RU2810975C2 - Diastereoisomerically enriched complex of gadolinium and chelating ligand based on pcta, method of its synthesis - Google Patents

Abstract

FIELD: pharmaceuticals.

SUBSTANCE: invention relates to a gadolinium-hexacid complex of formula (I), consisting of at least 80% diastereoisomeric excess of diastereoisomers I-RRR and I-SSS. Also the following is provided: a complex of formula (II) consisting of at least 80% diastereoisomeric excess of diastereoisomers II-RRR and II-SSS. The invention also relates to a method of preparing the said complex of formula (II) and an intermediate of formula (VI).

EFFECT: invention makes it possible to obtain a group of isomers, including a mixture of isomers II-RRR and II-SSS, which is thermodynamically stable and shows itself to be most advantageous as a contrast agent for medical imaging.

22 cl, 1 dwg, 6 tbl, 1 ex

Description

The present invention relates to a new method for the synthesis of a gadolinium chelating ligand complex based on PCTA, which allows the preferential preparation of stereoisomers of the said complex having physicochemical properties that are particularly advantageous for applications as a contrast agent in the field of medical imaging, especially for magnetic resonance imaging. -resonance tomography. The present invention also relates to the diastereoisomerically enriched complex itself, as well as two synthetic intermediates, optionally containing gadolinium.

There are many contrast agents based on lanthanide (paramagnetic metal) chelates, particularly gadolinium (Gd), such as those described in US Pat. No. 4,647,447. These products are often collectively referred to as GBCA (gadolinium-based contrast agent). There are several products on the market, including macrocyclic chelates, such as meglumine gadoterate based on DOTA (1,4,7,10-tetraazacyclododecane-N,N',N",N'"-tetraacetic acid), gadobutrol based on DO3A- butrol, HPDO3A-based gadoteridol, as well as linear chelates, especially those based on DTPA (diethylenetriaminepentaacetic acid) or DTPA-BMA (gadodiamide as a ligand).

Other products, some in development, represent the next generation of GBCA. Essentially, they are complexes of macrocyclic chelates, such as bicyclopolyazamacrocyclocarboxylic acid complexes (EP 0438206) or PCTA derivatives (i.e. derivatives characterized by at least the chemical structure 3,6,9,15-tetraazabicyclo[9,3,1 ]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid) described in EP 1931673.

Notably, the PCTA-based chelating ligand complexes described in EP 1931673 have the advantage of being relatively easy to synthesize chemically and, more importantly, being more relaxing than other GBCAs currently on the market ( relaxivity r ₁ , which can be no more than 11-12 mmol ^-1 .s ^-1 in water), while this relaxivity corresponds to the effectiveness of these products and, therefore, their contrasting ability.

In the body, lanthanide chelates (or complexes), and in particular gadolinium, are in a state of chemical equilibrium (characterized by its thermodynamic constant K _therm ), which can lead to the unwanted release of said lanthanide (see equation 1 below):

(Equation 1)

Chemical equilibrium of complexation between a chelate or ligand (Ch) and a lanthanide (Ln) to form a Ch-Ln complex.

Since 2006, a pathology called NSF (nephrogenic systemic fibrosis or fibrogenic dermopathy) has been at least partially associated with the release of free gadolinium in the body. This disease has attracted the attention of health authorities regarding gadolinium-based contrast agents marketed to certain patient populations.

Thus, strategies have been developed to solve the difficult problem of patient tolerance in a completely safe manner and limit or even eliminate the risk of unwanted release of the lanthanide after administration. This problem is difficult to resolve because the administration of contrast media is often repeated, either during diagnostic examinations or when adjusting doses and monitoring the effectiveness of therapeutic treatment.

Additionally, possible gadolinium deposition in the brain has been reported since 2014 following repeated administration of gadolinium-based products, particularly linear gadolinium chelates, with such deposition rarely or not at all associated with macrocyclic gadolinium chelates such as Dotarem ^® . As a result, various countries have decided to either withdraw most linear chelates from the market or significantly limit their indications for use given their stability, which is considered to be insufficient.

Thus, a strategy for limiting the risk of lanthanide release in the body is to give preference to complexes characterized by thermodynamic and/or kinetic stability, which should be as high as possible. This is because the more stable the complex, the more the amount of lanthanide released over time will be limited.

However, PCTA-based chelating ligand complexes having a picklene-type structure as described in EP 1931673, while having good kinetic stability, typically have a thermodynamic constant that is lower than that of other cyclen-based macrocycle complexes.

This is especially true in the case of the complex of formula (II), which is presented below:

(II).

More specifically, WO 2014/174120 details that the thermodynamic equilibrium constant corresponding to the formation reaction of the complex of formula (II), which is also called the stability constant, is 10 ^14.9 (i.e. log(K _term ) = 14. 9). For comparison, the stability constant of the complex of gadolinium and 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA-Gd) is 10 ^25.6 (i.e. log(K _therm. ) = 25.6).

However, it should be noted that the complex of formula (II) corresponds to several stereoisomers, in particular due to the presence of three asymmetric carbon atoms located on the side chains of the complex in the α-position relative to the nitrogen atoms of the macrocycle to which said side chains are grafted. These three asymmetric carbon atoms are marked with an asterisk (*) in formula (II) shown above.

Thus, the synthesis of the complex of formula (II) described in EP 1931673 leads to a mixture of stereoisomers.

The aminopropanediol groups of the side chains of the complex of formula (II) also include an asymmetric carbon atom. Thus, the complex of formula (II) contains a total of six asymmetric carbon atoms and thus exists as 64 configurational stereoisomers. However, in the remainder of the description, the only source of stereoisomerism considered for a given side chain, for the sake of simplicity of presentation, will be that corresponding to the asymmetric carbon atom bearing the carboxylate group, which is indicated by an asterisk (*) in formula (II) presented above.

Since each of these three asymmetric carbon atoms can have an absolute R- or S-configuration, the complex of formula (II) exists as eight families of stereoisomers, referred to hereinafter as II-RRR, II-SSS, II-RRS, II- SSR, II-RSS, II-SRR, II-RSR and II-SRS. Namely, according to conventional stereochemical nomenclature, the complex of formula (II) exists in the form of eight families of diastereoisomers.

The use of the term "family" is justified by the fact that each of these families includes several stereoisomers, especially due to the presence of an asymmetric carbon atom in the aminopropanediol group, as mentioned earlier.

However, since the remainder of the description will not deal with stereoisomerism associated with the asymmetric carbon atom of a given aminopropanediol group, the terms "isomers", "stereoisomers" or "diastereoisomers" II-RRR, II-SSS, II-RRS, II-SSR , II-RSS, II-SRR, II-RSR and II-SRS will be applied equally, without specifying whether each belongs to a stereoisomer family.

The inventors of the present invention have succeeded in separating and identifying by high performance liquid chromatography (HPLC) and ultra high performance liquid chromatography (UHPLC) four unresolved peaks or groups of isomers of the complex of formula (II), obtained according to the method of the prior art, which correspond to four different elution peaks, characterized by their retention time on the chromatogram, which in the remainder of the description of the present invention will be referred to as iso1, iso2, iso3 and iso4. When carrying out the method described in EP 1931673, the corresponding contents of iso1, iso2, iso3 and iso4 groups in the resulting mixture will be as follows: 20%, 20%, 40% and 20%.

The present inventors then discovered that these different groups of isomers have different physicochemical properties and determined that a group of isomers called iso4, which includes a mixture of II-RRR and II-SSS isomers of formula (II-RRR) and formula (II- SSS), presented below, shows itself to be the most advantageous as a contrast agent for medical imaging.

(II-SSS),

(II-RRR).

Thus, unexpectedly, iso4 has a thermodynamic stability that is significantly superior to that of the mixture of diastereoisomers in the form of which the complex of formula (II) is obtained by carrying out the method described in EP 1931673. More specifically, the thermodynamic equilibrium constant K _{therm. iso4} is equal to 10 ^18.7 (ie log(K _{term iso4} ) = 18.7), this value being determined by the method described in Pierrard et al., Contrast Media Mol. Imaging, 2008, 3, 243-252 and Moreau et al., Dalton Trans., 2007, 1611-1620.

Moreover, iso4 is a group of isomers having the best kinetic inertness (also called kinetic stability) among the four groups identified by the authors of the present invention. More specifically, the present inventors assessed the kinetic inertness of four groups of isomers by studying their decomplexation kinetics in an acidic aqueous solution (pH = 1.2) at 37°C. The half-lives ( _1/2 ) that were determined for each of the isomer groups are shown in Table 1 below, with the half-lives corresponding to the time at which 50% of the initially present amount of the complex has dissociated according to the following decomplex reaction (Equation 2 ):

(Equation 2)

Table 1. Kinetics of complex decomposition for groups of isomers iso1–iso4

Groups of isomers T _1/2 (pH 1.2 – 37°C) Iso1 18 hours ISO2 6 hours ISO3 8 days ISO4 27 days

In comparison, gadobutrol or gadoterate, which are macrocyclic gadolinium complexes, have a kinetic inertness of 18 hours and 4 days, respectively, under the same conditions, whereas linear gadolinium complexes such as gadodiamide or gadopentetate dissociate instantly.

