RU2828009C1 - Obtaining secure do3a - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a single-step method of producing a salt of protected DO3A of formula (I)

where X is an anion of chlorine, iodine or bromine; y is integer from 1 to 3; and R is C₁-C₆ alkyl or aryl group; wherein said method includes: 1) reaction of cyclene with activated acetic ester of formula XCH₂COOR in an organic solvent selected from the group consisting of DMF (N,N-dimethylformamide), DMSO (dimethylsulfoxide), MeCN (acetonitrile) and DMAC (dimethylacetamide) and in the presence of an auxiliary base selected from the group consisting of NaOAc, NaHCO₃, Na₂CO₃, KHCO₃, K₂CO₃, DIPEA (N,N-diisopropylethylamine), and triethylamine to obtain a mixture; 2) adding water to the mixture of step 1), in which the water added at step 2) is in amount of 2.0–10 times (wt./wt.) greater than the amount of cyclene at step 1), to obtain a suspension containing the protected DO3A in the form of a solid salt of formula (I); and 3) collecting and washing salt of protected DO3A.

EFFECT: novel method of producing a salt of protected DO3A of formula (I) is developed, which provides convenient production on a larger, for example, industrial scale and enables to isolate the corresponding triester of DO3A in form of a solid salt directly from the crude reaction mixture.

17 cl, 1 tbl, 8 ex

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The present invention relates to organic chemistry, in particular to the preparation of a protected compound DO3A. In particular, the present invention relates to a method for the one-step preparation and isolation of protected DO3A, such as DO3A-tri-tert-butyl ester (tri-tert-butyl ester of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) in the form of a salt.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging (MRI) is a well-known diagnostic imaging technique that is increasingly used in clinical diagnostics for a growing number of indications.

The significant expansion of medical MRI has benefited further from the development of a class of compounds, MRI contrast agents, which act by causing a dramatic change in the relaxation rates of nearby water protons in the tissues/organs/fluids in which they are distributed, thereby adding relevant physiological information to the impressive anatomical resolution typically obtained in unenhanced MRI images.

Examples of commercially available contrast agents for MRI include complexes of Gd ³⁺ ion with linear chelating agents such as DTPA ligand, marketed as MAGNEVIST ^® ; Gd ³⁺ ligand complex DTPA-BMA, marketed as OMNISCAN ^® ; Gd ³⁺ complex BOPTA, known as gadobenate dimeglumine and marketed as MultiHance ^™ ; and complexes of Gd ³⁺ with cyclic chelating agents such as DOTA ligand, marketed as DOTAREM ^® ; The Gd ³⁺ complex of the hydroxylated tetraazamacrocyclic ligand known as HPDO3A, long marketed under the name ProHance ^® , and the complex of the corresponding butyltriol derivative known as gadobutrol and marketed under the name Gadavist ^® .

Key intermediates in the preparation of many of these macrocyclic chelating ligands are DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) and its protected derivatives such as the tri-tert-butyl ester of formula

The preparation of DO3A-tri-tert-butyl ester is generally accomplished by following two main synthetic routes leading to the isolation of the product as the free base or, alternatively, as a salt with the anion of the alkylating agent.

Isolation of the product as the hydrobromide salt is advantageous in large-scale preparations because it allows the product to be collected in a stable form that can be conveniently stored even for long periods of time without deterioration in quality. If necessary, the hydrobromide salt can be further converted to the corresponding free base before its use in subsequent reactions.

Accordingly, much effort has been expended over a long period of time to optimize the synthesis and isolation of DO3A-tri-tert-butyl ester as the hydrobromide salt.

WO 93/02045 describes a procedure for the tris-alkylation of cyclen with tert-butyl bromoacetate in dimethylacetamide in the presence of NaOAc, requiring 19 days of reaction time to obtain the desired hydrobromide in an overall yield of 56%.

A procedure that reduces the reaction time to 60 h is disclosed by Moore in Org. Synth. 2008, 85, 10-14, which involves 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 diluting the feed with diethyl ether and cooling. A complex and laborious work-up procedure is then required, involving multiple dissolution, washing, and reprecipitation steps, leading to the final product in 65-80% yield.

Jagadish et al. (Tetrahedron Lett., 2011, 52(17), 2058-2061) disclose a procedure that involves tris-alkylation of 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate in DMAC (dimethylacetamide) and in the presence of NaOAc for 24 h at room temperature, pouring the reaction mixture into water to obtain a clear solution, and precipitating the hydrobromide salt by adding KHCO ₃ . The resulting solid is then collected, dissolved in CHCl ₃ , washed with water, concentrated, and recrystallized by adding ether to afford the desired product in about 80% yield.

