Preparation method of gadotetrol isomer

Abstract

The invention discloses a preparation method of a gadotetrol isomer. The method comprises the following steps: by taking cycleanine as an initial raw material, firstly reacting with tert-butyl bromoacetate to obtain a trisubstituted intermediate; then in the presence of a reducing agent, carrying out reductive amination reaction with hydroxyacetone, and directionally introducing a 1-hydroxy-propyl-2-yl side chain; and removing a protecting group through alkaline hydrolysis, carrying out chelation reaction with a gadolinium source, and purifying to obtain the gadotetrol isomer. The method adopts a reductive amination strategy to solve the problem of regioselectivity, is combined with an alkaline hydrolysis process, has the advantages of short synthesis route, high total yield, mild reaction conditions, simplicity and convenience in operation and the like, effectively avoids the use of high-pressure hydrogenation and highly toxic reagents, and is suitable for industrial preparation.

Description

Technical Field

[0001] This invention relates to a method for preparing pharmaceutical impurities, and more particularly to a method for preparing gadoterol isomers. Background Technology

[0002] Gadoteridol is a nonionic macrocyclic gadolinium chelate widely used clinically as a contrast agent in magnetic resonance imaging (MRI). During the synthesis of gadoteridol, due to differences in substitution sites on the macrocyclic amine and the steric effect of the reactants, a specific positional isomer impurity (i.e., the gadoteridol isomer described in this invention, with the structural formula shown below) is readily generated:

[0003] .

[0004] This isomer is listed and clearly defined in the United States Pharmacopeia (USP) and is a key related substance that must be strictly monitored in the quality control of gadoteryl alcohol raw materials and preparations. Therefore, the targeted synthesis and obtaining of high-purity gadoteryl alcohol isomer reference standards are of great significance for establishing accurate quality analysis methods and ensuring the safety of clinical medication.

[0005] However, existing methods for synthesizing this isomer have significant drawbacks, primarily in the difficulty of controlling regioselectivity:

[0006] The most conventional synthetic method in the prior art involves ring-opening reaction of DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) or its derivatives with propylene oxide, or nucleophilic substitution with 1-halo-2-propanol. However, due to steric hindrance and electronic effects, the secondary amine nitrogen atom on the macrocycle tends to attack the less sterically hindered terminal carbon atom, resulting in the formation of the main product, gadoterol (2-hydroxypropyl side chain), while the yield of the target isomer (1-hydroxy-propyl-2-yl side chain) is extremely low (typically less than 2%). Because these two isomers are structurally very similar, they are extremely difficult to effectively separate and enrich using conventional crystallization or chromatographic methods.

[0007] To circumvent the aforementioned selectivity issues, the literature (J. Chem. Soc. Perkin Trans. 1, 1991, 3329) employs a complex "full protection-deprotection" strategy: first, three benzyl protecting groups are introduced, then a side chain is introduced, and finally, the benzyl groups are removed and an acetate group is introduced. This route involves more than six steps, with an extremely low overall yield (<20%), and involves high-pressure hydrogenation and the use of highly toxic chloroacetic acid, which does not meet the safety and environmental protection requirements for industrial production. Existing technologies (such as CN118702638A) attempt to introduce the target structure by reducing the ethyl ester side chain with lithium borohydride, but this reduction reaction lacks chemoselectivity and easily leads to over-reduction, generating diol impurities, making purification difficult. Furthermore, although reductive amination can theoretically be used to introduce side chains, when directly reacting DO3A free acid with hydroxyacetone, the reaction conversion rate is extremely low and accompanied by numerous side reactions due to the poor solubility of the substrate in organic solvents and interference from intramolecular zwitterions, making it difficult to obtain the target product.

[0008] In summary, existing technologies lack a method for synthesizing gadoterol isomers that offers high regioselectivity, high yield, and simple operation. Therefore, developing a novel process that can directionally lock the isomer structure from the reaction mechanism while avoiding cumbersome protection steps and high-risk operations is a pressing technical challenge in this field. Summary of the Invention

[0009] To solve the above-mentioned technical problems, this invention prepares a certain amount of high-purity gadoterol isomers for scientific research, which can also be used to detect gadoterol and its products, control their quality, and use the high-purity gadoterol isomers as a reference standard in the detection process.

[0010] Therefore, the present invention provides a method for preparing gadoterol isomers, the preparation method of which is as follows:

[0011] Step 1: In the presence of a first basic reagent, cyclohexanetin is reacted with compound A in an N-alkylation reaction to obtain intermediate 1;

[0012] The general structural formula of compound A is: ;

[0013] Where X is selected from halogens, and R1 is a carboxyl protecting group;

[0014] Step 2: In the presence of a reducing agent, intermediate 1 is subjected to a reducing amination reaction with hydroxyacetone to obtain intermediate 2;

[0015] Step 3: The intermediate 2 is subjected to a hydrolysis reaction to remove the protecting group R1, thereby obtaining intermediate 3;

[0016] Step 4: The intermediate 3 is chelated with the gadolinium source to obtain the gadoterol isomer.

