RELATED APPLICATION:

This application claims the benefit of the earlier filing date of Indian Provisional Patent Application No. 201721019060 filed on May. 31, 2017 and Indian Provisional Patent Application No. 201821003970 filed on Feb. 02, 2018.

FIELD OF THE INVENTION

The present invention relates to an improved process for the preparation of compound of formula (A), which is an intermediate in the preparation of Obeticholic acid or its analogous compounds thereof.

Formula (A)

BACKGROUND OF THE INVENTION

Obeticholic acid is a semi- synthetic bile acid analogue also known as INT-747; and has a chemical name 3a, 7a-dihydroxy 6a-ethyl-5P-cholan-24-oic acid. Obeticholic acid exhibits stereoisomerism due to the presence of 11 chiral centers as shown in the below structure of formula (I)

Formula (I)

OCALIVA (Obeticholic acid) is a farnesoid X receptor (FXR) agonist indicated for treatment of primary biliary cholangitis (PBC) in combination with ursodeoxycholic acid (UDCA) in adults with an inadequate response to UDCA, or as monotherapy in adults unable to tolerate UDCA. WO 2002/072598 discloses Obeticholic acid and a process for the preparation of Obeticholic acid.

US 7,994,352 discloses a process for the preparation of Obeticholic acid as shown in the below Scheme-I

THF, LDA

Trimet ylc loro silane

Crude Obeticholic acid

Cooling, Crystallization

H ₂ 0, 30% Ammonia

Phosphoric acid

In the above process the epimerization is carried out under basic condition at high temperature such as 95-105°C.

Considering the importance of the Obeticholic acid in the pharmaceutical field there remains a need to identify a robust process for the preparation of Obeticholic acid in the commercial scale that yield the Obeticholic acid with good purity in terms of chemical as well as chiral purity.

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a process of epimerizing the compound Formula (B) to compound of Formula (A) in presence of lewis acid.

Formula (B) Formula (A)

Another aspect of the present invention is to provide a process of preparation of Obeticholic acid of Formula (I) or its analogous compounds thereof from compound of Formula (A).

DETAILED DESCRIPTION OF THE INVENTION

In stereochemistry, an epimer is one of a pair of stereoisomers. The two isomers differ in configuration at only one stereogenic center. The process of converting one member of the epimeric pair to the other, by changing the stereochemistry at only one asymmetric center, can be referred to as epimerization.

An aspect of the present invention relates to a process of epimerizing the compound Formula (B), to compound of Formula (A) in presence of lewis acid, wherein, R is a hydrogen or C ₁ -C5 alkyl group such as methyl, ethyl and the like, R ¹ is hydrogen or carboxy protecting group selected from alkyl, cycloalkyl, aryl, cycloaryl, hydrogen and the like and PG ₂ is hydrogen or hydroxyl protecting group selected from triethylsilyl, triisopropylsilyl, t- butyldimethylsilyl, t-butyldiphenylsilyl, methyl, t-butyl, allyl, benzyl, methoxymethyl, ethoxyethyl, 2-tetrahydropyranyl, formyl, acetyl, benzoyl, mesyl, tosyl, alkoxy or aryloxy carbonyl and the like.

In an embodiment of the present invention, the compound of Formula (B) may be essentially β-isomer, a mixture of a-isomer and β-isomer or a mixture of a-isomer and β- isomer with enriched β-isomer content at the C-6 position. In another embodiment of the present invention, the Formula (A) is essentially a- isomer or a mixture of a-isomer with reduced content of β-isomer at the C-6 position.

Still another embodiment of the present invention, the lewis acid is selected from metal halide such as boron trifluoride-etherate, iron chloride, aluminum chloride, aluminum bromide, titanium chloride, titanium tetrachloride, zinc chloride, zirconium(IV) oxydichloride octahydrate (ZrOCl ₂ - 8H ₂ 0) and zirconium(IV) tetrachloride (ZrCl ₄ ), Hafnium tetrachloride (HfCl ₄ ) and complex compounds thereof. The amount of lewis acid employed is not particularly limitative.

Yet another embodiment of the present invention the epimerization step is may be carried out in presence of any suitable solvent or mixture thereof that does not essentially obstruct the reaction. The epimerization is preferably conducted at a temperature in the range of 10 to 50° C, preferably at room temperature.

