FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention. -
PROCESS FOR THE PREPARATION OF DOXAZOSIN
AND SALTS THEREOF
2. Applicant(s)
(a) NAME : CIPLA LIMITED
(b) NATIONALITY : An Indian Company
(c) ADDRESS : Mumbai Central, Mumbai - 400 008, Maharashtra, India
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed :

Field of Invention
The present invention relates to a process for the preparation of doxazosin or salts thereof
Background and Prior Art
Doxazosin is chemically known as 4-amino-2-[4-(l,4-benzodioxan-2-carbonyl)piperazin-l-yl]-6,7-dimethoxyquinazoline and is represented by formula I,

Doxazosin, a quinazoline compound, is an alpha-1 adrenergic receptor blocker used to treat high blood pressure, benign prostatic hyperplasia and elevated serum low density lipoproteins. It inhibits the binding of norepinephrine to alpha receptors in the autonomic nervous system thus causing vasodilation, and a decrease in peripheral vascular resistance which further leads to decrease in blood pressure.
US4188390 discloses doxazosin, its pharmaceutically acceptable salts and process of preparation thereof. US'390 covers the preparation of doxazosin hydrochloride only and not of the doxazosin base.
The doxazosin hydrochloride is prepared by reacting 4-amino-2-chloro-6,7-dimethoxy quinazoline with N-(l,4-benzodioxan-2-carbonyl)piperazine as represented in following
reaction Scheme 1.


The major problem associated with the disclosed process is that reaction of piperazine with ethyl 2,3-dihydrobenzo[l,4]dioxin-2-carboxylate results in formation of the bis-amide impurity.
Organic Preparations and Procedures International (2003); 35; 603 - 608 describes the procedure for synthesizing doxazosin by treating (2,3-dihydrobenzo[b][l,4]dioxin-2-yl) (piperazin-l-yl)methanone with 2-chloro-6,7-dimethoxy quinazolin-4-amine in presence of n-butanol to obtain doxazosin hydrochloride; which on further treatment with aqueous ammonia solution yields doxazosin base.
However, the process described in Organic Preparations and Procedures International (2003) involves the use of highly toxic reagents like thionyl chloride which is not convenient to handle at industrial scale. Also the use of the reagent in the process leads to generation of unwanted side product such as dimeric impurity, formula A, which in turn affects the overall yield of the final product. Hence, the preparation of doxazosin hydrochloride by this process is not feasible at plant scale.


Other patents disclose preparation of doxazosin mesylate and its polymorphs.
From the prior art it is clear that there has been no disclosure of a process of preparing doxazosin. The known processes give the preparation of doxazosin salt which on treatment with a base yields doxazosin. The inventors have therefore felt a need to develop a process for the preparation of doxazosin that is simple, easy to scale up and industrially acceptable.
Object of the Invention
The main object of the present invention is to provide a process for the preparation of doxazosin base or its pharmaceutically acceptable salts.
Another object of the present invention is to provide an economical, eco-friendly and industrially acceptable process for the preparation of doxazosin.
Summary of the Invention
According to first aspect of the present invention, there is provided a process for preparing doxazosin base of formula (I), which process comprises reacting 2,3-dihydro-benzo[l,4]dioxine-2-carboxylic acid derivative of formula (II) with 6,7-dimethoxy-2-(piperazin-l-yl)quinazolin-4-amine (III), as represented in below reaction Scheme 2.


The doxazosin base of the process of present invention is prepared by carrying out the reaction in presence of suitable activating agent or coupling agent or mixtures thereof.
Accordingly in an aspect of the invention, when R = OH, the condensation reaction is carried out in presence of coupling agent.
In an embodiment, when R = OH, the doxazosin is prepared by process which comprises condensing 2,3-dihydro-benzo[l,4]dioxine-2-carboxylic acid (IV) with 6,7-dimethoxy-2-(piperazin-l-yl)quinazolin-4-amine (III) in presence of coupling agent to form the doxazosin of formula (I). The doxazosin may optionally be converted to a salt thereof.


