Allopurinol purification process
By introducing a pH-gated buffer and a seed-impurity anchor, the selective crystallization of allopurinol is achieved by utilizing the difference in ionization constants, solving the problem of irreconcilable purity and yield in traditional methods and achieving efficient allopurinol purification.
Patent Information
- Application Number
- CN202510939323.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-08
- Publication Date
- 2025-10-21
AI Technical Summary
In the existing technology, it is difficult to maintain high yield while ensuring high purity during the purification process of allopurinol, resulting in limited production efficiency and quality. Traditional methods require multiple crystallization steps, high energy consumption, and large solvent consumption.
By using a pH-gated buffer and a seed-impurity anchor, and taking advantage of the difference in ionization constants between allopurinol and impurities, the selective crystallization of allopurinol and the locking of impurities are achieved through precise control of the pH value and the introduction of a seed-impurity anchor.
The high purity (above 99.95%) and high yield (above 96%) of allopurinol were achieved in a single-step operation, significantly shortening the production cycle, reducing energy consumption and solvent consumption, and meeting the requirements of green and sustainable processes.
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Figure CN120817951A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the field of allopurinol purification, in particular to an allopurinol purification process. Background Art
[0002] Allopurinol is a key drug for the treatment of hyperuricemia and gout. During its synthesis, isomeric impurities, such as 3-aminopyrazole-4-carboxamide, are inevitably produced, sharing a strikingly similar molecular structure, polarity, and molecular weight. These impurities are isomers of allopurinol, with minimal differences in physical properties (such as solubility in common solvents). This creates significant bottlenecks in traditional purification methods (such as recrystallization and extraction).
[0003] Existing technologies typically rely on subtle differences in physical properties between the target product and impurities for separation. For example, repeated recrystallization utilizes a small solubility gradient to gradually enrich the target product. The fundamental flaw of this method is that yield and purity are an irreconcilable contradiction: to achieve higher purity, a large amount of mother liquor must be sacrificed, resulting in a sharp drop in yield; if the yield is to be guaranteed, a considerable portion of impurities will inevitably remain due to co-crystallization with the product, and pharmaceutical grade standards cannot be met. Therefore, industrial production often requires multi-step crystallization, which is a lengthy process, high energy consumption, large solvent consumption, and unsatisfactory overall yield, which directly limits the production efficiency and quality upper limit of allopurinol.
[0004] The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the Invention
[0005] The object of the present invention is to provide a process for purifying allopurinol to solve the problems raised in the above-mentioned background technology.
[0006] The technical solution of the present invention is: an allopurinol purification process for separating the target product from a crude product containing isomeric impurities such as 3-aminopyrazole-4-carboxamide, comprising the following steps: a) dissolving the crude allopurinol in an alkaline aqueous solution having a pH of 11.0-11.5, and filtering under pressure to obtain a clarified allopurinol sodium salt mother liquor; b) adding a pH-gating buffer to the allopurinol sodium salt mother solution to precisely and stably clamp the pH value of the system within the range of 8.4-8.6, and utilizing the difference in ionization constant ΔpKa between allopurinol and the isomeric impurities to convert most of the allopurinol into a neutral form with lower solubility, while maintaining most of the impurities in the form of ionized salts with higher solubility; c) adding a seed crystal-impurity anchoring agent to the system and performing ultrasonic dispersion; one end of the seed crystal-impurity anchoring agent serves as a nucleation site to accelerate the crystallization of the electrically neutral purinol molecules, and the other end anchors the dissolved impurities in the mother liquor through non-covalent bonding to prevent their co-precipitation; d) performing programmed temperature-controlled cooling and constant temperature aging on the system to allow the crystals to fully grow; e) separating the crystalline product by centrifugal filtration, washing and vacuum drying to obtain high-purity allopurinol.
[0007] A further improvement of the present invention is that the pH gating buffer is a functionalized mesoporous silica microsphere with a surface rich in glycine residues.
[0008] A further improvement of the present invention is that the preparation method of the pH gating buffer comprises: a) performing ultrasonic dispersion and reflux dehydration on the mesoporous silica microspheres in toluene to perform activation treatment; b) adding (3-aminopropyl)triethoxysilane to react with the activated microspheres to obtain amino-functionalized silica sphere intermediates; c) using N,N-dicyclohexylcarbodiimide (DCC) as a coupling agent to graft the amino-functionalized silica spheres with N-tert-butyloxycarbonylglycine; d) using a dioxane solution containing 4 M hydrochloric acid to degas the product by stirring to remove the Boc protecting group, thereby obtaining the pH gating buffer.
