMAC (37.5 mL) was added over 2.25 h. The mixture was stirred for 24 h at this temperature, then heated to 25 °C and stirred for 2 h at this temperature. After cooling to approximately 18-20 °C, water (80 mL) was added over 0.5 h and after 1 h the mixture was filtered through a porous septum P3 and washed with water (20 mL + 20 mL). The wet solid was dried under vacuum. Yield: 80.4% HPLC Area %: 98.1% Title NMR (vs. standard): 97.7% Bromide test: 103.6% Example 6: Synthesis of the tri-tert-butyl ester hydrobromide salt of DO3A ML / a / zuzz / uu ri ÜO 1x dilution water, 8x process water v 2(2x) wash water (w / w) on starting Cyclen Cyclon (10.0 g; 58.05 mmol) and sodium acetate (16.7 g; 203.17 mmol) in DMAC (72.5 mL) were loaded into a reactor. The resulting suspension was stirred (250 rpm) for 30 min at 25 °C. The suspension was then cooled to 0 °C and diluted with water (10 mL). Maintaining the temperature at 0 °C, a solution of tert-butyl bromoacetate (39.6 g; 203.17 mmol) in DMAC (37.5 mL) was added over 2.25 h. The mixture was stirred for 24 h at this temperature, then heated to 25 °C and stirred for 2 h at this temperature. After cooling to approximately 18-20 °C, water (80 mL) was added over 0.5 h and after 1 h the mixture was filtered through a porous septum P3 and washed with water (20 mL + 20 mL). The wet solid was dried under vacuum. Yield: 77.3% % HPLC Area: 98.3% Title NMR (vs. standard): 97.5% Bromide test: 106.9% Example 7: Synthesis of the tri-tert-butyl ester hydrobromide salt of DO3A 2x process water and 2(2x) wash water (w / w) on starting Cyclen Cyclon (10.0 g; 58.05 mmol) and sodium acetate (15.0 g; 182.85 mmol) in DMAC (72.5 mL) were loaded into a reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. The suspension was then cooled to 10 °C and a tert-butyl bromoacetate solution (35.7 g; 182.85 mmol) in DMAC (38.0 mL) was added at 11 °C for 2.5 h. The temperature was then raised to 25 °C for 1 h and the mixture was stirred for 24 h at this temperature. After cooling to approximately 18-20 °C, water (20 mL) was added in 0.5 h and after 2 h the mixture was filtered through a porous septum P3 and washed with water (20 mL + 20 mL). The wet solid was dried under vacuum. Yield: 72.6% HPLC Area %: 99.0% Title NMR (vs. standard): 95.9% Bromide test: 104.3% Example 8: Evaluation of the effect of the amount of water To determine the appropriate amounts of water added to the crude suspension obtained by the reaction of 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate in DMAc and NaOAc, tests were carried out using 10 g of starting Cyclen, the same amounts of DMAc (110 ml total amount) and tert-butyl bromoacetate, a fixed tert-butyl bromoacetate:Cyclen ratio, and varying amounts of process / wash water. Cyclon (10.0 g; 58.05 mmol) and sodium acetate (15.0 g; 182.85 mmol) in DMAC (72.5 mL) were loaded into a reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. The suspension was then cooled to 10 °C and a solution of tert-butyl bromoacetate (35.7 g; 182.85 mmol) in DMAC (37.5 mL) was added at 10 °C for 2.5 h. The temperature was then raised to 25 °C and the mixture was stirred for 24 h at this temperature. After cooling to approximately 18-20°C, the amount of water (see table below) was added in 0.5 h. After 2 h, the mixture was filtered through a porous septum P3 and washed with water. The wet solid was dried under vacuum.The results of the different tests carried out with different amounts of process and wash water (the amounts of water are provided as parts by weight, with reference to the amount of the starting Cyclen) are shown in Table 1 below, where the purity of the collected product is provided as HPLC (% area) and the NMR of the titer (vs standard). ML / a / ZUZZ / UU 11 uo Table 1 Test Process Water* Wash Water* Yield (%) HPLC (% of area) NMR Test 1 4 4x2 75.1 76.7b 99.4 100b 96.1a 98.8b 2 8 8x2 66.9 99.0 97.0a 3 8 2x2 67.6 99.2 96.4a 4 2 2x2 64.0 98.8 90.6a 5 6 3x2 73.8 98.4 96.0a 6 2 2x2 72.6 99.0 95.9a 7 3 4x2 68.3 98.8 98.0a at laboratory scale b pilot plant scale, example 3 *parts by weight with respect to the amount of Cyclen Results Although obtained through tests conducted on a laboratory scale (resulting in less efficient washing and filtration than those obtained in a pilot or industrial plant), the results reported in the table above confirm that the identified working conditions allow the desired product to be isolated with good performance and an optimal degree of purity, exceeding 95%, allowing its use as is, without requiring further processing or purification. NOVELTY OF THE INVENTION Having described the present invention, it is considered a novelty and, therefore, the contents of the following are claimed as property. CLAIMS 1. A one-step process for the preparation of a protected DO3A salt of the formula (I)H\ / \ / —COOH * (HX) N N. ] EITHER ROOC— / \__ / —COOfi where: X is an anion of chlorine, iodine, or bromine; and is an integer from 1 to 3; and