In more embodiment, the present invention relates to a process for epimerization of 3a-hydroxy-6-ethyl-7-keto-5P-cholanic acid of Formula (V) [R is methyl, R ¹ is hydrogen and PG ₂ is hydrogen in compound of formula (B)] having enriched content of β-isomer at the C- 6 position to 3a-hydroxy-6a -ethyl-7-keto-5P-cholanic acid of Formula (VI) [R is methyl, R ¹ is hydrogen and PG ₂ is hydrogen in compound of formula (A)] having less or no content of β-isomer.

Formula (V) Formula (VI) In still another embodiment, the present invention relates to a process of preparation of

Obeticholic acid comprising converting compound of Formula (VI) to Obeticholic acid of Formula (I) or its analogous compounds thereof.

Another embodiment of the present invention, the Formula (V) is optionally re- crystallized from solvents such as ethyl acetate or n- butyl acetate before subjecting into epimerization reaction. In few embodiments of the invention, the compound of Formula (VI) obtained after the epimerization is purified or crystallized using any known methods. Preferably the compound of Formula (VI) obtained after the epimerization is purified or crystallized using suitable solvent such as esters like ethyl acetate or n-butyl acetate, organic acids such as acetic acid, formic acid or propionic acid optionally with water or mixtures thereof. Inventors found the use of acetic acid /water yield the product in good purity.

In few embodiments, the compound of Formula (V) or Formula (A) can be prepared by any known methods reported in the literature and converted to Obeticholic acid of Formula (I) or analogous compounds or by following the process as depicted in the following Scheme-2:

Formula (II) Formula(III) Formula(IV)

Formula (B) or (V) Formula(IVa)

In the above scheme, PGi is any protected groups; preferably PGi is trimethylchlorosilane, tert-butyldimethylsilyl chloride, hexamethyldisilazane or diphenyl chlorosilane. PG ₂ is hydrogen or hydroxyl protecting group selected from triethylsilyl, triisopropylsilyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, methyl, t-butyl, allyl, benzyl, methoxymethyl, ethoxyethyl, 2-tetrahydropyranyl, formyl, acetyl, benzoyl, mesyl, tosyl, alkoxy or aryloxy carbonyl and the like. R ¹ is hydrogen, alkyl, cycloalkyl, aryl, cycloaryl. The compound of Formula (III), wherein R is benzyl, obtained as per the scheme-2 is purified by recrystallization in a suitable solvent or mixture thereof, preferably cyclohexane. In the above scheme, the product(s) obtained after each step may be directly used in next steps without isolating or purifying; or the product(s) may be purified or crystallized using any well-known techniques to one of skill in the art; and the reaction times, moles, and reaction temperature not particularly limitative.

In an embodiment of the invention the compound of Formula (A) or (VI) may be converted to Obeticholic acid by any process known in art, specifically in presence of sodium borohydride. In some embodiments of the invention, the analogous compound is 6a -ethyl- 3a,7a -23-trihydroxy-24-nor-5 P-cholan-23 -sulfate as represented by formula shown below, and its salts such as sodium salt.

In certain embodiments, the completion of the reaction may be monitored by known techniques, including, but not limited to, HPLC (high performance liquid chromatography), TLC (thin layer chromatography), LC-MS (Liquid chromatography-mass spectrometry) and gas chromatography (GC).

The invention is further exemplified by the following non-limiting examples, which are illustrative representing the preferred modes of carrying out the invention. The invention's scope is not limited to these specific embodiments only but should be read in conjunction with what is disclosed anywhere else in the specification together with those information and knowledge which are within the general understanding of the person skilled in the art.

Examples

Example 1: Preparation of 3a-hydroxy-6a-ethyl-7-keto-5p-cholanic acid of formula (VI) To THF, 3a-hydroxy-6-ethyl-7-keto-5P-cholanic acid (content of β isomer at C-6 position > 60 %) (100 g) and titanium tetrachloride (45.3 g) was added at 15-20 °C and stirred till completion of the reaction. After completion of the reaction, the reaction mass was treated with water and toluene then organic phase was separated and evaporated under vacuum. To the obtained crude product ethyl acetate was added and stirred for 2 hrs at 25-30 °C then filtered and suck dried. Then obtained wet product is re-crystallized from acetic acid and water to obtain pure 3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid. (70 g) (content of β isomer is < 0.5 % ).

