NOVEL CRYSTALLINE FORMS

Field of invention:

The present invention relates to novel crystalline forms of Candesartan cilexetil and intermediate thereof. Particularly, the present invention relates to two novel crystalline forms of candesartan cilexetil which are designated as form G and form H and a novel crystalline form of candesartan, tritylated candesartan and tritylated candesartan cilexetil and process for their preparation.

Background of the invention:

The chemical name of Candesartan cilexetil is 1-[[(cyclohexyloxy)carbonyl]oxy]ethyl 2-ethoxy-1 -[[2-(IH-tetazole-5-yl)[1 , 1 '-biphenyl-4-yl]methyl]-IH-benzimidazole-7- carboxylate. Its molecular formula is 0 ₃₃ Ha ₄ NeO ₆ and mol wt is 610.66. Candesartan cilexetil is represented by structural formula (I).

Candesartan cilexteil is an ester prodrug of 2-ethoxy-1-[[2-(IH-tetrazole-5-yl)[1 ,1'- biphenyl-4-yl]methyl]-1 H benzimidazole-7-carboxylic acid (candesartan), known as a potent Angiotensin Il receptor antagonist. It is useful in the treatment of cardiovascular complaints such as hypertension and heart failure. Candesartan cilexetil is a white to off-white powder and is sparingly soluble in water and in methanol. It is marketed by AstraZeneca under tradename ATACAND ^® .

U.S. Pat. No. 5,196,444 relates to crystal form of Candesartan Cilexetil i.e. C-type crystal (form I) and it describes a process of preparation of Candesartan cilexetil in which it is formed by reacting 2-ethoxy-1-[[2'-(N-triphenyImethyltetrazol-5-yl)biphenyl -4-yl]methyl]benzimidazole-7-carboxylic acid in dimethylformamide with cyclohexyl- 1-iodoethyl carbonate to form cilexetil trityl candesartan and its subsequent deprotection with a methanolic hydrochloric acid gives candesartan cilexetil in 47% yield after column chromatography. The yield of C-type crystal obtained by this process is very low. Moreover, the purification of final product by chromatography is commercially not suitable and is cumbersome at an industrial scale.

Chem. Pharm. Bull., 47(2), 182-186 (1999) discloses two crystalline forms of

Candesartan cilexetil such as form I and form Il and an amorphous form. In addition, candesartan cilexetil is heat sensitive and therefore grinding causes unwanted degradation and loss in purity.

WO2004085426 discloses 1 , 4-dioxane solvate and two crystalline forms of candesartan cilexetil.

WO2005077941 describes a process for the preparation of polymorphic forms of Candesartan cilexetil i.e. form-Ill, form-IV, form-V, form-VI, form-VII, form-VIII, form- IX, form-X, form-XI, form-XIII, form-XIV, form-XIV-1 , form-XV, form-XVI, form-XVII, form-VIII, form-XIX, form-XX, form-XXI, form-XXII or XXIII, having less than about 5% by weight of other polymorphic forms (form-l). These forms are hydrates and solvates of candesartan cilexetil.

WO2005123721 A1 describes a process for the preparation of two crystalline forms i.e. form A, form B and an amorphous form. The process disclosed hereinabove are tedious, time consuming and operationally difficult at industrial scale.

WO2006048237 A1 describes a process for the preparation of form 5, form 6, form 7, form 8 and amorphous form. These forms are prepared by dissolving candesartan cilexetil in a chlorinated solvent, optionally concentrate thus obtain solution then liquid hydrocarbon is added to the solution to precipitate out these forms.

Objects of the invention:

A primary object of the present invention is to provide novel crystalline forms of Candesartan cilexetil i.e. form G and form H.

Another object of the present invention is to provide a process for the preparation novel crystalline forms of Candesartan cilexetil i.e. form G and form H, which is simple and easy to handle at an industrial scale and cost effective.

