Method for preparing nitric oxide donating prostaglandin analogs

Abstract

The present invention relates to a method for preparing hexanoic acid, 6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester represented by formula (I). According to the present invention, Compound (I) can be efficiently prepared with high purity by coupling bimatoprost in a boronate-protected form with 6-(nitrooxy)hexanoyl chloride having high chemical purity and then removing the boronate protecting group. High-chemically pure 6-(nitrooxy)hexanoyl chloride is prepared from high-chemically pure 6-(nitrooxy)hexanoic acid. The present invention also relates to a method for preparing and purifying 6-(nitrooxy)hexanoic acid. TIFF2022544092000038.tif73165

Description

[Technical Field]

[0001] The present invention relates to an improved method suitable for the large-scale production of hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, represented by formula (I). This method makes it possible to obtain the said product with high chemical purity. The present invention also describes the production of high-purity 6-(nitrooxy)hexanoic acid (VIIa), which is an important intermediate in the synthesis. [Background technology]

[0002] Background of the Invention Equation (I) [ka] The hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, represented by , is a prostaglandin analog that has been proven effective as an IOP-reducing agent (Impagnatiello F, Toris CB, Batugo M, Prasanna G, Borghi V, Bastia E, Ongini E, Krauss AHP; Invest Ophthalmol Vis Sci. 2015; 56:6558-64).

[0003] A method for producing the compound represented by formula (I) is disclosed in International Publication No. 2009 / 136281.

[0004] International Publication No. 2009 / 136281 specifically discloses the synthesis of compound (I) and, more generally, the preparation of the 15-alkyl nitrate of bimatoprost.

[0005] International Publication No. 2009 / 136281 discloses the synthesis of the compound represented by formula (I) by reacting bimatoprost in its boronate-protected form (compound (II)) with 6-bromohexanoyl chloride to obtain the 15-(6-bromohexanoyl) ester of bimatoprost in its boronate-protected form (compound (XI)), converting this to a nitric acid derivative with silver nitrate in acetonitrile, and deprotecting / purifying it under reverse-phase chromatography to produce the compound represented by formula (I) (Example B-1). [ka]

[0006] Another major drawback of this method is the formation of impurities and by-products, such as 15-(6-bromohexanoyl) ester of bimatoprost (compound (IX)) and 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)). These are difficult to remove even after multiple purifications because they have similar chromatographic polarity, similar lipophilicity, and / or solubility to compound (I). Furthermore, compound (X) is predicted to be positive for bacterial in vitro mutagenicity. Removing these impurities requires repeated purification, which further reduces the yield and increases the cost of production on a commercial scale. [ka]

[0007] According to the method disclosed in International Publication No. 2009 / 136281, 15-(6-bromohexanoyl)ester of bimatoprost (compound (IX)) is an impurity derived from the incomplete reaction between compound (XI) after removal of the boronate protection and silver nitrate. 15-(6-chlorohexanoyl)ester of bimatoprost (compound (X)) is a byproduct formed by a halogen exchange reaction between the bromine atom of 15-(6-bromohexanoyl)ester of bimatoprost (compound (XI)) in the boronate-protected form and the free chloride anion of 4-dimethylaminopyridine hydrochloride formed during the esterification process. 15-(6-chlorohexanoyl)ester of bimatoprost (XIa) (Scheme 3) in the boronate-protected form does not react with silver nitrate and, after removal of the protecting group, produces compound (X).

[0008] Furthermore, International Publication No. 2009 / 136281 also discloses an alternative method for the preparation of 15-acylalkylnitrate bimatoprost derivatives (Examples N-1 and O-1). The synthesis involves reacting bimatoprost in a boronate-protected form (the compound represented by formula (II)) with a nitrate-alkylcarboxylic acid chloride in the presence of 4-dimethylaminopyridine (DMAP) (PS-DMAP) supported on a resin, followed by removal of the boronate protecting group and purification using silica gel chromatography.

[0009] The above method avoids the use of 6-bromohexanoyl chloride and the removal of silver salts from the final product. However, this method has another major drawback: the use of 4-dimethylaminopyridine supported on a resin, which makes it unsuitable for commercial scaling and expensive. Furthermore, nitrate-alkyl carboxylic acid chloride is added in high excess to the compound represented by formula (II) in two consecutive steps. In practice, the alkyl carboxylic acid chloride is added in an amount of approximately 2 to 4 equivalents.

[0010] Furthermore, International Publication No. 2009 / 136281 also discloses another method (Example Q1) for the preparation of a 15-acylalkylnitrate bimatoprost derivative. In this method, the compound was obtained by esterifying bimatoprost(II) in a boronate-protected form with an excess of alkyl-(p-nitrophenyl)carboxylate nitrate in the presence of 4-dimethylaminopyridine.

[0011] The main drawback of this method is the removal of unreacted nitrate-alkyl-(p-nitrophenyl)-carboxylate and the by-product p-nitrophenol, which are formed in equimolar amounts with the compound represented by formula (I), using chromatography.

[0012] International Publication No. 2016 / 155906 discloses a 15-nitrooxy derivative of fluprostenol and reports the synthesis of a 15-nitrooxy-hexyl ester of fluprostenol isopropyl ester. This compound was prepared by reacting fluprostenol isopropyl ester in its boronate-protected form with (4-nitrophenyl)-6-nitrooxyhexanoate in the presence of excess 4-dimethylaminopyridine. [Overview of the project] [Problems that the invention aims to solve]

[0013] As reported above, the main drawback of this method is the removal of unreacted alkyl-(p-nitrophenyl)carboxylate nitrate by chromatography, particularly the removal of p-nitrophenol byproducts.