In addition, iso4 in particular is more chemically stable than iso3. This is due to the fact that the amide functional groups of the complex of formula (II) are prone to hydrolysis. The hydrolysis reaction of the amide functionality (Equation 3) results in the formation of a double addition impurity, which is accompanied by the release of 3-amino-1,2-propanediol. The present inventors studied the kinetics of the hydrolysis reaction of the complex of formula (II) in aqueous solution at pH 13 and found that the amide functional groups in iso4 are more resistant to hydrolysis than the functional groups in iso3.

(Equation 3)

Regarding the relaxivity of different groups of isomers, i.e. their effectiveness as a contrast agent, the measurements taken demonstrate a contrast ability that is relatively equivalent for the iso1, iso2 and iso4 groups, and reduced efficiency for iso3 (see table 2).

Table 2. Relaxivity of isomer groups iso1–iso4 at 37°C

Groups of isomers r1, 20 MHz
(mmol ^-1 .s ^-1 ) r1, 60 MHz
(mmol ^-1 .s ^-1 ) Iso1 12.6 12.5 ISO2 13.3 12.9 ISO3 8.0 8.1 ISO4 12.9 13.0

The authors of the present invention have succeeded in developing a new method for the preparation and purification of the complex of formula (II), which makes it possible to preferentially obtain diastereoisomers II-RRR and II-SSS of the said complex, which have particularly advantageous physicochemical properties. The method according to the present invention includes an isomer enrichment step by converting the least stable stereoisomers into the most stable stereoisomers, which surprisingly, although carried out for the intermediate complex with a hexaacid and not with the final complex, allows preferentially to obtain the most stable isomers of the complex of formula (II).

Carrying out a process that advantageously produces the diastereoisomers of interest is certainly advantageous over the alternative of preparing a mixture of stereoisomers and then attempting to separate the diastereoisomers by conventional techniques and thereby isolate the isomers of interest. More specifically, in addition to the fact that it is easier to carry out a process that does not include a step for separating diastereoisomers on an industrial scale, the absence of separation, firstly, provides significant time savings, and secondly, it allows you to increase the overall yield of the process by limiting, to the extent possible, formation of undesired diastereoisomers, which will eventually be discarded. Moreover, conventional separation techniques typically involve extensive use of solvents, which is undesirable for environmental reasons, not to mention financial costs. In addition, silica chromatography in particular should be avoided given the health risks associated with occupational exposure to silica, which is classified as carcinogenic to humans by the International Agency for Research on Cancer (Group 1).

As stated above, the method for preparing the complex of formula (II) developed by the authors of the present invention is based on the step of isomeric enrichment of the intermediate complex of gadolinium and hexacid of formula (I), which is presented below:

(I).

The complex of formula (I) corresponds to several stereoisomers due to the presence of three asymmetric carbon atoms located on the side chains of the complex in the α-position relative to the nitrogen atoms of the macrocycle to which said side chains are grafted. These three asymmetric carbon atoms are marked with an asterisk (*) in formula (I) shown above.

Since each of the three asymmetric carbon atoms bearing the carboxylate functionality can have an absolute R- or S-configuration, the complex of formula (I) exists as eight stereoisomers, referred to hereinafter as I-RRR, I-SSS, I- RRS, I-SSR, I-RSS, I-SRR, I-RSR and I-SRS. Namely, according to conventional stereochemical nomenclature, the complex of formula (I) exists as four pairs of enantiomers, which are diastereoisomers with respect to each other.

The inventors of the present invention have succeeded in separating and identifying, using high performance liquid chromatography (HPLC) and ultra high performance liquid chromatography (UHPLC), four unresolved peaks or groups of isomers of the complex of formula (I), obtained according to the method described in EP 1931673, which correspond to four different elution peaks , characterized by their retention time in the chromatogram, which in the remainder of the description of the present invention will be referred to as isoA, isoB, isoC and isoD.

IsoD crystallizes from water. Using x-ray diffraction analysis, the present inventors were able to determine the crystal structure of this group of isomers and thus find that it includes the diastereoisomers I-RRR and I-SSS of the complex of formula (I), formula (I-RRR) and formula (I-SSS ) presented below.

(I-SSS),

(I-RRR).

It should be noted that the diastereoisomers I-RRR and I-SSS of the complex of formula (I) are enantiomers of each other.

The isomeric enrichment step of the method of the present invention is aimed at enriching the gadolinium hexacid intermediate complex of formula (I) with isoD.

In particular, the synthesis of the complex of formula (II) involves the conversion of the carboxylic acid functional groups of the intermediate hexacid complex of formula (I) into amide functional groups. This amidation reaction does not change the absolute configuration of the three asymmetric carbon atoms of the complex of formula (I).

Thus, carrying out the amidation reaction of the previously obtained complex with a hexacid of formula (I), enriched in isoD, makes it possible to obtain a complex of formula (II), enriched in iso4.

Gadolinium hexacid complex of formula (I)

Thus, the present invention relates primarily to the gadolinium hexacid complex of formula (I):

(I),

characterized by at least 80% diastereoisomeric excess of a mixture of isomers I-RRR and I-SSS of the formula:

(I-SSS),

(I-RRR).

In the context of the present invention, the term "diastereoisomeric excess" is meant to indicate, with respect to the gadolinium hexacid complex of formula (I), the fact that said complex is predominantly present as an isomer or group of isomers selected from the diastereoisomers I-RRR, I-SSS, I- RRS, I-SSR, I-RSS, I-SRR, I-RSR and I-SRS. Said diastereoisomeric excess is expressed as a percentage and corresponds to the amount represented by the predominant isomer or group of isomers relative to the total amount of the gadolinium hexacid complex of formula (I). It will be understood that this percentage may be molar or mass, since isomers by definition have the same molar mass.

In one particular embodiment, the complex of formula (I) according to the present invention is characterized by at least 85%, in particular at least 90%, in particular at least 95%, preferably at least 97%, advantageously at least 98 %, more preferably at least 99% diastereoisomeric excess of a mixture of I-RRR and I-SSS isomers.

Preferably, said diastereomeric excess comprises at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% diastereomeric excess of the mixture of I-RRR and I-SSS isomers.

Advantageously, said diastereoisomeric excess comprises a mixture of I-RRR and I-SSS isomers.

The term "mixture of I-RRR and I-SSS isomers" also covers, in a broader sense, the case where only one of the isomers: I-RRR or I-SSS is present. However, the term "mixture of I-RRR and I-SSS isomers" preferably refers to all cases in which each of the I-RRR and I-SSS isomers is present in varying amounts other than zero.

In a preferred embodiment, the I-RRR and I-SSS isomers are present in said mixture in a ratio of 65/35 to 35/65, in particular 60/40 to 40/60, in particular 55/45 to 45/55. Advantageously, the mixture of I-RRR/I-SSS isomers is a racemic mixture (50/50).

More specifically, the diastereoisomeric excess, as previously defined, corresponds to peak 4 on the HPLC chromatogram (i.e., the fourth peak in the elution order and corresponds to isoD), characterized by a retention time of 33.9 to 37.5 minutes, typically approximately 35.7 minutes, while the specified chromatogram was obtained using the HPLC technique described below.

For purposes of the present invention, the term "HPLC chromatogram" means the profile of concentrations measured by a detector after passing through and separating a mixture of compounds (in this case isomers of a compound) into a stationary phase, as a function of time for a given composition and a given eluent flow rate. An HPLC chromatogram consists of various peaks or unresolved peaks characteristic of a compound or mixture of compounds being analyzed.

HPLC technique

- Waters Symmetry® C18 column - 250 x 4.6 mm - 5 µm.

It is a reverse phase HPLC column containing spherical C18-grafted (octadecyl) silica particles whose silanol groups have been endcapped. It is also characterized by a length of 250 mm, an internal diameter of 4.6 mm, a particle size of 5 μm, a porosity of 100 Å and a carbon content of 19%.

Preferably, the stationary phase used should be compatible with aqueous mobile phases.

- Analytical conditions

Sample An aqueous solution of the complex of formula (I) with a concentration of 10 mg/ml Column temperature 25°C Sample temperature Room temperature (20-25°C) Flow rate 1.0 ml/min. Volume of administration 20 µl UV detection 200 nm

- Mobile phase gradient (by volume)

Time
(min.) % acetonitrile
(100%) _{% H2SO4}
(0.1% v/v aqueous solution) 0 1 99 10 5 95 40 10 90

Complex of formula (II)

Secondly, the present invention relates to a complex of formula (II):

(II),

characterized by at least 80% diastereoisomeric excess of a mixture of isomers II-RRR and II-SSS of the formula:

(II-SSS),

(II-RRR).

In the context of the present invention, the term "diastereoisomeric excess" is meant to indicate, with respect to a complex of formula (II), the fact that said complex is predominantly present as an isomer or group of isomers selected from the diastereoisomers II-RRR, II-SSS, II-RRS, II -SSR, II-RSS, II-SRR, II-RSR and II-SRS. Said diastereoisomeric excess is expressed as a percentage and corresponds to the amount represented by the predominant isomer or group of isomers relative to the total amount of the complex of formula (II). It will be understood that this percentage may be molar or mass, since isomers by definition have the same molar mass.

In one particular embodiment, the complex of formula (II) according to the present invention is characterized by at least 85%, in particular at least 90%, in particular at least 92%, preferably at least 94%, advantageously at least 97 %, more preferably at least 99% diastereoisomeric excess of a mixture of isomers II-RRR and II-SSS.

Preferably, said diastereomeric excess comprises at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% diastereomeric excess of the mixture of II-RRR and II-SSS isomers.

Advantageously, said diastereoisomeric excess comprises a mixture of II-RRR and II-SSS isomers.

