Field of the Invention:
The present invention relates to amorphous MTBE solvate of Tigecycline compound of formula-1, which is represented by the following structural formula:
The present invention also relates to an improved process for the preparation of Tigecycline compound of formula-1 and its polymorphs.
Background of the Invention:
Tigecycline is chemically known as (4S,4aS,5aR, 12aS)-9-[2-(tert-butylamino)acetamido] -4,7-bis(dimethylamino)-1,4,4a,5,5a,6,11,12a-octahydro-3,10,12,12a-tetrahydroxy-1,11 -dioxo-2-naphthacenecarboxamide. It is developed by^Wyeth Holdings, marketed under the brand name Tygacil® and used as an Antibiotic.
Tigecycline was firstly described in USRE40183E by Wyeth herein after referred as US' 183. The said patent also describes its process for the preparation.
USRE40183E discloses the process for preparation of Tigecycline as solid by the evaporation of methylene dichloride solution, which is further characterized in US7871993B2 as amorphous tigecycline. Amorphous tigecycline is reported by some other inventors as described in US20090275766, US20110124893A1, US20120028928A1 and CN101973906B.
US patent US8372995 describes crystalline forms-I, II, III, IV & V of Tigecycline. US patent US8207361 describes crystalline forms-I, II of Tigecycline.' US patent US8198469 describes crystalline forms-VI, VII, VIII, IX, X, XI, XV, XVIII, XX, XXI, XXII, XXIII, XXIV, XXV & XXVI of Tigecycline.
Because of drug compounds having, for example, improved stability, solubility, shelf life and in vivo pharmacology, are consistently sought, there is an ongoing1 need for new or pure

salts; hydrates, solvates and polymorphic forms of existing drug molecules. The amorphous form of Tigecycline with high purity described herein help meet this and other needs.

Polymorphic forms of Tigecycline obtained according to the prior know processes having C-4 epimer content in higher levels i.e., more than 0.5%. The higher level of C-4 epimer content is not suitable for pharmaceutical composition development.

The present inventors developed high pure compound of Tigecycline with lower levels of C-4 epimer content and with high yield.
Brief Description:
The first aspect of the present invention is to provide amorphous MTBE solvate of Tigecycline compound of formula-1.
The second aspect of the present invention is to provide process.for the preparation of
amorphous MTBE solvate of Tigecycline compound of formula-1. |
The third aspect of the present invention is to provide an improved process for the preparation of amorphous Tigecycline compound of formula-1.
The fourth aspect of the present invention is to provide an improved process for the preparation of Tigecycline compound of formula-1.
Brief description of the drawings:
Figure 1: Illustrates the PXRD pattern of amorphous MTBE solvate of Tigecycline compound of formula-1.
Figure 2: Illustrates the IR spectrum amorphous MTBE solvate of Tigecycline compound of
formula-1.
Figure 3: Illustrates the PXRD pattern of amorphous Tigecycline compound of formula-1.
Figure 4: Illustrates the IR spectrum of amorphous Tigecycline compound of formula-1.
Figure 5: Illustrates the TGA curve of amorphous MTBE solvate of Tigecycline compound of
formula-1.
i
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Detailed Description:
As used herein the term "suitable solvent" used in the present invention refers to j
"hydrocarbon solvents!' such as n-hexane, n-heptane, cyclohexane, pet ether, toluene, pentane,

