FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
'Process for impurity free production of Ractopamine hydrochloride'
APPLICANT
(a) Name: Embio Limited
(b) Nationality: Indian
(c) Address: #501, Sentinel, Central Avenue Road,
Hiranandani Gardens,
Powai, Mumbai- 400 076, India

PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed,
FIELD OF INVENTION
[0001] The present invention relates to a sympathomimetic agent
primarily used as an animal feed additive for promoting growth, improving the feed efficiency and improving the leanness of meat. More particularly, the invention relates to a new process for preparation of Ractopamine Hydrochloride wherein the ractopamine hydrochloride is substantially free from impurities and comprises a precisely controlled ratio of diastereomers and enantiomers.
BACKGROUND OF THE INVENTION
[0002] Ractopamine Hydrochloride was first described by J. van Dijk
and H.D.Moed (Rec. Trav. Chim. 92a 1281 (1973). It is a drug that is used as a feed additive to promote leanness in pigs raised for their meat and is structurally represented as Figure 1. Ractopamine characteristically contains as part of their chemical structure an ethanolamine or 2-amino-ethanoI moiety. Compounds such as ractopamine with two chiral centers have four isomers, which are the RR-, SS-, RS-, and SR-isomers. For the sake of simplicity, RR-ractopamine may herein be referred to as RR, SS ractopamine may herein be referred to as SS, RS ractopamine may herein be referred to as RS, and SR ractopamine may herein be referred to as SR. Ractopamine may exist in a number of forms i.e. in a single, pure RR or SS or RS or SR isomeric forms, or as mixtures of the compositions RR/SS, RR/SR, RR/RS or RS/SR, SR/SS or RS/SS. The compound ractopamine is a mixture of all four isomers. RR/SS/RS/SR ractopamine is a mixture of all four isomers in approximately similar concentrations. All four isomers usually exist in approximately the same concentrations of is approximately 25%. However, for the present

purpose, RR/SS/RS/SR ractopamine may contain from 23% to 27% of any of the four isomers.
[0003] Ractopamine has the molecular formula C18H23NO3 and is
typically prepared as a hydrochloride salt. The term ractopamine HC1 refers to the hydrochloride salt of RR/SS/RS/SR ractopamine. Thus, ractopamine HC1 is the hydrochloride salt of a mixture of all four isomers in approximately equal proportions.
[0004] The structure below depicts ractopamine. RR ractopamine has
the R-configuration at both chiral centres, while SR ractopamine has the S-configuration at one centre while R-configuration at the other.

[0005] Though several methods have been introduced for the production of ractopamine, however the resulting yields have presence of impurities that leads to adverse side effects. For illustrations, US patent 4,690,951 discloses the preparation of ractopamine hydrochloride by reacting 2-amino-l-(4-hydroxyphenyI) ethanol and 4-(4-hydroxyphenyl)-2-butanone by catalytic hydrogenation using a palladium on carbon catalyst as represented in Scheme 1.

Another patent, CN 1116620 describes a method wherein o-bromo-p-hydroxyacetophenone is reacted with l-methyl-3-(4-hydroxyphenyl) propyl

amine, and then the resulting amino ketone is hydrogenated using a palladium on carbon catalyst to produce ractopamine as depicted below in scheme 2.

Similar chemistry is followed in CN 1174830, CN 1315318 whereas in CN 1557804, instead of ω -bromo-p-hydroxyacetophenone and palladium on carbon catalyst, ω -chloro derivative and Raney nickel catalyst are used. CN 1660775 uses similar chemistry but makes use of sodium borohydride for reduction of the amino ketone to get ractopamine. In CN 101016249, p-hydroxyacetophenone is converted to dichloro derivative, hydrolyzed to dihydroxy derivative, reacted with l-methyI-3-(4-hydroxyphenyl) propylamine in the presence of sodium borohydride to give ractopamine as depicted in scheme 3.