In an embodiment, the term "coupling agent" may be taken to mean a chemical substance capable of activating the free carboxyl group through the formation of highly reactive intermediate, that are subject to facile nucleophilic attack by amino functional groups leading to peptide link formation. In a further embodiment, the term "coupling agent" may be taken to mean a chemical substance that couples compounds of formula (IV) and formula (III) to form doxazosin of formula (I).
The coupling agent used for the reaction may be selected from groups such as
carbodiimide, aminium or phosphonium salts. The coupling agents are selected from
dicyclohexylcarbodiimide, diisopropylcarbodiimide, 1 -ethyl-3 -(3 -dimethylaminopropy 1)
carbodiimide, hydroxy benzotriazole, [l,2,3]triazolo[4,5-b]pyridin-3-ol, 0-(benzotriazol-1-
yl)-N,N,N',N'-tetramethyluronium tetrafluoroborate, 0-benzotriazole-N,N,N',N'-
tetramethyl uraniumhexafluorophosphate, benzotriazole-1 -yl-oxy-
tris(dimethylamino)phosphonium hexafluorophosphate, (benzotriazol-1-
yloxy)tripyrrolidinophosphonium hexafluorophosphate. Preferably the coupling agent used in the process of present invention is dicyclohexylcarbodiimide.
According to another embodiment, when R is not -OH the compound of formula (IV) is treated with activating agent to obtain the corresponding compound of formula V.


wherein, R' represents the radicals as defined by R except for -OH group
According to an embodiment, when R is not -OH the compound of formula (IV) is treated with activating agent to obtain the corresponding compound of formula V, wherein R' is defined below.

According to another aspect there is provided a process for preparing doxazosin which
comprises:
a) condensing 2,3-dihydro-benzo[l,4]dioxine-2-carboxylic acid (IV) with an activating
agent to obtain corresponding intermediate compound of formula (V); and
(b) reacting intermediate compound of formula (V) with 6,7-dimethoxy-2-(piperazin-l-yl)
quinazolin-4-amine (III) to obtain doxazosin (I). The doxazosin may optionally be
converted to a salt thereof.


In an embodiment, the term "activating agent" may be taken to mean an agent that activates functional group for substitution or elimination reaction. In a further embodiment, the term "activating agent" may be taken to mean an agent that activates the compound of formula (IV) to form compound of formula (V).
The activating agent used for the preparation of doxazosin base may be selected from N,N'-carbonyldiimidazole, hydroxybenzotriazole, N-hydroxysuccinimide, N-hydroxy
phthalimide, pentafluorophenol, 2-chloro-4,6-dimethoxy-l,3,5-triazine or alkyl chloro formates.
In a preferred embodiment, R is imidazole ring and the compound of formula (V) has the formula (VA).

Suitably, the reaction is carried out without isolation of intermediate compound of formula (V). In other words, the intermediate is not isolated from the reaction mixture.

The solvent used in the process for the preparation of doxazosin base (I) is selected from acetonitrile, dioxane, tetrahydrofuran, ethyl acetate or dichloromethane. Preferably, the solvent used is tetrahydrofuran.
In an aspect, the doxazosin obtained from the above processes is isolated from the reaction mixture by replacing the solvent with another solvent.
In an embodiment, the second solvent used is selected from acetonitrile, acetone, dimethyl formamide or dichloromethane; preferably the solvent is acetone.
The use of coupling or activating agent in the process of present invention minimizes the dimeric impurity of formula (A) up to 0.01%.
In another embodiment, the solvent used for preparing doxazosin salt is a polar aprotic solvent.
According to yet another aspect of the present invention, there is provided (2,3-dihydrobenzo[b] [l,4]dioxin-2-yl)(lH-imidazol-l-yl)methanone of formula (VA).
According to still another aspect of the present invention, there is provided doxazosin base prepared according to a process described above.
According to another aspect of the present invention, there is provided doxazosin salt prepared according to a process described above.
According to a further aspect of the present invention, there is provided a pharmaceutical composition comprising doxazosin or salts thereof, prepared according to a process described above, together with one or more excipients.
According to another aspect of the present invention, there is provided doxazosin or salts thereof prepared according to process described above for use in the treatment of atherosclerosis.

