DESC:FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003

COMPLETE SPECIFICATION
(Section 10 and Rule 13)

AN IMPROVED PROCESS FOR THE PREPARATION OF PURE PYRIMETHAMINE

AUROBINDO PHARMA LTD HAVING CORPORATE OFFICE AT
THE WATER MARK BUILDING,
PLOT NO.11, SURVEY NO.9,
HITECH CITY, KONDAPUR,
HYDERABAD, 500 084,
TELANGANA, INDIA
AN INDIAN ORGANIZATION

The following specification particularly describes and ascertains the nature of this invention and the manner in which the same is to be performed:
FIELD OF THE INVENTION

The present invention relates to an improved process for the preparation of pure Pyrimethamine. The present invention also relates to a process for the preparation of intermediate of Pyrimethamine, 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II), which is free from deschloro impurity (Formula A):

BACKGROUND OF THE INVENTION

Pyrimethamine is chemically known as 5-(4-chlorophenyl)-6-ethyl-2,4-pyrimidinediamine and has the following structural Formula I:

Formula I

Pyrimethamine is an anti-parasitic medication and is indicated for the treatment of toxoplasmosis. Pyrimethamine, a folic acid antagonist, exhibits antimicrobial action against the causative agent of malaria and possesses sporontocidal action.

US 2,576,939 discloses Pyrimethamine and process for the preparation thereof. The process of US ‘939 involves coupling of (4-chlorophenyl)acetonitrile (IV) with ethyl propionate in presence of sodium ethoxide in ethanol to provide 2-(4-chlorophenyl)-3-oxopentanenitrile (III), which is reacted with diazomethane followed by guanidine and then recrystallized from ethanol to provide Pyrimethamine. The process is shown schematically below:

US 3,849,416 discloses a process for the preparation of Pyrimethamine, which involves reaction of 2-(4-chlorophenyl)-3-oxopentanenitrile (III) with ethylene glycol in presence of p-toluene sulfonic acid (PTSA) to produce 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II), which is recrystallized from methanol. The US ‘416 also discloses use of sulfuric acid instead of PTSA in example 8. The cyclic ketal is further reacted with guanidine to provide Pyrimethamine. However, the process uses excess molar equivalents of acid to prepare compound of Formula II. The process is shown schematically below:

Acta Crystallographica Section C: Crystal structure communications 2011, 67(11), 0428-0434, discloses DMSO solvate of Pyrimethamine.

Journal of Pharmacy and Pharmaceutical Sciences, 2014, 17(2), 190-206, discloses Pyrimethamine-MeOH solvate and anhydrous Pyrimethamine.

The inventors of the present invention observed that the prior art process of US ‘416 produces deschloro impurity of Formula A due to the use of excess amount of acid during the preparation of 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II), which is key intermediate in the process of Pyrimethamine.

The impurities formed during intermediates stage are carried forward in subsequent steps to Pyrimethamine. Further, the removal of these impurities requires tedious purification techniques such as column chromatography or repeated purifications that leads to loss of yield and makes the process of Pyrimethamine very expensive. Furthermore, the use of diazomethane as described in US ‘939 is hazardous and makes the invention expensive.

Therefore, there is a need to develop an improved process for intermediate of Formula II having deschloro impurity less than 0.1% and its conversion to Pyrimethamine wherein the purity is greater than 99.5% by HPLC.

OBJECTIVE OF THE INVENTION

The objective of the present invention is to provide pure Pyrimethamine by using 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II) having deschloro impurity of Formula A less than 0.1%.

Another object of the invention is to provide an improved process for the preparation of 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II), which is key intermediate of Pyrimethamine.

SUMMARY OF THE INVENTION

In an aspect of the present invention, there is provided an improved process for the preparation of pure Pyrimethamine (I), comprising of the following steps:

a) reacting 2-(4-chlorophenyl)-3-oxopentanenitrile (III) with ethylene glycol in presence of 0.2 to 0.5 molar equivalents of sulphuric acid to obtain 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II) having deschloro impurity of Formula A less than 0.1%; and

b) reacting the compound (II) of step (a) with guanidine or salt thereof to obtain Pyrimethamine;
c) optionally, purifying Pyrimethamine from DMSO and water.

In another aspect of the present invention, there is provided a process for the preparation of 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II) having purity greater than or equal to 99.9% by HPLC, which comprises condensation of 2-(4-chlorophenyl)-3-oxopentanenitrile (III) with ethylene glycol in presence of 0.2 to 0.5 molar equivalents of sulphuric acid to obtain 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II); and isolation the compound (II) from a mixture of alcohol and water.

