Process Research For The Synthesis Of Ambrisentan And Obeticholic Acid

The research contents of two parts are involved in the dissertation.Part I:Process research for the synthesis of ambrisentanAmbrisentan is a selective endothelin-A(ET_A)receptor antagonist,which is a preferred drug in the treatment for pulmonary arterial hypertension due to its good curative effect,lower side effect and oral administration once a day.Ambrisentan has a chiral center,which is constructed by the methodologies of chiral resolution,asymmetric synthesis and enzymatic catalysis at present.Considering the controllability of the quality of the drug substance,the operability of manufacturing scal-up and the materials cost,and an efficient synthetic route to ambrisentan was designed by recycling the unwanted isomer from the resolution mother liquors.The process route is as follows:A-3 was synthesized via the Darzens reaction with benzophenone and methyl chloroacetate in the presence of sodium methoxide in tetrahydrofuran and subsequent acidcatalyzed ring-opening with methanol to produce A-4,which underwent nucleophilic substitution with 4,6-dimethyl-2-(methylsulfonyl)pyrimidine to give A-18.The hydrolysis of A-18 to carboxylic acid A-19 followed by chiral resolution with(S)-1-phenylethylamine yielded diastereomeric salt A-20,which was dissociated with diluted hydrochloric acid in isopropanol,and then crystallized in isopropanol and water to obtain A-1.The unwanted isomer from the resolution mother liquors was recycled via the racemization to give A-19,and then A-1 was obtained again via repeating the procedures involved chiral resolution,dissociation and recrystallization.The results of this study were as follows:(1)Approximately 15%benzophenone remained on HPLC even though following the literature report,because the side reactions occurred between methyl chloroacetate and sodium methoxide.The formation of methyl 2-methoxyacetate was decreased at-10~-5°C,and the self-condensation of methyl 2-chloroacetate was avoided as far as possible via it was added slowly to the reaction mixture.Finally,the remained benzophenone was controlled no more than 3%on HPLC in the improved process.The yield was approximately 95%,and the purity was more than 97.0%.(2)Hydrochloric acid rather than boron trifluoride or p-toluenesulfonic acid as an acid catalyst was more environmentally friendly and safe in the preparation of A-4.The product was directely precipitated from methanol in the improved work-up process,however,the complex procedures involved quenching,extraction,solvent removal and crystallization would be performed in the literature report.The yield was approximately 90%,and the purity was more than 99.0%.(3)The elimination of methanol of A-18 were avoided using potassium carbonate rather than sodium hydride or sodium amide in N,N-dimethylformamide for preparing A-18,furthermore,the mixtures of sodium hydride or sodium amide and N,N-dimethylformamide have potential risks of explosion in the pilot plants.The crude A-18 was recrystallized with methanol and water,and the yield was approximately 94%,and the purity was more than 99.5%,the maximum impurity was no more than 0.1%.(4)The hydrolyzation of A-18 was carried out in different solvents to prepare A-19,and the amount of the elimination of methanol of A-19 was the least in dioxane.The yield was approximately 96%,and the purity was more than 99.0%.(5)The chiral resolution of A-19 was carried out with low-cost(S)-1-phenylethylamine in the mixed solvents of isopropanol and methyl tert-butyl ether in the preparation of A-20.The yield was approximately 40%after chiral resolution twice,and both the chemical purity and chiral purity were more than99.9%.(6)A-1 was dissociated from A-20 with diluted hydrochloric acid in isopropanol,and which avoided that A-1 was partly racemized using the aprotic solvent as extract solvent.The polymorph of the final product crystallized in isopropanol and water was consisted with that of the literature report.The yield was approximately 90%,and both the chemical purity and chiral purity were more than 99.9%.(7)In the process of recovering A-19,the chemical purity of the isomer mixture in the resolution mother liquor was increased from 95%to over 99%using the same dissociation method as described in(6).It is necessary that the solubility of racemic ambrisentan was less than that of ambrisentan in aprotic solvents such as acetonitrile,ethyl acetate and toluene for the sake of complete racemization of ambrisentan,and the rate of the racemization of ambrisentan was quikly in acetonitrile and the purity was almost unchanged.The dissociative isomer mixture was refluxed in acetonitrile to achieve racemization.The recovery yield of was approximately 90%,and the purity was more than 99.5%.The source,formation mechanism,purge pathway and control strategy of process-related substances generated in each reaction step were studied.The results showed that the quality of ambrisentan met ICH requirements.The stability of the synthetic process was validated by the small-scale experiments with three successive batches,and the quality of ambrisentan was preliminarily studied.The impurities with complex structures in the process were characterized in detail.After the above process optimization,using benzophenone as the starting material,when the unwanted isomer from the resolution mother liquors was recycled only once and the overall process yield was up to 47.5%in contrast to 21.5%for the original process,and the material cost was only one third of the original process.This synthetic process was suitable for manufacturing scale-up.Part II:Process research for the synthesis of obeticholic acidObeicholic acid is a farnesol X receptor(FXR)agonist,which is a drug in the treatment for the patients with primary biliary cholangitis(PBC)who respond poorly or intolerant to ursodeoxycholic acid and a potential treatment for nonalcoholic steatohepatitis(NASH).The selected process route is as follows:The 7-position hydroxyl of chenodeoxycholic acid(O-29)was selectively oxidized to produce O-2 and subsequent methylated to produce O-6.O-8 was synthesized by O-6 and acetaldehyde using Mukaiyama reaction,which was hydrolyzed to produce O-9.The hydrgenation of O-9 was preferentially generated to6?