Detailed Description of the Invention
The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.
The present invention provides a simple, economical and easy scale-up process for the synthesis of doxazosin in good yield and high purity.
In an embodiment of the present invention, there is provided a process for synthesis of doxazosin (I) as depicted below in reaction Scheme 2,



Accordingly in an embodiment, when R = OH, the condensation reaction is carried out in presence of coupling agent.
In an embodiment, when R = OH, the process of preparation of doxazosin (I) or salt
thereof, comprises:

with 6,7-dimethoxy-2-(piperazin-l-yl)quinazolin-4-amine (III)

(a) reacting 2,3-dihydro-benzo[l,4]dioxine-2-carboxylic acid (IV)
in presence of a coupling agent.
The coupling agent used for the preparation of doxazosin base may be selected from groups such as carbodiimide, aminium or phosphonium salts. The coupling agents are selected

from dicyclohexylcarbodiimide, diisopropylcarbodiimide, 1 -ethyl-3-(3-
dimethylaminopropyl)carbodiimide, hydroxy benzotriazole, [l,2,3]triazolo[4,5-b]pyridin-
3-ol, 0-(benzotriazol-l-yl)-N,N,N,,N'-tetramethyluronium tetrafluoroborate, O-
benzotriazole-NsN,N' ,N' -tetramethyluroniumhexafluorophosphate, benzotriazole-1 -yl-oxy-
tris(dimethyl amino)phosphonium hexafluorophosphate, (benzotriazol-1 -
yloxy)tripyrrolidino phosphonium hexafluorophosphate.
In preferred embodiment, the coupling agent used in the process of present invention is dicyclohexylcarbodiimide.
In another embodiment, when R is not OH, compound of formula (IV) is treated with an activating agent to give corresponding intermediate of formula (V).

with activating agent to obtain 2,3-dihydro-l,4-benzodioxine derivative of formula (V);

According to another aspect, the process of preparation of doxazosin (I), comprises (a) condensing 2,3-dihydro-l,4-benzodioxine-2-carboxylic acid (IV),
(b) reacting compound of formula (V) with 6,7-dimethoxy-2-(piperazin-l-yl) quinazolin-4-amine (III)


to obtain doxazosin of formula (I). The doxazosin may optionally be converted to a salt thereof.
The acid used for the preparation of doxazosin salt may be inorganic, like hydrochloric acid, or organic, like methanesulphonic acid.
The activating agent used for the process of present invention is selected from N,N'-carbonyldiimidazole, hydroxybenzotriazole, N-hydroxysuccinimide, N-hydroxy phthalimide, pentafluorophenol, 2-chloro-4,6-dimethoxy-1,3,5-triazine or alkyl chloro formates.
In a preferred embodiment, the activating agent used is N,N'-carbonyldiimidazole.
In a further embodiment, when the activating agent is N,N'-carbonyldiimidazole the compound of formula V has the formula,

Suitably, the reaction is carried out without isolation of intermediate compound of formula (V). In other words, the intermediate is not isolated from the reaction mixture.