[0006] Ractopamine Hydrochloride is official in USP35 as a 20%
suspension in water. The USP specification requires that the diastereomer ratio of (R*, S*) to (R*, R*) isomers be between 45:55 and 49:51. Besides, the impurities of l-methyl-3-(4-hydroxyphenyl) propylamine and deoxy ractopamine should be less than 0.5 % each.
[0007] The above mentioned patents use expensive raw materials,
production process involves lots of steps, low productivity hence not suitable for commercial production and most importantly the impurity levels are above the prescribed limits. Thus there exists a long standing technical need to provide a process that consistently gives ractopamine hydrochloride in high yield and with purity in accordance with the USP specifications.
[0008] A detailed study of the impurity profile detailed in USP35
monograph of Ractopamine Hydrochloride aqueous suspension revealed that
the following key impurities need to be rigorously controlled in the compendial
product:
Impurity 1: Octopamine [2-amino-l-(4-hydroxyphenyl) ethanol] : not more
Impurity 2: Tyramine [4-(2-aminoethyl) phenol] : not more than 0.5%
than 0.5%


Impurity 3: l-methyl-3-(4-hydroxyphenyl) propylamine [4-(3-aminobutyl) phenol] : not more than 0.5%

Impurity 4: 4-(4-hydroxyphenyl)-2-butanone: not more than 1.0%

Impurity 5: 4-(3-hydroxybutyl) phenol: not more than 0.5%

Impurity 6: Deoxy ractopamine [4-(2-{[4-(4-hydroxyphenyI) butan-2-yl] amino} ethyl) phenol]: not more than 0.5%

[0009] The following observations were made which helped in
overcoming the existing deficiencies in the ractopamine production process with respect to impurities:
[0010] 1. Octopamine HC1 [2-amino-l-(4-hydroxyphenyl) ethanol] is the
starting material. It is highly soluble in water. So it is possible to remove it from the product, if the product is crystallized from aqueous media.
[0011] 2, Tyramine may result from direct hydrogenolysis of the -OH
group of octopamine, especially if palladium on carbon catalyst is used for hydrogenation as in the case of US 4690951, CN 1116620, CN 1174830 and CN 1315318.

[0012] 3. l-Methyl-3-(4-hydroxyphenyl) propylamine: This is one of the
starting materials in the protocol followed by CN 1116620, CN 1174830, CN 1315318, CN 1557804 and CN 1660775. However if 4-(4-hydroxyphenyl)-2-butanone is used as one of the starting materials, this impurity may not remain as an artifact of the process in the final product. This impurity may still generate from the starting materials as described in the case of US 4690951 if hydrogenation speed is relatively slow. The formation of l-Methyl-3-(4-hydroxyphenyl) propylamine during hydrogenation of octopamine with 4-(4-hydroxyphenyl)-2-butanone can be explained as follows (scheme 4): Schiff base are known to isomerize by a 1, 3-proton shift. The initially formed Schiff base between octopamine and 4-(4-hydroxyphenyl)-2-butanone can isomerize under reaction conditions before it gets hydrogenated and then could undergo hydrolysis to give l-Methyl-3-(4-hydroxyphenyl) propylamine. It should be noted here that these reactions would take place only if the speed of hydrogenation of initially formed Schiff base is relatively slow for the side reactions to take place.

[0013] 4. 4-(4-hydroxyphenyl)-2-butanone is a starting material of the
process. So it may present itself as impurity in the final product. However, this compound is non-basic and water-insoluble substance, hence it should be possible to remove it by an extractive work-up in acidic aqueous media.