cycloheptane, methyl cyclohexane, m-, o-, or p-xylene and the like; "ether solvents" such as methyl tert-butyl ether (MTBE), cyclopentyl methyl ether, dimethoxymethane, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, furan, diethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, anisole, 1,2-dimethoxy ethane and the like; "ester solvents" such as methyl acetate, ethyl acetate, isopropyl acetate, n-butyl acetate and the like; "polar-aprotic solvents such as dimethylacetamide (DMA), dimethylformamide (DMF), dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP) and the like; "chloro solvents" such as dichloromethane, dichloroethane, chloroform, carbontetra chloride and the like; "ketone solvents" such as acetone, methyl ethyl ketone, methyl isobutyl ketone and the like; "nitrile solvents" such as acetonitrile, propionitrile, isobutyronitrile and the like; "alcoholic solvents" such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, 2-nitroethanol,< 2-fluoroethanol, 2,2,2-trifluoroethanol, ethylene glycol, 2-methoxyethanol, 1,2-ethoxyethanol, diethylene glycol, 1, 2, or 3-pentanol, neo-pentyl alcohol, t-pentyl alcohol, diethylene glycol monoethyl ether, cyclohexanol, benzyl alcohol, phenol, or glycerol and the like; "polar solvents" such as water or mixtures thereof.
The term "pure" refers to the compound of formula-1 having purity by HPLC is greater than 95%, preferably >99%, more preferably >99.5% and the said compound meets ICH guidelines.
The ■ first aspect of the present invention provides amorphous MTBE solvate of Tigecycline compound of formula-1. The amorphous MTBE solvate of the present invention is characterized by its powder X-Ray diffraction pattern as depicted in figure-1; by its IR spectrum as depicted in figure-2 and further by its TGA curve as depicted in figured.
The second aspect of the present invention provides a process for the preparation of amorphous MTBE solvate of Tigecycline compound of formula-1, comprising of;
a) Dissolving Tigecycline in methyl tertiary butyl ether [MTBE] or its mixture with another suitable solvent,
b) optionally filtering the reaction mixture,
c) cooling the reaction mixture to a suitable temperature,

d) stirring the reaction mixture,
e) filtering the precipitated solid to provide amorphous MTBE solvate of Tigecycline of formula-1.
Wherein in step-a) the suitable solvent is alcohol solvents, hydrocarbon solvents, ester solvents, chloro solvents, ketone solvents, polar solvents and water or mixtures thereof; preferably alcohol solvents, more preferably methanol; in step-c) the suitable temperature is about below 25°C, preferably -70°C to -20°C, more preferably -45°C to -35°C.
Preferred embodiment of the present invention provides a process for the preparation of amorphous MTBE solvate of Tigecycline compound of formula-1, comprising of;
a) Dissolving Tigecycline in the mixture of methyl tertiary butyl ether [MTBE] and methanol,
b) filtering the reaction mixture,
c) cooling the reaction mixture,
d) stirring the reaction mixture,
e) filtering the precipitated solid provides amorphous MTBE solvate of Tigecycline of formula-1.
The third aspect of the present invention provides an improved process for the preparation of an amorphous Tigecycline compound of formul-1, comprising of;
a) Dissolving the compound of formula-1 in water,
b) lyophilizing the obtained solution in step-a) by freeze drying at a suitable temperature,
c) drying the compound provides pure amorphous Tigecycline.
Wherein in step b) the suitable temperature is from -70°C to 0°C; preferably -70°C to -1'0°C and optionally under reduced pressure. Lyophilization process is performed by known techniques.
Preferred embodiment of the present invention provides an improved process for the preparation of an amorphous Tigecycline compound of formula-1, comprising of;
a) Dissolving Tigecycline compound of formula-1 in water,
b) lyophilizing the obtained solution in step-a) by freeze drying at below -30°C under reduced pressure,
c) drying the compound provides pure amorphous Tigecycline.

The fourth aspect of the present invention provides an improved process for the preparation of Tigecycline, comprising of:
a) Treating 2-(tert-butyamino)acetic acid compound of formula-2 or its acid addition salts
with thionyl chloride in a suitable solvent optionally in presence of dimethyl formamide provides 2-(tert-butyamino)acetyl chloride compound of formula-3 or its acid addition salts
JL 1UW1U1U -')
b) optionally purifying the compound using a suitable solvent,
c) reacting the compound of formula-3 or its acid addition salts with (4S, 4aS, 5aR, 12aS)-9-amino-4,7-bis(dimethylamino)-3, 10,12, 12a-tetrahydroxy-l,l 1-dioxo-l, 4, 4a, 5, 5a, 6, 11, 12a-octahydrotetracene-2- carboxamide compound of formula-6 or its acid addition salts
in presence of ammonia in a suitable solvent provides Tigecycline compound of formula-
Wherein in step-a) to c) the suitable solvent is selected from alcohol solvents, ether solvents, hydrocarbon solvents, ester solvents, chloro solvents, ketone solvents, polar solvents and water or their mixtures thereof. The term "acid addition salts" refers to acid salts of the said compounds. The acid is selected from organic or inorganic acids.
Preferred embodiment of the present invention provides a process for the preparation of Tigecycline, comprising of;
a) Treating 2-(tert-butyamino)acetic acid hydrochloride compound of formula-2a with