[0009] A further improvement of the present invention is that the seed-impurity anchor is a bifunctional molecule, one end of which is a pyrazole [3,4-d] pyrimidine analog structure, which serves as a seed; the other end is a polyethylene glycol diamine flexible chain, whose terminal amino group can form a hydrogen bond with the formamide group of the impurity, serving as an impurity anchoring segment.
[0010] A further improvement of the present invention is that the method for preparing the seed crystal-impurity anchoring agent comprises: a) heating pyrazolo[3,4-d]pyrimidine-4-thione and ethyl 6-bromohexanoate under reflux in ethanol to obtain the intermediate 4-(5-ethoxycarbonylpentylthio)pyrazolo[3,4-d]pyrimidine; b) hydrolyzing the 4-(5-ethoxycarbonylpentylthio)pyrazolo[3,4-d]pyrimidine to obtain the intermediate 6-((1H-pyrazolo[3,4-d]pyrimidin-4-yl)thio)hexanoic acid; c) using 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and hydroxybenzotriazole as coupling agents, standing and degassing before the reaction, and then coupling the 6-((1H-pyrazolo[3,4-d]pyrimidin-4-yl)thio)hexanoic acid with polyethylene glycol diamine, followed by purification by dialysis and freeze-drying to obtain the seed crystal-impurity anchoring agent.
[0011] A further improvement of the present invention is that in the step b), the pH gating buffer is added after the allopurinol sodium salt mother liquor is cooled to 50°C.
[0012] A further improvement of the present invention is that the programmed temperature-controlled cooling in step d) is specifically a gradual temperature reduction from 50° C. to 10° C. at a rate of 5° C. per hour.
[0013] A further improvement of the present invention is that the constant temperature aging in step d) is specifically constant temperature aging at 10° C. for 2 hours.
[0014] A further improvement of the present invention is that, based on 100 grams of crude allopurinol: The amount of the pH gating buffer is 10 grams; The amount of the seed crystal-impurity anchoring agent used is 1.0 gram.
[0015] The present invention provides an allopurinol purification process through improvement, which has the following improvements and advantages compared with the prior art: First, existing techniques, such as repeated recrystallization, rely on the subtle physical property differences between the target product and impurities for separation. This leads to an irreconcilable conflict between purity and yield: high purity must be achieved at the expense of significant yield, and vice versa. As shown in Comparative Example 3, the purity (98.61%) and yield (85.3%) achieved by the traditional method are both unsatisfactory. The present invention breaks away from the framework of relying on slight differences in physical properties and innovatively utilizes the often overlooked difference in ionization constant (pKa) between the target product and impurities. By introducing a core homemade material, a pH-gated buffer, the pH value of the crystallization system can be precisely clamped at a preset gating point (e.g., 8.4-8.6), thereby converting the microscopic pKa difference into a macroscopic, order-of-magnitude difference in solubility (>100 times). This allows most of the allopurinol to be selectively converted into a low-solubility neutral form and precipitated, while most impurities remain in the highly soluble ionic form and remain in the mother liquor. As shown in Example 1, the present invention can achieve a stable product purity of over 99.95% in a single-step operation, while achieving a process yield of over 96%, fundamentally solving the bottleneck problem of the prior art where purity and yield are in a balance.
[0016] Secondly, the present invention optimizes the traditional multi-step purification process into a single-step dissolution-regulation-crystallization process. By introducing another core homemade material, the seed crystal - the impurity anchor, the seed end can target and induce allopurinol to crystallize rapidly and uniformly, while the anchor end locks the impurities in the mother liquor through non-covalent bonds to prevent them from being encapsulated or co-precipitated. This design greatly accelerates the crystallization kinetics and ensures the quality of the crystals. The present invention significantly shortens the production cycle, reduces energy consumption, equipment occupancy and solvent consumption, and while improving economic value, it also meets the requirements of the modern pharmaceutical industry for green and sustainable processes. BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will be further explained below in conjunction with the accompanying drawings and examples: Figure 1 The present invention is a flow chart of the allopurinol purification process. DETAILED DESCRIPTION
[0018] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to specific embodiments. Example 1
[0019] This embodiment provides a purification process for allopurinol, which is used to separate the target product from a crude product containing isomeric impurities such as 3-aminopyrazole-4-carboxamide.
[0020] First, functionalized mesoporous silica microspheres with a surface rich in glycine residues are prepared as a pH-gated buffer. The preparation method includes: after activating the mesoporous silica microspheres, adding (3-aminopropyl)triethoxysilane to react to obtain amino-functionalized silica sphere intermediates; then, using N,N-dicyclohexylcarbodiimide as a coupling agent, grafting the amino-functionalized silica spheres with N-tert-butyloxycarbonylglycine; and removing the Boc protecting group using a dioxane solution containing 4M hydrochloric acid to obtain the pH-gated buffer.
