Technical Field:

The present invention relates to a new method for synthesis of (dl)-norepinephrine acid addition salts using hexamethylenetetramine. The present invention also relates to a novel intermediate formed during the synthesis.

Background and Prior art:

4-[(lR)-2-amino-l-hydroxyethyl]benzene-l,2-diol, commonly known as (R)-(-)- norepinephrine or noradrenaline is a catecholamine with multiple roles including as a hormone and a neurotransmitter. As a stress hormone, norepinephrine affects parts of the brain where attention and responding actions are controlled. Along with epinephrine, norepinephrine also underlies the fight-or-flight response, directly increasing heart rate, triggering the release of glucose from energy stores, and increasing blood flow to skeletal muscle. Norepinephrine also has a neurotransmitter role when released diffusely in the brain as an antiinflammatory agent.

When norepinephrine acts as a drug it increases blood pressure by increasing vascular tone through a-adrenergic receptor activation. The resulting increase in vascular resistance triggers a compensatory reflex that overcomes its direct stimulatory effects on the heart, called the baroreceptor reflex, which results in a drop in heart rate called reflex bradycardia.

(R)-(-)-Norepinephrine has a following structure:

(R)-(-)-Norepinephrine was first time disclosed in the US patent US2774789, where it was obtained by resolution of dl-norepinephrine, with optically active acids such as d- tartaric acid, 1-malic acid or N-benzoyl-l-threonine. The patent does not disclose the preparation of dl-norepinephrine. The patent GB747768 describes reduction of amino ketones where 3,4-dihydroxy-a- aminoacetophenone hydrochloride was converted into its d-tartrate salt; followed by reduction of the d-tartrate salt. This process leads to formation of excessive amount of d- adrenaline d-tartrate (which is a bi-product) as it crystallized first; whereas the desired 1- adrenaline d-tartrate crystallizes after 2 days and in smaller yield. Also the patent does not disclose the source of 3,4-dihydroxy-a-aminoacetophenone hydrochloride.

It has been unsuccessfully tried to treat dihydroxy-a-chloroacetophenone with hexamethylenetetramine (commonly known as hexamine) and to treat the reaction product with an acid to obtain arterenone (see Mannich, Hahn B., Berichte der deutschen chemischen Gesellschaft, volume 44, issue 2, Pages 1542 - 1552 (1911)). Mannich found that the treatment of this and similar halogen ketones with hexamine did not produce an addition compound but resulted in splitting of halogen acid which made the process impossible. Mannich also found that an addition compound of the halogen ketone and hexamine is formed only when the two phenolic hydroxyl groups are closed i.e. protected by acylation or etherification. Hence according to Mannich, the reaction is not at all possible for the compounds containing two unprotected phenolic hydroxyl groups. The US patent US 1680055 discloses the preparation of monohydroxy-a-substituted- aminoacetophenones either by reacting monohydroxy-a-bromoacetophenones with a substituted amine or by reacting protected monohydroxy-a-bromoacetophenones with a substituted amine followed by deprotection. The patent does not disclose the preparation of dihydroxy-a-aminoacetophenones (where amino group is unsubstituted).

It is disclosed in the US patent US2786871 that when chloroaceto pyrocatechol is treated with ammonia, arterenone is obtained in 50% yield. However when the reaction is carried out in basic medium, darkening of the reaction mass takes place which results in coloured product. The patent also discloses preparation of amino-methyl-(monohydroxyphenyl)- ketones by reacting halogen ketone with hexamine. It is also disclosed in the patent that the process is applicable only to the halogenomethyl-monohydroxyphenyl-ketones.

Following are some of the methods for preparation of 3,4-dihydroxy-a- aminoacetophenone, reported in the literature. J. Am. Pharm. Association (1946) 35, 306 - 309 discloses preparation of 3,4-dihydroxy- a-aminoacetophenone by reacting 3,4-dihydroxy-a-chloroacetophenone with dibenzyl amine followed by hydrogenation of resulting dibenzylamino ketone. The main disadvantage of this reaction is formation of derivatives of dibenzyl amines, which remain in the final product in the form of impurities.

Acta Chimica Academiae Scientiarum Hungaricae (1951), 1, 395-402, discloses preparation of 3,4-dihydroxy-a-aminoacetophenone from 3,4-dihydroxyphenyloxo acetaldehyde and benzyl amine followed by reduction of benzylamino ketone intermediate. The main disadvantage of this method is that the starting acetaldehyde derivative is very expensive and not easily available.