In another aspect of the present invention, there is provided a process for the purification of Pyrimethamine, which comprise in the following steps of:
i. providing a solution of Pyrimethamine in dimethyl sulfoxide at an elevated temperature;
ii. optionally, treating the solution of step i) with charcoal;
iii. cooling the solution of step (i) or (ii);
iv. adding water to the mixture of step iii); and
v. cooling the reaction mass of step iv) to obtain pure Pyrimethamine.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a powder X-ray power diffraction ("PXRD") pattern of crystalline Pyrimethamine obtained according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention refers to “pure” or “purity”, unless specified otherwise, that the compounds or salts thereof are having purity of about 99.5% or 99.90% by HPLC. Further, the “pure” or “purity” refers to the material of present invention is substantially free from impurities i.e. the impurities are less than about 1% or less than about 0.6% or less than about 0.3% or less than about 0.1% by HPLC analysis.

Further, the present invention refers to “salt” that may be obtained from inorganic acid or organic acid. The inorganic acid is selected from hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acid and the like; the organic acid may be selected from oxalic acid, acetic acid, maleic acid, succinic acid, methane sulfonic acid and the like.

In an aspect of the present invention, there is provided an improved process for the preparation of pure Pyrimethamine (I), comprising of the following steps:

a) reacting 2-(4-chlorophenyl)-3-oxopentanenitrile (III) with ethylene glycol in presence of 0.2 to 0.5 molar equivalents of sulphuric acid to obtain 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II) having deschloro impurity of Formula A less than 0.1%; and

b) reacting the compound (II) with guanidine or salt thereof to obtain Pyrimethamine;
c) optionally, purifying Pyrimethamine from dimethyl sulfoxide and water.

The compound, 2-(4-chlorophenyl)-3-oxopentanenitrile, of Formula III may be prepared by the known process disclosed in US 2,602,794 or prepared by the condensation of 4-chlorophenyl acetonitrile with ethyl propionate in presence of solvent like tetrahydrofuran and base like aqueous or alcoholic sodium methoxide at elevated temperature of above 30°C or about 45°C to about 70°C. The resultant compound of Formula III may be used in-situ in further steps without isolation.

The reaction of step a) is performed in presence of solvent comprises of hydrocarbons selected from hexane, heptane, toluene, or mixtures thereof; ether solvents such as diethyl ether, diisopropyl ether, tetrahydrofuran or mixtures thereof. The suitable temperature used instep (a) may be selected from above 30°C or reflux temperature depending on the solvent used.

Inventors of the present invention surprisingly found that the use of sulphuric acid in a molar equivalents of 0.2 to 0.5 instead of p-toluenesulphonic acid during the reaction of step (a) completes the reaction and avoids/reduces the formation of deschloro impurity (Formula A). Further, the isolation of compound (II) from the mixture of organic solvent and water improves the yield and purity of the compound of Formula II.

The molar equivalents of sulphuric acid used in step a) preferably in the range of 0.25 to 0.3 per the molar equivalent of 4-chlorophenyl acetonitrile (IV) to complete the consumption of starting material. The quantity of ethylene glycol used may be in the range of 1 to 5 molar equivalents or 3 to 4 molar equivalent per the molar equivalent of 4-chlorophenyl acetonitrile.

The compound of Formula II obtained from the present invention is having purity greater than 99.5% or greater than 99.95% by HPLC.

The resultant compound of Formula II is further reacted with guanidine or its salt in presence of an organic solvent and a base at a suitable temperature of above 30 to about 100°C or at reflux temperature depending on the solvent used.

The suitable organic solvent comprises an alcohol is selected from methanol, ethanol, n-propanol, isopropyl alcohol, n-butanol or combination thereof. The base comprises organic base or inorganic base. The organic base is selected from sodium methoxide, sodium ethoxide, tetramethyl ammonium hydroxide, tetrabutyl ammonium hydroxide, acetic acid, formic acid, oxalic acid, tartaric acid, maleic acid and the like; the inorganic base is selected from sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, sodium bicarbonate, ammonia, potassium bicarbonate and the like. The base may be used as an aqueous solution or an alcoholic solution. In an embodiment, the base used in the step b) is sodium methoxide in methanol.

After the completion of the reaction between the compound of Formula II and guanidine or salt thereof, the reaction mass is cooled and then subjected for precipitation of solid by the addition of water. The resultant Pyrimethamine may have purity greater than 99.5% determined by HPLC.