[0021] At the same time, a bifunctional molecule serving as a seed-impurity anchor is prepared. The preparation method includes: reacting pyrazolo[3,4-d]pyrimidine-4-thione with ethyl 6-bromohexanoate to obtain an intermediate 4-(5-ethoxycarbonylpentylthio)pyrazolo[3,4-d]pyrimidine; hydrolyzing the 4-(5-ethoxycarbonylpentylthio)pyrazolo[3,4-d]pyrimidine to obtain an intermediate 6-((1H-pyrazolo[3,4-d]pyrimidin-4-yl)thio)hexanoic acid (PPH-Acid); and finally coupling the 6-((1H-pyrazolo[3,4-d]pyrimidin-4-yl)thio)hexanoic acid with polyethylene glycol diamine using 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI) and hydroxybenzotriazole (HOBt) as coupling agents to obtain the seed-impurity anchor.
[0022] The specific steps of the allopurinol purification process are as follows: a) Dissolve 100 g of crude allopurinol (95% purity) in an alkaline aqueous solution at pH 11.0 and filter under pressure to obtain a clear mother liquor of allopurinol sodium salt.
[0023] b) After cooling the allopurinol sodium salt mother solution to 50° C., 10 g of the pH gating buffer was added to accurately and stably clamp the pH value of the system within the range of 8.5.
[0024] c) adding 1.0 g of the seed crystal-impurity anchoring agent to the system and performing ultrasonic dispersion.
[0025] d) performing programmed temperature-controlled cooling and constant temperature aging on the system, specifically, gradually cooling the temperature from 50° C. to 10° C. at a rate of 5° C. per hour, and aging at a constant temperature of 10° C. for 2 hours.
[0026] e) separating the crystalline product by centrifugal filtration, washing and vacuum drying to obtain high-purity allopurinol.
[0027] Existing techniques, such as repeated recrystallization, rely on the subtle physical property differences between the target product and impurities for separation. This leads to an irreconcilable conflict between purity and yield: pursuing high purity requires significant sacrifice of yield, and vice versa. As shown in Comparative Example 3, the purity (98.61%) and yield (85.3%) achieved by the traditional method are both unsatisfactory. The present invention breaks away from the framework of relying on slight differences in physical properties and innovatively utilizes the often overlooked difference in ionization constant (pKa) between the target product and impurities. By introducing a core homemade material, a pH-gated buffer, the pH value of the crystallization system can be precisely clamped at a preset gating point (e.g., 8.4-8.6), thereby converting the microscopic pKa difference into a macroscopic, order-of-magnitude difference in solubility (>100 times). This allows most of the allopurinol to be selectively converted into a low-solubility neutral form and precipitated, while most impurities remain in the highly soluble ionic form and remain in the mother liquor. As shown in Example 1, the present invention can achieve a stable product purity of over 99.95% in a single-step operation, while achieving a process yield of over 96%, fundamentally solving the bottleneck problem of the prior art where purity and yield are in a balance. Example 2
[0028] This example provides an allopurinol purification process, which has the same characteristics as Example 1, except that: in step b), the pH gating buffer accurately and stably clamps the system pH value within the range of 8.4; all other steps, including the preparation and dosage of the pH gating buffer and the seed crystal-impurity anchor, and the process parameters are consistent with Example 1. Example 3
[0029] This example provides an allopurinol purification process, which has the same characteristics as Example 1, except that: in step b), the pH gating buffer accurately and stably clamps the system pH value within the range of 8.6; all other steps, including the preparation and dosage of the pH gating buffer and the seed crystal-impurity anchor, and the process parameters are consistent with Example 1. Example 4
[0030] This example provides a purification process for allopurinol, which has substantially the same characteristics as Example 1, except that, in step d), the programmed temperature cooling is performed by gradually decreasing the temperature from 50°C to 10°C at a rate of 7°C per hour. All other steps, including the preparation and dosage of the pH gating buffer and seed crystal-impurity anchor, as well as the process parameters, remain consistent with those in Example 1. Example 5
[0031] This example provides an allopurinol purification process, which has substantially the same features as Example 1, except that, in step d), the constant temperature aging is specifically performed at 10°C for 1.5 hours. All other steps, including the preparation and dosage of the pH gating buffer and seed crystal-impurity anchor, as well as the process parameters, remain consistent with those in Example 1.
