Process for the preparation of Linezolid

Field of the invention

The present invention provides an improved process for the preparation of Linezolid of formula (I) from Linezolid amine of formula (Ia).

The present invention further provides process for the preparation of Form I of Linezolid of formula (I).

Background of the invention

Linezolid is chemically known as N-[[(5S)-3-[3-Fluoro-4-(4-morpholinyl)phenyl]-2-oxo-5-oxazolidinyl]methyl] acetamide and marketed by Pfizer in US under brand name Zyvox. Linezolid is a synthetic antibacterial agent of the oxazolidinone class. It is used for the treatment of infections caused by multi-resistant bacteria including streptococci and methicillin-resistant Staphylococcus aureus.

Linezolid was first disclosed in U.S. Pat. No. 5,688,792. The process for synthesis is as disclosed in Scheme-I

The process involves reduction of Linezolid azide of formula (IIb) in the presence of palladium/carbon in ethyl acetate solvent to obtain Linezolid amine (Ia), which is further treated with acetic anhydride in presence of pyridine to obtain Linezolid of formula (I). The purification process involves chromatography and separating the desired fraction, followed by evaporation and triturating the product to obtain pure Linezolid.

The polymorphic form obtained by following process disclosed in U.S. Pat. No. 5,688,792 is designated as Form I. Figure-1 depicts the PXRD graph of Form I obtained by following prior art process.

In the above described process such a method of treatment to obtain pure Linezolid results in very low yields. Further, such a process is difficult to follow at commercial level. Also, practice of chromatographic techniques requires large quantities of solvent and its subsequent recovery which increases the overall cost of production.

The process for the preparation of Linezolid is also disclosed in Journal of Medicinal Chemistry (1996), 39(3), 673-9, U.S. Pat. Nos. 6,492,555, 5,837,870, 6,887,995, 7,307,163, 7,429,661, etc.

None of the above mentioned prior arts offer simple and cost effective method for the production of compound of formula (I) from compound of formula (Ia). Nor any of the prior art process describes a process for preparation of Form I of Linezolid. Therefore, there is need to develop an efficient method, which is simple, cost-effective and commercially scalable for synthesis of Linezolid of formula (I) from Linezolid amine of formula (Ia). Further, it would be desirable to develop a process for preparation of Form I of Linezolid which is reproducible.

Summary of the invention

In one embodiment of the present invention is provided a process for the preparation of Linezolid of formula (I) comprising,

acylating Linezolid amine of formula (Ia) using acylating agent in the presence of ketonic solvent.

In another embodiment of the present invention is provided a process for preparing Form I of Linezolid of formula (I) comprising steps of:
(a) acylating Linezolid amine of formula (Ia) using acylating agent in the presence of ketonic solvent.

(b) crystallizing Linezolid obtained in step (a) from suitable solvent.

Linezolid obtained by the process of present invention has content of (R)-enantiomer less than about 0.1% and bis-Linezolid content less than 0.15%. Further the purity of Linezolid is more than 99% and the yield of the reaction is high.

Therefore, the process of present invention can be employed advantageously by avoiding the cumbersome and lengthy procedure of chromatography.

Brief description of the invention

Figure-1: PXRD graph of Form I obtained by following prior art process.
Figure-2: FTIR (Nujol) Form I obtained by following prior art process.
Figure-3: PXRD graph of Form I obtained by following Example 7.
Figure-4: FTIR (Nujol) Form I obtained by following Example 7.


Detailed description of the invention

As used herein, acylating agent refers to acetic anyhydride, acetyl chloride, acetic acid or any such reagent which is capable of introducing acetyl group.

As used herein, ketonic solvents refers to acetone, methyl iso-butyl ketone, methyl ethyl ketone, and the like or mixtures thereof.

In one embodiment of the present invention, Linezolid is prepared by process comprising,

acylating Linezolid amine of formula (Ia) using acylating agent in the presence of ketonic solvent.

The step of acylation is carried out at about 0°C to about room temperature, preferably at about 0°-5°C.

Linezolid amine of formula (Ia) can be used directly or without isolation after the step of reduction from Linezolid azide of formula (IIb). The step of reduction of Linezolid azide of formula (IIb) is preferably carried out using palladium on carbon in presence of ethyl acetate as solvent.

In a preferred embodiment of the present invention, a one pot process is provided wherein Linezolid amine of formula (Ia) is not isolated from the reduction mixture, but the residue obtained after removal of catalyst and solvent used for reduction step, is converted to Linezolid of formula (I) by acylation using acylating agent in the presence of ketonic solvent.