The Pyrimethamine optionally purified from dimethyl sulfoxide and water to obtain pure Pyrimethamine.

In another aspect of the present invention, there is provided a process for the preparation of 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II) having purity greater than or equal to 99.9% by HPLC, which comprises condensation of 2-(4-chlorophenyl)-3-oxopentanenitrile (III) with ethylene glycol in presence of 0.2 to 0.5 molar equivalents of sulphuric acid to obtain 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (II); and isolation of the compound of Formula II from a mixture of alcohol and water.

The condensation reaction may be performed between ethylene glycol and the compound of Formula III in presence of sulfuric acid and toluene at a reflux for a period of 1 to 4 hours or more without affecting the quality of compound of Formula II, particularly the formation of deschloro impurity (Formula A).

After completion of the reaction, the reaction mixture is treated with aqueous or alcoholic base to reach the pH to 7-9.0, separated the organic layer and then concentrated to obtain residue. The obtained residue is subjected for isolation of solid.

The isolation of the compound of Formula II is performed by dissolving the residue in an alcoholic solvent and then precipitating the solid by the addition of water. The obtained reaction mixture is stirred for a period of 30 minutes or more to enhance the precipitation. The alcoholic solvent used for dissolution comprises methanol, ethanol, n-propanol, isopropyl alcohol, n-butanol or mixtures thereof. The ratio of an alcohol and water may be in the range of 3:0.6-1.0 volumes.

The resultant compound of Formula II has purity greater than 99.9% or greater than or equal to 99.95% by HPLC.

In another aspect of the present invention, there is provided a process for the purification of Pyrimethamine, which comprise in the following steps of:
i. providing a solution of Pyrimethamine in dimethyl sulfoxide at an elevated temperature;
ii. optionally, treating the solution of step i) with charcoal;
iii. cooling the solution of step (i) or (ii);
iv. adding water to the mixture of step iii); and
v. cooling the reaction mass of step iv) to obtain pure Pyrimethamine.

The solution of Pyrimethamine in dimethyl sulfoxide (DMSO) may be obtained from the previous processing step or may be dissolved in DMSO at an elevated temperature. The elevated temperature is about 40°C to about 100°C or 70°C to 90°C or at a reflux temperature. The solution may be treated with charcoal/carbon at the same temperature to remove particles and color impurities and then cooled the solution to below 50°C. To the obtained reaction mixture, water is added and stirred for a period of 30 minutes to 1 hour or more to improve the precipitation of pure Pyrimethamine.

The obtained Pyrimethamine according to the process of the present invention is crystalline and having chemical purity by High Performance Liquid Chromatography (HPLC) of greater than about 99.95%.

The crystalline Pyrimethamine of the present invention is characterized by powder X-ray diffraction (“PXRD”) pattern substantially as illustrated by Fig. 1.

Having described the invention with reference to certain aspects embodiments, embodiments will become apparent to one skilled in the art from consideration of the specification. The invention is further defined by reference to the following examples describing the preparation of Pyrimethamine. It will be apparent to those skilled in the art that many modifications, both to materials and methods, may be practiced without departing from the scope of the invention.

EXAMPLES
EXAMPLE 1: 2-(4-CHLOROPHENYL)-2-(-2-ETHYL-1,3-DIOXOLAN-2-YL) ACETONITRILE OF FORMULA II:

30% NaOMe (357.16 g in MeOH) and THF (200 ml) were charged into RB flask and heated to 50-55°C to provide solution. The mixture of 4-Chloro benzyl cyanide (100 g), ethyl propionate (236.42g) and THF (200 ml) was added to the solution for a period of 30-45 minutes at the same temperature. The temperature of the reaction mixture was increased to 60-65°C and stirred for 3 hours. The obtained reaction mixture was cooled to 20-30°C. Toluene (600 ml) followed by water (500 ml) were charged to the reaction mixture and adjusted pH of the reaction mass to 5-7 with aq. HCl. The layers were separated. The obtained organic layer was washed with aq. NaCl solution, then concentrated completely at below 45°C and cooled to 20-30°C to obtain 2-(4-chlorophenyl)-3-oxopentanenitrile.