It is disclosed in Recueil des Travaux Chimiques des Pays-Bas et de la Belgique (1952), 71, 933-44, that 3,4-dihydroxy-a-aminoacetophenone hydrochloride is formed by demethylation of 3,4-dimethoxy-a-aminoacetophenone hydrochloride using 48% HBr. The reaction results in less than 10% yield of the aminoacetophenone.

Monatshefte fuer Chemie (1953), 84 1021-32, discloses preparation of 3,4-dihydroxy-a- aminoacetophenone by reacting 3,4-dihydroxy-a-chloroacetophenone with sodium azide followed by hydrogenation of azide intermediate using 4% palladium on carbon as a catalyst. In the hydrogenation step, 1.6 gm of azide intermediate requires 1.4 gm of catalyst, which is not economical and industrially feasible.

Preparation of 3,4-dihydroxy-a-aminoacetophenones hydrochloride is disclosed in J. Am. Chem. Soc, 1955, volume 77, issue 10, pages 2896 - 2897. The following scheme is disclosed in the article: It is clear from the above scheme that the process requires additional steps of protection and deprotection of hydroxyl and amino groups, and use of potassium phthalimide requires anhydrous reaction conditions. Therefore the process is time consuming and not economical.

Chinese patent CN101798271A describes reduction of 3,4-dihydroxy-a- aminoacetophenone hydrochloride in water as solvent followed by neutralization with aqueous ammonia. Since dl-norepinephrine has partial solubility in aqueous basic medium result in to loss of product. Also it is necessary to maintain low volume of solvent throughout the process for better yields making the process stringent.

European patent EP1930313 discloses preparation of a-amino ketones. The preparation is carried out by reacting an organic sulfide in a polar solvent with a compound containing a leaving group attached to a primary or secondary carbon atom to form a sulfonium salt, which is reacted with a ketone in presence of a base and a polar solvent. Oxiranes obtained are further converted into the corresponding aminoketone, by aminolysis followed by selective oxidation. The following scheme is disclosed in the patent.

It is clear from the above scheme that the process requires many steps and hence is time consuming. The patent does not exemplify the synthesis of dihydroxy-a- aminoacetophenones.

Thus, the search for a suitable manufacturing process for (R)-norepinephrine intermediates remains undoubtedly of interest. We were surprised to find that hardly any literature discloses the process for preparation of dihydroxy-a-aminoacetophenones acid addition salts. We have found that the reaction of dihydroxy-a-haloacetophenone with hexamine is feasible and results in high yield of product although both the hydroxyl groups on the phenyl ring of acetophenone are unprotected. Object of the invention:

It is therefore an object of the invention is to overcome or ameliorate at least one disadvantage of the prior art or to provide a useful alternative.

Another object of the invention is to provide a novel, safe, efficient, concise, ecological, high yielding, industrially feasible and simpler process for preparation of (R)-(-)- norepinephrine intermediates.

Another object of the invention is to provide a process for synthesis of 3,4-dihydroxy-a- aminoacetophenone salt, which is feasible without protecting both the hydroxyl group on the phenyl ring of acetophenone.

Yet another object of the invention is to provide an improved process for hydrogenation of 3,4-dihydroxy-a-aminoacetophenone salt to prepare (dl)-norepinephrine salt.

Summary of the invention:

In accordance with the above objectives, the present invention provides a process for preparation of (dl)-norepinephrine intermediate of formula (III) comprising reacting 3,4- dihydroxy-a-haloacetophenone of formula (I) with hexamine to provide a quaternary ammonium salt of formula (II); followed by hydrolyzing the quaternary ammonium salt of formula (II) with an acid.

In a second aspect, the present invention provides a novel quaternary ammonium salt of formula (II) and its preparation.

In a third aspect, the present invention provides a novel process for hydrogenation of 3,4- dihydroxy-a-aminoacetophenone acid salt to provide (dl)-norepinephrine acid addition salt.

Detailed description of the invention:

Unless specified otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art, to which this invention belongs. Although any methods and material or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. To describe the invention, certain terms are defined herein specified as follows:

Unless stated to the contrary, any of the words 'having', 'including', 'includes', 'comprising' and 'comprises' mean 'including without limitations' and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it. Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose illustration rather than limitation of the invention as set forth the appended claims.