[0032] Comparative Example 1 This comparative example provides an allopurinol purification process, which has substantially the same characteristics as Example 1, except that the process does not include step c), i.e., the seed crystals (impurity anchoring agent) are not added to the system; and the crystallization process relies on natural nucleation during slow cooling. All other steps remain the same as in Example 1.
[0033] Comparative Example 2 This comparative example provides an allopurinol purification process, which has essentially the same characteristics as Example 1, except that steps b) and c) are omitted, i.e., the pH-gating buffer and seed crystal-impurity anchor are not used. Instead, after cooling to 50°C, the pH is manually adjusted to approximately 8.5 by dropwise addition of 1M hydrochloric acid solution, followed by cooling and crystallization. All other steps remain the same as in Example 1.
[0034] Comparative Example 3 This comparative example provides a traditional recrystallization purification process. The steps are as follows: 100 g of crude allopurinol (95% purity) is dissolved in 2000 mL of boiling water, filtered while hot, and the filtrate is cooled to 10°C while stirring to allow crystallization. The filtrate is then filtered, washed with a small amount of cold water, and dried. This method does not utilize the pH-gating buffer or the seed crystal-impurity anchoring agent and simulates the prior art.
[0035] In Examples 1-5 and Comparative Examples 1-2, the mesoporous silica microspheres used were purchased from Suzhou Nanotechnology Co., Ltd. with a particle size of 50 μm; (3-aminopropyl)triethoxysilane, N,N-dicyclohexylcarbodiimide (DCC), N-tert-butyloxycarbonylglycine, ethyl 6-bromohexanoate, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI) and hydroxybenzotriazole (HOBt) were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; pyrazole [3,4-d] pyrimidine-4-thione was purchased from Jinan Bona Biotechnology Co., Ltd.; polyethylene glycol diamine (average molecular weight 2000 Da) was purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the crude allopurinol was an industrial product with a purity of 95%; other reagents used, such as toluene, dimethylformamide, dioxane, ethanol, sodium hydroxide, hydrochloric acid, etc., were all commercially available analytical grade products.
[0036] Performance testing and result analysis The allopurinol products prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to performance tests to examine their key process indicators and final product quality. The test results are shown in Table 1 below.
[0037] Table 1 Test items Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 pH fluctuation range (during crystallization) 8.5±0.1 8.4±0.1 8.6±0.1 8.5±0.1 8.5±0.1 8.5±0.1 8.5→9.2 not applicable Crystallization induction time (minutes) ~15 ~15 ~15 ~15 ~15 >45 >60 >120 Final product purity (%) 99.96 99.92 99.93 99.88 99.95 99.75 99.52 98.61 Process yield (%) 96.2 96.0 96.1 96.4 95.7 94.5 93.8 85.3 It can be seen from the test results in Table 1 that the present invention exhibits significant technical advantages over the comparative example by introducing a pH gating buffer and a seed-impurity anchor and optimizing process parameters.
[0038] Comparing Example 1 with Comparative Example 1, both utilized a pH-gated buffer, but Comparative Example 1 lacked a seed crystal-impurity anchor. The results showed that the crystallization induction time in Comparative Example 1 was significantly prolonged (>45 minutes), and the final purity (99.75%) and yield (94.5%) were significantly lower than those in Example 1. This demonstrates that the nucleation sites provided by the seed crystal-impurity anchor effectively promoted the homogeneous nucleation and crystallization of allopurinol, preventing impurity encapsulation caused by spontaneous nucleation. Furthermore, its anchoring effect on impurities inhibited coprecipitation, contributing to the high purity and high yield achieved.
[0039] Comparing Example 1 with Comparative Example 2, which did not use the two functional additives proposed in this invention and only employed conventional acid-base titration to control pH, the pH of the resulting product exhibited significant pH drift (from 8.5 to 9.2) during crystallization, significantly reducing the purity of the final product to 99.52%. This demonstrates that the pH-gated buffer can precisely and stably clamp the system pH within the target range, providing the basis for selective precipitation by exploiting the difference in ionization constants (ΔpKa) between allopurinol and impurities, effectively avoiding the co-precipitation of impurities caused by pH fluctuations.
[0040] Comparing Example 1 with Comparative Example 3, which simulates a traditional recrystallization process, the purity (98.61%) and yield (85.3%) of Comparative Example 3 are both relatively low, confirming the bottleneck mentioned in the background art that traditional methods have difficulty reconciling the contradiction between purity and yield. The process of the present invention uses chemical regulation to increase the purity to over 99.95% and the yield to over 96%, achieving a significant improvement in separation efficiency, greatly simplifying the process flow, and improving economic benefits.
