FIELD OF THE INVENTION

The present invention relates to an improved process for preparing crystalline Imipenem monohydrate of Formula I

containing Bis-Thienamycin formamidine impurity (Dimer-I) of Formula VIII and Imipenem thienamycin formamidine impurity (Dimer-II) of Formula IX, each less than 0.2%.

BACKGROUND OF THE INVENTION

Imipenem monohydrate i.e. [5R-[5a,6a(R*)]]-6-(l-Hydroxyethyl)-3-[[2-[iminomethyl)amino]ethyl]thio]-7-oxo-l-azabicyclo-[3,2,0]hept-2-ene-2-carboxylic acid monohydrate of Formula I is active against a wide range of gram-negative and gram-positive bacteria. Imipenem works by interfering with the bacterial ability to form cell-walls, and therefore break-up and die.

Imipenem is partially inactivated by an enzyme in the kidney which can reduce its effectiveness. Imipenem monohydrate is co-administered with Cilastatin sodium, which blocks the effect of the enzyme. Cilastatin does not have any antibacterial effects and does not affect the antibacterial activity of Imipenem. It is currently being marketed as PRIMAXIN®I.V.

In view of the importance of Imipenem monohydrate as an anti-bacterial compound, several synthetic methods have been reported in the literature, which are as summarized below:

Imipenem was first disclosed in US 4,194,047. Example 5 discloses a process to prepare Imipenem, which comprises reacting thienamycin with methyl formimidate hydrochloride in the presence of sodium hydroxide at pH 8.5. The solution of the reaction mass is chromatographed on a column of XAD-2 resin (150 cc), which is eluted with water, and the resulting solution is lyophilized to produce Imipenem as a white solid.

US 4,260,543 specifically discloses a process to prepare crystalline Imipenem monohydrate. The process comprises, dissolving lyophilized N-formimidoyl thienamycin in water and diluted with ethanol. The resulting solution is immersed in an ice-bath, seeded with N-formimidoyl thienamycin monohydrate crystals and stirred with a magnetic stirrer for 1.5 h to produce crystals of Imipenem monohydrate.

US 4,374,772 patent discloses a process comprising, reacting benzyl formimidate with thienamycin in presence of water at pH 7-8.5 to produce N-formimidoyl thienamycin.
US 4,894,450 patent discloses a process comprising, reacting protected bicyclic ketone with phosphorohalidate to give diprotected bicyclic ester in N-ethyl pyrrolidinone, which was reacted as such with cysteamine hydrochloride to give protected thienamycin, and then reacted with benzyl formimidate. The reaction mixture was poured into a mixture of butanol, water, ethyl acetate and N-methylmorpholine and performed hydrogenolysis over 20% Pd(OH)2 on carbon to yield Imipenem.

The disadvantage according to above processes is usage of chromatographic techniques, lyophilization technique, which are lengthy, time consuming and industrially not feasible.
US 7,241,885 discloses a process to prepare crystalline Imipenem monohydrate, which is as shown below:

US 7,332,600 discloses a process to prepare crystalline Imipenem monohydrate by dissolving crude Imipenem in warm water in the presence of base to obtain a solution and subjecting the resultant solution to activated carbon and thereafter adding an organic solvent selected from lower alcohol, a ketone or mixture thereof.

Further, using the above processes the product obtained has low purity and low assay due to formation of 5-15% of Thienamycin formimidine impurity (Dimer-I) and thienamycin impurity, which affects the yield of the final compound.

To develop Imipenem having high purity and assay the present inventors have developed a process for Imipenem, which contains Dimers -1 & II impurities each less than 0.2% and thienamycin impurity less than 1%, respectively; overall yield is 0.36 w/w with respect to bicyclic keto ester and purity greater than 98%.

OBJECTIVE

The objective of the present invention is to provide an improved process for the preparation of crystalline Imipenem monohydrate, which is simple and commercially feasible.
Yet another objective of the present invention is to provide an improved process for preparing crystalline Imipenem monohydrate having Dimers - I & II impurities each less than 0.2% and thienamycin impurity less than 1%, respectively.

Yet another objective of the present invention is to provide a process for preparing sterile crystalline Imipenem monohydrate having high purity and high assay.

