The present invention provides a process for the preparation of Silodosin of formula (I). More particularly, the present invention provides the process for preparation of tartrate salt of 3-[7-cyano-5[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy ] ethyl } amino)propyl] -2, 3 -dihydro- 1 H-indol- 1 -y 1 } propyl benzoate of formula (IV), which is a precursor in the preparation of Silodosin.

Background of the Invention:

A compound of 3-[7-cyano-5[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy) phenoxy] ethyl}amino)propyl]-2,3-dihydro-lH-indol-l-yl}propyl benzoate (IV) is a key intermediate for preparation of Silodosin. The chemical name of Silodosin is l-(3- hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)pheno xy]ethyl} amino) propyl]-2,3-dihydro-lH-indole-7-carboxamide and structurally represented as

(IV)

(I)

U.S.Pat. No. 5,387,603 discloses Silodosin as therapeutic agents for the treatment of dysuria, urinary disturbance associated with benign prostatic hyperplasia.

U.S.Pat. No. 6,310,086 discloses a process for preparing a Silodosin analogue compound from reaction of (R)-3-{5-(2-aminopropyl)-7-cyano-2,3- dihydro- 1 H-indol- 1 -yl jpropylbenzoate with 2-(2-Ethoxyphenoxy)ethyl methane sulfonate and finally isolated as residue and purified by column chromatography on silicagel. The said literature process has certain drawbacks like use of column chromatography.

U.S.Pat. No. 7,834,193 (IN 3178/DELNP/2007) discloses the process for preparation of monooxalate salt of 3-{7-cyano-5-[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy ] ethyl } amino)propyl] -2, 3 -dihydro- 1 H-indol- 1 -y 1 } propyl benzoate (IV). This patent specifically discloses the preparation of monooxalate salt of formula (IV) helps to remove N,N-dialkyl impurity to certain extend. CN 101993405 A discloses the reaction of (R)-5-(2-aminopropyl)-l-(3-(4- fluorobenzoyloxy)propyl)-7-cyanoindoline with 2-(2-(2,2,2-trifluoroethoxy) phenoxy)ethyl methane sulfonate followed by oxalic acid salt preparation.

The main drawback in the prior art process, the formation of N,N-dialkyl impurity compound of formula (VI), as disclosed in detailed description, in the preparation of Silodosin, during condensation of compound of formula (II) with compound of formula (III), the impurity which is not removable by crystallization method or precipitation technique and column chromatography purification is not suitable for commercial purpose. So considering the commercial importance of Silodosin, the present invention focus on the preparation of pure Silodosin, and surprisingly found that the isolation of formula (IV) as tartrate salt helps to prepare Silodosin having less than 0.2 % of N,N dialkyl impurity and with good yield. None of the prior arts teaches or motivates isolation of tartaric acid addition salt of formula (IV). The preparation of Silodosin from tartrate salt of 3-{7-cyano-5-[(2R)- 2-({2-[2-(2,2,2-trifluoroethoxy)phenoxy] ethyl}amino)propyl]-2,3-dihydro-lH- indol-l-yl} propyl benzoate (IV) or its freebase of the present invention has purity of greater than 99.6 %.

Objectives of the Invention:

The main objective of the present invention is to provide tartrate salt of 3-{7- cyano-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl} amino)propyl]-2,3- dihydro-lH-indol-l-yl}propyl benzoate, which is a precursor in the synthesis of Silodosin.

Another objective of the present invention is to provide a process for the preparation of tartrate salt of 3-{7-cyano-5-[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy ] ethyl } amino)propyl] -2, 3 -dihydro- 1 H-indol- 1 -y 1 } propyl benzoate, which is easy to implement in industry with good yield and high purity.

Still another objective of the present invention is to provide a process for preparation of highly pure Silodosin from tartrate salt of 3-{7-cyano-5-[(2R)-2-({2- [2-(2,2,2-trifluoroethoxy)phenoxy]etl yl} amino)propyl]-2,3-dihydro- lH-indol- 1 - yljpropyl benzoate.