[0014] European Patent Application Publication No. 3530649, filed prior to the relevant date of this invention and subsequently published, discloses a method for preparing hexanoic acid, 6-(nitrooxy),(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (compound (I)). Compound (I) is prepared by coupling bimatoprost in a boronate-protected form with 6-(nitrooxy)hexanoyl chloride and removing the boronate protecting group. The 6-(nitrooxy)hexanoyl chloride intermediate is prepared from 6-(nitrooxy)hexanoylic acid. 6-(nitrooxy)hexanoylic acid is prepared by the ring-opening reaction of 2-caprolactone, followed by the nitration of potassium 6-hydroxyhexanoate salt with a mixture of HNO3 and H2SO4 in dichloromethane. 6-(nitrooxy)hexanoic acid is used in the preparation of the corresponding asylloride without purification.

[0015] Over the past few years, various regulatory authorities have focused on purity requirements and the identification of impurities in active pharmaceutical ingredients (APIs). Currently, impurities other than APIs that may affect the efficacy and safety of pharmaceuticals are all considered organic substances. Therefore, the identification and quantification of each impurity, particularly impurities with structural concerns regarding mutagenicity, are essential regulatory requirements. In addition, because this product is intended for pharmaceutical use, the range of industrially acceptable reagents, solvents, catalysts, etc. that can be used in the synthesis of the active ingredient is limited to those acceptable in the pharmaceutical industry.

[0016] The compound represented by formula (I) is an oil, and its large-scale purification is difficult because it cannot be crystallized; therefore, the presence of impurities is a significant problem for large-scale production. Since the main sources of impurities are intermediates and by-products of synthesis, controlling the purity of the intermediates and reaction conditions is a crucial requirement for obtaining a final product with pharmaceutically acceptable purity.

[0017] In summary, the prior art methods for the preparation of the compounds represented by formula (I) have several drawbacks. For example, the use of 6-bromohexanoyl chloride and the reaction conditions result in the formation of the by-product 15-(6-chlorohexanoyl) ester of bimatoprost (Compound (X)). This by-product is predicted to be positive for in vitro mutagenicity in bacteria based on both statistical-based methods and expert rules required by regulatory authorities. The preparation of the intermediate alkyl carboxylic acid chloride nitrate or the use of silver nitrate for the nitration of 15-(6-bromohexanoyl) ester of bimatoprost (Compound (XI)) in the boronate protected form leads to the management of large amounts of silver nitrate wastewater. Furthermore, the metal content in the active pharmaceutical ingredient must meet specific acceptance criteria.

[0018] Therefore, there is a need to develop an industrially feasible method that provides high purity hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (Compound (I)) in good yield.

Means for Solving the Problems

[0019] Compound (I) can be efficiently prepared by coupling bimatoprost (Compound (II)) in its boronate protected form with 6-(nitrooxy)hexanoyl chloride. 6-(Nitrooxy)hexanoyl chloride is prepared by subjecting caprolactone to a ring-opening reaction with an alkali metal hydroxide solution and subsequently nitrating the alkali metal salt of 6-hydroxyhexanoic acid using a mixture of HNO3 and H2SO4 to afford 6-(nitrooxy)hexanoic acid. 6-(Nitrooxy)hexanoic acid is converted to 6-(nitrooxy)hexanoyl chloride. 6-(Nitrooxy)hexanoic acid and 6-(nitrooxy)hexanoyl chloride are used without further purification. However, from experiments conducted by the applicant, Compound (I) prepared using this method was shown to contain the 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester of bimatoprost (Compound (XII)), which is a "dimer impurity". [Chemical formula] was shown to contain.

[0020] Since Compound (I) and Compound (XII) have similar polarities in chromatography, repeated purification that reduces the yield of the process is required to remove Compound (XII).

[0021] Compound (XII) is formed when 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoyl chloride (Compound (IVb)) reacts with Compound (II) during the coupling step. The acidic reaction conditions of the nitration of the alkali salt of 6-hydroxyhexanoic acid using a mixture of HNO3 and H2SO4 result in the formation of 6-[6-hydroxyhexanoyl]oxy}hexanoic acid impurity, which, in the presence of HNO3, is the nitric acid derivative 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (Compound (VIIb)). [Chemical formula] It was found that it is converted to [this].

[0022] Therefore, when the crude product of the nitration reaction is reacted with a chlorinating agent without any purification, 6-(nitrooxy)hexanoyl chloride (IVa) and 6-[6-hydroxyhexanoyl]oxy}hexanoyl chloride (IVb) are obtained: [ka] Formation occurs.

[0023] A key factor in obtaining compound (I) in high yield and high purity is the high purity of the 6-(nitrooxy)hexanoic acid intermediate (VIIa). [ka] It was found that this intermediate was obtained by purifying the crude product of the nitration reaction using reverse-phase chromatography.

[0024] The present invention provides a method suitable for large-scale production that enables the high yield of hexanoic acid,6-(nitrooxy),(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (compound (I)), which contains 15-(6-chlorohexanoyl) ester (X) of bimatoprost, a predicted genotoxic impurity below safety levels, and 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester (compound (XII)) of bimatoprost, a "dimeric impurity" below 0.1% w / w.

