An improved process for the preparation of ensifentrine and its purification thereof

Abstract

The present invention relates to an improved process for the preparation of Ensifentrine of Formula-I. which comprises: reacting of 1-(3,4-dimethoxyphenethyl)urea compound of Formula II with dimethylmalonate of Formula III to obtain 1-(3,4-dimethoxyphenethyl)pyrimidine-2,4,6(1H,3H,5H)-trione of Formula IV; chlorination of compound of Formula-IV to obtain 2-chloro-9,10-dimethoxy-6,7- dihydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula V; reacting of compound of Formula V with 2,4,6-trimethyl aniline of Formula VI to obtain (E)-2-(mesitylimino)-9,10- dimethoxy-2,3,6,7-tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one compound of Formula VII; condensation of compound of Formula VII with compound of Formula VIII or a compound of Formula IX or a salt thereof to obtain (E)-1-(2-(2-(mesitylimino)-9,10- dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a] isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; optionally converting the compound of Formula I to its pharmaceutically acceptable salts thereof. The invention further comprises purification of crude Ensifentrine by reacting with salt forming agent in a suitable solvent followed by treating with a suitable base and a suitable solvent to obtain pure Ensifentrine freebase of Formula I.

Description

[0001] AN IMPROVED PROCESS FOR THE PREPARATION OF ENSIFENTRINE AND ITS PURIFICATION THEREOF CROSS-REFERENCE TO RELATED APPLICATIONS This PCT International application claims priority from Indian Patent Application No. IN202441048339 was filed as a provisional patent application on 24thJune 2024, Indian Patent Application No. IN202541012821 was filed as a provisional patent application on 14thFebruary 2025, and Indian Patent Application No. IN202541041215 was filed as a provisional patent application on 29thApril 2025. FIELD OF THE INVENTION The present invention relates to an improved process for the preparation of Ensifentrine having the structural formula (I), with higher yields and better purity. The present invention also provides a process for the preparation of Ensifentrine salt of Formula X, which is used in the purification of Ensifentrine freebase of Formula I. The present invention provides a novel crystalline Form N of Ensifentrine Fumarate of Formula-XI, which is used in the preparation of Ensifentrine of Formula I. BACKGROUND OF THE INVENTION ENSIFENTRINE [RPL-554 (LS-193,855)] is a first-in-class, inhaled, dual inhibitor of the phosphodiesterase 3 (PDE3) and phosphodiesterase 4 (PDE4) enzymes. Chemically known as 10-Dimethoxy-2-(2,4,6-trimethylphenylimino)-3-(N-carbamoyl-2-aminoethyl)-3,4,6,7- tetrahydro-2H-pyrimido[6,1-a]-isoquinolin-4-one. The molecular formula is C26H31N5O4 and the molecular weight is 477.565 grams per mole. Ensifentrine is used to treat Chronic obstructive pulmonary disease (COPD). Ensifentrine was first disclosed in US 6,794,391 B2 (Vernalis / Verona Pharma). This patent discloses a process for the preparation of Ensifentrine of Formula I, which involves chlorination of 9,10-dimethoxy-6,7-dihydro-2H-pyrimido[6,1-a]isoquinoline-2,4(3H)- dione of Formula II with POCl3 to obtain a compound of Formula III, then coupled with a compound of Formula IV in the presence of IPA to obtain a compound of Formula V, which is purified by column chromatography. Protection of the compound of Formula V with a compound of Formula VI in the presence of K2CO3, NaI, 2-butanone to obtain a compound of Formula VII, which is purified by column chromatography. Deprotection of compound of Formula VII with hydrazine hydrate, in the presence of chloroform and EtOH to obtain a compound of Formula VIII, followed by reacting with NaOCN in the presence of H2O and HCl to obtain a compound of Formula I, which is purified by column chromatography. The process is schematically shown below in Scheme I:

[0002] In the above process (Scheme-I), the sodium isocyanate is used in final step and involves purification by column chromatography, which affects the overall yield of the final product, and it is not suitable to perform in large-scale industrial production. Further, isocyanate is classified as potential human carcinogens. The synthesis disclosed in this prior art is difficult to operate, not environment friendly and expensive. Further, several routes to the synthesis of Ensifentrine compound of Formula I are known in the art, which are US 7,105,663 B2 and US 10,710,998 B2. The process disclosed in above references should undergo repeated purification procedures and also uses expensive reagents. Therefore, it would be desirable and of paramount importance to have a process for the preparation of Ensifentrine compound of Formula I, by employing inexpensive, readily available, easy to handle reagents. It would also be desirable to have a process that can be readily scaled up and which does not require a special purification step, thereby making it more suitable for industrial scale preparation. Though purification for Ensifentrine using column chromatography is known, none of the prior-art processes attain to get Ensifentrine with high purity. Hence there is a need in the art to develop a process the purification of Ensifentrine. SUMMARY OF THE INVENTION The main aspect of the present invention relates to an improved process for the preparation of Ensifentrine of Formula I with higher yield and better purity. In the first aspect, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which comprises: i) reacting 1-(3,4-dimethoxyphenethyl)urea of Formula II, with dimethylmalonate of Formula III, in the presence of sodium alkoxide and alcohol solvent to provide 1-(3,4- dimethoxyphenethyl)pyrimidine-2,4,6(1H,3H,5H)-trione of Formula IV; ii) chlorinating the compound of Formula IV in the presence of phosphorous oxychloride to provide 2-chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido[6,1- a]isoquinolin-4-one of Formula V; iii) reacting the compound of Formula V with 2,4,6-trimethyl aniline of Formula in the presence of suitable solvent to provide (E)-2-(mesitylimino)-9,10-dimethoxy- 2,3,6,7-tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII;

[0003] iv) condensing the compound of Formula VII with a compound of Formula VIII, in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10- dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; v) optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof. In the second aspect, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which comprises: i) condensing compound of Formula VII,

[0004] with a compound of Formula VIII, in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10- dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; ii) optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof. In the third aspect, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which comprises: i) reacting 1-(3,4-dimethoxyphenethyl)urea of Formula II, with dimethylmalonate of Formula III, in the presence of sodium alkoxide and alcohol solvent to provide 1-(3,4- dimethoxyphenethyl)pyrimidine-2,4,6(1H,3H,5H)-trione of Formula IV; ii) chlorinating the compound of Formula IV in the presence of phosphorous oxychloride to provide 2-chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido[6,1- a]isoquinolin-4-one of Formula V; iii) reacting the compound of Formula V with 2,4,6-trimethyl aniline of Formula VI, in the presence of suitable solvent to provide (E)-2-(mesitylimino)-9,10- dimethoxy-2,3,6,7-tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII;