Summary of the invention:

In first aspect of the present invention provides a process for the preparation of Silodosin which comprising steps of;

i) treating compound of formula (II) with compound formula (III) in a solvent and in the presence of base to provide compound of formula (IV); wherein R represents hydroxyl protecting group selected from group comprising benzoyl, benzyl, benzyloxycarbonyl, tetrahydropyran, tert-butoxycarbonyl and L is a leaving group selected from group comprising chloro, bromo, iodo, alkylsulphonyloxy, arylsulphonyloxy;

ii) converting the compound of formula (IV) to its tartrate salt in the presence of solvent;

iii) hydrolyzing the tartrate salt compound of formula (IV) in a solvent and in the presence of base to provide compound of formula (V); and

iv) treating compound of formula (V) with hydrogen peroxide in a solvent and in the presence of base to provide Silodosin.

The following scheme illustrates the present invention:

crude Silodosin Pure Silodosin

In second aspect of the present invention provides a process for the preparation of tartrate salt of compound of formula (IV) which comprising steps of: a) obtaining a solution of 3-{7-cyano-5-[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy] ethyl } amino)propyl] -2 ,3 -dihydro- 1 H-indol- 1 - yl} propyl benzoate (IV) in a solvent;

b) adding tartaric acid to step (a);

c) isolating tartrate salt of 3-{7-cyano-5-[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy] ethyljamino) propyl] -2,3-dihydro-lH-indol-l- yljpropyl benzoate (IV); and

d) optionally converting tartrate salt of formula (IV) into silodosin (I). Drawings of the invention:

Fig. 1 : illustrates the X-ray powder diffraction pattern of tartrate salt of 3-{7-cyano- 5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl}amino) propyl]-2,3-dihydro- lH-indol-l-yl} propyl benzoate (IV) of the present invention.

Fig. 2: illustrates the X-ray powder diffraction pattern of l-(3-hydroxypropyl)-5- [(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl-}amino)p ropyl]-2,3-dihydro- 1H- indole-7-carbonitrile (V) of the present invention.

The PXRD of said salt analyzed using following condition:

Detailed description of the invention:

In first embodiment of the present invention, the solvent used in step (i) is selected from group comprising of ethylacetate, methylacetate, butylacetate, isopropylacetate, methoxy ethylacetate, acetone, methylisobutylketone, 2- pentanone, ethylmethylketone, diethylketone, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutylalcohol, tert-butylalcohol, isoamylalcohol, 2- methoxyethanol, acetonitrile, dichloromethane, toluene, xylene or mixture thereof; preferably acetonitrile and the base used in step (i) is selected from an inorganic or organic base. Preferably the inorganic base is selected from group comprising of sodium bicarbonate, potassium bicarbonate, lithium carbonate, sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide or mixture thereof and the organic base is selected from group comprising of diisopropylamine, diisopropylethylamine triethylamine, dimethylamine, trimethyl amine, pyridine or mixtures thereof.

In second embodiment of the present invention, the solvent used in step (ii) & a) is selected from group comprising of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutylalcohol, tert-butylalcohol, isoamylalcohol, 2- methoxyethanol ethylacetate, methyl acetate, butyl acetate, isopropyl acetate, methoxyethylacetate, acetone, methylisobutylketone, 2-pentanone, ethylmethylketone, diethylketone, acetonitrile, water and the like or mixtures there of. The tartaric acid used in step (b) is either racemic or its stereoisomer preferably L (+)-tartaric acid.

In third embodiment of the present invention, process for preparation of tartrate salt of formula (IV), comprising treating compound of formula (IV) with an tartaric acid in a solvent at a temperature in the range of 20 °C to reflux of the used solvent. The isolation of acid addition salt is done either by cooling the solution or by addition anti-solvent which is selected from group comprising of hexane, heptane, cyclohexane, cycloheptane, diisopropylether, MTBE, diethyl ether or mixture thereof.

Following the prior art process for the preparation of Silodosin by reacting compound of formula (II) with compound of formula (III) resulted Silodosin or its precursor compounds like (IV), (V) having higher amount of N,N dialkyl compound of formula (VI) or dialkyl impurity as shown below:

R is a hydroxyl protecting group or H; and R ¹ is either -CN or -CONH ₂

Common method like crystallization and column chromatography are fails to remove the above said impurity (VI; particularly R is hydroxyl protecting group; and R ¹ is -CN) and hence not viable for commercial purpose. US 7,834,193 disclose monooxalate salt of compound of formula (IV) with 0.9 % dialkyl impurity (VI). None of the prior arts produces the compound of formula (IV) with less than 0.5 % dialkyl impurity, more particularly less than 0.2 %. The preparation of acid addition salt like HCl, sulphuric acid, DPTTA fails to produce salt of compound of formula (IV) as a solid and hence not suitable for purification. Applicant diligently did research to identify a process for compound (IV) with less or free of N,N dialkyl impurity and found that preparing [7-cyano-5[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy ] ethyl } amino)propyl] -2, 3 -dihydro- 1 H-indol- 1 -y 1 } propyl benzoate as its tartrate salt yielded compound of formula (IV) with reduced amount of N,N dialkyl impurity (VI; wherein R is hydroxyl protecting group; and R ¹ is - CN) from 15% to 0.5% preferably below 0.2%. No prior art teaches or motivates the invention provided in the present application, and constitute novelty of the present invention. Thus the present invention provides the robust process to eliminate the impurities particularly N,N-dialkyl impurity which is otherwise requires additional purification. Following table-I provides the results of reaction of 3-[7-cyano-5[(2R)-2- ( {2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl} amino)propyl]-2,3-dihydro-lH-indol- l -yl}propyl benzoate with different organic acids. From this table it is evident that the tartrate salt of compound of formula (IV) removes N,N dialkyl impurity to an extent less than 0.2%.

Table 1

Crude compound of formula (IV) having 13.5 - 15% of N, N dialkyl impurity (VI; wherein R is benzoyl; and R ¹ is -CN) was used as a starting material for the reaction.

NS - No solid formation

DPTTA- Di-p-toluyl tartaric acid; DBTA-Dibenzoyl-L-tartaric Acid

From this table, it is evident that the present invention eliminates dialkyl impurity from 15% to less than 0.2%. In fourth embodiment of the present invention, the solvent used in step (iii) is selected from group comprising of methanol, ethanol, n-propanol, isopropanol, n- butanol, isobutylalcohol, tert-butylalcohol, isoamylalcohol water or mixture thereof; and base is selected from group comprising of lithium hydroxide, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, or mixture thereof. The obtained l-(3- hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)pheno xy]ethyl- }amino)propyl]-2,3-dihydro-lH-indole-7-carbonitrile (V) is optionally isolated or used as such for next stage. The isolation of compound of formula (V) is done by treating the residue with solvents like diethyl ether, isopropyl ether, methyl tert- butyl ether, hexane, heptane, cyclohexane and the like. The isolation of compound of formula (V) as a solid form either in amorphous form or crystal form provides many advantages like i) storage ii) ease handling iii) storage stability iv) enhanced purity. And constitute one of the novelty of the present invention.

In fifth embodiment of the present invention, the solvent used in step (iv) is selected from group comprising of dimethylformamide, dimethylacetamide, dimethylsulphoxide, sulfolane, water or mixture thereof and base is selected from group comprising of sodium bicarbonate, potassium bicarbonate, lithium carbonate, sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide or mixture thereof and the isolation of Silodosin done by either cooling the solution or addition of anti- solvent.

In sixth embodiment of the present invention, the preparation of Silodosin by its tartrate salt of 3-{7-cyano-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)phenoxy] ethyl} amino) propyl] -2,3-dihydro-lH-indol-l-yl}propyl benzoate (IV) found to have advantages, as this process yields final compound in good purity, yield and cost effective as well as the obtained Silodosin is highly pure and suitable for pharmaceutical compositions.

In seventh embodiment of the present invention, the silodosin obtained according to the invention can be optionally purified by crystallization methods like re-crystallization, dissolution followed by precipitation using anti-solvent. The solvents used for crystallization includes but not limited to acetone, methylisobutylketone, ethylmethylketone, 2-pentanone, diethylketone, ethylacetate, n-butylacetate, tert-butylacetate, isopropylacetate, 2-methoxyethyl acetate, toluene, chlorobenzene, acetonitrile, dichloromethane, methanol, ethanol, isopropanol, n- butanol, tert-butyl alcohol, isoamyl alcohol, water, hexane, heptane, pentane, cyclohexane, petroleum ether or mixture thereof. This purification removes all impurities, especially the dehydro impurity from approximately in the range of 1.1 % to 0.1% as represented by eneral formula.