[0025] The "safe level" is calculated according to the Tolerable Threshold of Toxicological Concern (TTC) intake of 1.5 μg / day for chronic treatment.

[0026] Furthermore, the present invention provides high purity formula (VIIa) [ka] This relates to a method for preparing the 6-(nitrooxy)hexanoic acid intermediate shown in [the relevant formula]. [Modes for carrying out the invention]

[0027] Description of the present invention The object of the present invention is formula (I): [ka] A method for preparing hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, comprising the following steps: 1) Compound (II): [ka] 6-(nitrooxy)hexanoyl chloride (IVa) [ka] The reaction was carried out in the presence of free 4-dimethylaminopyridine to obtain compound (III). [ka] The process of obtaining, 2) A step of removing the boronate protecting group from compound (III) to obtain the compound represented by formula (I), 3) A step of purifying compound (I), 6-(nitrooxy)hexanoyl chloride (IVa) is produced by the following process: 4) 2-Caprolactone (V): [ka] Reacting with an inorganic base selected from KOH, NaOH, and LiOH yields formula (VI): [ka] [In the formula, M is K, Na, or Li] The process of obtaining the 6-hydroxyhexanoate shown, 5) The compound represented by formula (VI) is nitrated with a mixture of HNO3 and H2SO4 to obtain 6-(nitrooxy)hexanoic acid (VIIa). [ka] The process of obtaining, (The resulting product (VIIa) is 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (VIIb), a byproduct in a 1% w / w amount.) [ka] (May contain) 6) A step of purifying this nitration mixture by reverse-phase chromatography using formic acid (H2O + HCOOH) and acetonitrile in water as eluents to obtain pure 6-(nitrooxy)hexanoic acid represented by formula (VIIa) having compound (VIIb) in a content below the HPLC detection limit (<0.05%), 7) The method is characterized by being prepared by a step of reacting pure 6-(nitrooxy)hexanoic acid (VIIa) with a chlorinating agent to obtain 6-(nitrooxy)hexanoyl chloride (IVa).

[0028] Preferably, the 6-(nitrooxy)hexanoyl chloride obtained in step 7) is reacted directly with the compound represented by formula (II) in step 1) without further purification.

[0029] Step 1) is carried out at a temperature in the range of 0°C to room temperature, preferably in an aprotic organic solvent selected from methyl tert-butyl ether, N,N-dimethylformamide, or dichloromethane. Most preferably, the organic solvent is methyl tert-butyl ether. Free 4-dimethylaminopyridine (DMAP) means that DMAP is not bound to the resin.

[0030] The molar ratio of compound (II) to 6-(nitrooxy)hexanoyl chloride (IVa) is preferably in the range of 1:1.4 to 1:1.6.

[0031] The molar ratio of compound (II) to 4-dimethylaminopyridine is preferably in the range of 1:2.0 to 1:2.4.

[0032] In step 2), the removal of the boronate protecting group is preferably carried out using methanol at a temperature of 17°C to 24°C.

[0033] In step 4), the inorganic base used is preferably potassium hydroxide.

[0034] Step 4) is preferably carried out in a solvent selected from methanol, ethanol, or isopropanol, most preferably methanol.

[0035] Steps 5) and 7) are carried out in dichloromethane.

[0036] In step 6), the concentration of formic acid (H2O + HCOOH) in the water is 0.1%.

[0037] Preferably, pure 6-(nitrooxy)hexanoic acid represented by formula (VIIa) is obtained by extracting the fraction containing compound (VIIa) with CH2Cl2, drying with MgSO4, and evaporating the solvent.

[0038] The preferred chlorinating agent used in step 7) is oxalyl chloride.

[0039] Compound (II) is obtained by reacting bimatoprost with butylboronic acid. Preferably, this reaction is carried out in methyl tert butyl ether as the solvent.

[0040] A further object of the present invention is a method for preparing compound (VIIa) shown below in Scheme I (the reference numerals for the steps reported in the scheme correspond to those reported above), Step 4) involves reacting 2-caprolactone (V) with KOH in methanol to obtain potassium 6-hydroxyhexanoate salt (a compound represented by formula (VI) (wherein M is potassium), Step 5) involves reacting potassium 6-hydroxyhexanoate with a mixture of HNO3 and H2SO4 in dichloromethane, preferably at a temperature of about 5°C to 10°C, preferably 10°C, to obtain crude 6-(nitrooxy)hexanoic acid (VIIa) containing the by-product 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (VIIb), The method comprises the steps of: 6) purifying a crude mixture containing 6-(nitrooxy)hexanoic acid (VIIa) and 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (VIIb), preferably by reverse-phase chromatography using 90:10 to 75:25 H2O + 0.1% HCOOH / acetonitrile as the eluate, for example, by a C18, 25 μm-standard flash cartridge, and extracting the fraction containing compound (VIIa) with CH2Cl2 to obtain pure 6-(nitrooxy)hexanoic acid (VIIa) containing an amount of byproduct 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (VIIb) below the HPLC detection limit (<0.05%).

[0041] The pure compound (VIIa) obtained in step 6) above can be used in step 7) to react pure 6-(nitrooxy)hexanoyl chloride with oxalyl chloride to obtain 6-(nitrooxy)hexanoyl chloride (IVa). 6-(nitrooxy)hexanoyl chloride (IVa) is used without further purification in the method for obtaining the compound represented by formula (I), as described above.