SUMMARY OF THE INVENTION

The present invention relates to an improved process for preparing crystalline Imipenem monohydrate of Formula I

containing bis-Thienamycin formamidine impurity (Dimer-I) of Formula VIII and Imipenem thienamycin formamidine impurity (Dimer-II) of Formula IX, each less than 0.2%.
which comprises,

a) reacting bicyclic keto ester of Formula II,
with diphenyl chlorophosphate in a mixture of 1-methyl-2-pyrrolidinone and methylene chloride in the presence of diisopropylethylamine and .catalytic amount of 4-(dimethylamino)pyridine to give Enol phosphate of Formula III,

b) reacting Enol phosphate of Formula III in situ with cysteamine hydrochloride to give Thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate of Formula VI,

c) reacting thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate of Formula VI with benzyl formimidate hydrochloride in a mixture of methylene chloride and ethanol in the presence of diisopropylethylamine to form Imipenem p-nitrobenzyl ester hydrochloride of Formula VII,
d) extracting Imipenem p-nitrobenzyl ester hydrochloride of Formula VII into the aqueous layer by adding water and acidifying with hydrochloric acid to pH less than 7.0;
e) hydrogenating the aqueous layer containing Imipenem p-nitrobenzyl ester hydrochloride of Formula VII in situ in a mixture of alcohol, N-methylmorpholine, acetic acid, carbon enoanticromos and water with palladium on charcoal to give a reaction mixture containing Imipenem; and

f) washing the reaction mixture containing Imipenem with organic solvent and adding acetone to crystallize the crystalline Imipenem monohydrate of Formula I.
In another embodiment, present invention also relates to a process for preparing sterile crystalline Imipenem monohydrate of Formula I having purity and assay greater than 99%, which comprises,

a) dissolving crystalline Imipenem Monohydrate in a mixture of N-methylmorpholine, acetic acid and DM water at pH less than 7.0 to obtain aqueous solution of Imipenem;

b) optionally, treating the aqueous solution of Imipenem with carbon enoanticromos;

c) optionally, treating the aqueous solution of Imipenem with Dowex Resin and passing through 0.22|u filters; and

d) adding acetone to the aqueous solution of Imipenem to crystallize the sterile crystalline Imipenem monohydrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improved process for preparing crystalline Imipenem monohydrate of Formula I containing bis-Thienamycin formamidine impurity (Dimer-I) and Imipenem thienamycin formamidine impurity (Dimer-II), each less than 0.2%, reacting bicyclic keto ester with diphenyl chlorophosphate in a mixture of 1-methyl-2-pyrrolidinone and methylene chloride in the presence of diisopropylethylamine and catalytic amount of 4-(dimethylamino)pyridine to give Enol phosphate, which in situ reacted with cysteamine hydrochloride in the presence of diisopropylethylamine and isolating Thienamycin p-nitrobenzyl ester hydrochloride as a l-methyl-2-pyrrolidinone solvate of Formula VI.
Thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate is reacted with benzyl formimidate hydrochloride in a mixture of methylene chloride and ethanol in the presence of diisopropylethylamine to form Imipenem p-nitrobenzyl ester hydrochloride.
The reaction mixture of Imipenem p-nitrobenzyl ester hydrochloride is treated with water, acidified to pH less than 7.0 with hydrochloric acid to extract Imipenem p-nitrobenzyl ester hydrochloride of Formula VIII into the aqueous layer and hydrogenated in situ in a mixture of alcohol selected from ethanol, isopropropanol, N-methylmorpholine, acetic acid, carbon enoanticromos and water with palladium on charcoal to give a reaction mixture containing Imipenem.

The reaction mixture containing Imipenem is washed with organic solvent selected from methylenechloride or ethyl acetate and thereafter acetone is added to crystallize the crystalline imipenem monohydrate of Formula I.

The advantage of the present invention is isolation of Imipenem monohydrate with high yields, purity and assay, which is achieved by extracting Imipenem p-nitrobenzyl ester hydrochloride into the aqueous layer and performing hydrogenation in situ in a mixture of alcohol selected from ethanol, isopropanol, N-methylmorpholine, acetic acid and carbon enoanticromos below pH 7.0. Further, addition of extra carbon enoanticromos during hydrogenation reaction minimizes the formation of Dimer impurities.