Following comparison table indicates the content of (R)-enantiomer, bis-Linezolid impurity and purity of Linezolid when acylation is carried out in ethyl acetate and acetone:

Sr. No. Batch No. UPLC Purity Bis-Linezolid impurity (R)-enantiomer
Ethyl acetate
01 Batch-1 99.57 0.39 0.08
02 Batch-2 98.91 0.22 0.42
03 Batch-3 98.95 0.46 0.43
Acetone
01 Batch-1 99.62 0.12 0.03
02 Batch-2 99.70 0.07 0.02
03 Batch-3 99.95 ND 0.05

The data clearly indicates significant reduction in bis-Linezolid impurity and (R)-enantiomer when acylation reaction is carried out in the presence of acetone as solvent.

Therefore, Linezolid prepared by the process of the present invention has content of (R)-enantiomer less than about 0.1%, preferably less than 0.5%. Further, Linezolid prepared by the process of the present invention has content of bis-Linezolid less than about 0.15%. Also, the purity of Linezolid prepared by the process of the present invention is greater than 99%, preferably greater than 99.5%.

Further, embodiment of the present invention provides a process for preparing Form I of Linezolid of formula (I) comprising steps of:
(a) acylating Linezolid amine of formula (Ia) using acylating agent in the presence of ketonic solvent.

(b) crystallizing Linezolid obtained in step (a) from suitable solvent.

In a preferred embodiment, crystallization of linezolid is preferably carried out in the presence of n-propanol or methyl isobutyl ketone.

In another preferred embodiment of the present invention, Linezolid azide of formula (IIb) is reduced using palladium on carbon in presence of ethyl acetate as solvent. After completion of the reaction the reaction mass is filtered and ethyl acetate is removed from the filtrate. In the same pot i.e. without isolating or further purifying the Linezolid amine of formula (Ia), acetone is added followed by acetic anhydride and triethyl amine at about 0-5°C. Then, the reaction mass is heated to reflux at about 65-75°C followed by cooling at bout 0-5°C to obtain a solid which is isolated by conventional methods like filtration, centrifugation and the like and dried. The solid thus obtained in dissolved in n-propanol and treated with activated charcoal and filtered. The filtrate is concentrated and cooled to about 0-5°C to obtain Linezolid Form I.

The following examples illustrate the invention further. It should be understood however, that the invention is not confined to the specific limitations set forth in the individual example but rather to the scope of the appended claims.

Examples

Example 1: Preparation of 3-Fluoro-4-morpholinyl nitrobenzene.

To a solution of Methanol (90ml) and 3, 4-Difluoronitrobenzene (100g) at 25-30oC add Morpholine (115g) drop wise at 25-30oC in more than 1 hour under stirring. Stir the reaction mass at 25-30oC for 1-2 hours. Then add slowly water (400ml) with stirring the reaction mass at 25-30oC for 1 hour. Filter the solid & wash it with water. The solid is dried at 55-60oC. Yield: 1.408.; Percentage 99.0 %w/w.

Example 2: Preparation of N-Carbobenzoxy-3-fluoro-4-morpholinylaniline

Take 3-Fluoro-4-morpholinyl nitrobenzene (100g), Methanol (1000ml) and 10% palladium on carbon catalyst (2.0g 50% wet) in the autoclave at 20-30oC for 3-4 hrs at 1-2kg hydrogen pressure. Filter it and wash the hyflo bed by methanol. Apply vacuum to remove traces of methanol & add Acetone (100ml) to distill out completely below 70oC. Cool it & further add Acetone (400ml) and sodium carbonate (46.9g) to the residue. After cooling the mix at 0-5oC, 166g of Benzyl chloroformate (50% solution in Toluene) was added slowly at 0-5oC under stirring. Water (800ml) & n-Hexane (100ml) are added at 0-5oC for 1 hour at constant stirring. The mixture was filtered & solid was washed with water and n-Hexane (100ml). The solid is dried at 55-60oC. Yield: 1.43.; Percentage 97.9 %w/w.

Example 3: Preparation of (R)-[N-3-(3-Fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl] methanol.