The term "mixture of II-RRR and II-SSS isomers" also covers, in a broader sense, the case where only one of the isomers II-RRR or II-SSS is present. However, the term "mixture of II-RRR and II-SSS isomers" preferably refers to all cases in which each of the II-RRR and II-SSS isomers is present in varying amounts other than zero.

In a preferred embodiment, the isomers II-RRR and II-SSS are present in said mixture in a ratio of 65/35 to 35/65, in particular 60/40 to 40/60, in particular 55/45 to 45/55. Predominantly the isomers II-RRR and II-SSS are present in the mixture in a 50/50 ratio.

More specifically, the diastereoisomeric excess, as previously defined, corresponds to peak 4 on the UHPLC chromatogram (i.e., the fourth unresolved isomer peak in the elution order and corresponds to iso4), characterized by a retention time of 6.0 to 6.6 minutes, typically approximately 6.3 minutes, while the specified chromatogram was obtained using the UHPLC technique described below.

For the purposes of the present invention, the term "UHPLC chromatogram" means the profile of concentrations measured by a detector after passing and separating a mixture of compounds (in this case isomers of a compound) on a stationary phase, as a function of time for a given composition and a given eluent flow rate. A UHPLC chromatogram consists of various peaks or unresolved peaks characteristic of a compound or mixture of compounds being analyzed.

UHPLC technique

- Waters Cortecs® UPLC T3 column, 150 x 2.1 mm - 1.6 µm.

It is a reverse phase UPLC column containing spherical particles, which consists of a base that is preferably very rigid and made of silica, surrounded by porous trifunctional C18-grafted (octadecyl) silica, and whose silanol groups are treated with agents to blocking end groups (endcapped). It is also characterized by a length of 150 mm, an internal diameter of 2.1 mm, a particle size of 1.6 μm, a porosity of 120 Å and a carbon content of 4.7%.

Preferably, the stationary phase used should be compatible with aqueous mobile phases.

- Analytical conditions

Sample An aqueous solution of the complex of formula (II) with a concentration of 2.0 mg/ml Column temperature 40°C Sample temperature Room temperature (20-25°C) Flow rate 0.3 ml/min. Volume of administration 1 µl UV detection 200 nm

- Mobile phase gradient (% v/v)

Time (min.) Acetonitrile (100%) H ₂ SO ₄ (0.0005% v/v aqueous solution) 0 1 99 3 5 95 12 10 90

In a preferred embodiment, the complex of formula (II) in accordance with the present invention is prepared by amidation of the complex of formula (I) in accordance with the present invention, as defined above, by reaction with 3-amino-1,2-propanediol, racemic or enantiomeric pure form, preferably in racemic form.

For purposes of the present invention, the term "amidation" means the reaction of converting a carboxylic acid functional group into an amide functional group by reacting with an amine functional group.

In particular, such a reaction can be carried out after activation of the carboxylic acid functional groups, as described in detail in the following description of the present invention.

Method for obtaining complex of formula (II)

The present invention also relates to a method for preparing the complex of formula (II), comprising the following sequential steps:

a) providing complexation of the hexacid of formula (III) presented below:

(III),

with gadolinium to obtain the gadolinium-hexacid complex of formula (I) as defined above,

b) providing isomerization by heating the complex of gadolinium and a hexacid of formula (I) in aqueous solution at a pH value of from 2 to 4 to obtain a diastereomeric enriched complex characterized by at least 80% diastereomeric excess of a mixture of isomers I-RRR and I-SSS of said gadolinium complex and hexacids of formula (I), and

c) forming a complex of formula (II) by reacting the diastereoisomerically enriched complex obtained in step b) with 3-amino-1,2-propanediol.

As used herein, unless otherwise indicated, the terms "Gd", "gadolinium" and "Gd ³⁺ " are used interchangeably to refer to the Gd ³⁺ ion. More generally, a source of free gadolinium such as gadolinium chloride (GdCl ₃ ) or gadolinium oxide (Gd ₂ O ₃ ) may also be provided.

In the present invention, the term “free Gd” refers to non-complexed forms of gadolinium that are preferably available for complexation. Typically, a Gd ³⁺ ion dissolved in water is provided. More generally, a source of free gadolinium, such as gadolinium chloride (GdCl ₃ ) or gadolinium oxide, may also be provided.

▪ Stage a)

During this step, a complexation reaction occurs between the hexacid of formula (III) and gadolinium, resulting in the gadolinium-hexacid complex of formula (I) defined previously.

According to a particular embodiment, step a) involves reacting the hexacid of formula (III) with a source of free Gd in water.

In a preferred embodiment, the source of free Gd is GdCl ₃ or Gd ₂ O ₃ , preferably Gd ₂ O ₃ .

Preferably the reagents used in step a), i.e. the source of gadolinium (usually gadolinium oxide), the hexacid of formula (III) and water, should be as pure as possible, especially with regard to metal impurities.

Thus, the gadolinium source will advantageously be gadolinium oxide, preferably greater than 99.99% pure and even more preferably greater than 99.999%.

The water used in the process preferably contains less than 50 ppm calcium, more preferably less than 20 ppm, and most preferably less than 15 ppm calcium. Typically, the water used in the method is deionized water, water for injection (water of purity suitable for injection), or purified water.

Advantageously, the amounts of reactants (hexacids of formula (III) and gadolinium) used in this step a) correspond to or are close to stoichiometric proportions according to the balance equation of the complexation reaction that occurs during this step.

The term "close to stoichiometric proportions" means that the difference between the molar proportions in which the reactants are added and the stoichiometric proportions is less than 15%, in particular less than 10%, preferably less than 8%.

In particular, gadolinium can be administered in a slight excess relative to stoichiometric proportions. In such a case, the ratio of the amount of material administered as gadolinium to the amount of material administered as the hexacid of formula (III) is greater than 1, but typically less than 1.15, especially less than 1.10, advantageously less than 1 .08. In other words, the amount of gadolinium administered is more than 1 equivalent (eq), but typically less than 1.15 eq, especially less than 1.10 eq, preferably less than 1.08 eq. relative to the amount of hexacid of formula (III) introduced, which itself corresponds to 1 equivalent. In a preferred embodiment in which the source of free gadolinium is Gd ₂ O ₃ , the amount of Gd ₂ O ₃ administered is typically greater than 0.5 eq but less than 0.575 eq, especially less than 0.55 eq, advantageously less than 0.54 eq. relative to the amount of hexacid of formula (III) introduced (1 eq.).

According to a specific embodiment, step a) includes the following sequential steps:

a1) obtaining an aqueous solution of the hexacid of formula (III) and

a2) adding a source of free gadolinium to the aqueous solution obtained in step a1).

In this embodiment, the content of the hexacid of formula (III) in the aqueous solution obtained in step a1) is generally from 10% to 60%, in particular from 15% to 45%, preferably from 20% to 35%, preferably from 25% to 35% and even more preferably from 25% to 30% by weight relative to the total weight of the aqueous solution.

Preferably, steps a) and b) are carried out in accordance with the one-pot synthesis embodiment, i.e. in one reactor and without an intermediate isolation or purification step.

Thus, in this preferred embodiment, the gadolinium hexacid complex of formula (I) formed in step a) is directly subjected to step b) isomerization without isolation or purification and in the same reactor as in step a).

▪ Stage b)

The gadolinium hexacid complex of formula (I), formed by the complexation reaction between the hexacid of formula (III) and gadolinium in step a), is initially obtained in the form of a mixture of diastereoisomers.

Step b) is aimed at enriching the mixture of diastereoisomers with isomers I-RRR and I-SSS to obtain a diastereoisomerically enriched gadolinium-hexacid complex of formula (I) characterized by at least 85%, in particular at least 90%, in particular at least 95 %, preferably at least 97%, preferably at least 98%, more preferably at least 99% diastereoisomeric excess of a mixture of I-RRR and I-SSS isomers.

Preferably, said diastereomeric excess comprises at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% diastereomeric excess of the mixture of I-RRR and I-SSS isomers.

Advantageously, said diastereoisomeric excess comprises a mixture of I-RRR and I-SSS isomers.

As such, the present inventors have discovered that factors such as the pH and temperature of the solution of the gadolinium-hexacid complex of formula (I) obtained at the end of step a) influence the ratio in which the various isomers of the complex of formula (I) are present in the mixture diastereoisomers. Over time, the mixture tends to become enriched in a group of isomers, including isomers that are surprisingly the most thermodynamically stable as well as the most chemically stable, in this case the I-RRR and I-SSS isomers.

The term "mixture of I-RRR and I-SSS isomers" also covers, in a broader sense, the case where only one of the isomers: I-RRR or I-SSS is present. However, the term "mixture of I-RRR and I-SSS isomers" preferably refers to all cases in which each of the I-RRR and I-SSS isomers is present in varying amounts other than zero.

In a preferred embodiment, the I-RRR and I-SSS isomers are present in said mixture in a ratio of 65/35 to 35/65, in particular 60/40 to 40/60, in particular 55/45 to 45/55. Advantageously, the mixture of I-RRR/I-SSS isomers is a racemic mixture (50/50).

Step b) of isomerization of the gadolinium-hexacid complex of formula (I) in aqueous solution is generally carried out at a pH value of 2 to 4, in particular 2 to 3, preferably 2.2 to 2.8.

The pH is preferably adjusted with an acid, preferably an inorganic acid such as hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid or phosphoric acid, for example hydrochloric acid.