[0042] A preferred method for producing the compound represented by formula (I) is shown in Scheme 2, and the preferred method comprises the following steps: Step 1a) involves reacting bimatoprost with butylboronic acid (1.1-1.8 equivalents) in methyl tert-butyl ether (MTBE) at a temperature of approximately 40°C, and then removing water by azeotropic distillation to obtain bimatoprost boronate (II). Step 1) involves reacting bimatoprostboronate (II) with 6-(nitrooxy)hexanoyl chloride (IVa) (1.4-1.6 equivalents) in methyl tert butyl ether in the presence of 4-dimethylaminopyridine (2.0-2.4 equivalents) at a temperature in the range of 0°C to approximately room temperature to obtain (1S,2E)-3-{(6R,7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-(oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl 6-(nitrooxy)hexanoate (III), Step 2) involves reacting (1S,2E)-3{(6R,7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl 6-(nitrooxy)hexanoate(III) with methanol at room temperature to remove the protecting group and obtain crude hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, (HPLC quantitative analysis of the crude reaction mixture revealed compound (I) at approximately 70% content, 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)) at approximately 0.12%, and 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester of bimatoprost (compound (XII)) at a dimeric impurity below the HPLC detection limit at 0.05% (see Table 1).) Step 3) involves purifying the crude reaction mixture from Step 2 by chromatography to obtain hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (I) having a chemical purity of over 98%, approximately 0.12% of bimatoprost 15-(6-chlorohexanoyl) ester (compound (X)), and 0.05% of bimatoprost 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester (compound (XII)) which is a dimeric impurity below the HPLC detection limit.

[0043] [ka] [ka]

[0044] The experimental procedure for the steps of the present invention is described in detail below.

[0045] A further object of the present invention is to provide a compound having a chemical purity of over 98% and containing approximately 0.12% of 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)), [ka] Contains less than 0.05% of bimatoprost 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoate (compound (XII)), [ka] It is hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, represented by formula (I).

[0046] Another object of the present invention is to provide a hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester represented by formula (I), and at least a pharmaceutically acceptable excipient. Here, hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester is a pharmaceutical preparation having a chemical purity of over 98% and containing approximately 0.12% of bimatoprost 15-(6-chlorohexanoyl) ester (compound (X)) and less than 0.05% of the dimeric impurity bimatoprost 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester (compound (XII)).

[0047] General synthesis Preparation of 6-(nitrooxy)hexanoyl chloride (IVa) All processes are carried out under a nitrogen atmosphere.

[0048] Preparation of potassium 6-hydroxyhexanoate salt (compound (VI)) A solution of potassium hydroxide (1 equivalent) in methanol is added dropwise to a solution of 2-caprolactone (1 equivalent) in methanol. The mixture is cooled to approximately 5°C to 20°C. After the addition is complete, the mixture is stirred at 15°C to 20°C for approximately 5 hours to remove the solvent (the temperature is below 40°C). The crude product is slurryed in methyl tert-butyl ether, the potassium 6-hydroxyhexanoate salt is filtered, washed with methyl tert-butyl ether, and dried. The potassium 6-hydroxyhexanoate salt (VI) is obtained in 95% yield and 98.5% purity. 1 (Obtained by 1H-NMR and HCl assay)

[0049] Preparation of 6-(nitrooxy)hexanoic acid (compound (VIIa)) Potassium 6-hydroxyhexanoate (VI) (1 equivalent) is gradually added over approximately 30 minutes under nitrogen while maintaining a temperature below 10°C to a mixture of HNO3 (4.6 equivalents) and H2SO4 (3.1 equivalents) in dichloromethane cooled to 0°C to 5°C, and the resulting mixture is stirred at 0°C to 10°C for 2 to 3 hours. 1The reaction is monitored by 1H-NMR analysis, the mixture is cooled to 0°C–5°C, and saturated sodium chloride aqueous solution is added dropwise over approximately 20 minutes. The reaction mixture is maintained at a temperature below 10°C, the organic layer is dried over anhydrous sodium sulfate, and the solvent is removed to obtain 6-(nitrooxy)hexanoic acid (VIIa) in 86–88% yield and 96.2% HPLC purity. The main impurity is the dimeric compound 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (compound (VIIb)) [ka] (Approximately 1%)

[0050] Crude 6-(nitrooxy)hexanoic acid (compound (VIIa)) (1 equivalent) was loaded into a Purezza®-SpheraPlus Flash Cartridges-C18, 25 μL-standard flash cartridge, 48 g, and eluted with H2O + 0.1% HCOOH / acetonitrile at a ratio of 90:10 to 75:25. The fraction containing compound (VIIa) was collected, extracted with CH2Cl2, dried, and the solvent was evaporated to obtain pure 6-(nitrooxy)hexanoic acid in 80-90% yield. The HPLC purity of compound (VIIa) was 99%, and the content of compound (VIIb) was less than 0.05%.

[0051] Preparation of 6-(nitrooxy)hexanoyl chloride (compound (IVa)) N,N-dimethylformamide and oxalyl chloride are added dropwise to a solution of 6-(nitrooxy)hexanoic acid in dichloromethane for 1 hour while maintaining the solution temperature at 0°C to 5°C. Then, the mixture is stirred at 15°C to 30°C for 24 hours, and the solvent is evaporated to obtain 6-(nitrooxy)hexanoyl chloride in a yield of 88-97% w / w. This is used without further purification.

[0052] Preparation of hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (compound (I)) All processes are carried out under a nitrogen atmosphere.