Further, in the present invention the Thienamycin p-nitrobenzyl ester hydrochloride 1-methyl-2-pyrrolidinone solvate intermediate of Formula VI, is isolated with greater than 97% purity by HPLC by eliminating by-products such as diphenyl phosphate, excess cysteamine hydrochloride, diisopropylethylamine hydrochloride, un-reacted bicyclic keto ester of Formula II and un-reacted enol phosphate of Formula III into mother liquor.

In another embodiment, present invention relates to a process for preparing sterile crystalline Imipenem monohydrate having purity and assay greater than 99%, by dissolving the crystalline Imipenem monohydrate in a mixture of N-methylmorpholine, acetic acid and DM water at pH less than 7.0 to obtain aqueous solution of Imipenem and adding acetone to the aqueous solution of Imipenem to crystallize the sterile crystalline Imipenem monohydrate.
In yet another embodiment, present invention also relates to a process for preparing sterile crystalline Imipenem monohydrate having purity and assay greater than 99%, by dissolving the crystalline Imipenem monohydrate in a mixture of N-methylmorpholine, acetic acid and DM water at pH less than 7.0 to obtain aqueous solution of Imipenem, carbon treatment, thereafter adding Dowex Resin and passing through Q22\x filters and then adding acetone to crystallize the sterile crystalline Imipenem monohydrate.

During preparation of sterile crystalline Imipenem monohydrate, pH of the DM water is adjusted to less than 3.5 with acetic acid and re-adjusted to the pH of 5.5 - 6.9 with N-methylmorpholine and used for dissolution of Imipenem monohydrate to avoid degradation of Imipenem and to control thienamycin impurity.

The following examples illustrate the nature of the invention and are provided for illustrative purposes only and should not be construed to limit the scope of the invention.

EXAMPLE 1

a) PREPARATION OF ENOL PHOSPHATE

4-Nitrobenzyl (5R,6S)-6-[(lR)-l-hydroxyethyl]-3,7-dioxo-l-azabicyclo[3.2.0]heptane-2-carboxylate (bicyclic keto Ester, 100 g) was dissolved in a mixture of l-methyl-2-pyrrolidinone (500 ml), methylene chloride (200 ml) and 4-dimethylaminopyridine (DMAP, 1 g) and cooled to -45°C. Diisopropylethylamine (40.8 g) followed by diphenyl chlorophosphate (81 g) were added to the reaction mixture at -30 to -45°C was stirred for 1 h to complete the formation of Enol phosphate.

b) PREPARATION OF THIENAMYCIN P-NITROBENZYL ESTER
HYDROCHLORIDE l-METHYL-2-PYRROLIDINONE SOLVATE

Cysteamine hydrochloride (36 g) was dissolved in l-methyl-2-pyrrolidinone (300 ml) and added in situ to the reaction mixture of Example 1 (a), at -50 to -40°C. To the resulting reaction mixture, diisopropylethylamine (36.2 g) was added at -50 to -40°C and stirred to complete the reaction. After completion of reaction, acetonitrile (3000 ml) was added at 0-10°C, filtered the product and washed with methylene chloride (100 ml) to yield thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate Chromatographic purity: 98.78% (by HPLC)

c) PREPARATION OF IMIPENEM P-NITROBENZYL ESTER
HYDROCHLORIDE

Thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate obtained from Example 1 (b) was added to a mixture of methylene chloride (1500 ml) and ethanol (370 ml) at -40°C under nitrogen atmosphere. Diisopropylethylamine (23.8 g) was added to the reaction mixture at -45 to -35°C, stirred to obtain a clear solution and reacted with benzyl formamidine hydrochloride (54.4 g) at -60°C till

completion of reaction. After completion of reaction, water (1100 ml) was added to the reaction mixture and pH was adjusted to less than 7.0 with diluted hydrochloric acid at -10 to 20°C to extract Imipenem p-nitrobenzyl ester hydrochloride into aqueous layer.