It is quite surprising that under these pH conditions the mixture is enriched in specific isomers, in this case the I-RRR and I-SSS isomers, since gadolinium chelates are known in the art to have low kinetic inertness in acidic conditions. More specifically, the higher the concentration of H ⁺ ions in the medium, the greater the probability of proton transfer to one of the donor atoms of the ligand, which leads to dissociation of the complex. As a result, one skilled in the art would expect that placing a gadolinium hexacid complex of formula (I) in an aqueous solution at a pH of 2 to 4 would result in dissociation of said complex rather than its isomerization into I-RRR and I- SSS.

It should be noted that the pH range recommended in EP 1931673 for complexation of the hexacid of formula (III), namely 5.0-6.5, does not allow the preparation of a complex of formula (I) enriched in its isomers I-RRR and I-SSS.

Step b) is generally carried out at a temperature of from 80°C to 130°C, in particular from 90°C to 125°C, preferably from 98°C to 122°C, preferably from 100°C to 120°C, generally for a period of from 10 hours to 72 hours, in particular from 10 hours to 60 hours, preferably from 12 hours to 48 hours.

Contrary to all expectations, such temperature conditions, which in combination with the above pH conditions should contribute to the instability of the gadolinium chelate, do not lead to the dissolution of its complex or the formation of any other impurity, but to its isomerization into I-RRR and I-SSS.

In one particular embodiment, the aqueous solution in step b) contains acetic acid. Step b) is then advantageously carried out at a temperature of from 100°C to 120°C, in particular from 110°C to 118°C, generally for a period of from 12 hours to 48 hours, in particular from 20 hours to 30 hours, in particular from 24 hours to 26 hours.

Acetic acid is preferably added before heating the solution of the gadolinium hexacid complex of formula (I) obtained in step a) in such an amount that the acetic acid content is from 25% to 75%, in particular from 40% to 50% by weight relative to the weight hexacid of formula (III) used in step a).

When the aqueous solution is heated to a temperature preferably between 100°C and 120°C, typically between 110°C and 118°C, acetic acid is added gradually as the water evaporates to maintain a constant volume of the solution.

According to a preferred embodiment, upon completion of step b), the diastereoisomerically enriched complex is isolated by crystallization, preferably by crystallization using seed crystals.

In this embodiment, stage b) includes the following sequential stages:

b1) providing isomerization by heating the complex of gadolinium and a hexacid of formula (I) in aqueous solution at a pH value of from 2 to 4 to obtain a diastereomeric enriched complex characterized by at least 80% diastereomeric excess of a mixture of isomers I-RRR and I-SSS of said gadolinium complex and hexacids of formula (I), and

b2) isolating by crystallization of said diastereoisomerically enriched complex, preferably by crystallization using seed crystals.

Step b2) of crystallization aims, firstly, to remove any impurities present in the aqueous solution that may arise from the previous steps, to obtain a decolorized product of higher purity in crystal form, and, secondly, to continue the diastereoisomeric enrichment of the complex gadolinium and hexacids of formula (I) to obtain a higher diastereomeric excess of the mixture of isomers I-RRR and I-SSS of the said complex than that obtained at the end of step b1).

More specifically, the I-RRR and I-SSS isomers of the hexacid complex of formula (I) crystallize from water. On the other hand, a gadolinium-hexacid complex of formula (I), not enriched in the indicated isomers, does not crystallize.

A completely unexpected result is that the isomers I-RRR and I-SSS, in which the complex tends to be enriched during step b) (and, contrary to all expectations, due to the conditions under which it is carried out), are the only isomers of the complex that crystallize from water. Thus, isomerization and crystallization synergistically contribute to the enrichment of I-RRR and I-SSS isomers and, as a result, the overall efficiency of the method in accordance with the present invention.

Moreover, it should be noted that the crystallization in water of the isomers of interest of the gadolinium hexacid complex of formula (I) avoids the addition of a solvent as described in Example 7 of EP 1931673, which includes the step of precipitation from ethanol of the trisodium salt of the complex.

Step b2) is preferably carried out at a temperature of from 10°C to 70°C, in particular from 30°C to 65°C, in particular from 35°C to 60°C.

According to one embodiment, after lowering the temperature of the aqueous solution so that it is within the above ranges, the crystallization process is initiated by using seed crystals. “Seed crystallization,” also called “seeded crystallization,” involves introducing into a crystallization reactor (also called a crystallization vessel) a known quantity of crystals, called “seed crystals” or “seed crystals.” This allows you to reduce the crystallization time. Crystallization using seed crystals is well known to those skilled in the art. In the method of the present invention, the introduction of seed crystals using a seed, in this case crystals of a diastereoisomerically enriched complex of gadolinium and a hexacid of formula (I), added to an aqueous solution of the diastereoisomerically enriched complex, the temperature of which has previously been lowered, allows nucleation and thus initiation of crystallization . The duration of crystallization using seed crystals is advantageously from 2 hours to 20 hours and preferably from 6 hours to 18 hours; usually 16 hours.

Crystals of the diastereoisomerically enriched gadolinium hexacid complex of formula (I) are then typically isolated by filtration and drying using any technique well known to those skilled in the art.

Advantageously, the purity of the diastereoisomerically enriched gadolinium hexacid complex of formula (I) isolated at the end of step b2) is greater than 95%, in particular greater than 98%, preferably greater than 99%, said purity being expressed as the mass fraction of the complex of formula ( I) relative to the total mass obtained at the end of stage b2).

In a specific embodiment, the diastereoisomerically enriched complex from step b) isolated by crystallization is again purified by recrystallization to obtain a diastereoisomerically enriched and purified complex.

In this embodiment, step b) includes, in addition to the sequential steps b1) and b2) described previously, step b3) of purification by recrystallization of the isolated diastereoisomerically enriched gadolinium hexaacid complex of formula (I).

Stage b3) of recrystallization, like stage b2) of crystallization, is aimed, firstly, at obtaining a product of higher purity, and, secondly, at continuing the diastereomeric enrichment of the gadolinium-hexacid complex of formula (I) to obtain a higher diastereomeric excess of the mixture isomers I-RRR and I-SSS of the said complex than that obtained at the end of step b2).

Stage b3) typically includes the following sequential substages:

suspending the diastereoisomerically enriched gadolinium hexacid complex of formula (I) isolated in step b2) in an aqueous solution, preferably water,

dissolving said complex by heating to a temperature preferably from 80°C to 120°C, for example up to 100°C,

recrystallization, preferably by using seed crystals, at a temperature preferably from 10°C to 90°C, in particular from 20°C to 87°C, in particular from 55°C to 85°C, generally for a time of 2 hours to 20 hours, namely from 6 hours to 18 hours, and

isolating crystals of the diastereoisomerically enriched and purified gadolinium-hexacid complex of formula (I), for example, by filtration and drying.

The purity of the purified diastereoisomerically enriched gadolinium hexacid complex of formula (I) isolated at the end of step b3) is generally greater than 98%, in particular greater than 99%, advantageously greater than 99.5%, said purity being expressed as mass fraction of the complex of formula (I) relative to the total mass obtained at the end of step b2).

In another embodiment, the diastereoisomerically enriched complex of step b) is further enriched by allowing selective decomplexation of diastereoisomers of the complex of formula (I) other than the diastereoisomers I-RRR and I-SSS, i.e. by providing selective decomposition of the diastereoisomer complexes I-RSS, I-SRR, I-RSR, I-SRS, I-RRS and I-SSR.

In this embodiment, step b) includes, in addition to the sequential steps b1) and b2) described previously, step b4) of providing selective decomplexation of diastereoisomers of the complex of formula (I) other than the diastereoisomers I-RRR and I-SSS. In this embodiment, step b) may also include step b3) described previously, wherein said step b3) is carried out between steps b2) and b4) or after step b4).

The selective decomposition step of complex b4) is aimed at continuing the diastereomeric enrichment of the gadolinium hexaacid complex of formula (I) to obtain a higher diastereomeric excess of the mixture of I-RRR and I-SSS isomers of said complex than that obtained at the end of step b2) or at the end of step b3 ), when carrying out the specified stage to stage b4).

Stage b4) typically includes the following sequential substages:

suspending the diastereoisomerically enriched gadolinium hexacid complex of formula (I) isolated in step b2) or step b3) in water,

adding a base, such as sodium hydroxide,

heating to a temperature preferably from 30°C to 60°C, in particular from 35°C to 55°C, for example at 40°C, generally for a period of from 2 hours to 20 hours, in particular from 10 hours to 18 hours,

cooling to a temperature preferably from 10°C to 30°C, for example to 30°C, and

isolating the diastereoisomerically enriched and purified gadolinium hexacid complex of formula (I), for example by filtration and drying.

Carrying out step b4) was made possible due to the fact that the isomers I-RRR and I-SSS are the most stable in the basic environment. Such basic conditions promote the formation of gadolinium hydroxide and, as a result, the decomposition of complexes of the least stable isomers.

Thus, it should be noted that, unexpectedly, the I-RRR and I-SSS isomers are more stable both in an acidic environment, which allows step b1) of isomerization, and in a basic environment, which allows step b4) to ensure selective decomposition of the complex.

In a preferred embodiment, the diastereoisomerically enriched complex obtained at the end of step b) according to any of the above-described embodiments is characterized by at least 85%, in particular at least 90%, in particular at least 95%, preferably at least 97%, preferably at least 98%, more preferably at least 99% diastereoisomeric excess of a mixture of I-RRR and I-SSS isomers.