US 8138332 describes a process for the preparation which comprises reacting 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate in DMAC (dimethylacetamide) and in the presence of NaOAc at room temperature for 5 days, pouring the reaction slurry into a large excess of water (3.9:1 (w/w) relative to DMAC) to obtain a clear aqueous solution, adjusting the pH of the solution to 9 with solid NaHCO ₃ , and precipitating the hydrobromide salt of DO3A-tri-tert-butyl ester by adding a salt such as KBr to the solution. The hydrobromide salt is collected by filtration in about 73% yield.

The above procedures typically require a reaction time of several days, followed by expensive and labor-intensive purification of the raw materials, and are therefore unsuitable for large-scale production.

The growing interest in Gd-based macrocyclic contrast agents such as ProHance and gadobutrol, and in general in DO3A derivatives, makes it highly desirable to have optimized preparation procedures that overcome the above-mentioned drawbacks and provide convenient preparation of this important starting material on larger, e.g. industrial, scales.

ESSENCE OF THE INVENTION

The Applicant has surprisingly discovered that dilution with water of a raw material obtained by the reaction of 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 allows the corresponding triester DO3A to be isolated as a solid salt directly from the raw reaction mixture.

The present invention generally relates to an improved process for the preparation of protected DO3A, such as DO3A-tri-tert-butyl ester, wherein the compound is collected as a solid salt directly from an organic feed reaction mixture diluted with water.

More particularly, the present invention relates to an optimized process for the preparation of DO3A with ester-protected carboxyl groups, which essentially comprises diluting with water a feed mixture obtained by the reaction of 1,4,7,10-tetraazacyclododecane with an activated acetic acid ester, such as 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 method may include adding additional water to the organic solvent in which the reactants are mixed.

One aspect of the invention relates to a one-step process for producing a salt of protected DO3A of formula (I)

,

where X is a chlorine, iodine or, preferably, bromine anion; y is an integer from 1 to 3, such as 1, 2 or 3, and R is a C ₁ -C ₆ alkyl or aryl group;

wherein the method essentially includes:

1) the reaction of cyclen with an activated acetic acid ester of the formula _XCH2OOR in an organic solvent and in the presence of an auxiliary base, to obtain a raw reaction mixture;

2) adding water to the raw material mixture in step 1) to obtain a suspension containing protected DO3A in the form of a solid salt of formula (I); and

3) collection and washing of the salt of protected DO3A.

Organic solvents for use in the reaction in step 1) preferably include dipolar aprotic solvents such as DMF, DMSO, MeCN and DMAC. More preferably, the organic solvent is DMAC. The auxiliary base is preferably selected from weak bases such as NaOAc, NaHCO_3,Na₂CO₃, KHCO₃, K₂CO₃, DIPEA, triethylamine; more preferably NaOAc.

Preferably, in the above formula (I), X is bromine and y is 1 or 2; more preferably, y is 1. In one embodiment, in the above formula (I), R is benzyl, more preferably R is C ₁ -C ₆ alkyl.

C ₁ -C ₆ alkyls according to the invention include a straight or branched chain containing from 1 to 6 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl and the like. Most preferably, in formula (I), R is tert-butyl.

In an embodiment, the reaction mixture of stage 1 further comprises water.

In a preferred embodiment, step 1) of the process comprises reacting 1,4,7,10-tetraazacyclododecane with tert-butyl bromoacetate to produce protected DO3A as a tri-tert-butyl ester, such as the monohydrobromide salt of DO3A-tri-tert-butyl ester.

Preferably, the process of the invention allows the DO3A-tri-tert-butyl ester to be collected as a hydrobromide salt directly from the feed reaction mixture in good yield and optimum purity, which can accordingly be used as such (or conveniently stored), without the need for additional complex and time-consuming purifications or recrystallizations.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention relates to a one-step process for the preparation of DO3A tri-tert-butyl ester salt of formula (IA)

,

where y is an integer from 1 to 3, preferably from 1 to 2, and more preferably equal to 1, which includes:

1) the reaction of cyclen with tert-butyl bromoacetate in an organic solvent and in the presence of an auxiliary base, to obtain a mixture;

2) adding water to the mixture of step 1) to obtain a suspension containing DO3A-tri-tert-butyl ester as a solid salt of formula (IA); and

3) collection and washing of DO3A-tri-tert-butyl ester salt.

Stage 1

Step 1) of the process involves reacting cyclen with tert-butyl bromoacetate, essentially as schematically represented in the following synthesis scheme 1

Scheme 1

This step typically involves the addition of tert-butyl bromoacetate to the cyclen in an organic solvent and in the presence of an auxiliary base to produce a mixture containing the solid DO3A tri-tert-butyl ester salt.

For example, cyclen and an auxiliary base are first mixed in an organic solvent to form a mixture, to which tert-butyl bromoacetate is then added.

In one embodiment, tert-butyl bromoacetate is added to the mixture as is, without any prior dilution.