Example 2: Preparation of 3a-hydroxy-6a-ethyl-7-keto-5p-cholanic acid of formula (VI) To 3a-hydroxy-6-ethyl-7-keto-5P-cholanic acid (content of β isomer > 60 %) (100 g) in THF, ZrCl ₄ or (55.0 g) was added in to stirring apparatus at 20-30 °C and stirred till completion of the reaction. After completion of the reaction, the reaction mass was treated with water and ethyl acetate then organic phase was separated and evaporated under vacuum to obtain pure 3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid (105 gm). Example 3: Preparation of Obeticholic acid of formula (I)

To 3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid of formula (VI) (100 g) in water (450 ml), sodium hydroxide (14.33 g) was added, temperature was raised to 100+5°C, followed by addition of solution of sodium borohydride (13 g) in water and the reaction mass was maintained at 100+5°C till the completion of reaction. The obtained reaction mixture was cooled to room temperature and dichloromethane (MDC) was added, pH was adjusted to 2.5 using ortho phosphoric acid, and stirred till clear solution is obtained. Layers were separated, solvent (MDC) was distilled partially under vacuum, the product precipitated was filtered. The filtered product was dissolved in dichloromethane stirred and cooled. The product precipitated was filtered and washed to obtain pure Obeticholic acid (60-80g). Example 4: Preparation of Amorphous Obeticholic acid

To a clear solution of Obeticholic acid (100. Ogm) in a mixture of water (2000 ml) and liquid ammonia (20ml, 25% aq. solution), orthophosphoric acid (20 ml) was added at 25-30 °C to precipitate the product. The precipitated solid was stirred for 2 hrs. Filtered the solid and washed with water. Dried under vacuum at 45°C (80 g). Example 5: Preparation of sodium salt of Obeticholic acid

3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid of formula (VI) (5.0 g) was dissolved in a solution of water (45 ml) and NaOH (0.71 g) and treated with a solution of NaBH ₄ (0.67 g) in water (20 ml) at around 100°C. Ortho -phosphoric acid (95.0 ml) and MDC (75 ml) were added. Separate organic and aqueous layer, solvent was evaporated under vacuum to obtained residue. Methanol (50 ml) was added, stirred till clear solution was obtained, water (10ml) and Sodium hydroxide (0.695gm) were added and stirred at 40°C. Distill to dryness to obtain oil. Acetonitrile was added (50ml) and stirred for 8-10 hours, filtered, washed to obtain Sodium salt of Obeticholic acid. Example 6: Preparation of potassium salt of Obeticholic acid

3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid of formula (VI) (5.0 g) was dissolved in a solution of water (45 ml) and NaOH (0.71 g) and treated with a solution of NaBH ₄ (0.67 g) in water (20 ml) at around 100°C. Ortho -phosphoric acid (95.0 ml) and MDC (75 ml) were added. Separate organic and aqueous layer, solvent was evaporated under vacuum to obtained residue. Methanol (50 ml) was added, stirred till clear solution was obtained, water (10ml) and potassium hydroxide (0.695gm) were added and stirred at 40°C. Distill to dryness to obtain oil. Acetonitrile was added (50ml) and stirred for 8-10 hours, filtered, washed to obtain potassium salt of Obeticholic acid, potassium salt of Obeticholic acid (5.0g) in 100 ml DM water, added dropwise ortho-ophosphoric acid till pH below ~3, suspended solid was stirred for 2-3 hrs, the solid material was filtered, and washed to obtain amorphous form of Obeticholic acid.

Example 8: Preparation of Amorphous Obeticholic acid

3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid of formula (VI) (5.0 g) was dissolved in a solution of water (45 ml) and NaOH (0.71 g) and treated with a solution of NaBH ₄ (0.67 g) in water (20 ml) at around 100°C. Ortho -phosphoric acid (95.0 ml) and MDC (75 ml) were added. Separate organic and aqueous layer, solvent was evaporated under vacuum to obtained residue. Methanol (50 ml) was added, stirred till clear solution was obtained. Filtered, and calcium chloride solution was added (1.325g in 10 ml), reaction mass was stirred, filtered and washed to obtain calcium salt of Obeticholic acid. Calcium salt of Obeticholic acid (5.0g) in 100 ml DM water, added dropwise ortho-ophosphoric acid till pH below ~3, suspended solid was stirred for 2-3 hrs, the solid material was filtered, and washed to obtain amorphous form of Obeticholic acid.