Preferably, said diastereomeric excess comprises at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% diastereomeric excess of the mixture of I-RRR and I-SSS isomers.

Advantageously, said diastereoisomeric excess comprises a mixture of I-RRR and I-SSS isomers.

The term "mixture of I-RRR and I-SSS isomers" also covers, in a broader sense, the case where only one of the isomers: I-RRR or I-SSS is present. However, the term "mixture of I-RRR and I-SSS isomers" preferably refers to all cases in which each of the I-RRR and I-SSS isomers is present in varying amounts other than zero.

In a preferred embodiment, the I-RRR and I-SSS isomers are present in said mixture in a ratio of 65/35 to 35/65, in particular 60/40 to 40/60, in particular 55/45 to 45/55. Advantageously, the mixture of I-RRR/I-SSS isomers is a racemic mixture (50/50).

▪ Stage c)

Step c) aims to form a complex of formula (II) from its precursor, the diastereoisomerically enriched gadolinium hexacid complex of formula (I) obtained in step b).

During this step, the three carboxylic acid functional groups of the gadolinium complex and the hexacid of formula (I), carried by carbon atoms that are located on the side chains of the complex in the γ-position relative to the nitrogen atoms of the macrocycle to which these side chains are grafted, are converted into amide functional group by amidation reaction with 3-amino-1,2-propanediol, in racemic or enantiomerically pure form, preferably in racemic form.

This amidation reaction does not change the absolute configuration of the three asymmetric carbon atoms located on the side chains in the α-position relative to the nitrogen atoms of the macrocycle to which these side chains are grafted. As a result, step c) allows one to obtain a complex of formula (II) with a diastereomeric excess of a mixture of isomers II-RRR and II-SSS, identical to the diastereomeric excess of isomers I-RRR and I-SSS, with which a diastereomeric enriched gadolinium-hexacid complex of formula (I) is obtained ) obtained at the end of step b), which is at least 80%.

In a preferred embodiment, the complex of formula (II) obtained at the end of step c) is characterized by at least 85%, in particular at least 90%, in particular at least 92%, preferably at least 94%, advantageously at least at least 97%, more preferably at least 99% diastereoisomeric excess of a mixture of isomers II-RRR and II-SSS.

Preferably, said diastereomeric excess comprises at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% diastereomeric excess of the mixture of II-RRR and II-SSS isomers.

Advantageously, said diastereoisomeric excess comprises a mixture of II-RRR and II-SSS isomers.

The term "mixture of II-RRR and II-SSS isomers" also covers, in a broader sense, the case where only one of the isomers II-RRR or II-SSS is present. However, the term "mixture of I-RRR and I-SSS isomers" preferably refers to all cases in which each of the I-RRR and I-SSS isomers is present in varying amounts other than zero.

In a preferred embodiment, the isomers II-RRR and II-SSS are present in said mixture in a ratio of 65/35 to 35/65, in particular 60/40 to 40/60, in particular 55/45 to 45/55. Predominantly the isomers II-RRR and II-SSS are present in the mixture in a 50/50 ratio.

The amidation reaction can be carried out according to any method that is well known to those skilled in the art, especially in the presence of a reagent for activating carboxylic acid functional groups and/or by acid catalysis.

In particular, it can be carried out in accordance with the methods described in EP 1931673, in particular in paragraph [0027] of the said patent.

In one particular embodiment, step c) involves activating the carboxylic acid (COOH) functional groups of the hexacid complex of formula (I) carried by carbon atoms that are located on the side chains of the complex in the γ-position with respect to the nitrogen atoms of the macrocycle to which these are grafted side chains, in the form of derivatives of functional groups including a carbonyl group (C=O), in which the carbon atom of the carbonyl group is more electrophilic than the carbon atom of the carbonyl group of the carboxylic acid functional groups. Thus, according to this particular embodiment, said carboxylic acid functional groups may be particularly activated in the form of ester, acyl chloride or anhydride functional groups, or any activated form that can result in the formation of an amide bond. Activated forms that can result in the formation of an amide bond are well known to those skilled in the art and can be prepared, for example, using a variety of peptide bond creation techniques known in the art of peptide chemistry. Examples of such methods are given in the publication Synthesis of peptides and peptidomimetics, volume E22a, pages 425-588, Houben-Weyl et al., Goodman Editor, Thieme-Stuttgart-New York (2004), and among these examples one can particularly mention methods of activation carboxylic acids through the formation of an azide (acyl azide), for example, through the action of a reagent such as diphenylphosphoryl azide (commonly abbreviated DPPA), the use of carbodiimides alone or in the presence of catalysts (for example, N-hydroxysuccinimide and its derivatives), the use of carbonyldiimidazole (1,1' -carbonyldiimidazole, CDI), the use of phosphonium salts such as benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate (usually abbreviated BOP), or uronium such as 2-(1H-benzotriazol-1-yl)-1,1, 3,3-Tetramethyluronium hexafluorophosphate (commonly abbreviated HBTU).

Preferably, step c) involves activating the above-mentioned carboxylic acid (COOH) functional groups in the form of ester, acyl chloride or anhydride functional groups.

This embodiment is preferable to carrying out the peptide bond formation reaction by activating the carboxylic acid functionality using a coupling agent such as EDCI/HOBT as described in EP 1931673. More specifically, such a coupling reaction results in the formation of one equivalent of 1 -ethyl-3-[3–(dimethylamino)propyl]urea, which must be removed, especially by silica chromatography or liquid-phase extraction by adding a solvent. Regardless of the increase in process complexity caused by such an additional step, the use of such purification methods is not desirable, as discussed previously. In addition, the use of HOBT itself is problematic because it is an explosive product.

For the purposes of the present invention, the term “ester functional group” is intended to mean the group -C(O)O-. In particular, it may be a group -C(O)OR ₁ in which R ₁ corresponds to a (C ₁ -C ₆ )alkyl group.

For purposes of the present invention, the term "(C ₁ -C ₆ )alkyl group" means a straight or branched chain saturated hydrocarbon containing from 1 to 6 and preferably from 1 to 4 carbon atoms. Examples that may be given include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl and hexyl groups.

For the purposes of the present invention, the term "acyl chloride function", also known as "acid chloride function", refers to the group -CO-Cl.

For the purposes of the present invention, the term "anhydride functional group" means the group -CO-O-CO-. In particular, it may be a -CO-O-CO-R ₂ group, where R ₂ corresponds to a (C ₁ -C ₆ )alkyl group.

Reactions for converting a carboxylic acid functionality to an ester, acyl chloride, or anhydride functionality are well known to one skilled in the art and will be able to carry them out in any conventional manner familiar to him or her.

The complex of formula (II) is then obtained by aminolysis of carboxylic acid functional groups activated in the form of ester, acyl chloride or anhydride functional groups, in particular ester or anhydride functional groups, preferably ester functional groups, by reaction with 3-amino-1,2 -propanediol in racemic or enantiomerically pure form, preferably in racemic form.

Preferably, the carboxylic acid functional group activation and aminolysis steps are carried out in accordance with the one-pot synthesis embodiment, i.e. in a single reactor and without the intermediate step of isolating or purifying an intermediate comprising carboxylic acid functional groups activated in the form of ester, acyl chloride or anhydride functional groups, especially ester or anhydride functional groups, preferably ester functional groups.

According to a particular embodiment, step c) includes the following sequential steps:

c1) formation of an activated complex of formula (VII),

(VII),

where Y represents a chlorine atom, a -OR ₁ or -OC(O)-R ₂ group; preferably Y represents a -OR ₁ or -OC(O)-R ₂ group, wherein R ₁ and R ₂ independently correspond to a (C ₁ -C ₆ )alkyl group, and

c2) providing aminolysis of the activated complex of formula (VII) with 3-amino-1,2-propanediol.

As will be clear to one skilled in the art, the reaction to form the activated complex of formula (VII) does not change the absolute configuration of the three asymmetric carbon atoms located on the side chains in the α-position relative to the nitrogen atoms of the macrocycle to which said side chains are grafted. As a result, step c1) allows to obtain an activated complex of formula (VII) with a diastereomeric excess of a mixture of isomers VII-RRR and VII-SSS of formula (VII-RRR) and formula (VII-SSS) presented below, identical to the diastereomeric excess of a mixture of isomers I- RRR and I-SSS, with which a diastereoisomerically enriched complex of gadolinium and hexaacids of formula (I) is obtained after completion of step b), which is at least 80%.

(VII-SSS)

(VII-RRR)

In the case where Y represents a chlorine atom, step c1) is typically carried out by reacting the diastereoisomerically enriched gadolinium hexacid complex of formula (I) obtained in step b) with thionyl chloride (SOCl ₂ ).

In the case where Y represents a -OC(O)-CH ₃ group, step c1) is typically carried out by reacting the diastereoisomerically enriched gadolinium hexacid complex of formula (I) obtained in step b) with acetyl chloride.

In an advantageous embodiment, step c) involves activating the above-mentioned carboxylic acid (-COOH) functional groups in the form of ester functional groups.

According to this embodiment, step c) may in particular comprise the following sequential steps:

c1) formation of a triester of formula (VIII),

(VIII),

where R ₁ represents a (C ₁ -C ₆ )alkyl group, and

c2) providing aminolysis of the triester of formula (VIII) with 3-amino-1,2-propanediol.