[0017] Specifically, in compound A: X is selected from chlorine, bromine or iodine; R1 is selected from C1-C6 alkyl groups, or from tert-butyl, benzyl or p-methoxybenzyl.

[0018] Specifically, compound A is tert-butyl bromoacetate.

[0019] Specifically, the first alkaline reagent in step one is selected from sodium acetate, potassium acetate, or ammonium acetate; the reaction solvent in step one is N,N-dimethylformamide (DMF). Further, the first alkaline reagent is sodium acetate, potassium acetate, or ammonium acetate.

[0020] Specifically, the reductive amination reaction described in step two is performed as follows:

[0021] First, intermediate 1 is mixed and stirred with hydroxyacetone in a solvent to form an imine intermediate, and then a reducing agent is added to carry out the reaction.

[0022] The reducing agent is selected from sodium cyanoborohydride (NaBH3CN) or sodium triacetoxyborohydride (STAB).

[0023] The solvent is selected from methanol, ethanol or isopropanol.

[0024] Specifically, the hydrolysis reaction described in step three is an alkaline hydrolysis using an alkaline reagent; the alkaline reagent is selected from sodium hydroxide or potassium hydroxide.

[0025] Specifically, the gadolinium source mentioned in step four is selected from gadolinium oxide or gadolinium chloride; the purification method of the gadolinium alcohol isomer is as follows: the reaction solution is adjusted to neutral pH and then concentrated, and the residue is added to a mixed solvent of isopropanol and water for recrystallization.

[0026] Specifically, the structures of intermediate 2 and intermediate 3 are as follows: Intermediate 2: 1,4,7-tris(tert-butoxycarbonylmethyl)-10-(1-hydroxy-prop-2-yl)-1,4,7,10-tetraazacyclododecane;

[0027] Intermediate 3: 1,4,7-tris(carboxymethyl)-10-(1-hydroxy-propyl-2-yl)-1,4,7,10-tetraazacyclododecane.

[0028] A method for preparing gadoterol isomers, the synthetic route is as follows:

[0029] In a preferred embodiment of the present invention, the synthetic route is as follows:

[0030] In a preferred embodiment of the present invention, the synthetic route is as follows:

[0031]

[0032] Includes the following steps:

[0033] Step 1: Mix cyclohexane with sodium acetate in DMF, add tert-butyl bromoacetate dropwise, and react at 20~40℃. After the reaction is complete, add sodium bicarbonate solution dropwise to crystallize and obtain intermediate 1.

[0034] Step 2: Mix intermediate 1 with hydroxyacetone in methanol and stir, then add sodium cyanoborohydride and react at 20~30℃ to obtain intermediate 2;

[0035] Step 3: Add sodium hydroxide to intermediate 2 in a mixed solvent of water and methanol to carry out a hydrolysis reaction. After the reaction is completed, adjust the pH to acidic to obtain an aqueous solution of intermediate 3.

[0036] Step 4: Add gadolinium oxide to the aqueous solution of intermediate 3 and carry out a chelation reaction at 85~100℃. After the reaction is completed, adjust the pH to neutral, concentrate and recrystallize with isopropanol to obtain the gadoterol isomer.

[0037] Technical effects of the present invention

[0038] 1. Existing technologies typically employ ring-opening of propylene oxide or alkylation of 1-halo-2-propanol to introduce side chains. However, due to steric hindrance and electronic effects, the reaction tends to generate the main product (2-hydroxypropyl side chain), resulting in extremely low yields (<2%~8%) of the target isomer (1-hydroxy-propyl-2-yl side chain), which is also extremely difficult to separate. This invention creatively introduces a reductive amination strategy, utilizing the structural characteristic of the carbonyl group of the starting material 3 (hydroxyacetone) located at the C2 position, forcing the nitrogen atom of the macrocyclic secondary amine to undergo condensation and reduction only at the C2 position. This mechanism locks the connection between the nitrogen atom and the C2 position of the side chain from the source, avoiding the generation of regioisomers and allowing for the acquisition of high-purity target isomers without expensive preparative chromatographic separation.

[0039] 2. Compared with the fully protected-deprotected route (overall yield <20%) and the direct alkylation route (overall yield <12%) reported in existing literature, this invention significantly reduces side reactions through an optimized sodium acetate / DMF skeleton construction process and a highly efficient reductive amination system. Experimental data show that the overall yield of gadoterol isomers prepared by this invention can reach over 50% (up to ~59% in preferred embodiments), achieving a several-fold increase in yield and possessing significant industrial application value.

[0040] 3. This invention develops a mild alkaline hydrolysis process. Compared to acid hydrolysis (which requires repeated deacidification and desalination), the intermediate solution after alkaline hydrolysis can be directly pH-adjusted for subsequent gadolinium oxide chelation reaction (one-pot method). This process not only shortens the production cycle but also effectively avoids the interference of chloride ions, trifluoroacetate ions, and other anions on the purity of the final product, allowing the final product to achieve pharmacopoeia reference grade purity (HPLC>98%) after simple recrystallization.