[0014] 5. 4-(3-hydroxybutyl) phenol may result from direct
hydrogenation of 4-(4-hydroxyphenyl)-2-butanone especially if palladium catalyst is used as in the case of US 4690951.
[0015] 6. Deoxy ractopamine [4-(2-{[4-(4-hydroxyphenyl) butan-2-yl]
amino) ethyl) phenol] may result from either: i) reaction of tyramine (which itself may result from hydrogenolysis of the benzylic -OH group of octopamine) with 4-(4-hydroxyphenyl)-2-butanone, or ii) hydrogenolysis of the benzylic -OH group of the already formed ractopamine in a successive hydrogenation reaction. Such hydrogenolysis of benzylic -OH groups is known to occur if palladium catalysts are used in the case of US 4690951, CN 1116620, CN 1174830 and CN 1315318.
OBJECTS OF THE INVENTION
[0016] It is an object of the present invention to provide a new and
improved process for the preparation of ractopamine hydrochloride that gives a product with substantially low levels of impurities and has a precisely controlled ratio of diastereomers and enantiomers in the product. In a preferred embodiment the ratio of diastereomers, (RS,SR);(RR,SS) falls within the range of 45:55 and 49:51 and the ratio of individual enantiomers, RS:SR and RR:SS falls within the range of 99.5:100.5 and 100.5:99.5.
BRIEF DESCRPITION OF THE DIAGRAMS
[0017] In accordance with the preferred embodiments of the present
invention FIG. 1 depicts the JH NMR spectrum in presence of DMSO-d6
[0018] In accordance with the preferred embodiments of the present
invention FIG. 2 depicts the *H NMR spectrum in presence of Methanol-d^

BRIEF DESCRPITIQN OF THE TABLES
[0019] In accordance with the preferred embodiments of the present
invention Table 1 describes the interpretation and assignment of peaks as shown in FIG. 1.

Pea
k
No. Chemical shift (8, ppm) Peak type No. of protons Assignment
1 9.42 singlet 1H Phenolic -OH, A
2 8.7 broad singlet 2H NH2+, B
3 9.18 singlet 1H Phenolic -OH, C
4 6.9-7.15 doublet of doublet 4H Ar-H, D
5 6.55-6.75 doublet of doublet 4H Ar-H,E
6 5.9 broad singlet 1H Benzylic -OH, F
7 4.70-4.85 triplet 1H Benzylic H, G
8 3.0-3.1 multiplet 1H H
9 2.9-2.95 multiplet 2H I
10 2.5-2.6 multiplet 1H Benzylic H, J
11 2.3-2.45 multiplet 1H Benzylic H, K
12 1.9-2.05 multiplet 1H L
13 1.5-1.7 multiplet 1H M
13 1.2 doublet 3H -CH3, N
Table 1
[0020] In accordance with the preferred embodiments of the present
invention Table 2 describes the interpretation and assignment of peaks as shown in FIG. 2.

Pea k
No. Chemical shift (5, ppm) Peak type No. of protons Assignment
4 6.9-7.15 doublet of doublet 4H Ar-H,D
5 6.55-6.75 doublet of doublet 4H Ar-H, E
7 4.65-4.75 multiplet 1H Benzylic H, G
8 3.1-3.2 multiplet 1H H
9 2.9-3.05 multiplet 2H I
10 2.55-2.7 multiplet 1H Benzylic H, J
11 2.4-2.5 multiplet 1H Benzylic H, K
12 1.9-2.05 multiplet 1H L

13 1.65-1.8 multiplet IH M
13 1.27 doublet 3H -CH3, N
Table 2
SUMMARY OF THE INVENTION
[0021] The present invention discloses a process for the preparation of
ractopamine hydrochloride comprising; reacting 2-amino-l-(4-hydroxyphenyl) ethanol hydrochloride and 4-(4-hydroxyphenyi)-2-butanone in presence of a hydrogenation catalyst and hydrogen gas. The product of hydrogenation is suitably treated to isolate pure ractopamine hydrochloride.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The details of the invention, its objects, and its advantages are
further explained. The example/s given below are merely illustrative and do not limit the scope of this invention and it would be obvious that any modifications or changes in the steps by those skilled in the art without departing from the scope of the invention shall be consequently encompassed within the ambit and spirit of this approach and scope thereof.
[0023] In accordance with object of the present invention, Octopamine
HC1 [2-amino-l-(4-hydroxyphenyl) ethanol hydrochloride] and 4-(4-hydroxyphenyl)-2-butanone were used as starting materials. The reaction was conducted in a medium in which both the starting materials were soluble. The reaction mixture comprised a base to neutralize the hydrochloride salt of octopamine and generate octopamine base in-situ. The catalysts selected were such that it provided very high hydrogenation rates in order to avoid isomerization of Schiff base. The catalyst should not be palladium on carbon in order to avoid formation of hydrogenolysis-related impurities. Reaction workup included an extractive work-up step to remove non-basic water-insoluble impurities.