Claims

1) reacting Cyclen with an activated acetic ester of the formula XCH2OOR, in an organic solvent and in the presence of an auxiliary base, to give a mixture; 2) adding water to the mixture of step 1), to obtain a suspension comprising DO3A protected as a solid salt of formula (I); and 3) Collecting and washing the protected DO3A salt.

2. The process of claim 1, characterized in that the mixture of step 1) further comprises water.

3. The process of claim 2, characterized in that the water in the mixture of step 1) is in an amount of 0.1 to 2 times the amount of Cyclen (w / w).

4. The process of claims 1-3, characterized in that in formula (I) X is bromine, and R is tert-butyl.

5. The process of claims 1-4, characterized in that in step 1) the organic solvent is DMAC, and the auxiliary base is NaOAc.

6. The process of claim 5, characterized in that the reaction of step 1) is carried out using a Cyclen:tert-butyl bromoacetate and Cyclen:NaOAc ratio of 1:3 to 1:4 (mol / mol).

7. The process of claim 6, characterized in that the ratio of Cyclon:tert-butyl bromoacetate and Cyclon:NaOAc is from 1:3 to 1:3.

3. 8.The process of claims 5-7, characterized in that step 1) of the process comprises: i) obtaining a solution of tert-butyl bromoacetate in DMAC; ii) adding the obtained solution to a suspension of Cyclen and NaOAc in DMAC.

9. The process of claim 8, characterized in that the concentration of tert-butyl bromoacetate in the solution is 3 to 5 mol / L, and that of Cyclen in the suspension is 0.5 to 1.0 mol / L.

10. The process of claims 8-9, characterized in that the tert-butyl bromoacetate solution is added to the suspension of Cyclen and NaOAc and stirred at a temperature of 0-25 °C.

11. The process of claim 10, characterized in that the suspension of Cyclen and NaOAc in DMAC is stirred at a temperature of 0-15 °C.

12. The process according to any of the preceding claims 1-11, characterized in that the water added in step 2) is in an amount of 2.5-10 times (w / w) with respect to the amount of Cyclen in step 1).

13. The process of claims 1-12, characterized in that step 3) comprises collecting the protected DO3A salt from formula (I) obtained from step 2) and washing the collected salt with water.

14. The process of claim 13, characterized in that the protected DO3A salt is collected by filtration or centrifugation.

15. The process of claim 13 or 14, characterized in that the amount of washing water is 4 to 20 times the amount of Cyclen in step 1) (w / w).

16. The process of claims 1-15, characterized in that the collected protected DO3A salt is DO3A tri-tert-butyl ester monohydrobromohydrate.

17. The process of claim 16, characterized in that the DO3A tri-tert-butyl ester monohydrobromohydrate has a purity of at least 95%.