thionyl chloride in toluene in presence of dimethyl formamide provides 2-(tert-butyamino)acetyl chloride hydrochloride compound of formula-3a, b) reacting the compound of foimula-3a with (4S, 4aS, 5aR, 12aS)-9-amino-4,7-bis(dimethylamino)-3, 10, 12, 12a-tetrahydroxy-1,11-dioxo-l, 4, 4a, 5, 5a,-6, 11, 12a-octahydrotetracene-2- carboxamide bisulphate compound of formula-6a in presence of ammonia in water provides Tigecycline.
Further aspect of the present invention provides a pharmaceutical formulation comprising Tigecycline according to the present invention and a pharmaceutical^ acceptable carrier. Preferably, the formulation is an IV formulation.
Tigecycline obtained according to the present invention is having purity by HPLC >99% and with C4-epimer content <0.5%, preferably < 0.3%, more preferably <0.2%.
Tigecycline obtained according to the present invention optionally can be further micronized or milled to get the desired particle size to achieve desired solubility profile based on different forms of pharmaceutical composition requirements. Techniques that may be used for particle size reduction include, but not limited to ball, roller and hammer mills, and jet mills. Milling or micronization may be performed before drying, or after the completion of drying of the product.
PXRD analysis of compound of formula-1 produced by the present invention was carried out using BRUKER D8 ADVANCED/AXS X-Ray diffractometer using Cu Ka radiation of wavelength 1.5406 A° and continuous scan speed of 0.037min.
Tigecycline used in the present invention is synthesized by the prior known processes or. according to the present invention.
The present invention represented in the scheme-1.

The process described in the present invention was demonstrated in examples illustrated below. These examples are provided as illustration only and therefore should not be construed as limitation of the scope of the invention.
Examples:
Example-!: Preparation of compound of formula-6a
Minocycline hydrochloride compound of formula-4a (8.5 Kg) was slowjy added to pre-cooled sulfuric acid (46.9 Kg) at -5°C to 0°C and stirred the reaction mixture for 1 hour at same temperature. Nitric acid (1.5 Kg) was slowly added to the reaction mixture at -5°C to 0°C and stirred the reaction mixture for 2 hours at the same temperature. The obtained solution was slowly added to the pre-cooled mixture of isopropyl alcohol and n-heptane at -5°C to 0°C under nitrogen atmosphere. Raised the temperature of the reaction mixture to 2'5-30°C and further

stirred for 1 hour at the same temperature. The precipitated solid was filtered and washed with isopropyl alcohol. A solution of Pd/C (2.1 Kg) in methanol (8.5 Lt) was slowly added to a solution of above wet compound in methanol (68 Lt) at 25-30°C. Stirred the reaction mixture under 3 Kg/cm2 hydrogen pressure at 25-30°C for 4 hours. Filtered the reaction mixture and washed with methanol, filtrate was slowly added to n-butanol (255 Lt) at 25-30°C and stirred the reaction mxiture for 45 minutes at the same temperature. The precipitated solid was filtered and washed with n-butanol. The obtained wet solid was added to acetone (59.5 Lt) at 25-30°C and stirred it for 45 minutes at the same temperature. Filtered the solid, washed with acetone and dried to get the title compound. Yield: 7.5 Kg. M.R.: 140-150°C.
Example-2: Preparation of 2-(tert-butyamino)acetic acid hydroqhloride compound of formula-2a
Tert-butyl 2-bromo acetate (27 Kg) was slowly added to a solution of tert-butyl amine (20.2 Kg) in toluene at 25-30°C, then heated the reaction mixture to 65-70°C and stirred for 4 hours at the same temperature. The reaction mixture was cooled to 25-30°C; water was.added to it and stirred for 10 minutes. Both the organic and aqueous layers were separated. The aqueous layer was extracted with toluene. Combined the organic layers and hydrochloric acid (28.7 Lt) was slowly added to it at 25-30°C and stirred the reaction mixture for 4 hours at same temperature. Water was added to tjpe reaction mixture at 25-30°C, stirred for 10 minutes and separated the layers. The aqueous layer was distilled off under reduced pressure. Isopropanol (27 Lt) was added to the obtained compound at 25-30°C, raised the temperature of the reaction mixture to 80-85°C and stirred for 30 minutes at same temperature. The reaction mixture was cooled to 25-30°C and stirred for 30 minutes at the same temperature. Filtered the solid and dried to get the title compound. Yield: 18 Kg. M.R.: 190-200°C.
Example-3: Preparation of tigecycline crystalline form II.
Thionyl chloride (16 Kg) was slowly added to a solution of 2-(tert-butyamino)acetic acid hydrochloride compound of formula-2a (16.3 Kg) in toluene (81.3 Lt), N,N-dimethyl formamide (0.52 Lt) at 25^30°C. Heated the reaction mixture to 75-80°C and stirred it for 4 hours at same temperature. The reaction mixture was cooled to 15-20°C and stirred it for 45 minutes. Filtered