[0041] 4. This invention avoids high-pressure hydrogenation operations and the use of highly toxic reagents (such as chloroacetic acid and propylene oxide) throughout the entire process. All reactions are carried out at atmospheric pressure and within a mild temperature range of 0°C to 95°C. The reagents used (such as tert-butyl bromoacetate, hydroxyacetone, and sodium cyanoborohydride) are all low-toxicity, readily available, and conventional chemical raw materials, significantly reducing EHS (environmental, health, and safety) risks and meeting the requirements of modern green pharmaceutical manufacturing. Attached Figure Description

[0042] Figure 1 The image shows the 1H NMR spectrum (1H-NMR, solvent D2O) of intermediate 3 prepared in Example 1 of this invention. Detailed Implementation

[0043] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0044] In this invention, unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0045] In this invention, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0046] In this invention, unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0047] Example 1

[0048]

[0049] Step 1: Add 20 g (116 mmol) of cyclohexanetin and 100 ml of DMF to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for crystallization for 2–6 h. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 92%).

[0050] Step 2: Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and methanol (100 ml) were added to a reaction flask. After stirring for 1 h, sodium cyanoborohydride (1.9 g, 30 mmol) was added, and the reaction was maintained at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and the concentrate was subjected to column chromatography to obtain intermediate 2 (yield: 83%).

[0051] Specifically, the column chromatography method uses 200-300 mesh silica gel as packing material. After the concentrated solution is wet-loaded, it is eluted with eluent (dichloromethane / methanol = 50 / 1 → 20 / 1), and the corresponding fractions are collected and distilled under reduced pressure.

[0052] Step 3: Add intermediate 2 (8.6 g, 15 mmol), methanol (60 ml), and drinking water (40 ml) to the reaction flask, stir, then add alkali (90 mmol), and incubate at 25 ± 5 °C for 16 h before stopping the reaction. Slowly add hydrochloric acid to adjust the pH to ~3 to obtain intermediate 3 solution.

[0053] like Figure 1 As shown, the 1H-NMR spectrum of intermediate 3 reveals a distinct doublet (3H) in the high-field region (~1.1 ppm), attributed to the methyl group (-CH3) on the isomer side chain; multiple overlapping signal peaks appear in the mid-field region (2.9-3.8 ppm), with a total integrated area of ​​approximately 25 ppm. This corresponds to the methylene group (16H) on the ring-ring, the methylene group (6H) on the three acetic acid side chains, and the methylene and methine groups (3H) on the isomer side chains. The spectrum shows approximately 28 aliphatic protons, and no characteristic peak of the tert-butyl group (usually a singlet at 1.4-1.5 ppm, 9H or 27H) was observed, confirming that the tert-butyl ester protecting group has been completely hydrolyzed, and the structure of the resulting product is completely consistent with the theoretical structure of intermediate 3 (1,4,7-tricarboxymethyl-10-(1-hydroxy-propyl-2-yl)-1,4,7,10-tetraazacyclododecane).

[0054] Step 4: At a controlled temperature of 25±5℃, add gadolinium oxide (2.7 g, 7.5 mmol), and maintain the temperature at 95±5℃ for 8 hours. Stop the reaction when the content of intermediate 3 is <2% as monitored by HPLC. Cool to room temperature and adjust the pH to 7.0-7.5 by adding 10% sodium hydroxide solution dropwise. Filter, and distill the filtrate to dryness under reduced pressure. Add purified water (10 ml) and isopropanol (10 ml) to the residue, maintain the temperature at 75±5℃, stir until dissolved, and add isopropanol (100 ml) dropwise. After the addition is complete, continue stirring for 1 hour, slowly cool to 25±5℃, and continue stirring for 2 hours. Filter, wash the filter cake with isopropanol (20 ml), and dry the filter cake to obtain the isomer (2-step yield: 77%).

[0055] Example 2

[0056] Add 20 g (116 mmol) of cyclohexanetin and 100 ml of DMF to a reaction flask, stir until dissolved, then add anhydrous potassium carbonate (56.2 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for 2–6 h to allow crystals to crystallize. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 81%).

[0057] Example 3

[0058] Add 20 g (116 mmol) of cyclohexanetin and 100 ml of acetonitrile to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for 2–6 h to allow crystals to crystallize. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 73%).

[0059] Example 4

[0060] Add 20 g (116 mmol) of cyclohexanetin and 100 ml of N,N-dimethylacetamide to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for 2–6 h to allow crystals to crystallize. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 84%).

[0061] Example 5

[0062] Add 20 g (116 mmol) of cyclohexane and chloroform (100 ml) to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for 2–6 h to allow crystals to crystallize. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 54%).

[0063] Example 6

[0064] Add 20 g (116 mmol) of cyclohexanetin and 100 ml of DMF to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 60 ± 5 °C for 24 h, then stop the reaction. Control the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for crystallization for 2–6 h. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 86%).

