Claims:1. A substantially pure Salmeterol Impurity G compound of formula-II

Formula-II

2. The compound of claim 1, wherein Salmeterol Impurity G has purity more than 95%, when measured by HPLC.

3. A process a process for the preparation of Salmeterol Impurity G compound of formula-II

Formula-II
the process comprises the steps of
a) reacting Salmeterol Xinafoate with benzyl chloroformate in suitable base in a organic solvent to obtain Cbz protected Salmeterol,
b) adding manganese dioxide to Cbz protected Salmeterol obtained in step a) to obtain Cbz protected Salmeterolaldehyde,
c) condensing Cbz protected Salmeterolaldehyde with Salmeterol in alcoholic solvent in presence of acid and reducing agent to obtain Cbz-protected Salmeterol dimer,
d) deprotection of Cbz-protected Salmeterol dimer to obtain Salmeterol impurity G

4. The process of claim 3, wherein acid is selected from the group comprising one or more of hydrochloric acid, acetic acid and formic acid or mixtures thereof.

5. The process of claim 3, wherein suitable base is selected from one or more sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide and potassium hydroxide.

6. The process of claim 3, wherein organic solvent is aprotic solvent and water.

7. The process of claim 3, wherein aprotic solvent is selected from the group comprising one or more of N,N-dimethylformamide, N,N-diethylformamide, dimethyl sulfoxide, tetrahydrofuran and mixture thereof.

8. The process of claim 3, wherein reducing agent is selected from group comprising one or more of sodium borohydride, potassium borohydride, lithium borohydride, sodiumcyanoborohydride.

9. The process of claim 3, wherein alcoholic solvent is selected from the group comprising one or more of methanol, ethanol, propanol, isopropanol, 1-butanol, 2-butanol, isobutyl alcohol, t-butyl alcohol, or mixtures thereof.

10. The process of claim 3, wherein deprotection step (d) is carried out in presence of suitable hydrogenating agent, which is selected from group comprising one or more Raney Ni, Raney Co, Pt/C, Pd/C, Pd/CaCO3.
, Description:Field of Invention

The present invention provides Salmeterol Impurity G, process for preparing and isolating thereof. In further aspect of present provides the use of a Salmeterol Impurity G as reference markers and/or reference standards during the synthesis of Salmeterol or its pharmaceutically acceptable salt thereof.

Background of the invention

Salmeterol is approved under the brand name Serevent in the form of a Salmeterol Xinafoate. Salmeterol Xinafoate, is a selective ß2-adrenoreceptor agonist. It is clinically used as long-acting inhaled bronchodilator for maintenance treatment of asthma and to control nocturnal asthma.

Salmeterol Xinafoate is chemically known as 4-hydroxy-a'-[[[6-(4-phenylbutoxy)hexyl] amino]methyl]-1,3-benzenedimethanol, 1-hydroxy-2-naphthoate and is structurally represented by Formula (I)

Formula I

GB 2140800 describes the Salmeterol or its salt thereof and its process for the preparation. Other processes for the preparation of Salmeterol or its salt thereof also described in several patents including Patents Nos. US 4,992,474; US 5,795,594; US 6,680,345 B2; US 6,756,508 B2; US 6,613,307; GB Patent 2,176,476; ES 531722 and ES 539625
Researchers and developers in drug manufacturing understand that a compound in a relatively pure state can be used as a "reference standard" (a "reference marker" is similar to a reference standard but it is used for qualitative analysis) to quantify the amount of the compound in an unknown mixture.

There are several impurities of Salmeterol Xinafoate reported in European Pharmacopoeia 5.2, e.g. Impurity A, Impurity B, Impurity C, Impurity D, Impurity E, Impurity F and Impurity G.

The object of present invention is to provide a process for the preparation of 1-[4-hydroxy-3-[[[2-hydroxy-2-[4-hydroxy-3-(hydroxymethyl)phenyl]ethyl][6-(4-phenyl butoxy)hexyl]amino]methyl]phenyl]-2-[[6-(4-phenylbutoxy)hexyl]amino]ethanol (referred herein after “Salmeterol Impurity G”), which is very simple cost effective and may be employed at commercial scale. The further aspect of present invention is to provide use of “Salmeterol Impurity G” as a reference standard.

Summary of the Invention

The present invention provides a substantially pure Salmeterol Impurity G formula-II

Formula-II
having purity more than 95%, when measured by HPLC.

The present invention provides a use of Salmeterol Impurity G, as reference standards in a qualitative analysis of Salmeterol Xinafoate or its pharmaceutically acceptable salt thereof.

The present invention provides a process for the preparation Salmeterol Impurity G compound of formula-II

Formula-II
The present invention also provides a Salmeterol Impurity G, has purity more than 95 % when measured by HPLC.

Detailed description of the Invention

For purposes of the present invention, the following terms are defined below.