Another object of the present invention is to provide a process for the preparing candesartan form G comprising steps of i) dissolving candesartan cilexetil form C or mixture of forms in acetone and optionally heating until it becomes clear solution ii) cooling the said solution at O ⁰ C to 5 ⁰ C

Yet another object of the present invention is to provide a process for the preparing candesartan form H comprising a step of heating candesartan cilexetil form G at 75°C under reduced pressure.

A further object of the present invention is to provide novel crystalline form of candesartan, tritylated candesartan and tritylated candesartan cilexetil.

Summary of the invention

An aspect of the present invention is to provide novel crystalline forms of Candesartan cilexetil i.e. form G and form H.

Another aspect of the present invention is to provide a process for the preparing candesartan form G comprising steps of iii) dissolving candesartan cilexetil form C or mixture of forms in acetone and optionally heating until it becomes clear solution iv) cooling the said solution at 0 ⁰ C to 5°C

Yet another aspect of the present invention is to provide a process for the preparing candesartan form H comprising a step of heating candesartan cilexetil form G at 75°C under reduced pressure.

A further aspect of the present invention is to provide novel crystalline form of candesartan, tritylated candesartan and tritylated candesartan cilexetil.

Brief description of the drawings Figure-1 is an X-ray powder diffraction pattern of candesartan cilexetil form G Figure-2 is an X-ray powder diffraction pattern of candesartan cilexetil form H Figure-3 is an X-ray powder diffraction pattern of novel crystalline form of candesartan Figure-4 is an X-ray powder diffraction pattern of novel crystalline form of tritylated candesartan

Figure-5 is an X-ray powder diffraction pattern of novel crystalline form of tritylated candesartan cilexetil

Detailed description of the invention:

According to the present invention, it provides novel crystalline forms of

Candesartan cilexetil i.e. form G and form H.

One aspect of the present invention, there is provided a novel crystalline form of Candesartan cilexetil, designated as form G, characterized by an X-ray powder

diffraction spectrum having peaks at about 6.1 , 7.2, 9.1 , 10.9, 11.9, 12.6, 13.1 , 16.4, 20.0, 20.8 and 23.3 ±0.2 degree two-theta. Figure-1 depicts the X-ray powder diffraction spectrum of candesartan cilexetil form G.

According to another aspect of the present invention, there is provided a process for preparation of the form G of candesartan cilexetil comprising steps of i) dissolving candesartan cilexetil form C or mixture of forms in acetone and optionally heating until it becomes clear solution ii) cooling the said solution at 0 ⁰ C to 5°C

One aspect of the present invention, there is provided a novel crystalline form of Candesartan cilexetil, designated as form H, characterized by an X-ray powder diffraction spectrum having peaks at about 9.2, 12.0, 13.0, 15.3, 16.1, 16.7, 17.2, 20.7, 21.0, 25.7 and 32.8 ±0.2 degree two-theta. Figure-2 depicts the X-ray powder diffraction spectrum of candesartan cilexetil form H.

According to another aspect of the present invention, there is provided a process for preparation of the form H of candesartan cilexetil comprising a step of heating candesartan cilexetil form G at 75 ⁰ C under reduced pressure.

Yet another embodiment, the present invention is to provide a novel crystalline form of candesartan, tritylated candesartan and tritylated candesartan cilexetil.

One aspect of the present invention, there is provided a novel crystalline form of candesartan, characterized by an X-ray powder diffraction spectrum having peaks at about 10.4, 11.6, 13.8, 19.1 , 20.6, 20.8, 21.8, 22.6, 23.3, 26.1 , 28.3 and 30.3 ±0.2 degree two-theta. Figure-3 depicts the X-ray powder diffraction spectrum of a novel crystalline form of candesartan.

According to another aspect of the present invention, a novel crystalline form of candesartan is prepared by reacting methyl 1-[(2'-cyanobiphenyl-4-yl) methyl]-2- ethoxy-benzimidazole-7-carboxylate with sodium azide, tri butyl tin chloride in the presence of o-xylene at 135-15O ⁰ C to obtain methyl 2-ethoxy- 1-[(2'-{1H-tetrazole-5- yl} biphenyl-4-yl)-methyl] benzimidazole-7-carboxylate which is hydrolyzed in the presence of sodium hydroxide at 80-85 ⁰ C and recrystallized in acetone to obtain novel crystalline form of Candesartan (2-ethoxy- 1-[(2'-{1 H-tetrazole-5-yl} biphenyl-4- yl)-methyl] benzimidazole-7-carboxylic acid).