[0065] Example 7

[0066] Add 20 g (116 mmol) of cyclohexanetin and 100 ml of DMF to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 0 ± 5 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for 2–6 h to allow crystals to crystallize. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 26%).

[0067] Example 8

[0068] Add 20 g (116 mmol) of cyclohexanetin and 100 ml of DMF to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 67.7 g, 348 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for 2–6 h to allow crystals to crystallize. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 55%).

[0069] Example 9

[0070] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and 1,2-dichloroethane (100 mL) were added to a reaction flask. After stirring for 1 h, sodium triacetoxyborohydride (STAB, 6.36 g, 30 mmol) was added, and the reaction was maintained at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 78%).

[0071] Example 10

[0072] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and methanol (100 mL) were added to a reaction flask. After stirring for 1 h, sodium borohydride (1.13 g, 30 mmol) was added, and the reaction was maintained at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 45%).

[0073] Example 11

[0074] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and a methanol / acetic acid mixture (9:1, 100 mL) were added to a reaction flask. After stirring for 1 h, 2-methylpyridineborane (3.2 g, 30 mmol) was added, and the reaction was maintained at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared under TLC monitoring. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 53%).

[0075] Example 12

[0076] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), 10% Pd / C catalyst, and methanol (100 ml) were added to a high-pressure reactor. After purging with hydrogen, the reactor was purged with hydrogen to 5 bar and reacted at 25 ± 5 °C for 16 h. TLC monitoring showed extremely poor conversion. The catalyst was removed by filtration, and the concentrate was obtained by vacuum distillation. The concentrate was then subjected to column chromatography to obtain intermediate 2 (yield: 31%).

[0077] Example 13

[0078] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 5.8 g, 80 mmol), and methanol (100 mL) were added to a reaction flask. After stirring for 1 h, sodium cyanoborohydride (1.9 g, 30 mmol) was added, and the reaction was maintained at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and the concentrate was subjected to column chromatography to obtain intermediate 2 (yield: 72%).

[0079] Example 14

[0080] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and methanol (100 mL) were added to a reaction flask. After stirring for 1 h, sodium cyanoborohydride (1.9 g, 30 mmol) was added, and the mixture was cooled to 0 ± 5 °C and reacted for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 42%).

[0081] Example 15

[0082] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and methanol (100 mL) were added to a reaction flask. After stirring for 1 h, sodium cyanoborohydride (1.9 g, 30 mmol) was added, and the mixture was heated to 50 ± 5 °C and reacted for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 65%).

[0083] Example 16

[0084] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), sodium cyanoborohydride (1.9 g, 30 mmol), and methanol (100 mL) were added sequentially to a reaction flask, and the mixture was reacted at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 68%).

[0085] Example 17

[0086] Intermediate 1 (10.3 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and isopropanol (100 mL) were added to a reaction flask. After stirring for 1 h, sodium cyanoborohydride (1.9 g, 30 mmol) was added, and the reaction was maintained at 25 ± 5 °C for 16 h. The reaction was stopped after intermediate 1 disappeared as monitored by TLC. The concentrate was obtained by vacuum distillation, and column chromatography of the concentrate yielded intermediate 2 (yield: 77%).

[0087] Example 18

[0088] Intermediate 2 (8.6 g, 15 mmol) was dissolved in a mixed solution of dichloromethane (30 ml) and trifluoroacetic acid (30 ml), and the mixture was stirred at room temperature for 6 h. After the reaction was completed by TLC monitoring, the solvent and excess trifluoroacetic acid were removed by rotary evaporation under reduced pressure. The residue was added to drinking water (50 ml) and the pH was adjusted to 3 to obtain intermediate 3 solution.

[0089] At a controlled temperature of 25±5℃, gadolinium oxide (2.7 g, 7.5 mmol) was added, and the temperature was raised to 95±5℃ for 8 h. The reaction was stopped when the content of intermediate 3 was <0.1% by HPLC monitoring. The mixture was cooled to room temperature, and the pH was adjusted to 7.0-7.5 by adding 10% sodium hydroxide solution dropwise. The mixture was filtered, and the filtrate was distilled to dryness under reduced pressure. The residue was added to purified water (10 ml) and isopropanol (10 ml), and the mixture was kept at 75±5℃ with stirring until dissolved. Isopropanol (100 ml) was added dropwise, and stirring was continued for 1 h after the addition was complete. The temperature was then slowly lowered to 25±5℃, and stirring was continued for 2 h. The mixture was filtered, and the filter cake was washed with isopropanol (20 ml). The filter cake was dried to obtain the isomer (two-step yield: 56%).

[0090] Example 19

[0091] Intermediate 2 (8.6 g, 15 mmol) was added to 6 M hydrochloric acid / isopropanol solution (60 ml) and reacted at 60 ± 5 °C for 6 h. After the reaction was complete as monitored by TLC, the solution was concentrated under reduced pressure, and the residue was added to drinking water (50 ml) to adjust the pH to 3, yielding intermediate 3 solution. Subsequent operations (chelation and purification) were the same as in Example 18 (two-step yield: 61%).