[0005] iv) condensing the compound of Formula VII with oxazolidin-2-imine of Formula IX or salt thereof, in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10- dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; v) optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof. In the fourth aspect, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which comprises: i) condensing compound of Formula VII,

[0006] with a compound of Formula VIII or salt thereof, in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10- dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; ii) optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof. In the fifth aspect, the present invention provides a process for the purification of Ensifentrine freebase of Formula I, which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding a salt forming agent to the step i) solution at a suitable temperature to provide Ensifentrine salt of Formula X;

[0007] which is optionally isolated; iii. treating the Ensifentrine salt of Formula X obtained in step-ii) with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; iv. isolating the pure Ensifentrine freebase of Formula I. In the sixth aspect, the present invention provides a process for the purification of Ensifentrine freebase of Formula I, which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature to provide Ensifentrine Fumarate of Formula XI;

[0008] which is optionally isolated; iii. treating the Ensifentrine Fumarate of Formula XI obtained in step-ii) with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; iv. isolating the pure Ensifentrine freebase of Formula I. In the seventh aspect, the present invention provides a process for the preparation of Ensifentrine salt of Formula X, which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding a salt forming agent to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine salt of Formula X. In the eight aspect, the present invention provides a process for the preparation of Ensifentrine Fumarate of Formula XI,

[0009] which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine Fumarate of Formula XI. In the nineth aspect, the present invention provides a novel crystalline Form N of Ensifentrine Fumarate of Formula XI, characterized by its PXRD pattern having one or more peaks at about 6.6°, 6.8°, 13.3°, 13.6°, 22.4°, 23.8°, 24.0° and 24.8° ± 0.2° 2θ. In the tenth aspect, the present invention provides a process for the purification of Ensifentrine freebase of Formula I, which comprises: i. treating Ensifentrine salt of Formula X,

[0010] with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula I. In the eleventh aspect, the present invention provides a process for the purification of Ensifentrine freebase of Formula I, which comprises: i. treating Ensifentrine Fumarate of Formula XI, with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula I. DETAILED DESCRIPTION OF THE INVENTION The following description with accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness. The terms and words used in the following description and claims are not limited to the bibliographical meanings but are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the scope of the invention as defined by the appended claims and their equivalents. It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Features that are described and / or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and / or in combination with or instead of the features of the other embodiments. It should be emphasized that the term “comprises / comprising” when used in this specification is taken to specify the presence of stated features, steps or components but does not preclude the presence or addition of one or more other features, steps, components or groups thereof. The term “solvent / suitable solvent” used in the present invention is selected from the group comprising of water, alcohols, ethers, amides, esters, nitriles, sulfoxides, ketones, hydrocarbons and halogenated hydrocarbons; wherein alcohol is selected from the group consisting of methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol and the like; ester is selected from the group consisting of ethyl acetate, isopropyl acetate (IPAc); ketone is selected from the group consisting of acetone, methyl isobutyl ketone, methyl ethyl ketone; ether is selected from the group consisting of methyl tert-butyl ether, diisopropyl ether, diethyl ether tetrahydrofuran, 2-methyl tetrahydrofuran, cyclopentyl methyl ether, dioxane and the like; halogenated solvent is selected from the group consisting of methylene dichloride (MDC), chloroform, chlorobenzene, bromobenzene and the like; hydrocarbons is selected from the group consisting of heptane, hexane, cyclohexane, cycloheptane, toluene, xylene, cyclohexane and the like; nitrile is selected from the group consisting of acetonitrile (ACN), propionitrile and the like; amide is selected from the group consisting of N,N-dimethylformamide (DMF), N,N-dimethyl acetamide (DMAc) and the like; sulfoxide such as dimethyl sulfoxide; sulfone; or mixtures thereof. The term “base” used herein the present invention until unless specified is selected from inorganic bases like “alkali metal hydroxides” such as lithium hydroxide, sodium hydroxide, potassium hydroxide and the like; “alkali metal carbonates” such sodium carbonate, potassium carbonate, lithium carbonate and the like; “alkali metal bicarbonates” such as sodium bicarbonate, potassium bicarbonate, lithium bicarbonate and the like; “alkali metal hydrides” such as sodium hydride, potassium hydride, lithium hydride and the like; “alkali metal alkoxides” such as sodium methoxide, sodium ethoxide, sodium tert-butoxide, potassium methoxide, potassium ethoxide, potassium tert-butoxide and the like, ammonia and organic bases such as triethylamine, methylamine, ethylamine, 1,8- diazabicycle[5.4.0]undec7-ene (DBU), 1,5-diazabicyclo(4.3.0)non-5-ene (DBN), lithiumdiisopropyl-amine (LDA), n- butyl lithium, tribenzylamine, isopropyl amine, diisopropylamine (DIPA), diisopropylethyl amine (DIPEA), N- methylmorpholine (NMM), N-ethylmorpholine, piperidine, dimethyl amino pyridine (DMAP), morpholine, pyridine, 2,6-lutidine, 2,4,6-collidine, imidazole, 1- methylimidazole, 1,2,4-triazole, 1,4-diazabicyclo [2.2.2]octane (DABCO), cesium fluoride, cesium carbonate, potassium phosphate (tripotassium phosphate), potassium hydrogen phosphate and the like or mixtures thereof. The term “phase transfer catalyst” is used herein the present invention until unless specified is selected from tetramethyl ammonium, tetraethyl ammonium, benzyl triethyl ammonium salt, tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBAC)., tetrabutylammonium iodide (TBAI), bromidetetraphenylphosphonium bromide (TPPB), or tetrabutylphosphonium bromide (TBPB) and the like. The pharmaceutically acceptable salt is selected from acids of ethane-1,2- disulfonic, sulfuric, phosphoric, ethanesulfonic, hydrobromic, hydrochloric, methanesulfonic, naphthalene-1,5-disulfonic, p-toluenesulfonic or benzenesulfonic and the like. The “salt forming agent” as used in the present invention is selected from but is not limited to phosphoric acid, maleic acid, sulfuric acid, benzene sulfonic acid, oxalic acid, hydrobromic acid, hydrochloric acid, citric acid, succinic acid, fumaric acid, tartaric acid, p-toluenesulphonic acid, benzoic acid, benzenesulphonic acid, methanesulfonic acid, ethanesulphonic acid, ethane-1,2-disulfonic acid, naphthalene-1,5-disulfonic acid, 2-naphthalenesulphonic acid, 4- chlorobenzenesulphonic acid and the like. Accordingly, the present invention provides an improved process for the preparation of Ensifentrine of Formula I.