the precipitated solid and washed with toluene. The obtained wet solid was further purified in toluene. The cqmpound of formula-6a (6.5 Kg) was added to pre-cooled water at -5°C to 0°C. The reaction mixture pH was adjusted with aqueous ammonia solution at -5°C to 0°C. The above wet solid was added to the reaction mixture at -5 to 0°C and stirred for 1.5 hours at same temperature. Adjusted the reaction mixture pH using aqueous ammonia at -5 to 0°C and stirred the reaction mixture for 10 minutes. Raised the temperature of the reaction mixture to 25-30°C and dichloromethane was added to it. The reaction mixture was stirred for 10 minutes at same temperature and both the organic and aqueous layers were separated. The aqueous layer was extracted with dichloromethane, combined the organic layers and dried over sodium sulfate. The organic layer was treated with charcoal at 25-30°C. Distilled off the solvent completely from the organic layer under reduced pressure and co-distilled with methanol. Methanol (13 Lt) was added to the obtained residue, cooled to -5-0°C and stirred for 1 hour. Filtered the obtained solid and washed with methanol. Further recrystallized the obtained compound using methanol and dried to get the title compound.
Yield: 2.25 Kg. PXRD pattern of the obtained compound is matching with the PXRD pattern of Tigecycline form-II described in US8372995. Water content: 6.43% w/w. Purity by HPLC: 99.78%, C4-epimer content is 0.13%.
Example-4: Preparation of amorphous form of compound of formula-1
A solution of tigecycline (100 gm) in water (650 ml) is cooled to -35 to -30°C. Freeze dried the
obtained solution under high vacuum conditions at -35 to -30°C, and freeze dried up to complete
sublimation of the water to get amorphous tigecycline. . : .
Yield: 75gm; FfXRD of the obtained compound is depicted in figure-3. and IR is depicted in figure-4. Purity by HPLC: 99.5%, C4-epimer content is 0.29%.
F2xample-5: Preparation of amorphous MTBE solvate of compound of formula-1
Dissolved Tigecycline (75 gm) in the mixture of methyl tert-butyl ether (1350 ml) and methanol (150 ml) at 25-30°C. Filtered the reaction mixture through hyflow bed and washed with methyl tert-butyl ether. The obtained filtrate was cooled to -35 to -40°C and stirred for 60 minutes at same temperature. Filtered the precipitated solid, washed with methyl tert-butyl ether and dried to get the title compound as an orange color solid. Yield: 52 gm; MTBE content is 6.7 w/w.