In a preferred embodiment, step 1) of the process of the invention comprises i) preparing a solution of tert-butyl bromoacetate in an organic solvent, and ii) adding the resulting solution to a mixture of cyclen and an auxiliary base in the same organic solvent. Suitable organic solvents include dipolar aprotic solvents as defined 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 obtain a tert-butyl bromoacetate solution having a concentration of preferably 3-7 mol/L (where L refers to DMAC), more preferably 3-5 mol/L, and most preferably about 4-5 mol/L.

The resulting solution is then added to a mixture of cyclen and auxiliary base in DMAC. Preferably, the auxiliary base in the mixture is a weak base; in a preferred embodiment, this is NaOAc. Most often, the mixture is a suspension.

In some embodiments, the mixture of cyclen and auxiliary base in DMAC step 1) may further comprise an amount of water (or dilution water).

If present, the amount of dilution water is (by weight) 0.1-2, more preferably 0.5-1.5 times the amount of cyclen in the mixture. More preferably, it is substantially equal to the amount of cyclen by weight.

The method preferably comprises preparing a suspension of cyclen and NaOAc in DMAC (and optionally water) in which the final concentration of cyclen is 0.5-1.0, preferably 0.6-1.0, and most preferably 0.6-0.85 mol/L. The suspension is then charged with an amount of tert-butyl bromoacetate sufficient to provide the desired trialkylation of cyclen and prevent undesired further alkylation.

In one embodiment, the alkylation reaction of step 1) is carried out using a substantially stoichiometric ratio of cyclen:tert-butyl bromoacetate of 1:3 (mol/mol). In an alternative embodiment, an excess of tert-butyl bromoacetate may be used, for example about 1-30%, preferably 1-10%, which corresponds to a cyclen:tert-butyl bromoacetate ratio of 1:3 to 1:4 (mol/mol). Similarly, in one embodiment, a substantially stoichiometric ratio of cyclen to weak base NaOAc of 1:3 is used, while in alternative embodiments an excess of base of 1-30%, and preferably 1-10%, relative to the stoichiometric amount may be used.

A suitable amount of tert-butyl bromoacetate solution is preferably loaded into the resulting suspension of cyclen and NaOAc, maintained under stirring and at a temperature of 0-25°C.

It is convenient to add 1-4 hours before, preferably 2-3 hours before.

After addition, the reaction mixture is maintained under stirring until the alkylation reaction is complete, for example, for about 16-48 hours.

In one embodiment, the addition of tert-butyl bromoacetate and completion of the reaction are carried out at the same temperature, such as in the range of 0-25°C, preferably 5-15°C, more preferably 10-15°C, such as about 12°C; the resulting reaction mixture is then heated to (or maintained at) 25°C and stirred at this temperature for about 2-4 hours.

In an alternative embodiment, the addition of tert-butyl bromoacetate is carried out at a lower temperature, preferably 0-15°C, while completion of the alkylation reaction is carried out at a higher temperature, preferably 20-35°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 cyclen and NaOAc in the same solvent (and optionally water) at a temperature of about 10°C, which is carried out over a period of about 2.5 hours, during which the temperature of the reaction mixture is maintained at 10-15°C. The resulting reaction mixture is then maintained with stirring at this temperature until the alkylation is complete, such as for 20-48 hours, preferably 20-30 hours, more preferably 20-25 hours. After completion, the temperature of the reaction mixture is preferably raised to room temperature (e.g., about 25°C), and the mixture is maintained with stirring at this temperature for an additional 2-4 hours.

In an alternative embodiment, after adding the tert-butyl bromoacetate solution, the temperature of the resulting reaction mixture is raised to, for example, 23-35°C and the mixture is maintained under stirring at this temperature until the alkylation is complete. Preferably, the reaction mixture is maintained under stirring at a temperature of 25-30°C for 20-30 hours, preferably for 20-25 hours.

Under the above conditions, selective trialkylation of cyclen is carried out to form DO3A-tri-tert-butyl ester, which remains in suspension as a solid salt, for example as a mixture of hydrobromides, more typically as a monohydrobromide.

In this way, a feed mixture is obtained, usually a suspension or slurry, containing DO3A-tri-tert-butyl salts (e.g. in the form of hydrobromide), by-product salts (e.g. NaBr) and optional minor amounts of unreacted base and/or organic impurities in a solid suspended phase.

Stage 2

Step 2) is conveniently carried out by adding water (also referred to herein as “processing water”) to the raw material slurry obtained in step 1).

The Applicant has surprisingly found that suitable addition of water to the slurry collected in step 1) of the process allows the by-products and unreacted components to be solubilised, leaving the DO3A tri-tert-butyl ester salt (hydrobromide) as essentially the only remaining solid in the resulting feed slurry, which can then be collected.

The amount of water (by weight) to be added to the raw material mixture collected in step 1) can be suitably determined, for example, relative to the amount of cyclen (by weight) subjected to the alkylation reaction.