As used herein, the term “reference standard” refers to a compound that may be used both for quantitative and qualitative analysis of an active pharmaceutical ingredient. A reference marker” is used only for qualitative analysis, while a reference standard may be used for quantitative or qualitative analysis, or both. Hence, a reference marker is a subset of a reference standard, and is included within the definition of a reference standard.

As used herein, the term “Cbz” refers to Carboxybenzyl protecting group.

The present invention provides a substantially pure Salmeterol Impurity G compound of formula-II

Formula-II
having purity more than 95%, when measured by HPLC.

One another aspect of the present invention provides a use of Salmeterol Impurity G, as reference standards in a qualitative analysis of Salmeterol or its pharmaceutically acceptable salt thereof.

One another aspect of the present invention provides a process for the preparation of Salmeterol Impurity G compound of formula-II

Formula-II
the process comprises the steps of
a) reacting Salmeterol Xinafoate with benzyl chloroformate in suitable base in a organic solvent to obtain Cbz protected Salmeterol,
b) adding manganese dioxide to Cbz protected Salmeterol obtained in step a) to obtain Cbz protected Salmeterolaldehyde,
c) condensing Cbz protected Salmeterolaldehyde with Salmeterol in alcoholic solvent optionally in the presence of acid and reducing agent to obtain Cbz-protected Salmeterol dimer,
d) deprotection of Cbz-protected Salmeterol dimer to obtain Salmeterol impurity G

The step (a) of present invention involves adding Salmeterol Xinofoate to a solution of suitable base in organic solvent at temperature in between the range of 10°C to 15°C followed by slow addition of benzyl chloroformate at temperature in between the range of 10°C to 15°C, wherein suitable base is selected from group comprising one or more of sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide and potassium hydroxide.

The reaction mixture is stirred for a period of 1 hour. After completion of the reaction, the solvent is recovered under vacuum. The residue is further extracted with water and dichloromethane. The organic layer is washed with brine solution and dried over anhydrous sodium sulfate to obtain Cbz protected Salmeterol.

The organic solvent is aprotic solvent, water and mixture thereof, wherein aprotic solvent is selected from the group comprising one or more of N,N-dimethylformamide, N,N-diethylformamide, dimethyl sulfoxide, tetrahydrofuran and mixture thereof

The step b) of the present invention involves the addition of activated manganese dioxide to the organic layer obtained in step a) containing Cbz protected Salmeterol at temperature in between the range of 25°C to 35°C and stirring for a period of 24 hours. After completion of reaction, the reaction mass is filtered through celite, washed with dichloromethane and concentrate under vacuum to get Cbz protected Salmeterolaldehyde,

The step (c) of present invention involves condensing Cbz protected Salmeterolaldehyde with Salmeterol in alcoholic solvent followed by addition of acid and reducing agent at temperature in between the range of 25°C to 35°C for a period of 1 hour, wherein the acid is selected from the group comprising one or more of hydrochloric acid, acetic acid and formic acid or mixtures thereof. Preferably, acetic acid. The reaction mixture is stirred for a period of 12 hours. After completion of the reaction the solvent is concentrated under vacuum. The reaction mixture is quenched with water and extracted with ethyl acetate. The organic layers are collected, washed with brine, dried over anhydrous sodium sulfate and concentrated under vacuum to get Cbz protected Salmeterol dimer as an oily mass.

The alcoholic solvent is selected from group comprising one or more of methanol, ethanol, propanol, isopropanol, 1-butanol, 2-butanol, isobutyl alcohol, t-butyl alcohol, or mixtures thereof.

The suitable reducing agent is selected from group comprising one or more of sodium borohydride, potassium borohydride, lithium borohydride, Sodium cyanoborohydride, preferably Sodium cyanoborohydride.

The step (d) of the present invention involves deprotection of the Cbz protected Salmeterol dimer obtained in step c) by means of hydrogenation with suitable hydrogenating agent in alcoholic solvent at hydrogen pressure of 45 psi. The reaction is carried out for a period of 4 hours. After the completion of reaction, the reaction mixture is filtered to remove the catalyst and concentrated under vacuum to obtain the Salmeterol impurity G. The suitable hydrogenating agent is selected from group comprising one or more of Raney Ni, Raney Co, Pt/C, Pd/C, Pd/CaCO3, more preferable the catalyst used is Pd/C.

One another aspect of the present invention provides a Salmeterol Impurity G, has purity more than 95 % when measured by HPLC.