[0041] Comparing the internal data from Examples 1-5, it can be seen that when the pH value (8.4-8.6), cooling rate (e.g., 5-7°C / hour), and aging time (e.g., 1.5-2 hours) are adjusted within specific ranges, high-purity and high-yield products can be obtained, demonstrating the excellent stability and operating window of this process. Specifically, the parameter combination of Example 1 (pH clamped at 8.5, cooling rate 5°C / hour, aging for 2 hours) achieves a good balance between purity and yield, making it a preferred embodiment of this process.
[0042] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.
Claims
1. A process for the purification of allopurinol for separating the target product from a crude product containing isomeric impurities such as 3-aminopyrazole-4-carboxamide, characterized in that: The following steps are involved: a) dissolving the crude allopurinol in an alkaline aqueous solution having a pH of 11.0-11.5, and filtering under pressure to obtain a clarified allopurinol sodium salt mother liquor; b) adding a pH-gating buffer to the allopurinol sodium salt mother solution to precisely and stably clamp the pH value of the system within the range of 8.4-8.6, and utilizing the difference in ionization constant ΔpKa between allopurinol and the isomeric impurities to convert most of the allopurinol into a neutral form with lower solubility, while maintaining most of the impurities in the form of ionized salts with higher solubility; c) adding a seed crystal-impurity anchoring agent to the system and performing ultrasonic dispersion; one end of the seed crystal-impurity anchoring agent serves as a nucleation site to accelerate the crystallization of the electrically neutral purinol molecules, and the other end anchors the dissolved impurities in the mother liquor through non-covalent bonding to prevent their co-precipitation; d) performing programmed temperature-controlled cooling and constant temperature aging on the system to allow the crystals to fully grow; e) separating the crystalline product by centrifugal filtration, washing and vacuum drying to obtain high-purity allopurinol.
2. The allopurinol purification process according to claim 1, wherein The pH gating buffer is a functionalized mesoporous silica microsphere with a surface rich in glycine residues.
3. The allopurinol purification process according to claim 2, wherein The preparation method of the pH gating buffer comprises: a) performing ultrasonic dispersion and reflux dehydration on the mesoporous silica microspheres in toluene to perform activation treatment; b) adding (3-aminopropyl)triethoxysilane to react with the activated microspheres to obtain amino-functionalized silica sphere intermediates; c) using N,N-dicyclohexylcarbodiimide (DCC) as a coupling agent to graft the amino-functionalized silica spheres with N-tert-butyloxycarbonylglycine; d) using a dioxane solution containing 4 M hydrochloric acid to degas the product by stirring to remove the Boc protecting group, thereby obtaining the pH gating buffer.
4. The allopurinol purification process according to claim 1, wherein The seed-impurity anchor is a bifunctional molecule, one end of which is a pyrazole [3,4-d] pyrimidine analog structure, which serves as a seed; the other end is a polyethylene glycol diamine flexible chain, whose terminal amino group can form a hydrogen bond with the formamide group of the impurity, serving as an impurity anchoring segment.
5. The allopurinol purification process according to claim 4, wherein The preparation method of the seed crystal-impurity anchoring agent comprises: a) heating pyrazolo[3,4-d]pyrimidine-4-thione and ethyl 6-bromohexanoate under reflux in ethanol to obtain the intermediate 4-(5-ethoxycarbonylpentylthio)pyrazolo[3,4-d]pyrimidine; b) hydrolyzing the 4-(5-ethoxycarbonylpentylthio)pyrazolo[3,4-d]pyrimidine to obtain the intermediate 6-((1H-pyrazolo[3,4-d]pyrimidin-4-yl)thio)hexanoic acid; c) using 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and hydroxybenzotriazole as coupling agents, standing and degassing before the reaction, and then coupling the 6-((1H-pyrazolo[3,4-d]pyrimidin-4-yl)thio)hexanoic acid with polyethylene glycol diamine, followed by purification by dialysis and freeze-drying to obtain the seed crystal-impurity anchoring agent.
6. The allopurinol purification process according to claim 1, wherein In the step b), the allopurinol sodium salt mother solution is cooled to 50° C., and then the pH gating buffer is added.
7. The allopurinol purification process according to claim 1, wherein The programmed temperature-controlled cooling in step d) is specifically to gradually reduce the temperature from 50° C. to 10° C. at a rate of 5° C. per hour.
8. The allopurinol purification process according to claim 1, wherein The constant temperature aging in step d) is specifically aging at a constant temperature of 10° C. for 2 hours.
9. The allopurinol purification process according to claim 1, wherein Based on 100g of crude allopurinol: The amount of the pH gating buffer is 10 grams; The amount of the seed crystal-impurity anchoring agent used is 1.0 gram.