A suitable amount of process water (by weight) may, for example, be ten times or less relative to the amount of starting cyclen subjected to the alkylation reaction. Such an amount of process water is generally independent of the presence of optional amounts of water in the initial reaction mixture containing the cyclen and the auxiliary base in DMAC (these optional amounts, however, are not more than 2 times the amount of cyclen).

In a preferred embodiment, step 2) of the process is carried out by diluting the feed slurry obtained in step 1) with an amount of process water that is preferably more than twice (e.g. about 2.5 times) the amount of cyclen (w/w) subjected to the alkylation reaction. Preferably, the amount of water by weight is 3-10 times, more preferably 3-8 times, most preferably 3-6 times, particularly preferably 4-6 times, e.g. about 4, 5 or 6 times the amount of cyclen (w/w) under the reaction conditions.

Indeed, as confirmed by experimental results, dilution of the feedstock suspension obtained in step 1) with more than 2 times, for example 2.5-8 times, preferably 3-8 times the amount (by weight) of water of the corresponding starting cyclen (subjected to trialkylation) allows solubilization of most of the undesirable salts/impurities in the suspension, thereby eliminating the loss of the target product, which leads to obtaining the target product in good yield and purity.

Dilution is usually carried out with purified water, such as Milli-Q, or water purified, for example, by reverse osmosis, having a temperature of 15-20°C.

Preferably, the dilution is carried out at a temperature of 20-25°C, more preferably about 20°C, and for a time of about 0.5-1 hour, more preferably about 0.5 hour.

In a particularly preferred embodiment, step 2) of the process comprises diluting the suspension collected in step 1) with an amount of purified water as indicated above, preferably 3-8 times, more preferably 3-6 times and most preferably 4-6 times the amount (by weight) of the starting cyclen, for about 0.5 h and at a temperature of 20-25°C, and then maintaining the raw mixture with stirring at a temperature of about 20°C for 0.5-3 h, preferably about 2 h.

In this way a suspension is obtained in which the solid phase essentially consists of the hydrobromide salt of DO3A-tri-tert-butyl ester and, optionally, minor residual amounts of undesired reaction salts. The total amount of water in the suspension (i.e. including the 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.

Stage 3

Step 3) of the method comprises collecting and subsequently washing the solid product present in the suspension obtained as a result of step 2). The solid can be conveniently collected using procedures known to a person skilled in the art.

In one embodiment, the solid is collected by filtering the feed slurry.

In an alternative embodiment, which is particularly preferred when operating on an industrial scale, the suspension obtained in step 2) of the process is centrifuged to remove residual amounts of DMAC, water, solubilized reaction salts and optional liquid impurities in the liquid phase to obtain a wet solid containing the raw material as a solid salt, which is then collected.

Centrifugation is usually performed at high speed (e.g. 1800-2500 rpm).

The collected sediment containing the raw product is then washed.

Preferably, the collected raw material is washed with water (or "wash water").

In one preferred embodiment, the collected wet solid containing the raw material is washed with an amount of water that is 4 to 20 times the weight of the cyclen being reacted.

In this regard, the appropriate amount of water to allow dissolution and, accordingly, removal of unnecessary residual salts/impurities is preferably determined with reference to the amount of water added to the raw material suspension in step 2) of the process.

Indeed, it is clear to a person skilled in the art that smaller dilutions of the raw mixture in step 2) are preferably associated with washing the raw product from step 3) with higher amounts of water. Conversely, dilutions of the raw mixture using a larger amount of water allow the use of a smaller amount of washing water.

For example, as confirmed by the experimental results, when the raw material suspension obtained in step 1) of the method is diluted with an amount of processing water approximately 8 times (w/w) greater than the amount (by weight) of the starting cyclen, the raw material product collected in step 3) can be suitably washed twice with an amount of washing water each time 2 to 8 times by weight, or more preferably 2 to 4 times by weight, greater than the amount by weight of the cyclen, thereby obtaining a product with optimum purity.

When, instead, the raw material suspension is diluted with smaller amounts of processing water, for example, about 3-4 times the amount of cyclen by weight, the collected raw material is preferably washed twice with higher amounts of water, each of which is, for example, 4-10 times or, more preferably, 4-8 times the weight of the original cyclen.

In a preferred embodiment, the raw material suspension obtained in step 1) of the process is diluted with an amount of water that is approximately 4 times the amount (by weight) of cyclen, to obtain a crude product, which is then washed twice, each time with an amount of water that is approximately 4 times the amount (by weight) of cyclen, which corresponds, in this case, to an organic solvent:total water ratio of approximately 1:1-1:1.1 (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 a temperature above room temperature (RT) and/or under reduced pressure.

In one preferred embodiment, the product is dried at 35-40°C and reduced pressure (e.g. 5-25 mbar (0.5-2.5 kPa)) for 20-25 h to obtain DO3A-tri-tert-butyl ester as a monohydrobromide salt in 73-83% yield.