-ethyl intermediate,which was converted to O-10 in the heated aqueous solution of sodium hydroxide.The reduction of O-10 was performed with sodium borohydride to produce O-1a.Finally,the polymorphic O-1a was transformed into amorphous O-1.The results of this study were as follows:(1)The oxidation of O-29 was performed with hypochlorous acid to prepare O-2,and the reaction activity of hypochlorous acid was enhanced using hydrochloric acid rather than acetic acid,furthermore,the utilization efficiency of sodium hypochlorite was improved due to that the order of addition was changed.Sodium hypochlorite would not be added once more in the improved process.The purity of O-2 was improved by slurring in ethyl acetate.The yield was approximately 80%,and the purity was more than 99.0%.(2)In the preparation of O-6,a certain amount of water was added to the reaction mixture in batches to obtain product.However,O-6 would not be precipitated from the mixture due to the formation of the supersaturated solution of O-6 for the addition of excess water using the procedures in the literature report.The yield was approximately 90%,and the purity was over 99.5%.(3)The regioselectivity of the preparation of O-7 was significantly improved at-60~-65°C,which was better than that of the literature report using thin layer chromatography analysis.The dark colour impurity with large polarity in crude O-7was removed by silica gel adsorption to avoid adverse effects in the subsequent process.(4)The addition reaction of O-7 and acetaldehyde was carried out in the presence of boron trifluoride at-65~-60°C,and the elimination reaction to prepare O-8 went well at-10~-5°C.The reaction monitoring steps were proposed in process control,and the temperature of elimination reaction could be raised only when the addition reaction of O-7 and acetaldehyde was completed,which avoided the case that O-7 was converted to O-6 in the presence of boron trifluoride due to the incomplete reaction of O-7.The reaction was obviously better than that of the literature report by HPLC analysis.(5)O-9 was partially degraded to O-2 in aqueous solution of sodium hydroxide in the preparation of O-9,and the amount of the formation of O-2 was related to reaction temperature,and the level of O-2 was controlled well at 25°C.The amount of O-2 was controlled no more than 1.0%on HPLC after the crude O-9 was crystallized twice with ethanol and water.However,the complex procedures involved quenching,extraction,solvent removal,slurry and crystallization would be performed in the literature report.The three-step yield from O-6 was approximately 52%and the purity was over 97.0%,and the yield was only 31%in the literature report.(6)The hydrgenation of O-9 was preferentially generated to 6?-ethyl intermediate,which was converted to O-10 in the heated aqueous solution of sodium hydroxide.Approximately 2.7%6?-ethyl intermediate could not be converted to O-10due to that the configuration inversion was an equilibrium reaction.The oxide impurity of 3-hydroxyl of O-10 was found in the process of configuration inversion,and therefore the coexistence of palladium-carbon with air should be avoided for the configuration inversion after completing hydrogenation.The level of the key impurities were controlled well in O-10 after the crude was crystallized twice with ethanol and water.The yield was approximately 79%,and the purity is over 97.0%.(7)The amount of isomer impurity was controlled no more than 3.0%on HPLC in the presence of 1.1 equivalent sodium borohydride at 75°C for preparing O-1a.The content assay of the maximum impurity in O-1a was controlled no more than 0.1%by the crystallization twice with n-butyl acetate and n-heptane.However,the quality of O-1a could not meet reqirements if it was crystallized once with n-butyl acetate using the procedures in the literature report.The yield was approximately 70%,the content assay was over 99.5%,and the content assay of the maximum impurity in O-1a was no more than 0.1%.(8)The level of dimeric impurity from the self-esterification of O-1 was controlled no more than 0.1%in the preparation of amorphous O-1,and aqueous acetic acid solution rather than diluted hydrochloric acid would not catalyze the self-esterification of O-1,because the pH value of aqueous acetic acid solution could not be less than 4.The yield was approximately 90%,the content assay of O-1 was over 99.5%,and the content assay of the maximum impurity in O-1 was no more than0.1%.The source,formation mechanism,purge pathway and control strategy of process-related substances generated in each reaction step were studied.The results showed that the quality of obeticholic acid met ICH requirements.The stability of the synthetic process was validated by the small-scale experiments with three successive batches,and the quality of obeticholic acid was preliminarily studied.The structures of three epimeric impurities in obeticholic acid were characterized by nuclear magnetic resonance technique.After the above process optimization,using chenodeoxycholic acid as the starting material,and the total yield was up to 18.9%in contrast to 13.8%for the original process.This synthetic process was suitable for manufacturing scale-up.