[0092] Example 20

[0093] Intermediate 2 (8.6 g, 15 mmol), methanol (60 ml), and drinking water (40 ml) were added to a reaction flask. After stirring, sodium hydroxide (3.6 g, 90 mmol) was added, and the mixture was heated to 50 ± 5 °C and reacted for 6 h before stopping the reaction. Hydrochloric acid was slowly added dropwise to adjust the pH to ~3 to obtain intermediate 3 solution. Subsequent operations (chelation and purification) were the same as in Example 18 (two-step yield: 73%).

[0094] Example 21

[0095] Following the hydrolysis step in Example 1, after obtaining intermediate 3 solution, gadolinium oxide (2.7 g, 7.5 mmol) was added at a controlled temperature of 25 ± 5 °C, and the temperature was raised to 50 ± 5 °C for 24 h. The mixture was then cooled to room temperature, and the pH was adjusted to 7.0–7.5 by adding 10% sodium hydroxide solution dropwise. The mixture was filtered, and the filtrate was distilled to dryness under reduced pressure. The residue was added to purified water (10 ml) and isopropanol (10 ml), kept at 75 ± 5 °C, stirred until dissolved, and then isopropanol (100 ml) was added dropwise. After the addition was complete, stirring was continued for 1 h, and the temperature was slowly lowered to 25 ± 5 °C, with stirring continued for 2 h. The mixture was filtered, and the filter cake was washed with isopropanol (20 ml). The filter cake was dried to obtain the product (two-step yield: 59%).

[0096] Example 22

[0097] Following the hydrolysis step in Example 1, after obtaining the intermediate 3 solution, the temperature was controlled at 25±5℃, and gadolinium oxide (2.7 g, 7.5 mmol) was added. The temperature was raised to 95±5℃ and the reaction was carried out for 8 hours. The reaction was stopped when the content of intermediate 3 was <0.1% as monitored by HPLC. The temperature was lowered to room temperature, and the pH was adjusted to 7.0~7.5 by adding 10% sodium hydroxide solution dropwise. The mixture was filtered, and the filtrate was distilled to dryness under reduced pressure. The residue was added to purified water (10 ml) and ethanol (10 ml), kept at 75±5℃, stirred until dissolved, and ethanol (100 ml) was added dropwise. After the addition was complete, the mixture was stirred for 1 hour, slowly cooled to 25±5℃, and stirred for 2 hours. The mixture was filtered, and the filter cake was washed with ethanol (20 ml). The filter cake was dried to obtain the isomer (two-step yield: 55%).

[0098] Example 23

[0099] Following the hydrolysis step in Example 1, after obtaining the intermediate 3 solution, the temperature was controlled at 25±5℃, and gadolinium oxide (2.7 g, 7.5 mmol) was added. The temperature was raised to 95±5℃ and the reaction was carried out for 8 hours. The reaction was stopped when the content of intermediate 3 was <0.1% as monitored by HPLC. The temperature was lowered to room temperature, and the pH was adjusted to 7.0~7.5 by adding 10% sodium hydroxide solution dropwise. The mixture was filtered, and the filtrate was distilled to dryness under reduced pressure. The residue was added to purified water (10 ml) and acetone (10 ml), kept at 50±5℃, stirred until dissolved, and acetone (100 ml) was added dropwise. After the addition was complete, stirring was continued for 1 hour, and the temperature was slowly lowered to 25±5℃. Stirring was continued for 2 hours. The mixture was filtered, and the filter cake was washed with acetone (20 ml). The filter cake was dried to obtain the isomer (two-step yield: 49%).

[0100] Example 24

[0101] Following the hydrolysis step in Example 1, after obtaining the intermediate 3 solution, the temperature was controlled at 25±5℃, and gadolinium oxide (2.97 g, 8.25 mmol) was added. The temperature was raised to 95±5℃ and the reaction was carried out for 8 hours. The reaction was stopped when the content of intermediate 3 was <0.1% as monitored by HPLC. The temperature was lowered to room temperature, and the pH was adjusted to 7.0~7.5 by adding 10% sodium hydroxide solution dropwise. The mixture was filtered, and the filtrate was distilled to dryness under reduced pressure. The residue was added to purified water (10 ml) and isopropanol (10 ml), kept at 75±5℃, stirred until dissolved, and isopropanol (100 ml) was added dropwise. After the addition was complete, stirring was continued for 1 hour, and the temperature was slowly lowered to 25±5℃, and stirring was continued for 2 hours. The mixture was filtered, and the filter cake was washed with isopropanol (20 ml). The filter cake was dried to obtain the isomer (two-step yield: 75%).