Toluene (1000 ml) was added to the concentrated mass and then transferred the reaction mixture into a dean stark RB flask. Ethylene glycol (164.19 g) and H2SO4 (16.45 g) were charged to the reaction mixture, heated to 110-120°C and maintained for 4 hours. The reaction mixture was cooled to 20-30°C, charged water (500 ml) and adjusted pH to 7-9 with aq. NaOH. The layers were separated. The obtained organic layer was washed with aq. NaCl solution and concentrated completely at below 45°C. The resultant residue was cooled to below 35°C. Methanol (300 ml) was charged to the residue and stirred for 15 minutes. Water (60 ml) was charged to the reaction solution, stirred for 2 hours at 20-30°C, filtered and washed the solid with water. The obtained wet solid was slurred in mixture of methanol (300 ml) and water (60 ml) for 2 hours, filtered and dried to obtain titled compound.
Yield: 91 g
Purity by HPLC: 99.99%
Des chloro impurity: 0.01% by HPLC, area normalization

EXAMPLE 2: PREPARATION OF PYRIMETHAMINE
Guanidine hydrochloride (45.5 g) and 30% NaOMe in MeOH (32.1 g) were charged into ethanol (800 ml) at 20-30°C and stirred the reaction mass for 15 minutes. 2-(4-Chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile (100g) was charged to the reaction mixture, raised the temperature to 75- 80°C and stirred for 4 hours at reflux. The reaction mass was cooled to 20-30°C. Water (800 ml) was charged to the reaction mixture and stirred for 30 minutes. The resultant suspension was filtered and washed with water (200 ml). The resultant wet solid was slurred in ethanol (300 ml) for 30 minutes at 20-30°C, filtered, washed the solid with ethanol (100 ml) and dried to obtain Pyrimethamine.
Yield: (68.3 g)
Purity by HPLC: 99.98%

EXAMPLE 3: PURIFICATION OF PYRIMETHAMINE:
Pyrimethamine (100 g) was dissolved in DMSO (1000 ml) at a temperature of 75-85°C, charged carbon (5 g) to the solution and stirred for 30 minutes. The reaction mass was filtered on high flow bed at 75-85°C, washed the bed with DMSO (200 ml) and then cooled to 40-45°C. Water (1000 ml) was charged to the filtrate, cooled to 20-30°C and stirred for 1 hour. The resultant suspension was filtered and washed the solid with water (200 ml). The above wet solid obtained was slurred in water (1000 ml) at 55-65°C for 1 hour and then filtered the solid. The solid was washed with hot water (200 ml) and then dried to obtain pure Pyrimethamine.
Yield: (80 g)
Purity by HPLC: 99.98%
,CLAIMS:WE CLAIM:

1. An improved process for the preparation of pure Pyrimethamine of formula(I):

comprising the steps of:
a) reacting 2-(4-chlorophenyl)-3-oxopentanenitrile of formula (III) with ethylene glycol in presence of 0.2 to 0.5 molar equivalents of sulphuric acid to obtain 2-(4-chlorophenyl)-2-(-2-ethyl-1,3-dioxolan-2-yl)acetonitrile of formula (II) having less than 0.1% of deschloro impurity of Formula A; and

b) reacting the formula (II) obtained in step (a) with guanidine or its salt to obtain Pyrimethamine;
c) optionally, purifying Pyrimethamine from DMSO and water.

2. The process as claimed in claim-1, wherein the reaction of step-a) is carried out in presence of an organic solvent.

3. The process as claimed in claim-2, wherein the organic solvent is hydrocarbon, ether solvent or mixtures thereof.

4. The process as claimed in claim 1, wherein the compound of Formula II has a purity greater than 99.95% by HPLC.

5. The process as claimed in claim-1, wherein the sulphuric acid used in step a) is in the range of 0.25 to 0.3 molar ratio per molar equivalent of 4-chlorophenyl acetonitrile (IV).

6. The process as claimed in claim-1, wherein the ethylene glycol used in step-a) is in the range of 1 to 5 molar equivalents per molar equivalent of 4-chlorophenyl acetonitrile.

7. The process as claimed in claim-1, wherein the compound of formula (II) is crystallized from a mixture of methanol and water.

8. A process for the purification of Pyrimethamine, comprising the steps of:
i. providing a solution of Pyrimethamine in dimethyl sulfoxide at an elevated temperature;
ii. optionally, treating the solution of step i) with charcoal;
iii. cooling the solution of step (i) or (ii);
iv. adding water to the mixture of step iii); and
v. cooling the reaction mass of step iv) to obtain Pyrimethamine having purity greater than 99.9 % by HPLC.

9. The process as claimed in claim-8, wherein the elevated temperature is in the range of 75-85°C or at a reflux temperature.

10. The process as claimed in claim-8, wherein the solution of step iii) is cooled to 40-45°C.