One aspect of the present invention, there is provided a novel crystalline form of tritylated candesartan, characterized by an X-ray powder diffraction spectrum having peaks at about 8.7, 9.4, 9.7, 11.6, 14.0, 14.4, 14.9, 18.2, 20.8, 21.1, 21.7, 23.1 , 25.4, 26.5 and 27.0 ±0.2 degree two-theta. Figure-4 depicts the X-ray powder diffraction spectrum of novel crystalline form of tritylated candesartan.

According to another aspect of the present invention, a novel crystalline form of tritylated candesartan is prepared by reacting 2-ethoxy- 1 ~[(2'-{1 H-tetrazole-5-yl} biphenyl-4-yl)-methyl] benzimidazole-7-carboxylic acid with trityl chloride solution in the presence of acetone and triethylamine to obtain novel crystalline form of tritylated candesartan (2-ethoxy- 1-[(2'-{N-tri phenyl methyl tetrazole-5-yl} biphenyl- 4-yl)-methyl] benzimidazole-7-carboxylic acid ).

One aspect of the present invention, there is provided a novel crystalline form of tritylated candesartan cilexetil, characterized by an X-ray powder diffraction spectrum having peaks at about 8.6, 9.0, 9.7, 10.3, 12.0, 12.8, 15.2, 15.8, 16.2, 16.5, 17.3, 19.5, 19.8, 21.0, 21.4, 22.1 , 22.4 and 26.1 ±0.2 degree two-theta. Figure- 5 depicts the X-ray powder diffraction spectrum of novel crystalline form of tritylated candesartan cilexetil.

According to another aspect of the present invention, a novel crystalline form of tritylated candesartan cilexetil is prepared by reacting 2-ethoxy- 1-[(2'-{N-tri phenyl methyl tetrazole-5-yl}biphenyl-4-yl)-methyl] benzimidazole-7-carboxylic acid with cyclohexyl 1-chloroethylcarbonate in the presence of dimethylformamide, potassium ' carbonate to obtain tritylated candesartan cilexetil which is recrystallized in acetone to obtain novel crystalline form of tritylated candesartan cilexetil ((±)-1- (cyclohexyloxycarbonyloxy) ethyl-2-ethoxy- 1-[(2'-{N-tri phenyl methyl tetrazole-5- yl}biphenyl-4-yl)-methyl] benzimidazole-7-carboxylate ).

The process of the present invention is described by the following examples, which are illustrative only and should not be construed so as to limit the scope of the ' invention in any manner.

Example-1

Preparation of Form G of candesartan cilexetil

10 g form C type solid (form-l) or mix form of candesartan cilexetil was dissolved in acetone (60 ml) and heated it to get clear solution at reflux temperature 50-55° C. The clear solution was cooled to 25-35° C and then further chilled it to 0-5° C and stirred for 2 hours. The solution was filtered and washed with chilled acetone (10 ml) and suck dried to obtain form G. X-ray powder diffraction pattern is matching with Figure-1.

Example-2

Preparation of Form H of candesartan cilexetil Form G was dried at 73-75° C under reduced pressure for about 48-50 hours to obtain form H (7.5 g) X-ray powder diffraction pattern is matching with Figure-2.