Take n-Butanol (51.5g) and THF (100ml) at 20-30oC under Nitrogen atmosphere. After cooling the mix add slowly n- Butyl lithium (1.6M in hexane) (391.7g) at 10 to 20oC & maintain it for 45-60 minutes. Take THF (500ml) and N-Carbobenzoxy-3-fluoro-4-morpholinylaniline (100g) at 20-30oC under Nitrogen atmosphere. Cool the mix at -15 to -5oC under stirring. To this solution add slowly n- Butyl lithium solution & maintain for 45-60 minutes at -15 to -5oC, to this solution add slowly (R)-(-) Glycidyl butyrate (48.0g) & maintain for 1 hour at -10 to -5oC. After completing addition raise the temperature to 8-13oC and maintain for 1 hour & then take it to 13-15oC and maintain for 4-5 hours. Monitor the reaction by HPLC. Organic layer was separated by water (800ml) and Ethyl acetate (300ml). Filter & wash the solid with mix of Ethyl acetate- n-Hexane & dried in air tray dryer at 55-60oC. Yield: 0.765.: Percentage 85%w/w.

Example 4: Preparation of (R)-[N-3-(3-Fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl] methyl methane sulfonate

Triethyl amine (68.2g), Methane sulfonyl chloride (48.3g) are added to a flask containing Dichloromethane (1900ml) and (R)-[N-3-(3-Fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl] methanol (100g) at 20-30oC with constant stirring for 2-3 hours. After cooling & filtration wash the solid with Dichloromethane followed by water wash & dried in air tray dryer. Yield: 1.20.: Percentage 95%w/w.

Example 5: Synthesis of (R)-[N-3-(3-Fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl] methyl azide

Reflux the mix of Dimethyl formamide (250ml), (R)-[N-3-(3-Fluoro-4-morpholinyl phenyl)-2-oxo-5-oxazolidinyl] methyl methane sulfonate (100g) and Sodium azide (24.3g) at 60-65oC & maintain it for 6-7 hours. Cool the mix & add water (450ml) with constant stirring for one hour at 20-30oC. Filter it; wash the solid with mix of Dimethyl formamide - water (1:1) and with water & dried at 55-60oC. Yield: 0.82.: Percentage 95%w/w.

Example 6: Synthesis of Linezolid

Ethyl acetate (3500ml) and 10% palladium on carbon catalyst (6.0g) are added in autoclave having (R)-[N-3-(3-Fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl] methyl azide (100g) at 20-30oC. Cool the reaction mass & maintain 2-3kg hydrogen pressure at 15-20oC for 6-7 hrs. Filter it & wash the hyflo bed by Ethyl acetate. Distill out ethyl acetate at 75-90°C and then cool the reaction mass to 0-5°C. Add acetone (1000ml) & acetic anhydride (29.9g) at 0-5°C. Further, add Triethyl amine (37.8g) slowly at 0-5oC under stirring. Maintain the reaction mass at 0-5°C for 1-2 hrs. Heat the reaction mass to reflux at 65-75°C for 1 hr. Again cool the reaction mass to 0-5°C for1 hr. Filter the solid wash it with acetone and water and dry it at 55-60oC. Yield: 0.80.: Percentage 80%w/w.

Example 7: Synthesis of Linezolid Form I

Reflux n-propanol (400ml) and Linezolid (100g) at 95-100oC till all solid gets dissolved. Add activated charcoal (2.0g) and heat for 30 mins. Filter thro hyflo bed. Heat the filtrate and concentrate the solution by partially removing n-propanol. Cool to 0-5°C and filter the solid and dry it at 55-60oC under vacuum. Yield: 0.9.: Percentage 90%w/w.

We claim

1. A process for the preparation of Linezolid of formula (I) comprising,

acylating Linezolid amine of formula (Ia) using acylating agent in the presence of ketonic solvent.

2. A process for the preparation of Form I of Linezolid of formula (I) comprising steps of:
(a) acylating Linezolid amine of formula (Ia) using acylating agent in the presence of ketonic solvent,

(b) crystallizing Linezolid obtained in step (a) from suitable solvent.

3. A process claimed in claim 1 and 2, wherein said acylating agent is acetic anyhride or acetyl chloride.

4. A process claimed in claim 1 and 2, wherein said ketonic solvent is acetone, methyl iso-butyl ketone, methyl ethyl ketone or mixtures thereof.

5. A process claimed in claim 2, wherein said crystallization is carried out in n-propanol or methyl iso-butyl ketone.

6. Linezolid of formula (I) obtained by process of claim 1 and 2, having content of (R)-enantiomer less than about 0.1% and content of bis-Linezolid less than about 0.15%.

7. Linezolid of formula (I) obtained by process of claim 1 and 2, having purity greater than 99%.