[0053] Add bimatoprost to methyl tert butyl ether, cool the resulting solution to approximately 15°C-18°C, then add n-butylboronic acid (1.11-1.18 equivalents) all at once, and stir the mixture at 40°C for approximately 1-1.5 hours. The reaction is complete. 1 The reaction mixture is monitored by 1H NMR analysis, cooled to approximately 20°C-25°C, filtered, and the resulting water is removed by azeotropic distillation of methyl tert-butyl ether at a temperature of 40°C or below until the water content is 0.25% or less, yielding crude bimatoprostboronate (compound (II)). This is used in the next step without further purification.

[0054] Crude bimatoprostboronate (compound (II)) is added to methyl tert butyl ether, the resulting solution is cooled to approximately 0°C to 5°C, 4-dimethylaminopyridine (approximately 2.1 to 2.3 equivalents) is added, and 6-(nitrooxy)hexanoyl chloride (IVa) (1.5 equivalents) dissolved in methyl tert butyl ether is added dropwise while maintaining the temperature of the mixture at approximately 0°C to 5°C. After addition, the mixture was stirred at approximately 0°C to 5°C for a maximum of 4 hours, then stirred overnight at 15°C to 20°C, and the completion of the reaction was monitored by HPLC analysis. (1S,2E)-3-{(6R,7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl-6-(nitrooxy)hexanoate (compound (III)) was isolated by standard workup methods (an example of workup is described in Example 1).

[0055] (1S,2E)-3-{(6R,7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl-6-(nitrooxy)hexanoate (compound (III)) is dissolved in methanol, and the resulting solution is stirred at 17°C to 25°C for approximately 18 hours to convert compound (III) to compound (I). 1 The reaction is monitored by 1H NMR. If the reaction stops, the mixture is evaporated and redissolved in fresh methanol until completely converted. The reaction mixture is then concentrated under vacuum at a temperature below 40°C, and the crude hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (compound (I)) is isolated by standard work-up procedures.

[0056] HPLC quantitative analysis of the crude reaction mixture revealed compound (I) at approximately 70% content, 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)) at approximately 0.12%, and a dimeric impurity (compound (XII)) at a level below the HPLC detection limit at 0.05% (see Table 1).

[0057] The crude reaction mixture was purified by column chromatography using a silica gel column and a mixed solvent of dichloromethane and methanol to obtain compound (I) in total yield of over 60% w / w relative to bimatoprost.

[0058] The HPLC quantitative analysis of compound (I) showed a purity of over 98%, with the amount of bimatoprost 15-(6-chlorohexanoyl)ester (compound (X)) being less than 0.15%, and the content of dimeric impurities (compound (XII)) being less than 0.05%.

[0059] The method of the present invention produces by-products, in particular (S,E)-1-((1R,2R,3S,5R)-2-((Z)-7-(ethylamino)-7-oxohepta-2-enyl)-3,5-dihydroxycyclopentyl)-5-phenylpenta-1-en-3-yl 6-chlorohexanoate (compound (X)) and 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoate of bimatoprost (compound (XII)) (this compound is used in bacteria We provide hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (I) in high yield and high purity, which has a reduced amount of mutagenicity (predicted to be positive for in vitro mutagenicity).

[0060] Due to the above advantages, the method of the present invention is a cost-effective method that can be easily adapted to industrial scale. [Examples]

[0061] Experimental example All synthesis steps described below were carried out under a nitrogen atmosphere.

[0062] Example 1 Synthesis of hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (compound (I)) Synthesis of 6-(nitrooxy)hexanoyl chloride (compound (IV)) Step 1: Synthesis of potassium 6-hydroxyhexanoate salt (compound (VI)) A solution of potassium hydroxide (1.98 g, 1 equivalent) in methanol (10 ml) was prepared while cooling to 15°C-20°C, and this was added to a solution of 3.65 g of 2-caprolactone (1 equivalent) in methanol (6 ml) within 0.5 hours at 5°C-20°C. This mixture was stirred at 15°C-20°C for 4.5 hours. The reaction mixture was concentrated under vacuum (at a temperature below 40°C) to give crude potassium 6-hydroxyhexanoate salt (6.10 g). This crude product was re-slurried in methyl tert butyl ether (10 ml) at 20°C-25°C for 4 hours, filtered, washed with methyl tert butyl ether (2 × 25 ml), and dried under vacuum (at a temperature below 40°C) to obtain potassium 6-hydroxyhexanoate salt (5.16 g) in 93.7% yield. Melting point 208°C.

[0063] Step 2: Synthesis of 6-(nitrooxy)hexanoic acid (compound (VIIa)) Fuming HNO3 (4.6 equivalents) was added to concentrated H2SO4 (equivalent to 3.1) over 14 minutes at a temperature of 0°C to 5°C, and then CH2Cl2 (20 ml) was added over 12 minutes at a temperature of 0°C to 5°C. Potassium 6-hydroxyhexanoate salt (10.13 g, 1 equivalent) was added little by little over 28 minutes at a temperature below 10°C. This mixture was stirred at 0°C to 10°C for 2.2 hours to complete the reaction. 1 ¹H-NMR monitoring revealed a 99.9% conversion. The mixture was cooled to 0°C-5°C, and saturated sodium chloride aqueous solution was carefully added at a temperature below 10°C within 17 minutes. After filtration, the organic layer was decanted, dried over sodium sulfate, and concentrated under vacuum (at a temperature below 40°C) to obtain 9.15 g of 6-(nitrooxy)hexanoic acid (yield 87.7%) as 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (compound (VIIb)) with 96.2% HPLC purity and 0.75% content.