Example-3

Preparation of 2-Ethoxy- 1-[(2'-{1H-tetrazole-5-yl} biphenyl-4-yl)-methyl] benzimidazole-7-carboxylic acid (CANDESARTAN)

A solution of sodium azide (71.15 g) in D. M. water (214 ml) was prepared in round bottom flask and cooled to 0-10° C. Tri-n-butyl tin chloride (237.58 g) was added to the reaction mass at 0-10° C within 30-60 minutes. Reaction mixture was stirred for 2 hours at 0-10° C. O-xylene (1000 ml) was added to it and stirred. The o-xylene layer was separated and washed with 20 % brine solution. Methyl 1-[(2'- cyanobiphenyl-4-yl)methyl]-2-ethoxy-benzimidazole-7-carboxyl ate (100 g) and o- xylene layer were taken into a round bottom flask and the temperature of reaction mixture was raised to 140-145 ⁰ C & maintained for 8 hours. The reaction mass was cooled at 25-30° C and 5 % caustic solution was added to it. The reaction mass was heated to 80-85° C for 2 hours under stirring. After cooling the reaction mass, aqueous layer was separated. Acetone (700 ml) and sodium nitrite solution (41.95 gm sodium nitrite dissolved in 100 ml D. M. water) were added to aqueous layer and cooled to 0-15° C. The pH 3.7-4.3 was adjusted by dilute hydrochloric acid to obtain the desired compound. The product was filtered and washed with chilled D. M. water until neutral pH. Acetone (500 ml) was added to the wet cake and stirred for 1 hour at 25-35° C. The product was filtered, washed with acetone and dried at 25-35 ^° C to obtain 90.0 g of title compound.

X-ray powder diffraction pattern is matching with Figure-3.

Example-4

Preparation of 2-Ethoxy- 1-[(2'-{N-tri phenyl methyl tetrazole-5-yl} biphenyl-4- yl)-methyl] benzimidazole-7-carboxylic acid (TRITYLATED CANDESARTAN)

2-ethoxy- 1-[(2'-{1H-tetrazole-5-yl} biphenyl-4-yl)-methyl] benzimidazole-7-carboxylic acid (100 g), acetone (200 ml), triethylamine (27.55 g) were taken into round bottom flask and stirred. The reaction mass was refluxed at 55-6O ⁰ C and trityl chloride

solution (69.7 g trityl chloride dissolved in 400 ml acetone) was added within 1-2 hours and refluxed reaction mass for 7 hours at 55-60 ⁰ C. The reaction was cooled to 25-30 ⁰ C and stirred for 30 minutes. The product was filtered and washed with acetone (100 ml) and dried. D. M. water (500 ml) was added to wet cake and stirred it for 30 minutes. The product was filtered, washed with D. M. water (200 ml) and dried at 50-55 ⁰ C to obtain 117 g of title compound. X-ray powder diffraction pattern is matching with Figure-4.

Example-5 Preparation of (±)-i-(cyclohexyloxycarbonyloxy) ethyl-2-ethoxy- 1-[(2'-{N-tri phenyl methyl tetrazole-5-yl} biphenyl-4-yl)-methyl] benzimidazole-7- carboxylate (TRITYLATED CANDESARTAN CILEXETIL)

2-ethoxy- 1-[(2'-{N-tri phenyl methyl tetrazole-5-yl} biphenyl-4-yl)-methyl] benzimidazole-7-carboxylic acid (100 g), dimethylformamide (200 ml), potassium carbonate (24.28 g) were taken into round bottom flask and stirred. The temperature of reaction mixture was raised to 60 to 7O ⁰ C and cyclohexyl 1-chloro ethyl carbonate

(36.33 g) was added within 1-2 hours and maintained at this temperature for 3 hours. The reaction mixture was cooled to 25 to 3O ⁰ C. The reaction mass was poured in D. M. water (800 ml) at 0 to 10 ⁰ C and stirred for 30 minutes at this temperature. The product was filtered and washed with chilled D. M. water until neutral pH. Acetone (500 ml) was added to wet cake and raised the temperature of reaction mixture to 55 to 6O ⁰ C. The reaction mixture was stirred for 30 minutes at 55 to 6O ⁰ C. The reaction mixture was cooled to 25 to 3O ⁰ C and stirred for 30 minutes at 25 to 3O ⁰ C. The product was filtered, washed with acetone (100 ml) and dried at 25 to 30 ⁰ C to obtain 115 g of title compound.

X-ray powder diffraction pattern is matching with Figure-5.