PXRD of the obtained compound is depicted in figure-1, IR is depicted in figure-2 and TGA curve is depicted in figure-5. Purity by HPLC: 99.85%, C4-epimer content of 0.11%.
Example-6: Preparation of amorphous MTBE solvate of compound of formula-1
Dissolved Tigecycline (10 gm) in the mixture of methyl tert-butyl ether (180 mL) and isopropyl alcohol (20 mL) at 25-30°C. Filtered the reaction mixture through hyflow bed and washed with methyl tert-butyl ether. The obtained filtrate was cooled to -35 to -40°C and stirred for 60 minutes at same temperature. The precipitated solid was filtered, washed with methyl tert-butyl ether and dried to get the title compound as a solid. Yield: 7 gm.
Example-7: Preparation of amorphous MTBE solvate of compound of formula-1
Dissolved Tigecycline (250 gm) in the mixture of methyl tert-butyl ether (4500 ml) and methanol (500 ml) at 25-30°C. Filtered the reaction mixture through hyflow bed and washed with methyl tert-butyl ether. The obtained filtrate was cooled to -35 to -40°C and^sfirred for 60 minutes at same temperature. Filtered the precipitated solid, washed with methyl tert-butyl ether and dried to get the title compound as an orange color solid.
Yield: 173 gm; MTBE content is 6.6 w/w. Purity by HPLC: 99.83%, C4-epimer content of 0.13%. PXRD of the obtained compound is similar to the PXRD pattern of figure-1.

We Claim:
1. Amorphous MTBE solvate of Tigecycline.
2. Amorphous MTBE solvate according to claim 1, which is characterized by
a) its PXRD pattern as illustrated in figure-1,
b) its IR spectrum as illustrated in figure-2,
c) its TGA curve as illustrated in figure-5.
3. A process for the preparation of amorphous MTBE solvate of Tigecycline, whic
the steps of:
a) Dissolving Tigecycline in methyl tertiary butyl ether or its mixture with ano solvent,
b) cooling the reaction mixture to a suitable temperature,
c) stirring the reaction mixture,
d) filtering the precipitated solid,
e) drying the compound to provide amorphous MTBE solvate Tigecycline.
4. The process according to claim 3, wherein
in step-a) another suitable solvent-is selected from alcohol solvents, ketone so solvents, chloro solvents, polar solvents, water or mixtures thereof, in step-b) the suitable temperature is below 25°C, preferably -50°C to 20°C, more 45°C to 0°C.
5. A process for the preparation of amorphous MTBE solvate of Tigecycline, whic
the steps of:
a) Dissolving Tigecycline in the mixture of methyl tertiary butyl ether and methan
b) cooling the reaction mixture,
r
c) stirring the reaction mixture,
d) filtering the precipitated solid,
e) drying the obtained compound to provide amorphous MTBE solvate of Tigecyc
6. A process for the preparation of amorphous Tigecycline, comprising of;

!
a) Dissolving Tigecycline in water,
b) freeze drying the obtained reaction mixture and
c) drying the obtained compound to provide pure amorphous Tigecycline.
7. A process for the preparation of Tigecycline, comprising of; '
a) Treating 2-(tert-butyamino)acetic acid compound of formula-2 or its acid addition salt
with thionyl chloride in a suitable solvent optionally in presence of dimethyl formamide to provide 2-(tert-butyamino)acetyl chloride compound of formula-3 or its acid addition salts
b) reacting the compound of formula-3 or its acid addition salts with (4S, 4aS, 5aR, 12aS)-9-amino-4,7-bis(dimeihylamino)-3, 10,12, 12a-tetrahydroxy-l,l 1-dioxo-l, 4, 4a, 5, 5a, 6, 11, 12a-octahydrotetracene-2- carboxamide compound of formula-6 or its acid addition salts
in presence of ammonia in a suitable solvent to provide Tigecycline.
8. A process for the preparation of Tigecycline, comprising of;
a) Treating 2-(tert-butyamino)acetic acid hydrochloride compound of formula-2a
with thionyl chloride in toluene in presence of dimethyl formamide to provide 2-(tert-butyaminp)acetyl chloride hydrochloride compound of formula-3a

b) reacting the compound of formula-3a with (4S, 4aS, 5aR, 12aS)-9-amino-4,7-bis(dimethylamino)-3, 10,12, 12a-tetrahydroxy-l,l 1-dioxo-l, 4, 4a, 5, 5a, 6, 11, 12a-octahydrotetracene-2- carboxamide bisulphate compound of formula-6a
in presence of ammonia in water to provide tigecycline.
9. Tigecycline bbtained according to any of the preceding claims having purity greater than 99%
by HPLC and having C-4 epimer content less than 0.5%.
10. Amorphous MTBE solvate of Tigecycline having C-4 epimer content is less than 0.5%.