In an embodiment, the process is a one-pot process. In other words, the steps resulting to the preparation of doxazosin (I) of the present invention are carried out in a single reaction vessel.
In another embodiment, the solvent used in the process for the preparation of doxazosin of formula (I) is selected from acetonitrile, dioxane, tetrahydrofuran, ethyl acetate or dichloromethane. Preferably, the solvent used is tetrahydrofuran.
The process of present invention is preferably carried out at a temperature below 40°C.
In an embodiment, the doxazosin obtained from the above processes is isolated from the reaction mixture by replacing the solvent with another solvent.
In an embodiment, the second solvent is a polar aprotic solvent selected from acetonitrile, acetone, dimethyl formamide or dichloromethane. Preferably doxazosin is isolated from reaction mixture by using acetone solvent.
The doxazosin of the present invention may optionally be recrystallised by suspending doxazosin in a polar solvent such as C1 - C4 alcohol, such as methanol, and heating the mixture to reflux. The resulting suspension is cooled, filtered and dried at 50 - 65°C.
The use of coupling or activating agent in the process of present invention minimizes the dimeric impurity of formula (A) up to 0.01%.
The doxazosin salt of the present invention is prepared by dissolving doxazosin base (I) in a polar aprotic solvent which is selected from acetonitrile, acetone or dichloromethane. Preferably doxazosin (I) is dissolved in acetone.
The doxazosin salt of the process of present invention is purified using polar solvent like methanol, ethanol, n-butanol or isopropanol, mainly methanol.

In an embodiment, the process of present invention eliminates the wastage of product, reduces the use cost associated with multiple reactors, reduces cleanup thus making the process more industrially viable.
In most preferred embodiment, the process of the present invention is depicted below in Scheme 3,


Scheme 3
In a further aspect, the present invention provides a pharmaceutical composition comprising doxazosin or salts thereof prepared according to a process described above together with one or more excipients.
The invention also relates to the use of doxazosin or its salts prepared according to the process described above for the treatment of atherosclerosis.
The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention. It will be appreciated that the invention may be modified within the scope of the appended claims.
Examples
Example 1 -
2,3-dihydro-l,4-benzodioxine-2-carboxylic acid (0.38 mol; 68.51 g) and tetrahydrofuran (500 ml) were charged and contents were stirred at 20-25 C for 5-10 min to get a clear solution. N,N-carbonyl diimidazole (0.407 mol; 66.28 g) alongwith tetrahydrofuran (500 ml) was added to the solution and stirring continued at 20-25°C for 2 hr to obtain solution A. In another round bottom flask, 6,7-dimethoxy-2-(piperazin-l-yl)quinazolin-4-amine (0.346 mol; 100 g) and tetrahydrofuran (1000 ml) were charged and stirred at 26-28°C for 5-10 min to obtain a slurry. Solution A was added dropwise within 2-2.5 hr at 25-28°C to the slurry. The reaction was monitored by HPLC. To the resulting clear reaction mass charcoal was added and contents were stirred at 26-28 C for 45 min. The contents were filtered through celite and washed with tetrahydrofuran. The solvent was distilled out at 35 C under reduced pressure to obtain solid residue. Acetone (1000 ml) was added to the solid and contents were refluxed for 2 hr. On cooling to 25-28°C, the slurry was filtered. washed with acetone and dried at 60°C under reduced pressure to obtain 110 g of doxazosin base (HPLC purity - 99.4%).