Example 9: Preparation of Amorphous Obeticholic acid

3a-hydroxy-6a-ethyl-7-keto-5P-cholanic acid of Formula (VI) (5.0 g) was dissolved in a solution of water (45 ml) and NaOH (0.71 g) and treated with a solution of NaBH ₄ (0.67 g) in water (20 ml) at around 100°C. Ortho -phosphoric acid (95.0 ml) and MDC (75 ml) were added. Separate organic and aqueous layer, solvent was evaporated under vacuum to obtained residue. Methanol (50 ml) was added, stirred till clear solution was obtained. Filtered, and Magnesium sulfate solution was added (2.1gm in 7.5 ml), reaction mass was stirred, filtered and washed to obtain magnesium salt of Obeticholic acid. Magnesium salt of Obeticholic acid (5.0g) in 100 ml DM water, added dropwise ortho-ophosphoric acid till pH below ~3, suspended solid was stirred for 2-3 hrs. Dichloromethane was added and stirred till clear solution is obtained. Layers were separated and solid was isolated from organic solvent. Solid again dissolved in a mixture of water (100 ml) and liquid ammonia (10ml, 25% aq. solution). Orthophosphoric acid (20 ml) was added at 25-30 °C to precipitate the product. The precipitated solid was stirred for 2 hrs. Filtered the solid and washed with water. Dried under vacuum at 45°C.

Reference Example 1: Preparation of benzyl 3a-hydroxy-7-keto-5p-cholan-24-oate of formula (III) To 7-keto-lithocholic acid of Formula (II) (lOOg) in Ν,Ν-dimethylformaamide (DMF) (600ml), potassium carbonate (53.08 g) was added and temperature was raised to 50-60°C. To the reaction mass, solution of benzyl chloride (35.64 g) in DMF (50ml) was slowly added, temperature was raised to 50-60°C and reaction mixture was stirred at 50-60°C till completion of reaction. After completion of the reaction, the product was extracted with ethyl acetate and concentrated to get crude product. The resulting crude product was dissolved in cyclohexane and heated to 70-75 °C, then reaction mass was stirred, cooled, filtered, washed with cyclohexane and dried. (0.9-1.2 g, 75-95% yield).

Reference Example 2: Preparation of 3a-hydroxy-6-ethyl-7-keto-5p-cholanic acid of formula (V) To the solution of benzyl 3a-hydroxy-7-keto-5P-cholan-24-oate of Formula (IV) (lOOg) in tetrahydrofuran (THF) (300ml) 2.5M n-butyl lithium (79.94 gm), di-isopropyl amine (147.33 gm) and trimethylchlorosilane (TMSC1) (113 g) were added at -65 to -75 °C. To the resulting reaction mixture triethyl amine (TEA) was added drop wise and stirred for 1 hr at -78 °C then temperature was raised to 0 °C and stirred till completion of reaction. After completion of the reaction added sodium bicarbonate solution (500 ml) and the product was extracted with ethyl acetate and concentrated to get compound of Formula (IV) as a residue.

The residue was dissolved in dichloromethane (MDC) (1000 ml) followed by acetaldehyde (18.3 g) and boron trifluoride diethyl ether (147.6 g) were added at -75 °C and stirred for 2 hrs then temperature was raised to 25-30 °C. After completion of the reaction, the product was treated with sodium bicarbonate solution followed by extraction with MDC and concentrated to get compound of Formula (IVa).

The residue was dissolved in THF: toluene (1:1) (100 ml) followed by 10% Pd/C was added at 35-40 °C. The reaction solution was flushed with nitrogen and stirred at room temperature under hydrogen pressure (7-8 kg) for 24 hrs. The mixture was filtered through hyflo and the recovered filtrate was evaporated under vacuum to get crude compound of Formula (V) which was re-crystallized from n-butyl acetate. (55 g, 50 % yield) (content of β isomer > 60 %).