[0024] The next step is to crystallize the final product from acidic
aqueous media so as to remove any other basic water-soluble impurities in the mother liquors. The method is designed so as to achieve an overall impact on product quality to meet the pharmacopeial parameters.
[0025] Thus according to the present invention, octopamine
hydrochloride and 4-(4-hydroxyphenyl)-2-butanone are taken in a molar ratio of between 0.5 and 1.5; the suitable solvents for this reaction include water, Ci. 4 aliphatic alcohols or any mixtures thereof; the suitable base for the reaction is selected from a group comprising NaOH, KOH, pyridine, triethylamine, N-ethyl diisopropylamine, Na2C03, NaHC03 and K2C03. Catalyst used for the reaction is a platinum catalyst. Platinum may be used in the form of Pt02, platinum black, colloidal platinum or platinum metal supported on carbon or alumina, wherein the metal loading on support may be between 0.5 and 10%. The quantity of catalyst may be taken from 1% to 20% by weight of the quantity of octopamine hydrochloride. Hydrogenation may be carried out at a temperature between ambient and 100°C and a pressure of hydrogen between atmospheric and 15 kg/cm .
[0026] Still further, after the hydrogenation stage, the reaction mass may
be filtered to recover the catalyst. This catalyst is washed thoroughly with the solvent of reaction followed by acetic acid and water. The catalyst may be pyrophoric (except when alumina is the support) and so must be kept wet at all times. The catalyst so washed is reusable for at least 10 recycles without any drop in the yield.
[0027] In accordance with the present invention, the filtrate, after
removal of catalyst, is concentrated to remove the solvents. The residue is further diluted with water; made acidic and extracted with an organic solvent selected from a group comprising ethyl or isopropyl acetate, diethyl ether,

diisopropyl ether and methylene chloride. The organic phase may be concentrated to recover unreacted 4-(4-hydroxyphenyl)-2-butanone, which may be reused. The concentration of aqueous phase is suitably adjusted either by concentrating in vacuo or by further dilution with water. The aqueous phase is acidified by hydrochloric acid and product is allowed to crystallize. The product thus isolated has each individual impurity less than 0.5%; the ratio of (R*,S*) to (R*,R*) isomer by HPLC is between 45:55 and 49:51. The chiral HPLC analysis shows a 1:1 ratio for the RS,SR pair and the RR,SS pair.
[0028] In accordance with the present invention, the molar ratio of
octopamine hydrochloride to 4-(4-hydroxyphenyl)-2-butanone is between 0.8 and 1.2; solvent used for reaction is either methanol, ethanol or water of any mixture thereof; base used is preferably triethylamine, sodium or potassium hydroxide or sodium carbonate; catalyst used is either Pt02 or platinum on carbon. The preferable temperature range for the reaction is between 2-7 kg/cm2 and the operating temperature range is between 40° and 80°C.
[0029] ANALYTICAL METHODS: The impurity and diastereomeric
ratio is determined by HPLC methods given in the USP monograph.
Chiral HPLC is performed by following method:
Column: Chiralpak AD 250/4.6/10; Length x ID: 250 * 4.6
Wavelength: 210 nm;
Injection Volume: 10 JIL
Eluent; Hexane - isopropanol (80:20);
Solvent: Hexane - isopropanol (70:30)
Flow rate: 1 ml/min
[0030] SAMPLE PREPARATION: Treat 10 mg of sample with 0.01 ml
of (R)-(-)-menthyl chloroformate in mixture of 0.8 ml of dichloromethane and 0.5 ml of 5% Na2C03 solution. Allow the reaction to proceed by stirring at