Step c1) is typically carried out in an alcohol of the formula R ₁ OH, which acts as a solvent and reactant in the presence of an acid such as hydrochloric acid.

At the first stage, a gadolinium-hexacid complex of formula (I) and alcohol R ₁ OH are placed in the reactor. The reaction medium is then cooled to below 10°C, in particular below 5°C, typically to 0°C, and then an acidic alcohol solution R ₁ OH, typically a solution of hydrochloric acid in R ₁ OH, is gradually added. The reaction medium is continued to stir at room temperature (ie, 20 to 25° C.) for a period of time, typically more than 5 hours, preferably 10 hours to 20 hours. The reaction medium is cooled to a temperature below 10°C, in particular from 0°C to 5°C, before carrying out step c2).

Step c2) is typically also carried out in an alcohol of the formula R ₁ OH in the presence of an acid such as hydrochloric acid.

Thus, steps c1) and c2) can be easily carried out in accordance with the one-pot synthesis embodiment. Advantageously, the triester of formula (VII) is not isolated between steps c1) and c2).

However, to speed up the aminolysis reaction in step c2), the alcohol of formula R ₁ OH is preferably removed by vacuum distillation.

For the purposes of the present invention, the term "vacuum distillation" means distillation of a mixture carried out at a pressure of 10 to 500 mbar, in particular 10 to 350 mbar, preferably 10 to 150 mbar, in particular 50 to 100 mbar.

Similarly, to speed up the aminolysis reaction in step c2), 3-amino-1,2-propanediol is introduced in large excess. Typically, the amount of 3-amino-1,2-propanediol material introduced is more than 4 equivalents, especially more than 7 equivalents, preferably more than 10 equivalents. relative to the amount of diastereoisomerically enriched gadolinium hexacid complex material of formula (I) initially introduced in step c), which itself corresponds to 1 equivalent.

Surprisingly, despite the acidic conditions typically used in steps c1) and c2), which should increase the kinetic instability of the gadolinium complexes, no complex decomposition or isomerization of the triester of formula (VIII) was observed. The required triamide is obtained with a very good degree of conversion and preservation of the absolute configuration of three asymmetric carbon atoms located on the side chains in the α-position relative to the nitrogen atoms of the macrocycle.

Moreover, it should be noted that, as a rule, amidation reactions through direct reaction between an ester and an amine are hardly described in the literature (on this subject, see K.C. Nadimpally et al., Tetrahedron Letters, 2011, 52, 2579-2582).

In a preferred embodiment, step c) includes the following sequential steps:

c1) formation of a methyl triester of formula (IV),

(IV),

in particular, by carrying out the reaction in methanol in the presence of an acid such as hydrochloric acid, and

c2) causing aminolysis of the methyl triester of formula (IV) with 3-amino-1,2-propanediol, in particular in methanol in the presence of an acid such as hydrochloric acid.

Advantageously, the methyl triester of formula (IV) is not isolated between steps c1) and c2).

In a preferred embodiment, in step c2), the methanol is removed by vacuum distillation until the temperature generally exceeds 55°C, in particular does not exceed 60°C to 65°C, and the reaction medium is maintained at this temperature. temperature under vacuum for a period of time, usually more than 5 hours, especially 10 hours to 20 hours, before cooling to room temperature and diluting with water.

The present invention covers all combinations of the specific, advantageous or preferred embodiments described above in connection with each method step.

The present invention also relates to a gadolinium triester complex of formula (VIII):

(VIII),

characterized by at least 80% diastereoisomeric excess of a mixture of isomers VIII-RRR and VIII-SSS of the formulas:

(VIII-SSS),

(VIII-RRR).

In the context of the present invention, the term "diastereoisomeric excess" is meant to indicate, with respect to the gadolinium complex and triester of formula (VIII), the fact that said complex is predominantly present as an isomer or group of isomers selected from the diastereoisomers VIII-RRR, VIII-SSS, VIII -RRS, VIII-SSR, VIII-RSS, VIII-SRR, VIII-RSR and VIII-SRS. Said diastereomeric excess is expressed as a percentage and corresponds to the amount represented by the predominant isomer or group of isomers relative to the total amount of the triester complex of formula (VIII). It will be understood that this percentage may be molar or mass, since isomers by definition have the same molar mass.

In one particular embodiment, the gadolinium triester complex of formula (VIII) according to the present invention is characterized by at least 85%, in particular at least 90%, in particular at least 95%, preferably at least 97%, predominantly at least 98%, more preferably at least 99% diastereoisomeric excess of a mixture of isomers VIII-RRR and VIII-SSS.

Preferably, said diastereomeric excess comprises at least 70%, in particular at least 80%, advantageously at least 90%, preferably at least 95% diastereomeric excess of the mixture of VIII-RRR and VIII-SSS isomers.

Advantageously, said diastereoisomeric excess comprises a mixture of VIII-RRR and VIII-SSS isomers.

The term "mixture of isomers VIII-RRR and VIII-SSS" also covers the case where only one of the isomers VIII-RRR or VIII-SSS is present. However, the term "mixture of VIII-RRR and VIII-SSS isomers" preferably refers to all cases in which each of the VIII-RRR and VIII-SSS isomers is present in varying amounts other than zero.

In a preferred embodiment, isomers VIII-RRR and VIII-SSS are present in said mixture in a ratio of 65/35 to 35/65, in particular 60/40 to 40/60, in particular 55/45 to 45/55. Advantageously, the mixture of VIII-RRR/VIII-SSS isomers is a racemic mixture (50/50).

In a preferred embodiment, the gadolinium triester complex of formula (VIII) according to the present invention is a gadolinium trimethyl ester complex, i.e. gadolinium triesester complex of formula (VIII), where R ₁ represents a methyl group (CH ₃ ).

▪ Preparation of hexacid of formula (III)

The hexacid of formula (III) which participates in step a) of the process for preparing the complex of formula (II) according to the present invention can be prepared by any already known method, in particular by the methods described in EP 1931673.

However, according to a preferred embodiment, the hexacid of formula (III) is prepared by alkylation of picklene of formula (V):

(V),

a compound of the formula R ₃ OOC-CHG _p -(CH ₂ ) ₂ -COOR ₄ (IX),

Where

- R ₃ and R ₄ independently of each other represent a (C ₃ -C ₆ )alkyl group, in particular a (C ₄ -C ₆ )alkyl group such as butyl, isobutyl, sec-butyl, tert-butyl, pentyl or hexyl group, and

- G _p represents a leaving group such as a tosylate or triflate group, or a halogen atom, preferably a bromine atom,

to give the hexaester of formula (X),

(X),

followed by a hydrolysis step resulting in said hexacid of formula (III).

In a preferred embodiment, R ₃ and R ₄ are identical.

According to a preferred embodiment, the hexacid of formula (III) is prepared by alkylation of piclen of formula (V):

(V),

dibutyl-2-bromoglutarate to give the butyl hexaester of formula (VI):

(VI),

followed by a hydrolysis step resulting in said hexacid of formula (III).

The dibutyl 2-bromoglutarate used is in racemic or enantiomerically pure form, preferably in racemic form.

The use of dibutyl 2-bromoglutarate is particularly advantageous compared to the use of ethyl 2-bromoglutarate described in EP 1931673. More specifically, commercial diethyl 2-bromoglutarate is a relatively unstable compound that decomposes over time and under the influence of temperature. Namely, this ester tends to undergo hydrolysis or cyclization and thus lose its bromine atom. Attempts to purify commercial diethyl 2-bromoglutarate or to develop new synthetic routes to obtain it with improved purity and thus prevent its degradation have been unsuccessful.

The alkylation reaction is typically carried out in a polar solvent, preferably water, in particular deionized water, advantageously in the presence of a base such as potassium or sodium carbonate.

In particular, the use of water is preferred over the use of acetonitrile described in EP 1931673 for obvious reasons.

The reaction is advantageously carried out at a temperature of from 40°C to 80°C, generally from 50°C to 70°C and in particular from 55°C to 60°C, for a period of from 5 hours to 20 hours, in particular from 8 o'clock to 15 o'clock.

The hydrolysis step is advantageously carried out in the presence of an acid or a base, preferably a base such as sodium hydroxide. The hydrolysis solvent may be water, an alcohol such as ethanol, or a water/alcohol mixture. This step is advantageously carried out at a temperature of from 40°C to 80°C, typically from 40°C to 70°C and in particular from 50°C to 60°C, typically for a period of from 10 hours to 30 hours, especially from 15 hours to 25 hours.

The present invention also relates to a butyl hexaester of formula (VI):

(VI).

In particular, this hexaester has significantly improved stability compared to esters with a shorter alkyl chain, especially compared to the ethyl hexaester described in EP 1931673.

Graphic materials

Fig. 1: decomposition under alkaline conditions of the iso1–iso4 isomer groups of the complex of formula (II), expressed as the change in the percentage of peak area of a given isomer group over time.

Examples

The following examples are presented in the form of illustrations and do not limit the present invention.

Separation of the isoA, isoB, isoC and isoD isomer groups of the gadolinium hexacid complex of formula (I) by HPLC

An HPLC system is used, consisting of a pump, an injector, a chromatographic column, a spectrophotometric UV detector, and a data processing and control system. The chromatography column used is a C18 - 250 x 4.6 mm - 5 µm column (Waters Symmetry® series column).