[0064] Step 3: Purification of crude 6-(nitrooxy)hexanoic acid (compound (VIIa)) containing 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (compound (VIIb)) 1.5g Crude 6-(nitrooxy)hexanoic acid (VIIa) obtained in step 2 is processed into Purezza®-Sphera Plus The compound was loaded into a 48g Flash Cartridges-C18, 25μ-standard flash cartridge and eluted with H2O + 0.1% HCOOH / acetonitrile at a ratio of 90:10 to 75:25. The fraction containing compound (VIIa) eluted at 78:22 was collected, extracted with CH2Cl2 (3 × 150 mL), dried with sodium sulfate, and concentrated under vacuum at a temperature below 40°C to give 1.04 g of 6-(nitrooxy)hexanoic acid (yield 69%) with an HPLC purity of over 99.5% and less than 0.05% of compound (VIIb).

[0065] Step 4: Synthesis of 6-(nitrooxy)hexanoyl chloride (compound (IVa)) 6-(nitrooxy)hexanoic acid (2.61 g, 1 equivalent) was dissolved in dichloromethane (15 ml) and cooled to 0°C to 5°C under nitrogen. Then, N,N-dimethylformamide (0.16 ml) and oxalyl chloride (1.93 g) were added within 34 minutes at 0°C to 5°C. The reaction mixture was stirred at 0°C to 5°C for 3.5 hours, and then at 15°C to 20°C for 14 hours. Then, the reaction mixture was concentrated under vacuum (at a temperature of 40°C or less) and co-evaporated with dichloromethane to obtain 6-(nitrooxy)hexanoyl chloride (compound (IVa)) (2.8 g) in 97% yield.

[0066] Step 5: Synthesis of (Z)-7-[(1S,5R,6R,7R)-3-butyl-6-[((E,3S)-3-hydroxy-5-phenyl-penta-1-enyl]-2,4-dioxa-3-borabicyclo[3.2.1)octan-7-yl]-N-ethylhepta-5-enamide (compound (II)) Bimatoprost (2.3 g, 1 equivalent) was dissolved in methyl tert butyl ether (30 ml), and butylboronic acid (0.62 g, 1.13 equivalents) was added. This mixture was heated at 40°C for 1 hour. This reaction was continued until the conversion rate reached 97%. 1 The data was monitored using 1H NMR.

[0067] The reaction mixture was rinsed with methyl tert butyl ether (10 ml), and the mixture was heated under vacuum at approximately 40°C for azeotropic distillation. Rinsing with methyl tert butyl ether and azeotropic distillation were continued until the water content of (Z)-7-[(1S,5R,6R,7R)-3-butyl-6-[((E,3S)-3-hydroxy-5-phenyl-penta-1-enyl]-2,4-dioxa-3-borabicyclo[3.2.1)octan-7-yl]-N-ethyl-hepta-5-enamide (compound (II)) was 0.25% or less. Compound (II) was obtained in quantitative yield (2.84 g).

[0068] Step 6: Synthesis of (1S,2E)-3-{(6R,7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl 6-(nitrooxy)hexanoate (compound (III)) (Z)-7-[(1S,5R,6R,7R)-3-butyl-6-[((E,3S)-3-hydroxy-5-phenyl-penta-1-enyl]-2,4-dioxa-3-borabicyclo[3.2.1)octan-7-yl]-N-ethyl-hepta-5-enamide (compound (II)) (3.21 g, 1 equivalent) was dissolved in methyl tert butyl ether (30 ml) and cooled to 0°C to 5°C. 4-dimethylamino Pyridine (1.85 g, 2.27 equivalents) was added all at once. A solution of 6-(nitrooxy)hexanoyl chloride (compound (IVa)) (1.96, 1.5 equivalents) in methyl tert-butyl ether (5 ml) was added dropwise at 0°C to 5°C within 1 hour. After stirring at 0°C to 5°C for 24 minutes and at 15°C to 20°C for 14.5 hours, the reaction mixture was cooled at 0°C to 5°C, and deionized water was added at a maximum temperature of 15°C within 20 minutes.

[0069] The mixture was stirred for 5 minutes. The aqueous layer was discarded. The organic layer was washed with 1N hydrochloric acid solution, then deionized water, and finally brine.

[0070] The organic layer was dried over sodium sulfate and concentrated under vacuum to obtain (1S,2E)-3-{(6R,7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl 6-(nitroxy)hexanoate (compound (III)) (4.1 g, yield 94%).

[0071] Step 7: Synthesis of Hexanoic Acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (I) (compound (I)) (1S, 2E)-3-{(6R, 7R)-3-butyl-7[(2Z)-7-(ethylamino)-7-(ethylamino)-7-oxohepta-2-en-1-yl]-2,4-dioxa-3-borabicyclo[3.2.1]octa-6-yl}-1-(2-phenylethyl)-propa-2-en-1-yl 6-(nitroxy)hexanoate (compound (III)) (4g crude product, 1 equivalent) was dissolved in methanol (30ml), and the resulting solution was prepared. 1 The reaction was monitored by 1H-NMR while the mixture was stirred at room temperature for 24 hours. Methanol was then removed under vacuum at 35°C to 40°C. The residue was dissolved in methyl tert-butyl ether and washed with deionized water, followed by brine. The organic layer was dried over sodium sulfate and concentrated under vacuum at a temperature below 40°C to give a crude reaction mixture (3.8 g). After HPLC (% area) reverse-phase quantitative analysis, it was shown that the mixture contained 73% compound (I), 0.11% bimatoprost 15-(6-chlorohexanoyl) ester (compound (X)), and less than 0.05% bimatoprost 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester (compound (XII)), a dimeric impurity (see Table 1).