d) PREPARATION OF IMIPENEM MONOHYDRATE

pH of the DM water (400 ml) was adjusted to less than 3.5 with acetic acid and then re-adjusted to pH 6.7 with N-methylmorpholine. The resulting solution was diluted with isopropyl alcohol (1500 ml) and then added to the aqueous portion obtained from Example I (c) at 0-20°C. To the reaction mixture, carbon enoanticromos and palladium on charcoal were added and hydrogenated at 8-12 Kg pressure for 3 h at 0-20°C. The mixture was filtered and the filtrate was washed with dichloromethane (700 ml). To the aqueous portion, acetone (4400 ml) was added and stirred for 15 h to crystalline product. The product was filtered and washed with acetone (40 ml) and dried at 20-35°C to yield crystalline Imipenem monohydrate Yield: 37 g Chromatographic Purity: 98.87% (by HPLC) Dimer I impurity: 0.12% Dimer II impurity: Not Detected Thienamycin: 0.73%

EXAMPLE 2

PREPARATION OF STERILE CRYSTALLINE IMIPENEM MONOHYDRATE

pH of the DM water (400 ml) was adjusted to less than 3.5 with acetic acid and re¬adjusted to the pH 6.8 with N-methylmorpholine and then heated to 48-52°C under nitrogen atmosphere. To the above solution, crystalline Imipenem monohydrate (65 g) was added at 48-52°C and stirred for about 5 min for dissolution. To the aqueous Imipenem solution, carbon enoanticromos (6.5 g) was added at 45-52°C and rapidly cooled to 0-10°C. The carbon was filtered, washed with DM water, the filtrate was stirred with Dowex Resin (1 x 2-200, 100-200 Mesh; 10 g) at 10-20°C for 15 min and passed through 0.22u filters to obtain clear aqueous Imipenem solution.

Acetone (5.5 1) was added to the clear aqueous Imipenem solution obtained above at 20-25 °C and stirred for 30 min to crystallize the product. Further, the slurry was cooled to 0-5°C and stirred for 4 h. The product was filtered, washed with acetone and dried under reduced pressure at 30-40°C for 3 h to obtain crystalline Imipenem monohydrate.

WE CLAIM;

1. A process for preparing crystalline Imipenem monohydrate of Formula I
which comprises,

a) reacting bicyclic keto ester of Formula II, with diphenyl chlorophosphate in a mixture of l-methyl-2-pyrrolidinone and methylene chloride in the presence of diisopropylethylamine and catalyst to give Enol phosphate of Formula HI,

b) reacting Enol phosphate of Formula III in situ with cysteamine hydrochloride to give Thienamycin p-nitrobenzyl ester hydrochloride 1-methyl-2-pyrrolidinone solvate of Formula VI,

c) reacting thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate of Formula VI with benzyl formimidate hydrochloride in a mixture of methylene chloride and ethanol in the presence of diisopropylethylamine to form Imipenem p-nitrobenzyl ester hydrochloride of Formula VII,

d) extracting Imipenem p-nitrobenzyl ester hydrochloride of Formula VII into the aqueous layer by adding water and acidifying with hydrochloric acid to pH less than 7.0;

e) hydrogenating the aqueous layer containing Imipenem p-nitrobenzyl ester hydrochloride of Formula VII in situ in a mixture of alcohol, N-methylmorpholine, acetic acid, carbon enoanticromos and water with palladium on charcoal to give a reaction mixture containing Imipenem; and

f) washing the reaction mixture containing Imipenem with organic solvent and adding acetone to crystallize the crystalline imipenem monohydrate of Formula I..

2. The process according to claim 1, wherein catalyst is 4-(dimethylamino)pyridine.

3. The process according to claim 1, wherein thienamycin p-nitrobenzyl ester hydrochloride l-methyl-2-pyrrolidinone solvate of formula VI is isolated.

4. A process for preparing sterile crystalline Imipenem monohydrate of Formula I
having purity and assay greater than 99%, which comprises,

a) dissolving crystalline Imipenem monohydrate in a mixture of N-
methylmorpholine, acetic acid and DM water at pH less than 7.0 to obtain
aqueous solution of Imipenem;

b) optionally treating the aqueous solution of Imipenem with carbon enoanticromos;

c) optionally treating with Dowex Resin and passing through 0.22JJ, filters; and

d) adding acetone to the aqueous solution of Imipenem to crystallize the sterile crystalline Imipenem monohydrate.

5. Imipenem monohydrate containing bis-Thienamycin formamidine impurity (Dimer-I) of Formula VIII and Imipenem thienamycin formamidine impurity (Dimer-II) of Formula IX, each less than 0.2%

6. A process for preparing Imipenem monohydrate of formula I substantially as described herein.