Accordingly, the present invention provides a process for preparation of (R)- norepinephrine intermediate of formula (III),

Formula (III)

Wherein X' is anion selected from the group consisting of CI, Br, I, HSO ₄ , alkyl sulfonate, aryl sulfonate, C10 ₄ or I0 ₄ , comprising the following steps:

a) reacting 3,4-dihydroxy-a-haloacetophenone of formula (I) with hexamine to provide a quaternary ammonium of formula (II)

Formula (I) Formula (II)

where X is CI, Br or I; and

b) hydrolyzing the quaternary ammonium salt of formula (II) with

Hexamine is an easily accessible reactant; and highly soluble in water and polar organic solvents. The reaction provides selective access to 3,4-dihydroxy-a-aminoacetophenone without side reactions, in short reaction time and relatively mild reaction conditions. Hexamine is not hygroscopic and not subject to change during storage and can be used even after several months. It has very low toxicity and diluted solution can be broken down biologically.

Hexamine is conveniently used in an amount relative to 3,4-dihydroxy-a- haloacetophenone, preferably in a range between 1 to 10 equivalents, preferably between 1 to 4 equivalents.

The step (a) is carried out in presence of at least one solvent. The solvent is selected from alcohol, chlorinated solvents or mixture thereof. The preferred solvent for the step a, is a mixture of isopropanol and chloroform. The step (a) is carried out for 1 to 7 hours. The time required for the step (a) is preferably 3 to 5 hours. The step (a) is carried out at 40 to 80°C. Preferably the step (a) is carried out at 55 to 65°C.

Advantageously the step (a) is carried out by reacting 3,4-dihydroxy-a- chloroacetophenone with hexamine.

The quaternary ammonium salt of formula (II) can be isolated from the reaction mixture or directly used in the subsequent step without isolation. The quaternary ammonium salt is preferably isolated from the reaction mixture. The quaternary ammonium salt is not hygroscopic and not subject to change during storage and can be used even after several months.

The acid used for the step (b) is an aqueous acid selected from hydrochloric acid, sulfuric acid, hydrobromic acid, perchloric acid, para-toluenesulfonic acid and hydroiodic acid. The acid used for hydrolysis is preferably hydrochloric acid. The acid is conveniently used in an amount, relative to quaternary ammonium salt of formula (II), preferably in a range between 2 to 20 equivalents, more preferably between 4 to 12 equivalents. The most preferred quantity of the acid is 5 to 8 equivalents.

The step (b) is advantageously carried out in a solvent selected from alcohols. The solvent used for the step (b) is preferably alcohol containing 1 to 6 carbon atoms. The most preferred solvent for the step (b) is methanol. The step (b) is carried out for 10 to 25 hours. The time required for the step (b) is preferably 15 to 20 hours. The most preferred time required for the step (b) is 16 to 18 hours. The step (b) is carried out at 30 to 65°C. The preferred temperature for hydrolysis is 40 to 60°C.

In a second aspect, the present invention provides a novel quaternary ammonium salt of formula (II).

Formula (II)

where X is CI, Br or I.

The process for preparation of a quaternary ammonium salt of formula (II) comprises reaction of 3,4-dihydroxy-a-haloacetophenone of formula (I) with hexamine.

Hexamine is conveniently used in an amount relative to 3,4-dihydroxy-a- haloacetophenone, preferably in a range between 1 to 10 equivalents, preferably between 1 to 4 equivalents.

The preparation of the quaternary ammonium salt of formula (II) is advantageously carried out in a solvent selected from alcohol, chlorinated solvents or mixture thereof. The preferred solvent is a mixture of isopropanol and chloroform. The reaction of hexamine and 3,4-dihydroxy-a-haloacetophenone of formula (I) is carried out for 1 to 7 hours. The time required for the reaction is preferably 3 to 5 hours. The reaction of hexamine and 3,4-dihydroxy-a-haloacetophenone of formula (I) is carried out at 40 to 80°C, preferably at 55 to 65°C.

In a third aspect, the present invention provides a convenient method for hydrogenation of 3,4-dihydroxy-a-aminoacetophenone acid addition salts to provide (dl)-norepinephrine of formula IV; wherein, X' is anion selected from the group consisting of CI, Br, I, HS0 ₄ , alkyl sulfonate, aryl sulfonate, C10 ₄ or IO4,

Formula IV (dl)-norepinephrine is sparingly soluble in many solvents other than water even in basic pH. Thus when the reaction is carried out in solvents other than water the product can be isolated in maximum yields and purity.