[0102] Comparative Example 1

[0103] DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, 10.3 g, 20 mmol) was dissolved in drinking water (50 ml), and the pH was adjusted to 10.0 ± 0.2 by adding 10% sodium hydroxide solution dropwise with stirring. The temperature was maintained at 45 ± 5 °C, and propylene oxide (3.5 g, 60 mmol) was slowly added dropwise. After the addition was complete, the reaction was maintained at this temperature for 24 h. Sodium hydroxide solution was added continuously during the reaction to maintain the pH at approximately 10. After complete conversion of the starting material DO3A by HPLC, gadolinium oxide (3.6 g, 10 mmol) was added, and the temperature was raised to 95 ± 5 °C for 6 h for chelation. After the reaction was complete, the reaction solution was cooled to room temperature, and unreacted gadolinium oxide was removed by filtration. The filtrate was concentrated to dryness under reduced pressure.

[0104] HPLC analysis of residues: The peak area of ​​the main product gadoterol (2-hydroxypropyl side chain) accounted for 98.5%, while the peak area of ​​the target isomer (1-hydroxypropyl-2-yl side chain) accounted for only 1.4%. Due to the low content of the isomer and its retention time being very close to that of the main peak, it was impossible to obtain a reference standard with a purity >95%.

[0105] Comparative Example 2

[0106] Step 1: Dissolve 10.0 g (58 mmol) of cyclohexane in chloroform (100 ml), add trichloroacetaldehyde (25.6 g, 174 mmol), and stir at room temperature for 24 h. Reduce the solvent by rotary evaporation, and purify the residue by silica gel column chromatography (eluent: dichloromethane / methanol 20:1) to obtain a triformyl intermediate (11.5 g, yield 77%).

[0107] Step 2: Dissolve the above intermediate (5.1 g, 20 mmol) in acetonitrile (50 ml), add potassium carbonate (5.5 g, 40 mmol) and 1-bromo-2-propanol (3.3 g, 24 mmol), and reflux for 48 h. Filter, evaporate the filtrate to dryness, and perform column chromatography purification again to obtain the monosubstituted intermediate (3.8 g, yield 60%).

[0108] Step 3: Dissolve the product from Step 2 in 6M hydrochloric acid (30 ml) and reflux for 12 h to remove the formyl group. Adjust the pH to alkaline, extract, dry, and evaporate to dryness. Dissolve the resulting oil in DMF (40 ml), add potassium carbonate (6.2 g, 45 mmol) and tert-butyl bromoacetate (8.8 g, 45 mmol), and react at 50 °C for 24 h.

[0109] Step 4: Process as in Example 1.

[0110] The process was lengthy and involved two column chromatography purification steps with extremely low yields. Finally, 1.2 g of the target isomer gadolinium complex was obtained, with a total yield (based on cyclotinocyanine) of 11.8%.

[0111] Comparative Example 3

[0112] The precursor compound was prepared based on CN118702638A: 1,4,7-tris(tert-butoxycarbonylmethyl)-10-(ethoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (i.e., DO3A-ethyl ester derivative, 12.0 g, 20 mmol).

[0113] The precursor was dissolved in anhydrous tetrahydrofuran (120 ml) and cooled to 0 ± 5 °C under nitrogen protection. Lithium borohydride (LiBH4, 0.88 g, 40 mmol) was added in portions, and the mixture was allowed to rise naturally to 25 °C for 12 h. TLC showed that the starting material spot did not completely disappear and several impurity spots appeared. The reaction was quenched by slowly adding saturated ammonium chloride solution (50 ml) and stirring vigorously for 30 min. The mixture was extracted with ethyl acetate (50 ml × 3). The organic phases were combined, dried over anhydrous sodium sulfate, filtered, and concentrated to give a pale yellow oil (9.5 g).

[0114] The target alcohol (i.e., the product of ethyl ester reduction) accounted for only 35% of the product. The main byproducts included diol impurities (approximately 25%) generated from the reduction of tert-butyl ester, and intramolecular amidation products (approximately 20%) formed by macrocyclic amines attacking ethyl ester. Despite repeated purification, the crude product could not yield an intermediate of acceptable purity, indicating that LiBH4 lacked sufficient chemoselectivity.

[0115] Comparative Example 4

[0116] Step 1: Add 20 g (116 mmol) of cyclohexanetin and 100 ml of DMF to a reaction flask, stir until dissolved, then add sodium acetate (33.3 g, 407 mmol), stir for 10 min, and then add starting material 2 (tert-butyl bromoacetate, 79.0 g, 407 mmol) dropwise. Maintain the reaction temperature at 30 ± 10 °C for 24 h, then stop the reaction. Maintain the temperature at 10 ± 10 °C, add 43% sodium bicarbonate solution (400 ml) dropwise, and maintain the temperature for crystallization for 2–6 h. Filter, wash the filter cake with drinking water (400 ml), and dry the filter cake to obtain intermediate 1 (yield: 92%).

[0117] Step 2: Add intermediate 1 (10.3 g, 20 mmol), acetonitrile (100 ml), and anhydrous potassium carbonate (5.5 g, 40 mmol) to a reaction flask, and add 1-bromo-2-propanol (3.3 g, 24 mmol) dropwise with stirring. Heat to reflux (80 ± 5 °C) and react for 24 h. Stop the reaction after TLC monitoring. Filter to remove inorganic salts, and distill the filtrate under reduced pressure to obtain a concentrated solution (at this point, it is a mixture of isomers; no column chromatography separation was performed, and it was directly added to the next step).