In eighth embodiment of the present invention, the Silodosin obtained as per present invention can be further micronized, milled or sieved to get the desired particle size (D90 less than 200 preferably less than 100 more preferably less than 20 microns). Silodosin prepared by the present invention is a free flow solid and suitable for pharmaceutical composition. The starting material of 2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl methansulfonate (II) is prepared by conventional method of prior art processes or by following Scheme-I OR II; SCHEME-I

PG - (Tetrahydropyran)THP, trityl, acetyl, benzoyl, trimethyl silyl, pivaloyi

L - Leaving group

SCHEME-II

Another starting material 5[(2R)-(2-aminopropyl)-l-(3-benzoyloxy propyl)]2,3-dihydro-7-carbonitrile-lH-indole (2R, 3R)-2,3-dihydroxy butanediate is prepared by conventional method or by following the Scheme-Ill; SCHEME-III

While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are included within the scope of the present invention. The examples are provided to illustrate particular aspects of the disclosure and do not limit the scope of the present invention. Example 1: Preparation of tartrate salt of 3-[7-cyano-5[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy) phenoxy] ethyl}amino)propyl]-2,3-dihydro-lH-indol-l- yl}propyl benzoate (IV) Method A: To the solution of 3-{7-cyano-5-[(2R)-2-aminopropyl]-2,3-dihydro-lH- indol-l-yl} propyl benzoate (II) (1 mole) in acetonitrile were added sodium carbonate (41.3 grams) and 2-(2-(2,2,2-trifluoroethoxy)phenoxy)ethyl methane sulfonate of formula (III) (79.6 grams) and heated to reflux temperature till reaction completion. After completion, reaction mass cooled to room temperature, water and ethylacetate added over it, stirred and layer separated. The obtained organic layer was washed with brine solution. To the organic layer, L (+) tartaric acid (29.2 grams) was added at room temperature and stirred. The solid obtained was filtered the solid and dried under vacuum at 50 ° - 55 °C; Yield: 110 grams. Method B: A mixture of acetonitrile, 3-{7-cyano-5-[(2R)-2-aminopropyl]-2,3- dihydro-lH-indol-l-yl}propyl benzoate tartrate salt (II) (10 grams), potassium carbonate (13.4 grams) and 2-(2-(2,2,2-trifluoroethoxy)phenoxy)ethyl methane sulfonate (7.9 grams) of formula (III) was heated to reflux temperature till reaction completion. After reaction completion, the reaction mass quenched with water and extracted using ethylacetate twice. The combined ethylacetate layer was dried with sodium sulphate, filtered and treated with L (+) tartaric acid (2.9 grams) at room temperature under stirring and maintained for reaction completion. The obtained solid was filtered, washed with ethylacetate and dried under vacuum. Yield: 10 grams.

Method C: To the solution 3-{7-cyano-5-[(2R)-2-aminopropyl]-2,3-dihydro-lH- indol-l-yl} propyl benzoate of formula (II) in tert-butanol, 2-(2-(2,2,2- trifluoroethoxy)phenoxy)ethyl methanesulfonate of formula (III) and sodium carbonate were added and heated to reflux temperature. After completion, reaction mass gradually cooled to 25-35°C. DM water, saturated sodium bicarbonate and ethylacetate was added over it, stirred and layer separated. The obtained ethylacetate layer was washed with water, dried with sodium sulphate and distilled out completely under vacuum. To the residue ethyl acetate added followed by tartaric acid was added and stirred at room temperature. The obtained solid was filtered, washed with ethylacetate and dried under vacuum.

Table 3

It is clearly derivable from the above table-3 that the present process helps to removal of N,N dialkyl impurity from 15% to less than 0.2%.

HPLC condition

Mobile Phase -A (Dissolve about 2.72 g of potassium dihydrogen orthophosphate in 1000 ml of water. Adjust the pH 3.0 ± 0.05 using Orthophosphoric acid, Filter the solution through 0.2 μ porosity membrane filter and degas); Mobile Phase-B (Use filtered acetonitrile as mobile phase B); Diluent-Mix thoroughly water and Acetonitrile in the ratio 1 : 1;

Chromatographic Parameters

Use mobile phase -A and mobile phase -B as per the following elution program.