The process of hydrogenation is preferably carried out in presence of a catalyst. The catalyst is selected from platinum or palladium on carbon. The preferred catalyst used for hydrogenation is palladium on carbon. Hydrogenation is carried out for 3 to 20 hours. The time required for hydrogenation is preferably 5 to 20 hours. Hydrogenation is carried out at 25 - 80°C. The preferred temperature for hydrogenation is 40 - 50°C. Hydrogenation is carried out in presence of a solvent. The solvent is selected from polar solvents. The most preferred solvent for hydrogenation is methanol.

The compound of formula (IV) is converted to (R)-(-)-norepinephrine bitartrate by methods known in the art.

The pure (R)-(-)-norepinephrine bitartrate obtained by the process of the invention may be formulated into a dosage form by combining with one or more pharmaceutically acceptable excipients using known techniques. Further the dosage form may be immediate release or extended release.

Further details of the process of the present invention will be apparent from the examples presented below. Examples presented are purely illustrative and are not limited to the particular embodiments illustrated herein but include the permutations, which are obvious as set forth in the description.

Example 1

Preparation of quaternary ammonium salt

A 5000 ml four neck round bottom flask with water condenser and calcium chloride tube was charged with Hexamine (210.28 gm), chloroform (1200 ml), 3,4-dihydroxy-a- chloroacetophenone (250 gm) and isopropanol (1000 ml) at room temperature. The reaction mass was gently heated at 63°C for 4 hours. The reaction was monitored by TLC. The reaction mass was cooled to room temperature and filtered to get solid. The solid was washed with acetone and dried at 50°C for 4 hours to obtain quaternary ammonium salt which was used in the next step without purification.

Yield - 410 gm (93.65%)

Nature - off white solid

m.p. - 180 to l82°C

NMR (DMSO-d6): - δ =4.51 - 4.75 (m, 8H), 5.39 (s, 6H), 6.92 (d, 1H, J= 7.5 Hz), 7.37 - 7.42 (m, 2H), 9.67 (s, br, 1H), 10.44 (s, br, 1H)

Example 2

Preparation of 3,4-dihydroxy-a-aminoacetophenone hydrochloride

A 2000 ml four neck round bottom flask with water condenser and calcium chloride tube was charged with the quaternary ammonium salt obtained in the example 1 (120 gm), methanol (862.5 ml) and cone, hydrochloric acid (194.4 ml). The reaction mixture was heated to 60 to 65°C and aged at same temperature for 3 to 4 hours. The reaction was monitored by TLC. The reaction mass was cooled and neutralized using base to give 3,4- dihydroxy-a-aminoacetophenone. The solid was filtered, washed with water and dried at 50°C. This base was further converted in to its hydrochloride salt with IPA-HC1 mixture. Yield - 72 gm (96.3%)

Nature - off white solid

HPLC - 99.7%

1H NMR(CD30D) - 5 = 3.62(s, 1H), 6.80 (d, J = 8 Hz, 1H), 7.38 (d, J = 1.3 Hz, 1H), 7.63 (d, J = 8 Hz, 1H).

Example 3

Preparation of (dl)-norepinephrine hydrochloride

A 500 ml hydrogenation flask was charged with 3,4-dihydroxy-a-aminoacetophenone hydrochloride obtained in the example 2 (55 gm), 10% palladium on carbon (5 gm) and methanol (300 ml). The reaction mixture was heated to 45°C with hydrogen gas pressure of 4 to 5 kg m2. The reaction mixture was stirred at 45°C for 5 hours. The catalyst was removed by filtration. The filtrate was cooled to 5 to 10 °C and ammonia gas was passed through the solvent for 2 h till the pH of the solution was around 9. The solid obtained was filtered, washed with methanol and dried in air to obtain (dl)-norepinephrine. Yield - 43.5 gm (96.7%)

Nature white crystalline solid

HPLC 99.6%

Example 4

Preparation of (dl)-norepinephrine hydrochloride

A 500 ml hydrogenation flask was charged with 3,4-dihydroxy-a-aminoacetophenone hydrochloride obtained from process similar to example 2 (55 gm), 10% palladium on carbon (5 gm) and methanol (300 ml). The reaction mixture was aged at 25 °C with hydrogen gas pressure of 4 to 3 kg/m2. The reaction mixture was stirred at 25°C for 15 hours. The reaction was monitored by TLC. The catalyst was removed by filtration. The filtrate was cooled to 5 to 10 °C and ammonia solution was added to the reaction mixture till the pH of the solution around 9. The solid obtained was filtered, washed with methanol and dried in air to obtain (dl)-norepinephrine.

Yield - 41.5 gm (92.2%)

Nature - white crystalline solid

HPLC - 99.5%