Example 2 -
Step A - 0.0172 mol (3.096 g) of 2,3-dihydro-l,4-benzodioxine-2-carboxylic acid and 25 ml of tetrahydrofuran were charged in a round bottom flask. The reaction mixture was stirred at 20-25°C for 5-10 min. To the clear solution N,N-carbonyl diimidazole (0.018 mol; 2.92 g) and 25 ml of tetrahydrofuran was added and stirred at 20-25°C for 2 hr. The solution was concentrated to obtain l-(2,3-dihydro-l,4-benzodioxin-2-yIcarbonyI)-lH-imidazole.
Step B - In another round bottom flask, 6,7-dimethoxy-2-(piperazin-l-yl)quinazolin-4-amine (0.017 mol; 4.913 g) and tetrahydrofuran (50 ml) were added and stirred at 26-28°C for 5-10 min. To this slurry solution of l-(2,3-dihydro-l,4-benzodioxin-2-ylcarbonyl)-lH-imidazole in tetrahydrofuran was added dropwise within 2-2.5 hr at room temperature. The resulting clear reaction mass was charcoalized and stirred at 26-28 C for 45 min. The slurry was filtered through celite and washed with tetrahydrofuran. The solvent was distilled out at 35°C under reduced pressure and solid was obtained. Acetone (1000 ml) was added to the solid and refluxed for 2 hr. On cooling to room temperature, the slurry was filtered, washed with acetone and dried at 60°C under reduced pressure to obtain solid. Step C - To the solid, obtained from step B, methanol (52 ml) was added and contents heated to reflux for 1 hr. The slurry was cooled to room temperature, filtered, washed with methanol and dried at 60°C under reduced pressure to obtain 5.5 g of pure doxazosin base (HPLC purity-99.26%).
Example 3 -
In a reaction vessel, 0.25 mol (45.0 g) of 2,3-dihydro-l,4-benzodioxine-2-carboxylic acid, tetrahydrofuran (328 ml) and 6,7-dimethoxy-2-(piperazin-l-yl)quinazolin-4-amine (0.22 mol; 63.58 g) were charged and contents were stirred at 20-25 C for 5-10 min. To the resulting mixture dicyclohexylcarbodiimide (0.26 mol; 54.0 g) was added and stirring continued at 20-25 C for 1.5 hr. To the resulting reaction mass charcoal was added and contents were stirred at 26-28 C for 45 min. The contents were filtered through celite and washed with tetrahydrofuran. The solvent was distilled out at 35°C under reduced pressure to obtain solid residue. Acetone (1000 ml) was added to the solid and contents were refluxed for 2 hr. On cooling to 25-28°C, the slurry was filtered, washed with acetone and dried at 60 C under reduced pressure to obtain 93 g of doxazosin base (HPLC purity -99.0%).

Example 4 -
Doxazosin base (0.277 mol; 125 g) was charged in acetone (1875 ml) and stirred at 25°C to obtain slurry. On cooling the slurry to 10-15°C, methanesulphonic acid (0.33 mol; 31.96 g) was added to it. The contents were stirred for 10-15 min at 10-15°C and then stirring continued at 25°C for 2 hr. The solid obtained was filtered, washed with acetone and dried at 50-55°C to obtain solid. The solid was purified by refluxing in methanol (1500 ml) for 1 hr. The slurry was cooled to room temperature, filtered, washed with methanol and then dried at 60 C under reduced pressure to obtain 135 g of pure doxazosin mesylate (HPTC purity-99.81%).
Example 5 -
Doxazosin base (0.110 mol; 49.61 g) was charged in acetone (750 ml) and stirred at 25 C to obtain slurry. On cooling the slurry to 10-15°C, hydrochloric acid solution was added till the pH was 2-3. The contents were stirred for 10-15 min at 10-15°C and then stirring continued at 25°C for 2 hr. The solid obtained was filtered, washed with acetone and dried at 50-55°C to obtain solid product. The solid was purified by refluxing with methanol (600 ml) for 1 hr. The slurry was cooled to room temperature, filtered, washed with methanol and then dried at 60°C under reduced pressure to obtain 51 g of pure doxazosin hydrochloride (HPLC purity - 99.77%).

WE CLAIMS

with 6,7-dimethoxy-2-(piperazin-l-yl) quinazolin-4-amine of formula (III)

1. A process for preparing doxazosin of the formula (I) comprising condensing compound of formula (II),
2. A process for preparing doxazosin according to claim 1, wherein when R = OH, condensation is carried out in presence of coupling agent.

3. A process for preparing doxazosin according to claim 2, wherein the coupling agent is selected from carbodiimide, aminium or phosphonium salts.
4. A process for preparing doxazosin according to claim 2 or 3, wherein the coupling agent is selected from dicyclohexylcarbodiimide, diisopropylcarbodiimide, l-ethyl-3-(3-dimethylaminopropyl) carbodiimide, hydroxy benzotriazole, [1,2,3]triazolo[4,5-b]pyridin-3 -ol, 0-(benzotriazol-1 -yI)-N,N,N',N'-tetramethyluronium tetrafluoroborate, O-benzotriazole-N,N,N',N'-tetramethyluroniumhexafluorophosphate, benzotriazole-1-yl-oxy-tris(dimethylamino)phosphonium hexafluorophosphate, (benzotriazol-l-yloxy)tripyrrolidinophosphoniumhexafluorophosphate.
5. A process for preparing doxazosin according to any of claims 2 to 4, wherein the coupling agent is dicyclohexylcarbodiimide.
6. A process for preparing doxazosin according to claim 1, wherein R is not -OH, compound of formula (IV) is reacted with an activating agent to corresponding intermediate (V).