ambient temperature for 10 min. Take 0.1 ml of organic phase, remove the solvent in N2 flow and dissolve the residue in 1 ml of solvent. Integration: 1-50 min
Retention times: RR-isomer: 14 min; SR-isomer: 17 min; SS-isomer: 18 min; RS-isomer. 36 min. Example 1
[0031] 100 g (0.527 m. 1.0 eq) of octopamine hydrochloride, 96 g (0.584
m, 1.1 eq) of 4-(4-hydroxyphenyl)-2-butanone and S5 g (117 ml, 0.841 m, 1.6 eq) of triethylamine is dissolved in 2 L of methanol. The mixture is charged to a 5 L SS autoclave. 0.5 g of platinum dioxide is added to the mixture. The autoclave is evacuated and then filled with hydrogen to 2 kg/cm . The cycle of evacuation and filling with hydrogen is repeated two times. Finally the autoclave is pressurized to 6 kg/cm1 with hydrogen. The temperature rapidly rises to 40-45°C. It is maintained at this level and hydrogen pressure maintained between 5 and 6 kg/cm2 until tendency to drop the pressure ceases. After this phase, the autoclave is heated to raise the temperature to 75-80°C. The hydrogen pressure maintained between 5 and 6 kg/cm until tendency to drop the pressure ceases. The autoclave is cooled to room temperature and vented. The reaction mass is filtered and the catalyst is washed with 50 ml methanol. The filtrate is concentrated in vacuo.
Example 2
[0032] The residue is dissolved in 1500 ml of water. The pH is adjusted
to 3-4 with sulphuric acid and the mass is extracted with 3 □ 500 ml of ethyl acetate. The aqueous phase is acidified with 50 ml of concentrated hydrochloric acid. The mass is stirred at room temperature for 24 hours to complete precipitation of crystals. The crystals are filtered and washed with 2200 ml of chilled water and dried in oven at 75-85°C. The Yield is 107 g.

Analysis: 1. Impurities:
i. Octopamine: Nil;
ii. Tyramine: Nil;
iii. 1-Methy 1-3-(4-hydroxyphenyl) propylamine: 0.09%;
iv. 4-(4-hydroxyphenyl)-2-butanone: Nil;
v. 4-(3-hydroxybutyl)phenol: Nil;
vi. Deoxyractopamine: 0.14%;
vii. Ractopamine dimer: 0.15%
2. Diastereomeric ratio:
RS,SR isomer: 48.97%; RR,SS isomer: 51.03%
3. Enantiomers:
RS isomer: 24.5% SR isomer: 24.3% RR isomer: 25.4% SS isomer: 25.7%
4. Assay (non-aqueous titration): 98.97% w/w (dried basis)
5. NMR Data: H NMR Spectra with interpretation and assignment of peaks as depicted below;

The lH NMR spectrum in presence of DMSO-d6 as depicted in FIG. 1 is explained in Table 1. As shown in the table, the values are 6 9.42 (s, 1H, phenolic -OH); 5 8.7 (bs, 2H NH/); 6 9.18 (s, 1H, phenolic -OH); 5 6.9-7.15 (d of d, 4H, Ar-H); 5 6.55-6.75 (d of d, 4H, Ar-H); 5 5.9 (bs, 1H, benzylic -OH); 6 4.70-4.85 (t, 1H, benzylic CH-OH); 8 3.0-3.1 (m, 1H, -NH2+-CH-); 5 2.9-2.95 (m, 2H, -NH2+-CH2-): 5 2.5-2.6 (m, 1H, benzylic CHj); 5 2.3-2.45

(m, 1H, benzylic CHJ; 5 1.9-2.05 (m, 1H, C-CH2-C); 8 1.5-1.7 (m, 1H, C-CH2-C);5 1.2(d,3H,CH3).