[0011] In one embodiment, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which is outlined below in Scheme II: The starting compound 1-(3,4-dimethoxyphenethyl)urea of Formula II is known in the art and is commercially available. The same can be obtained from the commercial sources or can be prepared by the processes known in the art. In step i), reaction of 1-(3,4-dimethoxyphenethyl)urea of Formula II with dimethylmalonate of Formula III in the presence of sodium alkoxide and alcohol solvent to provide 1-(3,4-dimethoxyphenethyl)pyrimidine-2,4,6(1H,3H,5H)-trione of Formula IV. The sodium alkoxide is selected from the group consisting of sodium methoxide, sodium ethoxide, sodium propoxide, sodium isoproxide, or sodium tert-butoxide; preferably, sodium methoxide; the alcohol solvent is selected from the group consisting of methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, methanol (MeOH). The reaction was carried out at a suitable temperature of about 20 °C to about 75 °C; preferably, at about 60 °C to about 70 °C for sufficient period of time; preferably, for about 24 to 26 hours. The above wet material is suspended in alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, methanol (MeOH). The reaction is stirred for sufficient period of time; preferably, for about 1 to 2 hours; most preferably, for about 1 hour. In step ii), chlorination of 1-(3,4-dimethoxyphenethyl)pyrimidine-2,4,6 (1H,3H,5H)-trione of Formula IV with phosphorous oxychloride to obtain 2- chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula V. The reaction is carried out at a suitable temperature of about 90 °C to about 120 °C; preferably, at about 100 °C to about 110 °C for sufficient period of time; preferably, for about 8 to 14 hours; most preferably, for about 8 to 10 hours. In step iii), condensation of 2-chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido [6,1-a]isoquinolin-4-one of Formula V with 2,4,6-trimethyl aniline of Formula VI in the presence of suitable solvent to obtain (E)-2-(mesitylimino)-9,10-dimethoxy- 2,3,6,7-tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII. The suitable solvent used in the process is as defined above and preferably, alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; most preferably, iso-propanol (IPA). The reaction is carried out at a suitable temperature of about 20 °C to about 100 °C; preferably, at about 85 °C to about 95 °C for sufficient period of time; preferably, for about 8 to 14 hours; most preferably, for about 12 to 14 hours. The above crude material is suspended in alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, methanol (MeOH) or iso-propanol (IPA) or mixture thereof. The reaction is carried out at a suitable temperature of about 25 °C to about 65 °C; preferably, at about 45 °C to about 55 °C. The reaction is stirred for sufficient period of time; preferably, for about 30 minutes to 90 minutes; most preferably, for about 30 minutes to 45 minutes. In step iva), reaction of (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7-tetrahydro- 4H-pyrimido[6,1-a]isoquinolin-4-one compound of Formula VII with compound of Formula VIII in the presence of suitable base and suitable solvent, optionally in presence of suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)- 9,10-dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl) ethyl)urea (Ensifentrine) of Formula I. The suitable base used in the process is as defined above and preferably, potassium phosphate (tripotassium phosphate); The suitable solvent used in the process is as defined above and preferably, amide solvent such as N,N-dimethylformamide (DMF), N,N-dimethyl acetamide (DMAc) and the like; most preferably, N,N-dimethylformamide (DMF). The reaction is carried out at a suitable temperature of about 20 °C to about 120 °C; preferably, at about 85 °C to about 95 °C for sufficient period of time; preferably, for about 36 to 72 hours; most preferably, for about 44 to 48 hours. The above crude material is suspended in alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, iso-propanol (IPA) and water followed by DMF / DMAc and acetonitrile mixture. The reaction is carried out at a suitable temperature of about 25 °C to about 100 °C; preferably, at about 80 °C to about 95 °C. The reaction is stirred for sufficient period of time; preferably, for about 12 hours to 18 hours; most preferably, for about 12 hours to 14 hours. In step ivb), reaction of (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7-tetrahydro- 4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII with a compound of Formula IX or salt thereof, in the presence of suitable base and suitable solvent, optionally in presence of suitable phase transfer catalyst to obtain (E)-1-(2-(2-(mesitylimino)- 9,10-dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl) ethyl)urea (Ensifentrine) of Formula I. The suitable base used in the process is as defined above and preferably, potassium phosphate (tripotassium phosphate); The suitable solvent used in the process is as defined above and preferably, amide solvent such as N,N-dimethylformamide (DMF), N,N-dimethyl acetamide (DMAc) and the like; most preferably, N,N-dimethylformamide (DMF). The reaction is carried out at a suitable temperature of about 20 °C to about 120 °C; preferably, at about 85 °C to about 95 °C for sufficient period of time; preferably, for about 36 to 72 hours; most preferably, for about 44 to 48 hours. Alternatively, the compound of formula IX may be used in the form of suitable salt such as hydrochloride, hydrobromide, hydroiodide and like. The above crude material is suspended in alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, iso-propanol (IPA) and water followed by DMF / DMAc and acetonitrile mixture. The reaction is carried out at a suitable temperature of about 25 °C to about 100 °C; preferably, at about 80 °C to about 95 °C. The reaction is stirred for sufficient period of time; preferably, for about 12 hours to 18 hours; most preferably, for about 12 hours to 14 hours. In the second embodiment, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which is outlined below in Scheme III:

[0012] The starting compound (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7-tetrahydro- 4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII is known in the art and is commercially available. The same can be obtained from the commercial sources or can be prepared by the processes known in the art. In scheme-III, reaction of (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7- tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one compound of Formula VII with compound of Formula VIII in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to obtain (E)-1-(2-(2- (mesitylimino)-9,10-dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a] isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I. The suitable base used in the process is as defined above and preferably, potassium phosphate (tripotassium phosphate); the suitable solvent used in the process is as defined above and preferably, amide solvent such as N,N-dimethylformamide (DMF), N,N- dimethyl acetamide (DMAc) and the like; most preferably, N,N-dimethylformamide (DMF). The reaction is carried out at a suitable temperature of about 20 °C to about 120 °C; preferably, at about 85 °C to about 95 °C for sufficient period of time; preferably, for about 36 to 72 hours; most preferably, for about 44 to 48 hours. The above crude material is suspended in alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, iso-propanol (IPA) and water followed by DMF / DMAc and acetonitrile mixture. The reaction is carried out at a suitable temperature of about 25 °C to about 100 °C; preferably, at about 80 °C to about 95 °C. The reaction is stirred for sufficient period of time; preferably, for about 12 hours to 18 hours; most preferably, for about 12 hours to 14 hours. In the third embodiment, the present invention provides an improved process for the preparation of Ensifentrine of Formula I, which is outlined below in Scheme IV: The starting compound (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7-tetrahydro- 4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII is known in the art and is commercially available. The same can be obtained from the commercial sources or can be prepared by the processes known in the art. In scheme IV, reaction of (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7-tetrahydro- 4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII with compound of Formula IX or salt thereof, in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to obtain (E)-1-(2-(2- (mesitylimino)-9,10-dimethoxy-4-oxo-6,7-dihydro-2H-pyrimido[6,1-a] isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I. The suitable base used in the process is as defined above and preferably, potassium phosphate (tripotassium phosphate); the suitable solvent used in the process is as defined above and preferably, amide solvent such as N,N-dimethylformamide (DMF), N,N- dimethyl acetamide (DMAc) and the like; most preferably, N,N-dimethylformamide (DMF). The reaction is carried out at a suitable temperature of about 20 °C to about 120 °C; preferably, at about 85 °C to about 95 °C for sufficient period of time; preferably, for about 36 to 72 hours; most preferably, for about 44 to 48 hours. Alternatively, the compound of formula IX may be used in the form of suitable salt such as hydrochloride, hydrobromide, hydroiodide and like. The above crude material is suspended in alcohol solvent such as methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, iso-butanol; preferably, iso-propanol (IPA) and water followed by DMF / DMAc and acetonitrile mixture. The reaction is carried out at a suitable temperature of about 25 °C to about 100 °C; preferably, at about 80 °C to about 95 °C. The reaction is stirred for sufficient period of time; preferably, for about 12 hours to 18 hours; most preferably, for about 12 hours to 14 hours. In the fourth embodiment, the present invention provides a process for the purification of Ensifentrine freebase of Formula I, which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding a salt forming agent to the step i) solution at a suitable temperature to provide Ensifentrine salt of Formula X;