The process of the present invention is depicted in the following scheme 1:

Scheme 1
The present invention is further illustrated by the following example, which does not limit the scope of the invention. Certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present application.
EXAMPLES

Example 1: Synthesis of Cbz Salmeterol

Charged Salmeterol Xinofoate (5g) to a solution of sodium bicarbonate (2.06g) in tetrahydrofuran (30 ml ) and water (40 ml) at temperature in between the range of 10°C to 15°C followed by slow addition of benzyl chloroformate (3.10 g) at temperature in between the range of 10°C to 15°C. The reaction mixture was stirred for a period of 1 hour. After completion of the reaction, tetrahydrofuran was recovered under vacuum. The residue was further extracted with water (50 ml) and dichloromethane (100 ml). The organic layer is washed with brine solution (150 ml) and dried over anhydrous sodium sulfate. To the organic layer in dichloromethane containing Cbz protected Salmeterol activated manganese dioxide (5.21 g) was added at temperature in between the range of 25°C to 35°C for a period of 24 hours. After completion of reaction, the reaction mass was filtered through celite, washed with dichloromethane and concentrated under vacuum to get titled product.
Yield: 3.6 g
Mass: 546.6 (M-1)
1H NMR (400 MHz, CDCl3): 1.25-1.71 (m, 12H), 2.62 (t, 2H), 3.52 (d, 1H), 3.12-3.41 (m, 8H), 4.90-4.95 (m, 1H), 5.17 (s, 2H), 6.95 (d, 1H), 7.14-7.39 (m, 10H), 7.49 (d, 1H), 7.60 (s, 1H), 9.85 (s, 1H), 10.96 (s, 1H).
13C NMR (CDCl3):
25.87, 26.47, 27.98, 28.35, 29.31, 29.54, 35.64, 49.10, 53.36, 56.29, 67.53, 70.63, 72.97, 117.61, 120.24, 125.61, 127.84, 128.14, 128.18, 128.32, 128.49, 130.80, 133.90, 134.48, 136.29, 142.39, 158.29, 160.98, 196.50.

Example 2: Preparation of Cbz Salmeterol dimer

Condensed Cbz protected Salmeterolaldehyde (3.6 g) with Salmeterol (3.0 g) in ethanol (50 ml) followed by slow addition of acetic acid (0.1 g) and Sodium cyanoborohydride ( 0.64 g) at temperature in between the range of 25°C to 35°C for a period of 1 hour. The reaction was stirred for a period of 12 hours at temperature 25°C to 35°C. After completion of the reaction the solvent is concentrated under vacuum. The reaction mixture is quenched with water (50 ml) and extracted with ethyl acetate (100 ml). The organic layers are collected, washed with brine (50 ml), dried over anhydrous sodium sulfate and concentrated under vacuum to get titled compound as an oily mass.
Yield: 7.2 g
Mass: 947.9 (M+1)
1H NMR (400 MHz, CDCl3): 1.19-1.29 (m, 10H), 1.41-1.49 (m, 6H), 1.55-1.69 (m , 8H), 2.50-2.85 (m, 10H), 3.16 (m, 2H), 3.34-3.42 (m, 6H), 3.68-3.80 (m, 2H), 4.77-4.82 (m, 6H), 5.14 (m, 2H), 6.73-6.76 (m , 3H), 6.81-6.83 (d, 1H), 6.90-6.92 (m, 1H), 7.09-7.11 (m, 3H), 7.16-7.18 (m, 3H), 7.24-7.27 (m, 6H), 7.31-7.36 (m, 4H).
13C NMR (CDCl3):
158.13, 157.00, 155.57, 142.37, 136.40, 133.80, 132.68, 128.46, 128.34 128.20, 128.02, 127.79, 126.60, 126.24, 125.62, 125.44, 122.25, 116.18, 115.99, 73.45, 71.13, 70.78, 70.67, 67.44, 65.79, 63.40, 61.26, 58.28, 56.06, 54.14, 48.77, 35.64, 29.62, 29.52, 29.43, 29.27, 29.22, 28.30, 27.96, 27.93, 26.90, 26.51, 25.86, 15.18.

Example 3: Preparation of Salmeterol impurity G

Charged ethanol (50ml) to Cbz-Salmeterol dimer (7.2g in 500ml par reactor). 5% Pd/C catalyst (1.0g) was added and the reaction is stirred at hydrogen pressure (45psi) for 4 hours. After completion of reaction catalyst was filtered and the filtrate was concentrated under vacuum to get entitled product.
Yield: 4.7g
1H NMR (400 MHz, DMSO-d6): 1.21-1.27 (m, 8H), 1.45-1.48 ( m, 8H), 1.55-1.58 (m, 8H), 2.48-2.57 (m, 4H), 2.63-2.88 (m, 6H), 3.34-3.95 (m, 8H), 4.43-4.47 (m, 4H), 4.71-4.75 (m, 2H), 4.93-5.0 (m, 2H), 5.94 (m, 1H), 6.69-6.75 (m,3H), 6.96-7.04 (m, 3H), 7.11-7.16 (m, 5H), 7.22-7.30 (m, 5H), 8.55 (m, 3H), 9.30 (s, 1H), 9.38 (s, 1H).
Mass: 814.9 (M+1).
HPLC purity: 95%.