While the above description relates in particular to the preparation of the hydrobromide salt of DO3A-tri-tert-butyl ester, i.e. the compound of formula (I) wherein X is a bromine anion and R is tert-butyl, it is, however, known to the skilled person that the disclosed process can be similarly carried out to prepare compounds of formula (I)

,

where X is a chlorine, fluorine or iodine anion; y is 2 or 3, and R is benzyl or, preferably, C ₁ -C ₆ alkyl other than tert-butyl.

The use of the method of the invention effectively allows for a significant reduction in both the reaction time and the processing time.

For example, the method of the invention allows obtaining the target product in a yield of about 83% or even higher when operating on an industrial scale, with a reaction time of about 24 hours, which is significantly lower than the reaction time required in known procedures.

More particularly, the proposed process allows to obtain DO3A-tri-tert-butyl ester as a hydrobromide salt in the above yield and in a purity of at least 95%, preferably at least 97%, more preferably at least 98%, most preferably at least 99%, such as up to 99.5% directly from the feed reaction mixture (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 feed mixture with water, collecting and washing the solid hydrobromide salt with water. Accordingly, the collected product can be effectively used as is, without requiring any additional expensive and/or time-consuming additional purification step or reprecipitation or recrystallization, which makes the process particularly advantageous for large-scale production, such as in industrial production.

The purity of the collected hydrobromide can be determined by various analytical methods, including, for example, NMR against a standard to confirm the structure and analysis 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 method for preparing a macrocyclic chelating ligand of formula (II)

,

in which R ₁ and R ₂ are both -CH ₂ OH, or R ₁ is H and R ₂ is -CH ₃ , or a chelate complex thereof with a paramagnetic metal ion selected from the group consisting of Fe ²⁺ , Fe ³⁺ , Cu ²⁺ , Cr ³⁺ , Gd ³⁺ , Eu ³⁺ , Dy ³⁺ , La ³⁺ , Yb ³⁺ or Mn ²⁺ , or an alkaline earth metal ion, or a salt thereof, wherein said method comprises:

a) obtaining the hydrobromide salt of DO3A-tri-tert-butyl ester in accordance with the synthesis method described above;

b) converting the resulting hydrobromide salt into the desired ligand of formula (II); and,

c) if applicable, complexing the resulting ligand with a metal ion and isolating the complex compound.

Preferably, the paramagnetic metal ion is Gd3 ⁺ and the alkaline earth metal ion is Ca2 ⁺ and the resulting chelate complex is gadobutrol, gadoteridol or calteridol.

More preferably, the metal ion is Gd ³⁺ , and the resulting chelate complex is gadoteridol or gadobutrol of the formula

, .

More specifically, in a further aspect, the invention relates to a novel process for the synthetic preparation of gadoteridol or gadobutrol, which comprises:

a) obtaining DO3A-tri-tert-butyl ester in the form of a hydrobromide salt according to the process of the present invention;

b) converting the resulting hydrobromide salt into the desired ligand of formula (II); and

c) complex formation of the obtained ligand with the metal ion Gd ³⁺ and isolation of the corresponding complex of gadoteridol or gadobutrol.

In the above process, step a), comprising obtaining DO3A-tri-tert-butyl ester as a hydrobromide salt, is carried out using the preparation process of the present invention as described in detail above, while steps b) and c), comprising the experimental conditions and further variations thereof, are carried out in accordance with a procedure known in the art.

For example, step b) may comprise i) neutralizing the collected hydrobromide salt and deprotecting it to obtain the DO3A ligand in which the carboxyl groups are in the unprotected, acidic form; and ii) alkylating DO3A to obtain the target ligand in the acidic form.

The neutralization and removal of the protecting groups from the hydrobromide salt collected in step a) can be carried out according to known methods, for example by hydrolysis in the presence of a base, or by treatment with trifluoroacetic acid. Alkylation of the resulting DO3A can then be carried out, for example using a suitable epoxide, as described for example in EP 0 988 294, to obtain the desired 10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (HPDO3A) or the ligand [10-[2,3-dihydroxy-1-(hydroxymethyl)propyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A butyltriol).

Alternatively, the ligand of formula (II) can be prepared by alkylation in the presence of a base of the hydrobromide salt collected in step a) or from DO3A-tri-tert-butyl ester collected after neutralization of the salt carried out in a separate step. The alkylation reaction can be carried out by a conventional procedure, for example, involving the reaction of the ester with an optionally protected alkylating group or with an epoxide, for example, as schematically shown in the following scheme 1

Scheme 1

The resulting alkylated ester is then deprotected according to conventional techniques, for example using trifluoroacetic acid to obtain the corresponding deprotected ligand.

Step c) of the process comprising complexing with gadolinium the ligand obtained in step b) can be carried out, for example, by stoichiometrically adding a suitable Gd(III) derivative, in particular a Gd(III) salt or oxide, to a solution of the ligand, for example by treatment according to well-known experimental methods, for example as described in EP 230893.