[0013] which is optionally isolated; iii. treating the Ensifentrine salt of Formula-X obtained in step-ii) with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; iv. isolating the pure Ensifentrine freebase of Formula I. The starting material crude Ensifentrine is known in the art and can be prepared by the known methods or procured from the available commercial sources. The step i) of the aforementioned process involves dissolution of crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature. The suitable solvent or mixture of solvents are as defined above. The amount of solvent used for dissolution can range from about 15 to about 28 times, or about 15 times, to the initial weight of the crude Ensifentrine. Suitable temperature for forming a solution range from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step ii) of the aforementioned process involves addition of a salt forming agent to the step i) solution at a suitable temperature to provide a corresponding salt of Formula-X. Suitable temperature ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 10 hours to 14 hours, preferably between 12 hours and 13 hours. The salt forming agent is as defined above. The step iii) of the aforementioned process involves optional isolation of Ensifentrine salt of Formula-X which can be carried out by the methods known in the art; for example in case of isolation, cooling the reaction mass temperature to less than 35 °C, maintaining for sufficient period of time till complete formation of material followed by filtration. The step iv) of the aforementioned process involves desalination of Ensifentrine salt of Formula-X with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature. The suitable base and a suitable solvent or mixture of solvents are as defined above. Suitable temperature for desalination ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step iv) of the aforementioned process involves isolation of pure Ensifentrine freebase of Formula I which can be carried out by the methods known in the art; for example, removing of the solvent from step iii) solution or optionally adding suitable anti-solvent to step iii) solution followed by filtration and drying of the obtained solid. The removal of solvent from the step iii) solution is carried out by known techniques such as distillation, spray drying, agitated thin film drying (“ATFD”), and freeze drying or optionally adding anti-solvent; for example, the removal of solvent is carried out by distillation of the solvent completely or some portion from the reaction mass in a rotary evaporator, optionally adding a second solvent; optional cooling of the reaction mass, then filtering off the solid followed by drying. In the fifth embodiment, the present invention provides a process for the purification of Ensifentrine freebase of Formula I,

[0014] which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature to provide Ensifentrine Fumarate of Formula XI; which is optionally isolated; iii. treating the Ensifentrine Fumarate of Formula XI obtained in step-ii) with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; iv. isolating the pure Ensifentrine freebase of Formula I. The step i) of the aforementioned process involves dissolution of crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature. The suitable solvent or mixture of solvents are as defined above. The amount of solvent used for dissolution can range from about 15 to about 28 times, or about 15 times, to the initial weight of the crude Ensifentrine. Suitable temperature for forming a solution range from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step ii) of the aforementioned process involves addition of fumaric acid to the step i) solution at a suitable temperature to provide Ensifentrine Fumarate of Formula-XI. Suitable temperature ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 10 hours to 14 hours, preferably between 12 hours and 13 hours. The salt forming agent is as defined above. The step iii) of the aforementioned process involves optional isolation of Ensifentrine Fumarate of Formula XI, which can be carried out by the methods known in the art; for example in case of isolation, cooling the reaction mass temperature to less than 35 °C, maintaining for sufficient period of time till complete formation of material followed by filtration. The step iv) of the aforementioned process involves desalination of Ensifentrine Fumarate of Formula-XI with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature. The suitable base and a suitable solvent or mixture of solvents are as defined above. Suitable temperature for desalination ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step iv) of the aforementioned process involves isolation of pure Ensifentrine freebase of Formula I which can be carried out by the methods known in the art; for example, removing of the solvent from step iii)solution or optionally adding suitable anti-solvent to step iv) solution followed by filtration and drying of the obtained solid. The removal of solvent from the step iii) solution is carried out by known techniques such as distillation, spray drying, agitated thin film drying (“ATFD”), and freeze drying or optionally adding anti-solvent; for example, the removal of solvent is carried out by distillation of the solvent completely or some portion from the reaction mass in a rotary evaporator, optionally adding a second solvent; optionally cooling the reaction mass, then filtering off the solid followed by drying. In the sixth embodiment, the present invention provides a process for the preparation of Ensifentrine salt of Formula X, which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding a salt forming agent to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine salt of Formula X. The step i) of the aforementioned process involves dissolution of crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature. The suitable solvent or mixture of solvents are as defined above. The amount of solvent used for dissolution can range from about 15 to 28 times, or about 15 times, to the initial weight of the crude Ensifentrine. Suitable temperature for forming a solution range from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step ii) of the aforementioned process involves addition of a salt forming agent to the step i) solution at a suitable temperature to provide Ensifentrine salt of Formula-X. Suitable temperature ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 10 hours to 14 hours, preferably between 12 hours and 13 hours. The salt forming agent is as defined above. The step iii) of the aforementioned process involves isolation of Ensifentrine salt of Formula X which can be carried out by the methods known in the art; for example, cooling the reaction mass temperature to less than 35°C, maintaining for sufficient period of time till complete formation of material followed by filtration. In the seventh embodiment, the present invention provides a process for the preparation of Ensifentrine Fumarate of Formula XI, which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine Fumarate of Formula XI. The step i) of the aforementioned process involves dissolution of crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature. The suitable solvent or mixture of solvents are as defined above. The amount of solvent used for dissolution can range from about 15 to about 28 times, or about 15 times, to the initial weight of the crude Ensifentrine. Suitable temperature for forming a solution range from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step ii) of the aforementioned process involves addition of fumaric acid to the step i) solution at a suitable temperature to provide Ensifentrine Fumarate of Formula XI. Suitable temperature ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 10 hours to 14 hours, preferably between 12 hours and 13 hours. The step iii) of the aforementioned process involves isolation of Ensifentrine Fumarate of Formula XI, which can be carried out by the methods known in the art; for example, cooling the reaction mass temperature to less than 35 °C, maintaining for sufficient period of time till complete formation of material followed by filtration. In the eight embodiment, the present invention provides a novel crystalline Form N of Ensifentrine Fumarate of Formula XI, characterized by its PXRD pattern having one or more peaks at about 6.6°, 6.8°, 13.3°, 13.6°, 22.4°, 23.8°, 24.0° and 24.8° ± 0.2° 2θ. In the ninth embodiment, there is provided the use of crystalline Form N of Ensifentrine Fumarate of Formula XI in the preparation of Ensifentrine freebase of Formula-I. In the tenth embodiment, the present invention provides a process for the purification of Ensifentrine freebase of Formula I,