[0118] Step 3: Add the concentrated solution obtained in Step 2, methanol (60 ml), and drinking water (40 ml) to the reaction flask, stir, and then add sodium hydroxide (3.6 g, 90 mmol). Keep the mixture at 50 ± 5 °C for 6 hours and then stop the reaction. Slowly add hydrochloric acid to adjust the pH to ~3 to obtain intermediate 3 solution (mixture).

[0119] Step 4: Maintain a temperature of 25±5℃, add gadolinium oxide (2.7 g, 7.5 mmol), and react at 95±5℃ for 8 hours. Cool to room temperature, and adjust the pH to 7.0-7.5 by adding 10% sodium hydroxide solution dropwise. Filter, and distill the filtrate under reduced pressure to dryness. Add purified water (10 ml) and isopropanol (10 ml) to the residue, maintain a temperature of 75±5℃, stir until dissolved, and add isopropanol (100 ml) dropwise to induce crystallization. Filter and dry to obtain the solid product.

[0120] HPLC analysis showed that the main product, gadoterol (2-hydroxypropyl side chain), accounted for >90% of the solid product, while the target isomer (1-hydroxypropyl-2-yl side chain) accounted for <8%. This confirms that the direct alkylation method is sterically hindered and cannot effectively prepare the target isomer.

[0121] Comparative Example 5

[0122] Step 1: Add DO3A (free acid, 6.9 g, 20 mmol), starting material 3 (hydroxyacetone, 2.9 g, 40 mmol), and a methanol / water mixture (1:1, 100 ml) to a reaction flask, and adjust the pH to 6.0–6.5 with acetic acid. After stirring for 1 h, add sodium cyanoborohydride (1.9 g, 30 mmol), and incubate at 25 ± 5 °C for 24 h.

[0123] Step 2: HPLC and TLC monitoring showed that >70% of the DO3A in the raw material remained, and a large amount of 1,2-propanediol impurities were generated. The solvent was removed by vacuum distillation, and the residue was attempted to be purified by ion exchange resin. Due to the extremely low reaction conversion rate and the similar properties of the byproducts, an intermediate of acceptable purity could not be obtained, and subsequent chelation reactions could not be carried out. Esterification protection is necessary to improve the lipid solubility of intermediate 1 in order to achieve efficient reductive amination.

[0124] To verify the process stability of the preparation method of the present invention and to confirm the key factors affecting the reaction yield and regioselectivity, the inventors systematically compared and explored the mechanism of the data from Examples 1 to 24 and Comparative Examples 1 to 5.

[0125] Leveraging the regioselectivity advantage of its core synthetic strategy, this invention aims to solve the challenge of targeted synthesis of specific isomers (1-hydroxy-propyl-2-yl side chain). Comparing existing technologies, Comparative Example 1, employing the traditional "propylene oxide ring-opening" route, suffers from steric hindrance and electronic effects, resulting in nucleophilic attack primarily occurring on the terminal carbon of the epoxide, leading to extremely low yields of the target isomer (<1.4%). Comparative Example 4, using the "direct alkylation" route, similarly suffers from steric hindrance due to the significant secondary alkyl halides, resulting in a target product yield of less than 8%. In contrast, this invention creatively employs a reductive amination strategy (step 2), utilizing the C2 position of the carbonyl group in starting material 3 (hydroxyacetone) to force the secondary amine nitrogen atom on the macroring to undergo Schiff base condensation only at the C2 position, subsequently being reduced. This strategy completely locks the connection between the nitrogen atom and the C2 position of the side chain from a reaction mechanism perspective, fundamentally preventing the generation of regioisomers. This not only solves the subsequent problem of isomer separation but also significantly improves atom utilization.

[0126] Regarding the screening and optimization of skeletal construction conditions, comparative experiments showed that the sodium acetate / DMF system (Example 1) was significantly better than potassium carbonate / DMF (Example 2) or other solvent systems in constructing trisubstituted macrocyclic skeletons. Sodium acetate, as a weak base, effectively neutralizes the acid produced in the reaction while avoiding the "tetrasubstituted" side reaction caused by excessive alkalinity. A mild condition of around 30°C is most favorable for monosubstituted reactions; excessively high or low temperatures lead to a decrease in yield. For the critical reductive amination step, the choice of reduction system is crucial. Comparing Example 1 and Example 10, sodium cyanoborohydride exhibited the best effect (yield 83%) due to its selectivity in preferentially reducing imines (C=N) under weak acid / neutral conditions. In contrast, the strong reducing agent sodium borohydride easily reduced the raw material hydroxyacetone directly to propylene glycol impurities, resulting in a significant decrease in yield (45%). Furthermore, the operation mode of pre-stirring to form imines before adding the reducing agent is significantly better than the "one-pot" method of direct addition.