[0072] The residue was chromatographed on a silica gel column using 95:5 dichloromethane / methanol as the eluent. The fractions were monitored by TLC, and only the pure fractions were combined and concentrated under vacuum at a temperature of 40 °C or lower to produce Compound (I) with an HPLC purity of over 98%. The content of the 15-(6-chlorohexanoyl) ester of bimatoprost (Compound (X)) was 0.11%, and the content of the 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester of bimatoprost (Compound (XII)), a dimer impurity, was less than 0.05%.

[0073] Example 2 (Comparative Example) Synthesis of hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (Compound (I)) according to the method disclosed in International Publication No. WO 2009 / 136281 (Scheme 3 below) Step A: Preparation of (Z)-7-[(1S,5R,6R,7R)-3-butyl-6-[((E,3S)-3-hydroxy-5-phenyl-penta-1-enyl]-2,4-dioxo-3-borabicyclo[3.2.1]octan-7-yl]-N-ethyl-hept-5-enamide (Compound (II)) Butylboronic acid (1.129 equivalents) was added to a solution of bimatoprost (1 g, 1 equivalent) in dichloromethane (16 ml). The mixture was heated at 40 °C for 1 hour, 1 and the progress of the reaction was monitored by 1H-NMR. The solvent was removed under reduced pressure for 2 hours. Dichloromethane (16 ml) was added, and the mixture was heated at 40 °C for an additional 1 hour. The solvent was removed under reduced pressure for 40 minutes. Dichloromethane (16 ml) was added, and the mixture was heated at 40 °C for 16 hours. The solvent was evaporated, and the crude product was dried at 40 °C under high vacuum for 3 hours to produce Compound (II) in quantitative yield. This was used in the next step without any further purification. MS: m / z = 438 [M + H]+

[0074] Step B: Synthesis of (S,E)-1-((1S,5R,6R,7S)-3-butyl-7-((Z)-7-(ethylamino)-7-oxohepta-2-en-1-yl)-2,4-dioxa-3-borabicyclo[3.2.1]octan-6-yl)-5-phenylpenta-1-en-3-yl 6-bromohexanoate (compound (XI)) 4-dimethylaminopyridine (1.1 equivalents) and 6-bromohexanoyl chloride (1.15 equivalents) were added to a solution of compound (II) (0.8 g, 1 equivalent) in dichloromethane (15.3 ml) cooled to 0-5°C. This mixture was stirred at 0-5°C for 0.5 hours and at 20-25°C for 16 hours.

[0075] 4-dimethylaminopyridine (0.25 equivalents) and 6-bromohexanoyl chloride (0.25 equivalents) were added, and the mixture was stirred for a further 19 hours. This reaction was continued until the transformation was complete. 1 The mixture was monitored by 1H-NMR. The mixture was diluted with dichloromethane (15.3 ml), and the organic solution was washed with deionized water (6.25 ml) and brine (6.25 ml). The organic phase was dried over Na2SO4 and concentrated under vacuum to give compound (XI) as a pale yellow oily substance (calculated as quantitative yield). This was used in the next step without further purification.

[0076] Step C: Synthesis of hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester (compound (I)) Silver nitrate (3.72 equivalents) was added to a solution of compound (VIII) (0.8 g, 1 equivalent) in acetonitrile (9.4 ml). This mixture was stirred at 20°C to 25°C for 18 hours. The conversion was carried out in DMSO. 1 The monitoring was performed using 1H-NMR.

[0077] Silver nitrate (0.5 equivalents) was added, and the mixture was then stirred for a further 20 hours until HPLC analysis showed a 99.7% conversion.

[0078] The mixture was filtered through a Whatman filter. The filtrate was concentrated under vacuum. The residue was dissolved in ethyl acetate (30 ml). The organic phase was washed with deionized water (5 ml) and brine (5 ml). After drying with Na2SO4, the layers were concentrated under vacuum. The residue was chromatographed using a silica gel column with a 95:5 dichloromethane / methanol eluent. The fractions were monitored by TLC, and the pure fractions were mixed and concentrated under vacuum at a temperature below 40°C to produce compound (I) in 86% overall yield with 4.27% bimatoprost.

[0079] HPLC (% area) reverse-phase quantitative analysis revealed that the purity of compound (I) after chromatography was 77%, and the content of bimatoprost 15-(6-chlorohexanoyl) ester (compound (X)) was 8.34%.

[0080] [Table 1]

[0081] Table 1 reports the results of HPLC quantitative analysis of compound (I) and the main impurities formed during the preparation of compound (I) by the method of the present invention (Example 1) and the method disclosed in International Publication No. 2009 / 136281 (Example 2).

[0082] Compound (X) is 15-(6-chlorohexanoyl)ester of bimatoprost, which is predicted to be positive for bacterial in vitro mutagenicity. The results show that the method of the present invention provides compound (I), comprising compound (X) in a content of 0.11% and 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid ester of bimatoprost (compound (XII)) in a subdetector-limit "dimeric impurity" of 0.05%. Methods disclosed in the prior art result in compound (I) having a lower chemical purity and a content of 8.34%, i.e., more than 30 times greater than the amount of compound (X) formed by the method of the present invention.