[0015] which comprises: i. treating Ensifentrine salt of Formula-X, with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula I. The step i) of the aforementioned process involves desalination of Ensifentrine salt of Formula X with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature. The suitable base and a suitable solvent or mixture of solvents are as defined above. Suitable temperature for desalination ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step ii) of the aforementioned process involves isolation of pure Ensifentrine freebase of Formula I can be carried out by the methods known in the art; for example, removing of the solvent from step i)solution or optionally adding suitable anti-solvent to step iv) solution followed by filtration and drying of the obtained solid. The removal of solvent from the step i) solution is carried out by known techniques such as distillation, spray drying, agitated thin film drying (“ATFD”), and freeze drying or optionally adding anti-solvent; for example, the removal of solvent is carried out by distillation of the solvent completely or some portion from the reaction mass in a rotary evaporator, optionally adding a second solvent; then filtering off the solid followed by drying. In the eleventh embodiment, the present invention provides a process for the purification of Ensifentrine freebase of Formula I, which comprises: i. treating Ensifentrine Fumarate of Formula XI, with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula I. The starting material Ensifentrine Fumarate of Formula XI in the above invention is prepared by the method described herein the present invention process. The step i) of the aforementioned process involves desalination of Ensifentrine Fumarate of Formula XI with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature. The suitable base and a suitable solvent or mixture of solvents are as defined above. Suitable temperature for desalination ranges from about 25 °C to about 100 °C, or the reflux temperature of the solvent used, preferably at about 25 °C to about 45 °C; and it is maintained for 15 minutes to 60 minutes, preferably between 20 minutes and 40 minutes. The step ii) of the aforementioned process involves isolation of pure Ensifentrine freebase of Formula-I can be carried out by the methods known in the art; for example, removing of the solvent from step i)solution or optionally adding suitable anti-solvent to step iv) solution followed by filtration and drying of the obtained solid. The removal of solvent from the step i)solution is carried out by known techniques such as distillation, spray drying, agitated thin film drying (“ATFD”), and freeze drying or optionally adding anti-solvent; for example, the removal of solvent is carried out by distillation of the solvent completely or some portion from the reaction mass in a rotary evaporator, adding a second solvent; optionally cooling the reaction mass, then filtering off the solid followed by drying. The crude Ensifentrine obtained by the known methods is having purity by HPLC is about 60% to about 95%. The purity by HPLC of Ensifentrine freebase obtained in any of the above processes is having more than 98%, preferably more than 99% and most preferably 99.5% and above. All the impurities present in the Ensifentrine freebased obtained in the above processes are well within the ICH limits. The pure Ensifentrine freebase of Formula-I of the present invention may be administered as part of a pharmaceutical composition for the treatment of Chronic Obstructive Pulmonary Disease (COPD). Accordingly, in a further embodiment, there is provided a pharmaceutical composition comprising the pure Ensifentrine freebase of Formula I of the present invention, one or more pharmaceutically acceptable carriers, diluents, or excipients, and optionally other therapeutic ingredients. Pharmaceutical compositions comprising the pure Ensifentrine freebase of Formula I of the present invention may be administered orally, topically, parenterally, by inhalation or spray, rectally, or in the form of injectables. EXPERIMENTAL SECTION: The details of the invention are given in the examples provided below, which are given to illustrate the invention only and therefore should not be construed to limit the scope of the invention. EXAMPLES: Example-1: Preparation of 1-(3,4-dimethoxyphenethyl)pyrimidine-2,4,6 (1H,3H,5H)-trione. To a clean and dry RBF, 1-(3,4-dimethoxyphenethyl)urea (50.0 grams, 0.223 moles, 1.0 equivalent), dimethylmalonate (58.9 grams, 0.446 moles, 2.0 equivalents) and methanol (300.0 mL, 6.0 volumes) were added at 25 to 35 °C under nitrogen atmosphere. Subsequently, 30% sodium methoxide solution in methanol (24.08 grams, 0.446 moles, 2.0 equiv.) was added. The temperature of reaction mass was raised to 60 to 70 °C and the mixture was stirred at 60 to70 °C for 24 to 26 hours. The progress of the reaction was monitored by using HPLC, after completion of the reaction, the mixture was cooled to 25 to35 °C and concentrated under reduced pressure, diluted by water (250.0 mL, 5.0 volumes), quenched with 5N aqueous hydrochloride solution (50.0 mL, 1.0 volumes) and stirred for 0.5 hours at 25 to 35 °C. Then, the reaction mass was cooled to 0 to10 °C and stirred for 0.5 hour. Finally, filtered the solid and solid bed was washed with water (50.0 mL, 1.0 volumes). The wet material is suspended in methanol (150.0 mL, 3.0 volumes) and stirred for 1.0 hour at 25 to 35 °C. Finally, the solid was filtered and washed with methanol (50.0 mL), and dried at 50 to 60 °C for 10-12 hours in hot air oven to get the titled compound. Yield: 82.0%. Example-2: Preparation of 2-chloro-9,10-dimethoxy-6,7-dihydro-4H- pyrimido[6,1-a]isoquinolin-4-one. To a clean and dry RBF,, 1-(3,4-dimethoxyphenethyl)pyrimidine-2,4,6 (1H,3H,5H)-trione (25.0 grams; 0.0855 moles; 1 equivalent) and POCl3(125.0 mL, 5.0 volumes) were added at 25 to35 °C. Next, the reaction mass is heated to 100 to110 °C and stirred for 8 to 10 hours, and progress of the reaction was monitored by using HPLC. After completion of the reaction, the reaction mixture was cooled 25 to 35 °C and toluene (100.0 mL, 4.0 volumes) was added, excess POCl3removed under vacuum distillation. This residue was added to the water (750.0 mL, 30.0 volumes) at 5 to15 °C and basified by adding 30% NaOH solution. Reaction mass temperature was raised to 25 to35 °C and extracted with dichloromethane (2 X 1000 mL), combined the organic layers and washed with water (120.0 mL, 5.0 vol.). Then, the organic layer was concentrated under reduced pressure up to 3 to 4 volumes. Finally, methyl tertiary butyl ether (125.0 mL, 5.0 volumes) was added and stirred at 25 to 35 °C for 1 hour, filtered the solid and washed solid bed with methyl tertiary butyl ether (25.0 mL, 1.0 volumes) and dried at 50 to 60 °C for 10 to 14 hours in hot air oven to get the titled compound. Yield: 80.0 to 85.0%. Example-3: Preparation of (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7- tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one To a clean and dry RBF,, 2-chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido[6,1- a]isoquinolin-4-one (18.0 grams, 0.615 moles, 1.0 equivalents), isopropyl alcohol (180.0 mL, 10.0 volumes) and 2,4,6-trimethyl aniline were added at 25 to 35 °C. Next, the reaction mass was heated to 85 to 95 °C and stirred for 12 to 14 hours, and progress of the reaction was monitored by using HPLC. After completion of the reaction, the mixture