Column-: Symmetry C18 (150*4.6mm, 5μ); Flow rate: 1.2 ml/min; Detection: UV at 225 nm; Injection volume: 10 mL; Run time: 52 min; Column oven temperature: 38 °C. (Gradient)

RRT of N, N dialkyl impurity is 1.37

Example 2: Preparation of l-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy]ethyl-}amino)propyl]-2,3-dihydro-lH- indole-7- carbonitrile of formula (V)

Method A: To tartrate salt of 3-[7-cyano-5[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy) phenoxy] ethyl}amino)propyl]-2,3-dihydro-lH-indol-l-yl}propyl benzoate (IV) (100 grams) in methanol (400 ml) was added aqueous potassium hydroxide solution (38.38 grams) and stirred at room temperature. The compound of formula (V) was extracted in ethylacetate. Organic layer separated, washed with sodium bicarbonate, brine solution and dried with sodium sulphate. Finally the obtained organic layer distilled under vacuum. Yield: 65 g Method B: To the solution of potassium carbonate in water, tartrate salt of 3-[7- cyano-5[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy) phenoxy] ethyl}amino)propyl]-2,3- dihydro-lH-indol-l-yl}propyl benzoate (IV) was added and stirred. The free base of formula (IV) was extracted in ethylacetate. The separated ethylacetate was washed with water and distilled, to the residue methanol was added followed by solution of potassium hydroxide in water. The reaction mass stirred till completion. After completion, reaction mass quenched with water and extracted in ethylacetate, washed with water, brine and dried. The obtained ethylacetate layer completely concentrated under vacuum. Residue yield 48 grams.

To the residue, isopropyl ether (550 ml) was added under stirring at room temperature and maintained for 1 hour. The obtained solid is filtered, washed with isopropyl ether and dried under vacuum. Yield: 33g.

Example 3

Preparation of l-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy) phenoxy] ethyl-} amino) propyl]-2,3-dihydro-lH-indole-7-carboxamide (Silodosin)

Method A: The compound of l-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy)phenoxy] ethyl- } amino)propyl] -2,3 -dihydro- 1 H-indole-7- carbonitrile of formula (V) in dimethylsulfoxide was treated with 48% hydrogen peroxide and 20% sodium hydroxide solution and stirred at room temperature till completion of reaction. After completion of reaction, reaction mass quenched with 5% sodium bisulphite solution and ethylacetate was added over it. The ethylacetate layer was separated and treated with 20 % aqueous hydrochloric acid. The aqueous layer separated, neutralized with sodium bicarbonate solution and extracted with ethylacetate. The separated organic layer was washed with 10% sodium bicarbonate solution, brine solution and dried under vacuum. The organic layer distilled upto residue under vacuum at 50-55°C. The obtained residue was crystallized in ethylacetate.

Method B: To the tartrate salt of 3-[7-cyano-5[(2R)-2-({2-[2-(2,2,2- trifluoroethoxy) phenoxy] ethyl}amino)propyl]-2,3-dihydro-lH-indol-l-yl}propyl benzoate (IV) (100 grams) in methanol, aqueous potassium hydroxide solution (38.38 grams) was added and stirred at room temperature till reaction completion. After completion of reaction, DM water and dichloromethane was added over it under stirring. Organic layer separated, washed with brine solution distilled under vacuum upto less than 1 volume. To the solution, dimethyl sulphoxide, 20% sodium hydroxide and hydrogen peroxide was added and stirred till completion of reaction. After completion of reaction, water containing sodium bisulfite was added to the reaction mass. The pH of the reaction mixture adjusted to about 8.5 using 10% sodium hydroxide and extracted in dichloromethane twice, washed with water, dried and concentrated upto 1-2 volume under vacuum. To the obtained solution, toluene was added over it at room temperature under stirring. The reaction mixture maintained for complete solid formation, filtered and dried under vacuum. Yield 58 grams. Example 4

Purification of Silodosin:

Method A: To the mixture of toluene and acetonitrile solvent, Silodosin was added over it and heated to 50° - 55 °C for complete dissolution. The reaction mass gradually cooled to room temperature and maintained for completion of solid formation. The obtained solid is filtered, washed with toluene and dried under vacuum. Method B: To the mixture of ethyl acetate and toluene solvent, Silodosin was added over it and heated to 60° - 65 °C for complete dissolution. The reaction mass gradually cooled to room temperature and maintained for completion of solid formation. The obtained solid is filtered, washed with toluene and dried under vacuum.