wherein, R' represents the radicals as defined by R except for -OH group
7. A process for preparing doxazosin according to claim 6, wherein the activating agent used is selected from N,N'-carbonyldiimidazole, hydroxybenzotriazole, N-hydroxysuccinimide, N-hydroxy phthalimide, pentafluorophenol, 2-chloro-4,6-dimethoxy-1,3,5-triazine or alkyl chloro formates.

8. A process for preparing doxazosin according to claims 6 or 7, wherein the activating
agent used is N,N'-carbonyldiimidazole.
9. A process according to any of claims 6, 7 and 8 wherein the intermediate of formula (V)
is not isolated before it is reacted with the compound of formula (III).
10. A process for preparing doxazosin salt, comprising dissolving the doxazosin base according to any preceding claim in a solvent and then treating with an acid.
11. A process according to claim 10, wherein the solvent is selected from acetonitrile, acetone, dichloromethane or a mixture thereof.
12. A process according to claim 10 or 11, wherein the acid used is inorganic, preferably hydrochloric acid, or organic, preferably methanesulphonic acid.
13. A process for preparing doxazosin comprising:
(a) condensing 2,3-dihydro-benzo[l,4]dioxine-2-carboxylic acid of formula (IV),

with 6,7-dimethoxy-2-(piperazin-l-yl) quinazolin-4-amine of formula (III)

in presence of dicyclohexylcarbodiimide to obtain doxazosin; and (b) optionally converting doxazosin to a salt thereof.

14. A process for preparing doxazosin comprising:
(a) condensing 2,3-dihydro-benzo[l,4]dioxine-2-carboxylic acid of formula (IV),

(b) reacting compound of formufa (VA) with 6,7-dimethoxy-2-(piperazin-l-yl) quinazolin-4-amine of formula (III)

to obtain doxazosin of formula (I); and
(c) optionally converting doxazosin to a salt thereof.
15. A process according to claim 13 or 14, when the doxazosin is converted to a salt thereof, the doxazosin salt is an acid addition salt.
16. A process according to claim 15, wherein acid addition salt is inorganic, preferably hydrochloric acid, or organic, preferably methanesulphonic acid.


25. A process for preparing compound of formula (VA) substantially as herein described with reference to the examples.
17. A process according to any of claims 6, 7, 8 and 14, wherein the process is a one-pot process.
18. A process according to claim 14, wherein the (2,3-dihydro-benzo[l,4]dioxin-2-yl)-imidazol-1-yl-methanone of formula (VA) is not isolated before reacting with 6.7-dimethoxy-2-(piperazin-l-yl) quinazolin-4-amine of formula (III).
19. A process according to any preceding claim, wherein the condensation reaction is carried out in the presence of a solvent selected from acetonitrile, dioxane. tetrahydrofuran, ethyl acetate, dichloromethane or a mixture thereof.
20. A process according to claim 19. wherein the solvent used is tetrahydrofuran.
21. A process according to claims 19 or 20, further comprises isolating doxazosin by replacing solvent with a second solvent.
22. A process according to claim 21, wherein the second solvent is a polar aprotic solvent.
23. A process according to claim 21 or 22, wherein the second solvent is selected from acetonitrile, acetone, dimethyl formamide, dichloromethane or a mixture thereof.
24. (2,3-dihydro-benzo[l,4]dioxin-2-yl)-imidazol-l-yl-methanone of formula (VA)

25. A process for preparing doxazosin substantially as herein described with reference to the examples.
26. A process for preparing doxazosin salt substantially as herein described with reference to the examples.