- Mobile phase

Phase A: 100% acetonitrile, and phase B: 0.1% v/v. aqueous solution of H ₂ SO ₄ (96%)

- Obtaining test solutions

A solution of the complex of gadolinium and hexacid of formula (I) in purified water with a concentration of 10 mg/ml

- Analytical conditions

Column temperature 25°C Sample temperature Room temperature (20-25°C) Flow rate 1.0 ml/min. Volume of administration 20 µl UV detection 200 nm Duration of analysis 60 min.

- Gradient

Time %Acn % 0.1% H ₂ SO ₄ 0 1 99 10 5 95 40 10 90 50 25 75 55 1 99 60 1 99

% Acn: % v/v acetonitrile in the mobile phase

% 0.1% H ₂ SO ₄ : % v/v H ₂ SO ₄ solution with a concentration of 0.1% v/v. in the mobile phase

Four main peaks are obtained. Peak 4 in the HPLC chromatogram, namely isoD, corresponds to a retention time of 35.7 minutes.

Separation of the isoA, isoB, isoC and isoD isomer groups of the gadolinium hexacid complex of formula (I) using UHPLC

A UHPLC system is used, consisting of a pump, an injector, a chromatography column, a UV detector, and a data acquisition and processing system. The chromatography column used is a 150 × 2.1 mm - 1.8 µm UHPLC column (Waters Acquity UPLC HSS T3 column). It is a reverse phase UPLC column that contains spherical C18-grafted (octadecyl) silica particles whose silanol groups have been endcapped. It is also characterized by a length of 150 mm, an internal diameter of 2.1 mm, a particle size of 1.8 μm, a porosity of 100 Å and a carbon content of 11%.

Preferably, the stationary phase used should be compatible with aqueous mobile phases.

- Mobile phase

Phase A: 100% acetonitrile, and phase B: 0.1% v/v. aqueous solution of H ₂ SO ₄ (96%)

- Obtaining test solutions

A solution of the complex of gadolinium and hexacid of formula (I) in purified water with a concentration of 0.8 mg/ml

- Analytical conditions

Column temperature 35°C Sample temperature Room temperature (20-25°C) Flow rate 0.4 ml/min. Volume of administration 10 µl UV detection 200 nm Duration of analysis 32 min.

- Gradient

Time %Acn % 0.1% H ₂ SO ₄ 0 1 99 14 8 92 20 eleven 89 25 25 75 27 1 99 32 1 99

Four main peaks are obtained. Peak 4 in the UHPLC chromatogram, namely isoD, corresponds to a retention time of 17.4 minutes.

Separation of the iso1, iso2, iso3 and iso4 groups of isomers of the complex of formula (II) using UHPLC

A UHPLC system is used, consisting of a pump, an injector, a chromatography column, a UV detector, and a data acquisition and processing system. The chromatography column used is a 150 × 2.1 mm - 1.6 µm UHPLC column (Waters Cortecs® UPLC T3 column).

- Mobile phase

Phase A: 100% acetonitrile, and phase B: 0.0005% v/v. aqueous solution of H ₂ SO ₄ (96%)

- Obtaining test solutions

A solution of the complex of formula (II) in purified water with a concentration of 2 mg/ml

- Analytical conditions

Column temperature 40°C Sample temperature Room temperature (20-25°C) Flow rate 0.3 ml/min. Volume of administration 1 µl UV detection 200 nm Duration of analysis 20 minutes.

- Gradient

Time %Acn % _0.0005 % _H2SO4 0 1 99 3 5 95 12 10 90 15 25 75 16 1 99 20 1 99

Four main peaks are obtained. Peak 4 in the UHPLC chromatogram, namely iso4, corresponds to a retention time of 6.3 minutes.

Dimensions of relaxation

Relaxation times T ₁ and T ₂ were determined according to standard procedures on a Minispec® mq20 instrument (Brüker) at 20 MHz (0.47 T), 60 MHz (1.41 T) and 37°C. Longitudinal relaxation time T ₁ is measured using an inversion recovery sequence, and transverse relaxation time T ₂ is measured using the CPMG (Carr-Purcell-Meiboom-Gill) technique.

Relaxation rates R ₁ (= 1/T ₁ ) and R ₂ (= 1/T ₂ ) were calculated for various total metal concentrations (from 0.5 x 10 ^-3 to 5 x 10 ^-3 mol/l) in aqueous solution at 37°C. The correlation between R ₁ or R ₂ as a function of concentration is linear, and the slope of the curve represents the relaxivity r ₁ (R ₁ /C) or r ₂ (R ₂ /C) expressed in (1/second) x (1/mmol /l), i.e. in (mM ^-1 .s ^-1 ).

Measurement of the kinetic inertness of isomer groups of the complex of formula (II) in an acidic medium

The dissociation of gadolinium complexes present in the four unresolved peaks of isomers iso1–iso4 (C = 8 × 10 ^-6 M) was studied at 37°C, pH 1.2 in hydrochloric acid solution under pseudo-first order reaction kinetics conditions without control of ionic strength by monitoring release of gadolinium in solution. The amount of free gadolinium was determined spectrometrically at 654 nm after adding arsenazo III solution (C = 5.3 × 10 ^-4 M).

The half-life values (T _1/2 ) that were determined for each of the isomer groups are summarized in the table below:

Groups of isomers T _1/2 (pH 1.2 – 37°C) Iso1 18 hours ISO2 6 hours ISO3 8 days ISO4 27 days

Study of the decomposition under alkaline conditions of groups of isomers of the complex of formula (II)

The complex of formula (II) will be referred to as AP in the remainder of this example.

The degradation kinetics of unresolved isomer peaks iso1–iso4, referred to collectively as isoX, was assessed by measuring purity by HPLC and monitoring the area of each unresolved isomer peak over time. The measured values are as follows:

- P _HPLC (time) and

- [Matt. 1]

The selected degradation conditions are as follows: [AP] = 1 mM in 0.1 N. sodium hydroxide solution. The impact of AP degradation on the experimental environment under these dilution conditions is low. Decomposition products do not change the pH of the medium, and this parameter is critical when studying the kinetics of decomposition. This is confirmed experimentally by measuring the initial pH and the pH at the end of digestion (72 hours, 37°C).

AP solution pH 0.1 N. NaOH( _T0 ) 12.9 pH (after 72 hours at 37°C) 12.8

The method for preparing solutions is described below:

- weigh approximately 0.05 g of each product, add the required amount of water mQ up to 10 ml to obtain a solution A c [AP] _A = 5 mM,

- dilution: dilute 2 ml of solution A by adding the required amount of NaOH (0.1 N) to 10 ml to obtain solution B with [AP] _B = 1 mM and [NaOH] = 0.08 M,

- transfer aliquots of solutions into HPLC vials and

- incubate HPLC vials containing solutions of AP in NaOH at the test temperature (37°C).

For each point, an aliquot is removed and analyzed by HPLC without diluting the sample (ammonium acetate method).

The results obtained are presented in Fig. 1.

Preparation of butyl hexaester of formula (VI)

184 kg (570 mol) of dibutyl 2-bromoglutarate and 89 kg (644 mol) of potassium carbonate are mixed in the reactor and heated to 55-60°C. An aqueous solution of 29.4 kg (143 mol) of picklene in 24 kg of water is added to the previous mixture. The reaction mixture is kept at 55-60°C and then refluxed for approximately 10 hours. After the reaction, the medium is cooled, diluted with 155 kg of toluene and then washed with 300 liters of water. The butyl hexaester is extracted with an aqueous phase consisting of 175 kg (1340 mol) phosphoric acid (75%). The resulting mixture is then washed three times with 150 kg of toluene. The butyl hexaester is re-extracted into the toluene phase by dilution with 145 kg of toluene and 165 kg of water, followed by alkalization with 30% sodium hydroxide solution (w/w) to achieve a pH of 5-5.5. The lower aqueous phase is removed. The butyl hexaester is obtained by concentration to dryness in vacuo at 60°C in approximately 85% yield.

Preparation of hexacid of formula (III)

113 kg (121 mol) of butyl hexaester are placed in the reactor along with 8 kg of ethanol. The medium is brought to 55 ± 5°C and then 161 kg (1207.5 mol) of 30% sodium hydroxide solution (w/w) is added over 3 hours. The reaction mixture is maintained at this temperature for approximately 20 hours. The butanol is then removed by decanting the reaction medium. The hexacid of formula (III), obtained in the form of a sodium salt, is diluted with water to obtain an aqueous solution with a concentration of approximately 10% (w/w). This solution is treated with an acidic cation exchange resin. An aqueous solution of the hexacid of formula (III) is obtained in approximately 90% yield and 95% purity.

Preparation of gadolinium hexacid complex of formula (I)

▪ Description of the experiment

• Complexation and isomerization

- Without acetic acid

418 kg (117 kg pure hexacid of formula (III)/196 mol) of an aqueous solution of hexacid of formula (III) with a concentration of 28% by weight are placed in the reactor. The pH of the solution is adjusted to 2.7 by adding hydrochloric acid and then 37 kg (103.2 mol) of gadolinium oxide is added. The reaction medium is heated at 100-102°C for 48 hours to achieve the expected isomeric distribution of the hexacid of formula (III).

- With acetic acid

Suspend gadolinium oxide (0.525 molar equivalent) in a solution of hexacid of formula (III) with a concentration of 28.1% by weight.

99-100% acetic acid (50% by weight/pure hexacid of formula (III)) is poured into the medium at room temperature.