In another embodiment, the invention relates to a new process in which the DO3A-tri-tert-butyl ester hydrobromide salt obtained according to the process of the invention is used as a starting material for the preparation of compounds described in WO 2017/098038, WO 2017/098044 or in WO 2018/108780.

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 of the inventive method are further provided in the following experimental section, which provides a general reference to the operating conditions employed in the inventive method.

EXPERIMENTAL PART

Abbreviations and definitions of terms

DO3A: 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid DO3A tri-tert-butyl ester: 1,4,7-tris(1,1-dimethylethyl) ester of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid DO3A tri-tert-butyl ester-(HBr) _y : hydrobromide salt of 1,4,7-tris(1,1-dimethylethyl) ester of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid Cyclene: 1,4,7,10-tetraazacyclododecane HPDO3A: 10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid NMR: nuclear magnetic resonance MRV: magnetic resonance imaging NaOAc: sodium acetate DMAC: N,N-dimethylacetamide DMF: N,N-dimethylformamide DMSO dimethyl sulfoxide MeCN acetonitrile _CHCl3 chloroform _NaHCO3 sodium bicarbonate _{Na2CO3 ​} sodium carbonate _KHCO3 potassium bicarbonate _{K2CO3 ​} potassium carbonate DIPEA N,N-diisopropylethylamine rpm number of revolutions per minute

HPLC Analysis for Determination of DO3A-Tri-tert-Butyl-HBr

General methodology

HPLC characterization of collected DO3A-tri-tert-butyl ester hydrobromide salt

HPLC characterization of the DO3A-tri-tert-butyl ester hydrobromide salt obtained by the method of the invention was performed using an Agilent 1100 liquid chromatography system. The experimental setup of the HPLC measurements is given below.

Analytical conditions

HPLC system: HPLC equipped with a solvent delivery system, autosampler, column thermostat, degasser and diode array or variable wavelength detector (or equivalent). Stationary phase: Zorbax Eclipse XDB-C8, 5 µm, 150×4.6 mm Column temperature: 45°C Mobile phase: A: 0.01 MK ₂ HPO ₄ , 0.017 MH ₃ PO ₄ B: Acetonitrile Elution gradient Time (min) % IN 0 5 30 80 35 80 38 5 45 5 Flow 1 ml/min Temperature 45°C Detection UV, 210 nm, Bw=8 nm; Standard 360 nm, Bw=100 nm Volume injected 10 µl Stop time 35 min Peak of comparison DO3A 3tBu Retention time

Analysis for the determination of bromides in DO3A-tri-tert-butyl-HBr.

General methodology

The content of bromide ions in the collected DO3A tri-t-butyl-HBr was determined by potentiometric titration using a 0.1 N silver nitrate solution with a combined Ag/AgCl electrode according to known methods.

Example 1: Synthesis of DO3A-tri-tert-butyl ester monohydrobromide salt: Process water 4× and wash water 2(4×) (w/w) relative to starting cyclen

The synthesis was carried out according to the following synthesis scheme

To 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) was added 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) at 10 °C over 2.5 h. Then, the temperature was raised to 25 °C and the mixture was stirred for 24 h. Then, after 0.5 h, water (60.0 kg; 4:1 w/w relative to the starting cyclen) was added. After 2 h, the mixture was centrifuged and the collected precipitate was washed with water (2 x 60.0 kg = 2 x 4:1 w/w relative to the starting cyclen). The wet solid was dried under vacuum to give 38.43 kg of the desired hydrobromide salt as a white powder.

Yield: 73.5%.

Title compound by HPLC: (relative to standard) is 100%

The title compound by NMR: (relative to the standard) is 99.86%.

Bromine Analysis: 100.7%

Example 2: Synthesis of DO3A-tri-tert-butyl ester hydrobromide salt

Treatment water 4× and wash water 5.3× + 4× (w/w) relative to the initial cyclen

Cyclene (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 charged into the reactor. The resulting suspension was kept under stirring for 30 min at 25 °C. Then the suspension was cooled at 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 over 3 h. The tube was flushed 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 relative to the starting cyclen) was then added after 0.5 h and after 2 h the mixture was centrifuged and the precipitate was washed with water (47.4 kg + 36.5 kg). The wet solid was dried under vacuum to yield 22.70 kg of the hydrobromide salt.

Yield: 72.8%.

Title compound by HPLC: (relative to standard) is 100%

The title compound by NMR: (relative to the standard) is 99.86%.

Bromine Analysis: 100.6%

Example 3: Synthesis of DO3A-tri-tert-butyl ester hydrobromide salt

Treatment water 4× and wash water 2(4×) (w/w) relative to the initial cyclen

Cyclene (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 charged into the reactor. The resulting suspension was kept under stirring for 30 min at 25 °C. Then the suspension was cooled at 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 over 3 h. The tube was flushed 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 after 0.5 h and after 2 h the mixture was centrifuged and the precipitate was washed with water (36.7 kg + 36.8 kg). The wet solid was dried under vacuum to give 24.16 kg of the hydrobromide salt.