was cooled 25 to 35 °C and added methyl tertiary butyl ether (540.0 mL, 30.0 volumes) at the same temperature and stirred for 1.0 hour. Finally, filtered the solid and washed solid bed with methyl tertiary butyl ether (36.0 mL, 2.0 volume) and dried at 55 to65 °C for 4 hours in hot air oven to get the titled compound. Yield: 50.0 to 60.0%. Example-4: Purification of (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7- tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one. To a clean and dry RBF, crude (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7- tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one (80.0 g; 1.0 equivalents) and methanol (1000.0 mL) were added at 25-35 °C. Reaction mass was heated to 45 to 55 °C and stirred for 30 to 45 minutes. Next, 10.0 g of activated carbon was added at 50 to 60 °C and stirred for 30 to 45 minutes. Reaction mass was passed through celite bed and washed with hot methanol (100.0 mL, 1.0 volume). Reaction mass was concentrated under reduced pressure up to 1 to 2 volumes and added isopropyl alcohol (300.0 mL). The reaction mass was cooled to 0 to 10 °C and stirred for 1.0 hour at the same temperature. Finally, filtered the solid and solid bed was washed with isopropyl alcohol (100.0 mL, 1.0 volume) and dried at 55 to 65 °C for 12 to 14 hours to get the titled compound. Yield: 55.0%. Example-5: Preparation of E)-1-(2-(2-(mesitylimino)-9,10-dimethoxy-4-oxo- 6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea(Ensifentrine) Method-1: To a clean and dry RBF, (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7- tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one (50.0 grams, 0.128 moles, 1.0 equivalents), chloroethyl-urea (94.12 grams, 0.768 moles, 6.0 equivalents) and N, N-dimethyl formamide (300.0 mL, 6.0 volumes) were added at 25-35 °C under nitrogen atmosphere. Subsequently, tripotassium phosphate (81.34 grams, 0.3832 moles, 3.0 equivalents) and tetra butyl ammonium iodide (4.73 grams, 0.0128 moles, 0.1 equivalents) were added at 25-35 °C and temperature raised to 85 to 95 °C and stirred for 36 to 72 hours at the same temperature. The progress of the reaction was monitored by using HPLC. After completion of the reaction, the mixture was cooled 25-35 °C and diluted with water (750.0 mL; 15.0 volumes) and neutralized with 7% sodium bicarbonate solution (800.0 mL, 16.0 volumes) and stirred for 1 to 2 hours at 25-35 °C. Finally, filtered the solid and solid bed was washed with purified water (50.0 mL, 1.0 volumes) and dried at 65 to 75 °C for 12 hours to get the titled compound. Yield: 82.0%. Method-2: To a clean and dry RBF, (E)-2-(mesitylimino)-9,10-dimethoxy-2,3,6,7- tetrahydro-4H-pyrimido[6,1-a] isoquinolin-4-one (10.0 grams, 0.0255 moles, 1.0 equivalents), oxazolidin-2-imine hydrochloride (18.75 grams, 0.153 moles, 6.0 equivalents) and N, N-dimethyl acetamide (60.0 mL, 6.0 volumes) were added at 25to 35 °C. Subsequently, tripotassium phosphate (10.84 grams, 0.051 moles, 2.0 equivalents) and tetra butyl ammonium iodide (0.95 grams, 0.00255 moles, 0.1 equivalents) were added at 25 to 35 °C and the reaction mass temperature was raised to 85 to 95 °C and stirred for 44 to 48 hours. The progress of the reaction was monitored by using HPLC. After completion of the reaction, the mixture was cooled to 25 to 35 °C, diluted with water (150.0 mL, 15.0 volumes), neutralized with 7% sodium bicarbonate solution (60.0 mL, 16.0 volumes) and the reaction mass was stirred for 1 to 2 hours at 25 to 35 °C. Finally, filtered the solid and solid bed is washed with purified water (500.0 mL, 2.0 volumes) and dried at 65to 75 °C for 4 to 6 hours in hot air oven. Yield: 82.0%. Example-6: Purification process of Ensifentrine To a clean and dry RBF, crude Ensifentrine (50.0 grams; 1 equivalents), isopropanol and water mixture (100.0 mL; 2 volumes; 7:3) were added at 25-35 °C. Next, the temperature was raised to 80 to 85 °C and stirred for 1.0 hour at the same temperature. Then, the reaction mass was cooled to 25 to 35 °C and stirred for 12.0 to 14.0 hours at the same temperature. Finally, the obtained solid was filtered and washed with isopropanol and water mixture (50.0 mL; 1 volume; 7:3), followed by isopropanol (100.0 mL; 2 volumes). This cake was suspended in DMF / DMAc and acetonitrile mixture (250.0 mL; 5 volumes; 1:1) at 25 to 35 °C. Next, the temperature was raised to 85 to 95 °C and stirred for 1.0 hour at the same temperature. Reaction mass was cooled to 25 to 35 °C and filtered the solid, washed with DMF / DMAc and acetonitrile mixture (25.0 mL; 5 volumes; 1:1), followed by acetonitrile (25.0 mL; 0.5 volume). Finally, the obtained crude product was purified by column chromatography using methanol and dichloromethane to get the titled compound. Yield: 20.0 to 30.0%. Example-7: Process for the purification of Ensifentrine freebase: To a clean and dry RBF, crude Ensifentrine (Purity by HPLC: ^60%; 100.0 grams; 1.0 equivalent) and ^9% v / v methanol in dichloromethane (800.0 mL) were added at 25 °C to 35 °C and the reaction mass was stirred for 15 to 30 minutes at the same temperature to obtain the clear solution. Fumaric acid (36.46 grams, 1.5 equivalents) was added at 25 °C to 35 °C and the reaction mass was stirred for 14to 16 hours at the same temperature. Next, the reaction mass was cooled to 5 to 15 °C and stirred for 1 to 2 hours at the same temperature. The obtained solid was filtered and washed with dichloromethane (200.0 mL; 2 volumes). This material was suspended in dichloromethane (800.0 mL; 8.0 volume) and cooled to 5 °C to 15 °C, then pH of the reaction mass was adjusted to ^7 to 8 using 10% w / w aqueous sodium bicarbonate solution (650.0 mL; 6.5 volumes) and stirred for 30.0 minutes at the same temperature. The organic and aqueous layers were separated, and the aqueous layer was extracted with dichloromethane (200.0 mL, 2.0 volumes). Combined the organic layers and washed with water (2 x 500.0 mL) followed by 10% brine solution (1 x 1000.0 mL). The organic layer was distilled up to 7to 8 volumes under vacuum. Methanol (50.0 mL) and fumaric acid (18.2 g, 0.75 equiv.) were added at 25to 35 °C and the reaction mass was stirred for 14 to 16 hours at the same temperature. Next, the reaction mass was cooled to 5 to 15 °C and stirred for 1 to 2 hours at the same temperature. The obtained solid was filtered and washed with dichloromethane (100.0 mL; 1.0 volume). This material was suspended in dichloromethane (400.0 mL; 4.0 volume) and cooled to 5 °C to 15 °C, then pH of the reaction mass was adjusted to ^7 to 8 using 10% w / w aqueous sodium bicarbonate solution (325.0 mL; 3.25 volumes) and stirred for 30.0 minutes at the same temperature. The organic and aqueous layers were separated, and the aqueous layer was extracted with dichloromethane (100.0 mL, 1.0 volume). Combined the organic layers and washed with water (2 x 250.0 mL) followed by 10% brine solution (1 x 500.0 mL). The organic layer was distilled up to 3to 4 volumes under vacuum and methanol (400.0 mL) was added. The reaction mass was further distilled up to 3to 4 volumes under vacuum at 50 °C and co-distilled with methanol (500.0 mL) up to 1to 2 volumes under vacuum at 50 °C. Finally, methyl tertiary butyl ether (500.0 mL) was added and stirred for 30to 40 minutes at 25to 35 °C. Filtered the solid and washed with methyl tertiary butyl ether (200.0 mL) and dried the material under vacuum at 55to 65 °C to get the title compound. Yield: ~29%; Purity: >99.50 by HPLC.