[0083] The results demonstrate that the method of the present invention shows improvement over the methods described in the prior art.

[0084] [ka]

Claims

1. Equation (I): 【Chemistry 1】 A method for preparing hexanoic acid, 6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, comprising the following steps: 1) Compound (II): 【Chemistry 2】 6-(nitrooxy)hexanoyl chloride (IVa) 【Transformation 3】 Then, reacting in the presence of free 4-dimethylaminopyridine, compound (III) is obtained. 【Chemistry 4】 The process of obtaining, 2) A step of removing the boronate protecting group from compound (III) to obtain the compound represented by formula (I), 3) A step of purifying compound (I), 6-(nitrooxy)hexanoyl chloride (IVa) is produced by the following process: 4) 2-Caprolactone (V) 【Transformation 5】 This is reacted with an inorganic base selected from KOH, NaOH, and LiOH to obtain formula (VI). 【Transformation 6】 [In the formula, M is K, Na, or Li] The process of obtaining the 6-hydroxyhexanoate shown, 5) The compound represented by formula (VI) is HNO ３ and H ２ SO ４ Esterilized with a mixture of the following, 6-(nitrooxy)hexanoic acid (VIIa): 【Transformation 7】 The process of obtaining, 6) Formic acid (H) in water ２ The process involves purifying this nitrate esterified mixture by reverse-phase chromatography using O + HCOOH and acetonitrile as eluents to obtain pure 6-(nitrooxy)hexanoic acid represented by formula (VIIa), 7) The preparation is characterized by comprising the step of reacting pure 6-(nitrooxy)hexanoic acid (VIIa) with a chlorinating agent to obtain 6-(nitrooxy)hexanoyl chloride (IVa), method.

2. The method according to claim 1, wherein step 1) is carried out in an aprotic organic solvent at a temperature in the range of 0°C to room temperature.

3. The method according to claim 2, wherein the aprotic organic solvent is methyl tert-butyl ether.

4. The method according to any one of claims 1 to 3, wherein in step 1), the molar ratio of compound (II) to 6-(nitrooxy)hexanoyl chloride (IVa) is 1:1.4 to 1:1.6, and the molar ratio of compound (II) to 4-dimethylaminopyridine is 1:2.0 to 1:2.

4.

5. The method according to any one of claims 1 to 4, wherein in step 4) the inorganic base is potassium hydroxide.

6. The method according to any one of claims 1 to 5, wherein step 4) is carried out in a solvent selected from methanol, ethanol, or isopropanol.

7. The method according to claim 6, wherein the solvent is methanol.

8. The method according to any one of claims 1 to 7, wherein step 5) is carried out in dichloromethane.

9. In step 6), the formic acid (H) in the water ２ The method according to any one of claims 1 to 8, wherein the concentration of (O + HCOOH) is 0.1% w / w.

10. In step 6), the chromatographic fraction containing compound (VIIa) is subjected to CH ２ Cl ２ The method according to any one of claims 1 to 9, comprising extracting, drying, and evaporating the solvent.

11. The method according to any one of claims 1 to 10, wherein step 7) is carried out in dichloromethane, and the chlorinating agent is oxalyl chloride.

12. The method according to any one of claims 1 to 11, wherein the 6-(nitrooxy)hexanoyl chloride obtained in step 7) is used without further purification.

13. A method for preparing 6-(nitrooxy)hexanoic acid (compound (VIIa)) with a purity of 99%, comprising the following steps: 4) 2-Caprolactone (V) 【Transformation 8】 When reacted with KOH in methanol, formula (VI) is obtained. 【Chemistry 9】 [In the formula, M is K.] The process of obtaining the 6-hydroxyhexanoate shown, 5) A step of subjecting the compound represented by formula (VI) to nitrate esterification in dichloromethane at a temperature of 5°C to 10°C using a mixture of HNO ３ and H ２ SO ４ 5)式(VI)で示される化合物をHNO ３ とH ２ SO ４ との混合物により、ジクロロメタン中において、5℃~10℃の温度で硝酸エステル化する工程と、 5) A step of subjecting the compound represented by formula (VI) to nitrate esterification in dichloromethane at a temperature of 5°C to 10°C using a mixture of HNO ３ and H ２ SO ４ and performing nitrate esterification at a temperature of 5°C to 10°C in dichloromethane with a mixture thereof; 6) The nitrate esterified mixture is purified by reverse-phase chromatography using a solution of 0.1% formic acid and acetonitrile in water as the eluent, and this fraction is then extracted with dichloromethane to obtain 6-(nitrooxy)hexanoic acid (VIIa). 【Chemistry 10】 The process includes obtaining method.

14. The method according to claim 13, wherein the nitrate esterification of the compound represented by formula (VI) in step 5) is carried out at 10°C.

15. It has a chemical purity of over 98%, It contains 0.12% of 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)), 【Chemistry 11】 The hexanoic acid, 6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester represented by formula (I).

16. Equation (I): 【Chemistry 12】 It contains hexanoic acid, 6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester, and at least a pharmaceutically acceptable excipient. Here, hexanoic acid,6-(nitrooxy)-,(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester has a chemical purity of over 98% and contains 0.12% of bimatoprost 15-(6-chlorohexanoyl) ester (compound (X)). Pharmaceutical preparations.

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