The medium is heated under reflux followed by distillation to 113°C by weight, gradually adding acetic acid to the medium as water is removed. Once the temperature reaches 113°C, add enough acetic acid to reach the initial volume.

The medium is maintained at 113°C overnight.

• Crystallization, recrystallization

- Crystallization

The solution of the gadolinium hexacid complex of formula (I) is cooled to 40° C., the seed is added and left in contact with each other for at least 2 hours. The product is then isolated by filtration at 40°C and washed with purified water by osmosis.

- Recrystallization

180 kg of the gadolinium hexacid complex of formula (I) obtained previously (dry matter content approximately 72%) are suspended in 390 kg of water. The medium is heated to 100°C to dissolve the product and then cooled to 80°C and a small amount of seed is added. After cooling to room temperature, the gadolinium hexacid complex of formula (I) is isolated by filtration and drying.

• Selective breakdown of the complex

Place the dry product in a reactor with water purified by osmosis at 20°C. The mass of added water is equal to twice the theoretical mass of the gadolinium-hexacid complex of formula (I). Pour 30.5% w/w sodium hydroxide solution (6.5 eq) into the medium at 20°C. Once the NaOH addition is complete, the media are left in contact with each other at 50°C for 16 hours. The medium is cooled to 25°C and the product is filtered through a pad of Clarcel.

▪ The value of diastereoisomeric excess of a mixture of isomers I-RRR and I-SSS

The ratio in which the various isomers of the complex of formula (I) are present in the mixture of diastereoisomers depends on the conditions under which the complexation and isomerization steps are carried out, as shown in Table 3 below.

Table 3. Content of I-RRR and I-SSS mixture depending on complexation/isomerization conditions

pH value Temperature Content of hexacid of formula (III) Time Diastereoisomeric excess of mixture
I-RRR and I-SSS 5.7 80°C 40% 3 hours 19% 3.5 90°C 50% 10 hours 49% 3.0 101°C 40% 10 hours 68% 2.7 101°C 28% 48 hours 98.04%

Additional steps of recrystallization and selective decomposition of the complex make it possible to increase the diastereomeric excess of the mixture of I-RRR and I-SSS (see table 4).

Table 4. Diastereoisomeric excess value of the mixture of I-RRR and I-SSS after crystallization/recrystallization/selective decomposition of the complex

After the first crystallization After recrystallization After selective disintegration of the complex Diastereoisomeric excess of mixture
I-RRR and I-SSS 98.04% 99.12% 99.75%

Preparation of the complex of formula (II)

90 kg (119 mol) of the hexacid complex of formula (I) and 650 kg of methanol are placed in the reactor. The mixture is cooled to approximately 0°C and then 111 kg (252 mol) of a methanolic hydrochloric acid solution (8.25% HCl in methanol) is poured into it, maintaining the temperature at 0°C. The reaction medium is brought to room temperature and then stirring is continued for 16 hours. After cooling to 0-5°C, 120 kg (1319 mol) of 3-amino-1,2-propanediol are added. Then the reaction medium is heated, distilling off methanol in vacuum, until a temperature of 60-65°C is reached. The concentrate is kept for 16 hours at this temperature under vacuum. After the reaction is complete, the medium is diluted with 607 kg of water while cooling to room temperature. The solution of the crude complex of formula (II) is neutralized with 20% hydrochloric acid solution (w/w). Thus, 978.6 kg of solution with a concentration of 10.3% are obtained, which is 101 kg of material. The obtained yield is 86.5%.

Tests for the conversion of isomers from complexes of formula (II)

Isomers of the complex of formula (II) were synthesized from the isoA, isoB, isoC and isoD groups of isomers of the hexacid complex of formula (I), isolated by preparative HPLC. Four groups of isomers were isolated and then amidated with 3-amino-1,2-propanediol (APD) with R- and S-configuration. Eight isomers were thus obtained:

isoA + APD(R) and isoA + APD(S),

isoB + APD(R) and isoB + APD(S),

isoC + APD(R) and isoC + APD(S) and

isoD + APD(R) and isoD + APD(S).

Each of these isomers was placed under conditions that ensure isomerization of the complex of gadolinium and hexacid of formula (I).

Thus, an HCl solution with pH 3 is prepared by diluting 1 ml of 1 N. HCl in 1 liter of water. The isomers are added to an HCl solution with pH 3 at a concentration of 1 mM. 10 mg of powder is dissolved in 10 ml of this solution. The resulting eight solutions were heated to 100°C and then analyzed at T ₀ and T ₀ + 23 hours using HPLC.

The percentage purity values measured by HPLC are given in the table below.

isoA+APD(S) isoB + APD(S) isoC+APD(S) ISOD + APD(S) _T0 95.4% 92.3% 91% 98.6% T ₀ + 23 h. 86% 83% 84% 92%

Loss of purity occurs due to chemical decomposition (hydrolysis of amide functional groups) of the product due to the conditions imposed by the isomerization reaction.

Since the conditions allowing the isomerization of the various compounds lead to significant chemical decomposition of the products due to the hydrolysis of the amide functional groups, the isomerization cannot be carried out in a pure and selective manner directly on the complex of formula (II) obtained in accordance with the method described in EP 1931673.

Claims (51)

1. Gadolinium hexacid complex of formula (I) (I), consisting of at least 80% diastereoisomeric excess of the diastereoisomers I-RRR and I-SSS of the formulas: (I-SSS), (I-RRR). 2. The complex according to claim 1, wherein it is characterized by at least 90% of the specified diastereoisomeric excess. 3. Complex according to claim 1 or 2, where the ratio of isomers I-RRR and I-SSS is in the range from 55/45 to 45/55. 4. Complex of formula (II) (II), consisting of at least 80% diastereoisomeric excess of diastereoisomers II-RRR and II-SSS of the formulas: (II-SSS), (II-RRR). 5. The complex according to claim 4, characterized in that it consists of at least 92% of the specified diastereoisomeric excess. 6. Complex according to claim 4 or 5, where the ratio of isomers II-RRR and II-SSS is in the range from 55/45 to 45/55. 7. Complex according to paragraphs. 4-6, obtained by amidation of a complex of gadolinium and hexacid of formula (I) according to any one of paragraphs. 1-3 3-amino-1,2-propanediol. 8. Method for preparing the complex of formula (II) presented below: (II), including the following sequential stages: a) providing complexation of the hexacid of formula (III) presented below: (III), with gadolinium to obtain a complex of gadolinium and hexacid of formula (I) according to claim 1, b) providing isomerization by heating the complex of gadolinium and a hexacid of formula (I) in aqueous solution at a pH value of from 2 to 4 to obtain a diastereomeric enriched complex characterized by at least 80% diastereomeric excess of a mixture of isomers I-RRR and I-SSS of said gadolinium complex and hexacids of formula (I), and c) forming a complex of formula (II) by reacting the diastereoisomerically enriched complex obtained in step b) with 3-amino-1,2-propanediol. 9. Method according to claim 8, characterized in that the aqueous solution in step b) contains acetic acid. 10. The method according to claim 8 or 9, where step b) is carried out at a temperature between 80°C and 130°C for a period of time from 12 hours to 48 hours. 11. The method according to claim 10, where step b) is carried out at a temperature between 100°C and 120°C for a period of time from 12 to 48 hours. 12. Method according to claim 8 or 9, characterized in that upon completion of step b) the diastereoisomerically enriched complex is isolated by crystallization. 13. The method according to claim 12, characterized in that the diastereoisomerically enriched complex from step b) isolated by crystallization is purified by recrystallization to obtain a diastereoisomerically enriched and purified complex. 14. The method according to claim 12 or 13, characterized in that the diastereoisomerically enriched complex from step b) is further enriched by ensuring selective decomposition of complexes of diastereoisomers of the complex of formula (I), other than the diastereoisomers I-RRR and I-SSS, i.e. by providing selective decomposition of the diastereoisomer complexes I-RSS, I-SRR, I-RSR, I-SRS, I-RRS and I-SSR. 15. Method according to any one of paragraphs. 8-14, characterized in that stage c) includes the following sequential stages: c1) formation of a triester of formula (VIII) (VIII), where R ₁ represents a (C ₁ -C ₆ )alkyl group, in particular by reacting in an alcohol of the formula R ₁ OH in the presence of an acid such as hydrochloric acid, and c2) causing aminolysis of the triester of formula (VIII) with 3-amino-1,2-propanediol, in particular in an alcohol of formula R ₁ OH in the presence of an acid such as hydrochloric acid. 16. Method according to claim 15, characterized in that the triester of formula (VIII) is not isolated between steps c1) and c2). 17. Method according to claim 15 or 16, characterized in that R ₁ represents a methyl group. 18. Method according to any one of paragraphs. 8-17, characterized in that the hexacid of formula (III) defined in claim 8 is obtained by alkylation of picklene of formula (V) (V) dibutyl-2-bromoglutarate to give butyl hexaester of formula (VI) (VI) followed by a hydrolysis step resulting in said hexacid of formula (III). 19. Complex of formula (II) (II), consisting of at least 80% diastereoisomeric excess of diastereoisomers II-RRR and II-SSS of the formulas: (II-SSS), (II-RRR), which can be obtained by the method according to any one of claims. 8-18. 20. The complex according to claim 19, characterized in that it consists of at least 92% of the specified diastereoisomeric excess. 21. Complex according to claim 19 or 20, where the ratio of isomers II-RRR and II-SSS is in the range from 55/45 to 45/55. 22. Butyl hexaester of formula (VI) given below: (VI).