Claims

We claim:

1. An improved process for the preparation of Ensifentrine of Formula I,which comprises: i. condensing compound of Formula VII,with a compound of Formula VIII,in the presence of a suitable base and a suitable solvent, optionally in the presence of a suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10-dimethoxy- 4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; ii. optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof.

2. An improved process for the preparation of Ensifentrine of Formula I,which comprises: i. condensing compound of Formula VII,with a compound of Formula IX or a salt thereof,in the presence of a suitable base and a suitable solvent, optionally in the presence of a suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10-dimethoxy- 4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; ii. optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof.

3. An improved process for the preparation of Ensifentrine of Formula I,which comprises: i. reacting 1-(3,4-dimethoxyphenethyl)urea of Formula II,with dimethylmalonate of Formula III,in the presence of sodium alkoxide and alcohol solvent to provide 1-(3,4- dimethoxyphenethyl)pyrimidine-2,4,6(1H,3H,5H)-trione of Formula IV;ii. chlorinating compound of Formula IV in the presence of phosphorous oxychloride to provide 2-chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido[6,1-a]isoquinolin- 4-one of Formula V;iii. reacting the compound of Formula V with 2,4,6-trimethylaniline of Formula VI,in the presence of suitable solvent to provide (E)-2-(mesitylimino)-9,10-dimethoxy- 2,3,6,7-tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII;iv. condensing the compound of Formula VII with a compound of Formula VIII,in the presence of suitable base and suitable solvent, optionally in the presence of suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10-dimethoxy- 4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; v. optionally, converting the compound of Formula I to its pharmaceutically acceptable salts.

4. An improved process for the preparation of Ensifentrine of Formula I,which comprises: i. reacting 1-(3,4-dimethoxyphenethyl)urea of Formula II,with dimethylmalonate of Formula III,in the presence of sodium alkoxide and alcohol solvent to provide 1-(3,4- dimethoxyphenethyl)pyrimidine-2,4,6(1H,3H,5H)-trione of Formula IV;ii. chlorinating the compound of Formula IV in the presence of phosphorous oxychloride to provide 2-chloro-9,10-dimethoxy-6,7-dihydro-4H-pyrimido[6,1- a]isoquinolin-4-one of Formula V;iii. reacting the compound of Formula V with 2,4,6-trimethylaniline of Formula VI,in the presence of a suitable solvent to provide (E)-2-(mesitylimino)-9,10-dimethoxy- 2,3,6,7-tetrahydro-4H-pyrimido[6,1-a]isoquinolin-4-one of Formula VII;iv. condensing the compound of Formula VII with oxazolidin-2-imine of Formula IX or salt thereof,in the presence of suitable base and suitable solvent, optionally in the presence of a suitable phase transfer catalyst to provide (E)-1-(2-(2-(mesitylimino)-9,10-dimethoxy- 4-oxo-6,7-dihydro-2H-pyrimido[6,1-a]isoquinolin-3(4H)-yl)ethyl)urea (Ensifentrine) of Formula I; v. optionally, converting the compound of Formula I to its pharmaceutically acceptable salts thereof.

5. The process as claimed in claims 1 to 4, wherein the base is selected from inorganic bases such as alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, alkali metal hydrides, alkali metal alkoxides, ammonia or organic bases such as triethylamine, 1,8-diazabicycle[5.4.0]undec7-ene (DBU), lithiumdiisopropyl- amine (LDA), n-butyl lithium, isopropyl amine, diisopropylamine (DIPA), diisopropylethyl amine (DIPEA), N-methylmorpholine (NMM), piperidine, dimethyl amino pyridine (DMAP), morpholine, pyridine, 2,6-lutidine, 1,4-diazabicyclo [2.2.2]octane (DABCO), cesium fluoride, cesium carbonate, potassium phosphate(tripotassium phosphate), potassium hydrogen phosphate, hydrates of potassium phosphate or mixtures thereof.

6. The process as claimed in claims 1 to 4, wherein the suitable solvent is selected from alcohols, ethers, amides, esters, nitriles, sulfoxides, ketones, hydrocarbons, halogenated hydrocarbons, water or mixtures thereof.