Pea
k No. Chemical shift (6, ppm) Peak type No. of protons Assignment
1 9.42 singlet 1H Phenolic -OH, A
2 8.7 broad singlet 2H NH2+, B
3 9.18 singlet 1H Phenolic -OH, C
4 6.9-7.15 doublet of doublet 4H Ar-R D
5 6.55-6.75 doublet of doublet 4H Ar-H, E
6 5.9 broad singlet 1H Benzylic -OH, F
7 4.70-4.85 triplet 1H Benzylic H, G
8 3.0-3.1 multiplet 1H H
9 2.9-2.95 multiplet 2H I
10 2.5-2.6 multiplet 1H Benzylic H, J
11 2.3-2.45 multiplet 1H Benzylic H, K
12 1.9-2.05 multiplet 1H L
13 1.5-1.7 multiplet 1H M
13 1.2 doublet 3H -CH3, N
In case of ]H NMR spectrum of methanol-d4 as depicted in FIG. 2, the protons at A, B, C and F get exchanged with deuterium in methanol-d4, hence the corresponding peaks have disappeared in this spectrum. Therefore, the values corresponding to FIG. 2 are depicted in Table 2 which are 5 6.9-7.15 (d of d, 4H, Ar-H); S 6.55-6.75 (d of d, 4H, Ar-H): 6 4.65-4.75 (m, 1H, benzylic CH-OH); 6 3.1-3.2 (m, 1H, -NH2+-CH-); 6 2.9-3.05 (m, 2H, -NH/-CH2-); 5 2.55-2.7 (m, 1H, benzylic CH2); 8 2.4-2.5 (m, 1H, benzylic CH2); 5 1.9-2.05 (m, 1H, C-CH2-C); 5 }.65-1.8 (m, 1H, C-CH2-C); 8 1.27 (d, 3H, CHj).

CLAIMS
We claim:
1. A process to prepare ractopamine hydrochloride to give a product
substantially free from impurities, the steps comprising of;
a. a precisely controlled ratio of diastereomers by reacting 2-amino-l-(4-
hydroxyphenyl) ethanol hydrochloride and 4-(4-hydroxyphenyl)-2-
butanone dissolved in a solvent;
b. a base;
c. a hydrogenation catalyst; and
d. hydrogen gas.
2. The process as mentioned in claim 1 wherein the individual impurities
comprises of octopamine, tyramine, l-methyl-3-(4-hydroxyphenyl)
propylamine, 4-(4-hydroxyphenyl)-2-butanone, 4-(3-hydroxybutyl)
phenol, deoxyractopamine and ractopamine dimer.
3. The impurities as mentioned in claim 2 wherein the impurities are
within the range of 0.01 to 0.2%.
4. The ratio of diastereomers, (RS,SR):(RR,SS) as mentioned in claim 1 wherein said ratio falls within the range of 45:55 and 49:51.
5. The ratio of individual enantiomers, RS:SR and RR:SS as mentioned in claim 4 wherein the said ratio falls within the range of 99.5:100.5 and 100.5:99.5.
6. The molar ratio of 2-amino-l-(4-hydroxyphenyl) ethanol hydrochloride to 4-(4-hydroxyphenyl)-2-butanone as mentioned in step a of claim 1 wherein said ratio falls within the range of 0.5 and 1.5.

7. The process as mentioned in claim 1 wherein the solvent used is selected from a group comprising of water, C1-4 aliphatic alcohols or any combinations thereof.
8. The process as mentioned in claim 1, wherein the suitable base for the reaction is selected from a group comprising of NaOH, KOH, pyridine, triethylamine, N-ethyl diisopropylamine, Na2C03, NaHC03 and K2C03.
9. The process as mentioned in claim 1 wherein the catalyst used for the reaction is selected from the group comprising of a platinum catalyst in the form of Pt02, platinum black, colloidal platinum or platinum metal supported on carbon or alumina, wherein the metal loading on support is within the range of 0.5 and 10% and the quantity of catalyst may range from 1% to 20% of the quantity of an impurity.
10. The process as mentioned in claim 1 has a reaction temperature in the range of 25andl00°C.
11. The process as mentioned in claim 1 wherein the pressure of hydrogen is between atmospheric and 15 kg/cm .
12. The catalyst as mentioned in claims 1 and 9, wherein said catalyst is a recovered and washed catalyst from a previous run.