Output: 76.7%

Title compound by HPLC: (relative to standard) is 100%

The title compound by NMR: (relative to the standard) is 98.84%.

Bromine Analysis: 99.9%

Example 4: Synthesis of DO3A-tri-tert-butyl ester hydrobromide salt

Treatment water 4× (w/w) and wash water 2(4×) (w/w) relative to the initial cyclen

Cyclene (10.0 g, 58.05 mmol) and sodium acetate (17.9 g, 217.68 mmol) in DMAC (72.5 mL) were charged into the reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. Then the suspension was cooled at 12 °C and a solution of tert-butyl bromoacetate (42.5 g, 217.68 mmol) in DMAC (37.5 mL) was added at 12 °C over 1.5 h. The mixture was stirred for 24 h at this temperature, then it was warmed to 25 °C and stirred for 2 h at this temperature. After cooling at about 18-20°C, water (40 ml) was added after 0.25 h and after 1 h the mixture was filtered through a P3 porous membrane and washed with water (40 ml+40 ml). The wet solid was dried under vacuum.

Output: 82.9%

HPLC, area %: 99.2%

Title compound by NMR: (relative to standard): 97.6%

Bromine Analysis: 102.0%

Example 5: Synthesis of DO3A-tri-tert-butyl ester hydrobromide salt

Dilution water 1×, treatment water 8× and wash water 2(2×) (w/w) relative to the original cyclen

Cyclene (10.0 g, 58.05 mmol) and sodium acetate (17.9 g, 217.68 mmol) in DMAC (72.5 mL) were charged into the reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. Then the suspension was cooled at 12 °C and water (10 mL) was added. 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 it was warmed to 25 °C and stirred for 2 h at this temperature. After cooling at about 18-20°C, water (80 ml) was added after 0.5 h and after 1 h the mixture was filtered through a P3 porous membrane and washed with water (20 ml+20 ml). The wet solid was dried under vacuum.

Output: 80.4%

HPLC, area %: 98.1%

Title compound by NMR: (relative to standard): 97.7%

Bromine Analysis: 103.6%

Example 6: Synthesis of DO3A-tri-tert-butyl ester hydrobromide salt

Dilution water 1×, treatment water 8× and wash water 2(2×) (w/w) relative to the original cyclen

Cyclene (10.0 g, 58.05 mmol) and sodium acetate (16.7 g, 203.17 mmol) in DMAC (72.5 mL) were charged into the reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. Then the suspension was cooled at 0 °C and water (10 mL) was added. 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 it was warmed to 25 °C and stirred for 2 h at this temperature. After cooling at about 18-20°C, water (80 ml) was added after 0.5 h and after 1 h the mixture was filtered through a P3 porous membrane and washed with water (20 ml+20 ml). The wet solid was dried under vacuum.

Output: 77.3%

HPLC, area %: 98.3%

Title compound by NMR: (relative to standard): 97.5%

Bromine Analysis: 106.9%

Example 7: Synthesis of DO3A-tri-tert-butyl ester hydrobromide salt

Treatment water 2× and wash water 2(2×) (w/w) relative to the initial cyclen

Cyclene (10.0 g, 58.05 mmol) and sodium acetate (15.0 g, 182.85 mmol) in DMAC (72.5 mL) were charged into the reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. The suspension was then cooled at 10 °C and a solution of tert-butyl bromoacetate (35.7 g, 182.85 mmol) in DMAC (38.0 mL) was added at 11 °C over 2.5 h. The temperature was then raised to 25 °C over 1 h and the mixture was stirred for 24 h at this temperature. After cooling at about 18-20°C, water (20 ml) was added after 0.5 h and after 2 h the mixture was filtered through a porous membrane P3 and washed with water (20 ml+20 ml). The wet solid was dried under vacuum.

Output: 72.6%

HPLC, area %: 99.0%

Title compound by NMR: (relative to standard): 95.9%

Bromine Analysis: 104.3%

Example 8: Assessing the impact of water quantity

To determine suitable amounts of water to be added to the feed slurry 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 feed cyclen, the same amounts of DMAC (total 110 ml) and tert-butyl bromoacetate, a fixed ratio of tert-butyl bromoacetate:cyclen, and variable amounts of process/rinse water.