7. The process as claimed in claims 1 to 4, wherein the phase transfer catalyst is selected from tetramethyl ammonium salt, tetraethyl ammonium salt, benzyl triethyl ammonium salt, tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBAC)., tetrabutylammonium iodide (TBAI), bromoditetraphenylphosphonium bromide (TPPB), or tetrabutylphosphonium bromide (TBPB).

8. The process as claimed in claims 1 to 4, wherein the pharmaceutically acceptable salt is selected from acids of ethane-1,2-disulfonic, sulfuric, phosphoric, ethanesulfonic, hydrobromic, hydrochloric, methanesulfonic, naphthalene-1,5-disulfonic, p- toluenesulfonic or benzenesulfonic.

9. The process as claimed in claims 3 to 4, wherein in step i) the sodium alkoxide is selected from sodium methoxide, sodium ethoxide, sodium propoxide, sodium isopropylate, n-butanol sodium or sodium tert-butoxide; and the alcohol solvent is selected from the group consisting of methanol (MeOH), ethanol (EtOH), iso-propanol (IPA), n-butanol, or iso-butanol or mixtures thereof.

10. A process for the purification of Ensifentrine freebase of Formula-I,which comprises:i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding a salt forming agent to the step i) solution at a suitable temperature to provide Ensifentrine salt of Formula-X;which is optionally isolated; iii. treating the Ensifentrine salt of Formula-X obtained in step-ii) with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; iv. isolating the pure Ensifentrine freebase of Formula I.

11. A process for the purification of Ensifentrine freebase of Formula-I,which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature to provide Ensifentrine Fumarate of Formula-XI;which is optionally isolated; iii. treating the Ensifentrine Fumarate of Formula-XI obtained in step-ii) with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; iv. isolating the pure Ensifentrine freebase of Formula I.

12. A process for the preparation of Ensifentrine salt of Formula-X,which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding a salt forming agent to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine salt of Formula-X.

13. A process for the preparation of Ensifentrine Fumarate of Formula-XI,which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine Fumarate of Formula-XI.

14. A process for the purification of Ensifentrine freebase of Formula-I,which comprises: i. treating Ensifentrine salt of Formula-X,with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula-I.

15. A process for the purification of Ensifentrine freebase of Formula-I,which comprises: i. treating Ensifentrine Fumarate of Formula-XI,with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula-I.

16. The process as claimed in claims 10 to 15, wherein the solvent or mixture of solvents are selected from alcohol solvents, ester solvents, ether solvents, ketone solvents, halogenated solvents, hydrocarbon solvents, nitrile solvents, amide solvents, sulfoxide, polar solvents, formic acid, acetic acid or mixtures thereof.

17. The process as claimed in claims 10 to 15, wherein the base is selected from inorganic bases like alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, alkali metal hydrides, ammonia; and organic bases such as alkali metal alkoxides, triethylamine, methylamine, ethylamine, 1,8-diaza bicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo(4.3.0)non-5-ene (DBN), lithiumdiiso propylamide (LDA), n-butyl lithium, tribenzylamine, isopropyl amine, diisopropylamine, diisopropylethylamine,N-methylmorpholine, N-ethylmorpholine, piperidine, dimethylamino pyridine, morpholine, pyridine, 2,6-lutidine, 2,4,6-collidine, imidazole, 1-methyl imidazole, 1,2,4-triazole, 1,4-diazabicyclo[2.2.2]octane (DABCO) or mixtures thereof.

18. The process as claimed in claims 10 to 15, wherein the salt forming agent is selected from phosphoric acid, maleic acid, sulfuric acid, benzene sulfonic acid, oxalic acid, hydrobromic acid, hydrochloric acid, citric acid, succinic acid, fumaric acid, tartaric acid, p-toluenesulphonic acid, benzoic acid, benzenesulphonic acid, methanesulfonic acid, ethanesulphonic acid, ethane-1,2-disulfonic acid, naphthalene-1,5-disulfonic, 2- naphthalenesulphonic acid, 4-chlorobenzenesulphonic acid.

19. A novel crystalline Form N of Ensifentrine Fumarate of Formula-XI, characterized by its PXRD pattern having one or more peaks at about 6.6°, 6.8°, 13.3°, 13.6°, 22.4°, 23.8°, 24.0° and 24.8° ± 0.2° 2θ.

20. A process for the preparation of crystalline Form N of Ensifentrine Fumarate of Formula-XI,characterized by its PXRD pattern having one or more peaks at about 6.6°, 6.8°, 13.3°, 13.6°, 22.4°, 23.8°, 24.0° and 24.8° ± 0.2° 2θ; which comprises: i. dissolving crude Ensifentrine in a suitable solvent or mixture of solvents at a suitable temperature; ii. adding Fumaric acid to the step i) solution at a suitable temperature; iii. isolating the Ensifentrine Fumarate of Formula-XI.

21. A process for the purification of Ensifentrine freebase of Formula-I,which comprises: i. treating crystalline Form N of Ensifentrine Fumarate of Formula-XI,characterized by its PXRD pattern having one or more peaks at about 6.6°, 6.8°, 13.3°, 13.6°, 22.4°, 23.8°, 24.0° and 24.8° ± 0.2° 2θ; with a suitable base in a suitable solvent or mixture of solvents at a suitable temperature; ii. isolating the pure Ensifentrine freebase of Formula-I.

22. The process as claimed in claims 20 to 21, wherein the solvent or mixture of solvents are selected from alcohol solvents, ester solvents, ether solvents, ketone solvents, halogenated solvents, hydrocarbon solvents, nitrile solvents, amide solvents, sulfoxide, polar solvents, formic acid, acetic acid or mixtures thereof.

23. The process as claimed in claims 20 to 21, wherein the base is selected from inorganic bases like alkali metal hydroxides, alkali metal carbonates, alkali metal bicarbonates, alkali metal hydrides, ammonia; and organic bases such as alkali metal alkoxides, triethylamine, methylamine, ethylamine, 1,8-diaza bicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo(4.3.0)non-5-ene (DBN), lithiumdiiso propylamide (LDA), n-butyl lithium, tribenzylamine, isopropyl amine, diisopropylamine, diisopropylethylamine,N-methylmorpholine, N-ethylmorpholine, piperidine, dimethylamino pyridine, morpholine, pyridine, 2,6-lutidine, 2,4,6-collidine, imidazole, 1-methyl imidazole, 1,2,4-triazole, 1,4-diazabicyclo[2.2.2]octane (DABCO) or mixtures thereof.

24. The process as claimed in any of the proceeding claims, wherein the purity by HPLC of Ensifentrine freebase is having more than 98%, preferably more than 99% and most preferably 99.5% and above.

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