[0127] Regarding the necessity of esterification protection and the improvement of the hydrolysis process, a comparison of Example 1 and Comparative Example 5 revealed that if the carboxyl group is not pre-protected to an ester, direct reductive amination on a free acid substrate results in extremely low feed conversion and generates a large number of impurities, demonstrating the necessity of the "esterification protection first - reductive amination then - hydrolysis last" strategy. In the deprotection step, this invention found that alkaline hydrolysis using sodium hydroxide (Example 1) yielded a better two-step yield (77%) than acidic hydrolysis (Examples 18-19). This is because the TFA or chloride ions introduced by acid hydrolysis interfere with subsequent gadolinium oxide chelation or increase the difficulty of desalination, while alkaline hydrolysis allows for direct pH adjustment for "one-pot" chelation, which is simpler and yields higher results. Finally, in the crystallization purification system, the isopropanol / water system exhibited better crystallization performance than the ethanol or acetone system, effectively removing inorganic salt residues and obtaining a high-purity product. In summary, this invention successfully overcomes the regioselectivity challenge through a reductive amination strategy, and by combining optimized process parameters, it achieves a new preparation route with high yield, high selectivity, and suitability for industrial production.

Claims

1. A method for preparing a gadoterol isomer, characterized in that, Includes the following steps: Step 1: In the presence of a first basic reagent, cyclohexanetin is reacted with compound A in an N-alkylation reaction to obtain intermediate 1; The general structural formula of compound A is: ; Where X is selected from halogens, and R1 is a carboxyl protecting group; Step 2: In the presence of a reducing agent, intermediate 1 is subjected to a reducing amination reaction with hydroxyacetone to obtain intermediate 2; Step 3: The intermediate 2 is subjected to a hydrolysis reaction to remove the protecting group R1, thereby obtaining intermediate 3; Step 4: The intermediate 3 is chelated with the gadolinium source to obtain the gadoterol isomer.

2. The method according to claim 1, characterized in that, In compound A: X is selected from chlorine, bromine or iodine; R1 is selected from C1-C6 alkyl groups, or from tert-butyl, benzyl or p-methoxybenzyl.

3. The method according to claim 2, characterized in that, Compound A is tert-butyl bromoacetate.

4. The method according to claim 1, characterized in that, The first alkaline reagent mentioned in step one is selected from sodium acetate, potassium acetate, or ammonium acetate; the reaction solvent in step one is N,N-dimethylformamide (DMF). Further, the first alkaline reagent is sodium acetate, potassium acetate, or ammonium acetate.

5. The method according to claim 1, characterized in that, The reductive amination reaction described in step two is as follows: First, intermediate 1 is mixed and stirred with hydroxyacetone in a solvent to form an imine intermediate, and then a reducing agent is added to carry out the reaction. The reducing agent is selected from sodium cyanoborohydride (NaBH3CN) or sodium triacetoxyborohydride (STAB). The solvent is selected from methanol, ethanol or isopropanol.

6. The method according to claim 1, characterized in that, The hydrolysis reaction described in step three is an alkaline hydrolysis using an alkaline reagent; the alkaline reagent is selected from sodium hydroxide or potassium hydroxide.

7. The method according to claim 1, characterized in that, The gadolinium source mentioned in step four is selected from gadolinium oxide or gadolinium chloride; the purification method of the gadolinium alcohol isomer is as follows: after adjusting the pH of the reaction solution to neutral, it is concentrated, and the residue is added to a mixed solvent of isopropanol and water for recrystallization.

8. The method according to claim 1, characterized in that, The structures of intermediates 2 and 3 are as follows: Intermediate 2: 1,4,7-tris(tert-butoxycarbonylmethyl)-10-(1-hydroxy-prop-2-yl)-1,4,7,10-tetraazacyclododecane; Intermediate 3: 1,4,7-tris(carboxymethyl)-10-(1-hydroxy-propyl-2-yl)-1,4,7,10-tetraazacyclododecane.

9. The method for preparing gadoterol isomers according to claim 1, characterized in that, The synthesis route is as follows: The operation steps are as follows: Step 1: Mix cyclohexane with sodium acetate in DMF, add tert-butyl bromoacetate dropwise, and react at 20~40℃. After the reaction is complete, add sodium bicarbonate solution dropwise to crystallize and obtain intermediate 1. Step 2: Mix intermediate 1 with hydroxyacetone in methanol and stir, then add sodium cyanoborohydride and react at 20~30℃ to obtain intermediate 2; Step 3: Add sodium hydroxide to intermediate 2 in a mixed solvent of water and methanol to carry out a hydrolysis reaction. After the reaction is completed, adjust the pH to acidic to obtain an aqueous solution of intermediate 3. Step 4: Add gadolinium oxide to the aqueous solution of intermediate 3 and carry out a chelation reaction at 85~100℃. After the reaction is completed, adjust the pH to neutral, concentrate and recrystallize with isopropanol to obtain the gadoterol isomer.

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