Cyclene (10.0 g, 58.05 mmol) and sodium acetate (15.0 g, 182.85 mmol) in DMAC (72.5 mL) were charged into the reactor. The resulting suspension was kept under stirring (250 rpm) for 30 min at 25 °C. The suspension was then cooled at 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 over 2.5 h. The temperature was then raised to 25 °C over 1 h and the mixture was stirred for 24 h at this temperature. After cooling at about 18-20 °C, an amount of water (see the following table) was added after 0.5 h and after 2 h the mixture was filtered through a P3 porous membrane and washed with water. The wet solid was dried under vacuum. The results of various tests carried out with different amounts of processing and washing water (amounts of water are given as parts by weight, relative to the amount of starting cyclen) are given in the following Table 1, which gives the purity of the collected product by HPLC (area %) and NMR (relative to standard).

Table 1

Trial Water treatment* Rinse water* Exit (%) HPLC (area %) NMR 1 4 4×2 75.1
76.7 ^b 99.4
100 ^b 96.1 ^a
98.8 ^b 2 8 8×2 66.9 99.0 97.0 ^a 3 8 2×2 67.6 99.2 96.4 ^a 4 2 2×2 64.0 98.8 90.6 ^a 5 6 3×2 73.8 98.4 96.0 ^a 6 2 2×2 72.6 99.0 95.9 ^a 7 3 4×2 68.3 98.8 98.0 ^a

a - on a laboratory scale

b - on the scale of the pilot plant, example 3

*mass parts relative to the amount of cyclen

Results

Although the results were obtained using tests carried out on a laboratory scale (with less efficient washing and filtration than in a pilot or industrial plant), the results presented in the table above confirm that the operating conditions identified allow the target product to be isolated in good yield and with an optimum purity greater than 95%, allowing it to be used as is, without the need for further processing or purification.

Claims (26)

1. One-step method for obtaining a salt of protected DO3A of formula (I) , where X is an anion of chlorine, iodine or bromine; y is an integer between 1 and 3; and R is a C ₁ -C ₆ alkyl or aryl group; said method comprising: 1) reaction of cyclen with an activated acetic ester of formula _XCH2COOR in an organic solvent selected from the group consisting of DMF (N,N-dimethylformamide), DMSO (dimethyl sulfoxide), MeCN (acetonitrile) and DMAC (dimethylacetamide), and in the presence of an auxiliary base selected from the group consisting of NaOAc, _NaHCO3 , _Na2CO3 , _{KHCO3 , K2CO3} , _DIPEA (N,N-diisopropylethylamine), and triethylamine to obtain a mixture; 2) adding water to the mixture of step 1), wherein the water added in step 2) is in an amount 2.0-10 times (w/w) greater than the amount of cyclen in step 1), to obtain a suspension containing protected DO3A as a solid salt of formula (I); and 3) collection and washing of the salt of protected DO3A. 2. The method according to claim 1, wherein the mixture of step 1) additionally contains water. 3. The method according to claim 2, wherein the water in the mixture of step 1) is in an amount that is a multiple of 0.1-2 relative to the amount of cyclen (w/w). 4. The method according to claims 1-3, wherein in formula (I) X is bromine and R is tert-butyl. 5. The method according to claims 1-4, wherein in step 1) the organic solvent is DMAC and the auxiliary base is NaOAc. 6. The method according to claim 5, wherein the reaction in step 1) is carried out using a ratio of cyclen:tert-butyl bromoacetate and cyclen:NaOAc from 1:3 to 1:4 (mol/mol). 7. The method according to claim 6, wherein the ratio of cyclen:tert-butyl bromoacetate and cyclen:NaOAc is from 1:3 to 1:3.3. 8. The method according to paragraphs 5-7, in which stage 1) of the method includes: i) obtaining a solution of tert-butyl bromoacetate in DMAC; ii) adding the resulting solution to a suspension of cyclen and NaOAc in DMAC. 9. The method according to claim 8, wherein the concentration of tert-butyl bromoacetate in the solution is 3-5 mol/l, and the concentration of cyclen in the suspension is 0.5-1.0 mol/l. 10. The method according to paragraphs 8, 9, in which the solution of tert-butyl bromoacetate is added to the suspension of cyclen and NaOAc, stirring at a temperature of 0-25°C. 11. The method according to claim 10, wherein the suspension of cyclen and NaOAc in DMAC is stirred at a temperature of 0-15°C. 12. The method according to any of the preceding claims 1-11, wherein the water added in step 2) is in an amount 2.5-10 times (w/w) greater relative to the amount of cyclen in step 1). 13. The method according to claims 1-12, in which step 3) comprises collecting the salt of protected DO3A of formula (I) obtained in step 2) and washing the collected salt with water. 14. The method according to claim 13, wherein the salt of protected DO3A is collected by filtration or centrifugation. 15. The method according to claim 13 or 14, wherein the amount of wash water is 4-20 times greater than the amount of cyclen in step 1) (w/w). 16. The method according to claims 1-15, wherein the collected salt of protected DO3A is DO3A-tri-tert-butyl ester monohydrobromide. 17. The method according to claim 16, wherein the DO3A-tri-tert-butyl ester monohydrobromide has a purity of at least 95%.