FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
[Section 10, and Rule 13]
AN IMPROVED PROCESS FOR THE PREPARATION OF LINEZOLID AND
INTERMEDIATE THEREOF
Applicant
Name: Torrent Pharmaceuticals Limited
Nationality: Indian
Address: Torrent House, Off Ashram Road,
Near Dinesh Hall,
Ahmadabad 380 009.
Gujarat, India
The following specification particularly describes the nature of the invention and the manner in which it is to be performed:

AN IMPROVED PROCESS FOR THE PREPARATION OF LINEZOLID AND INTERMEDIATE THEREOF
FIELD OF THE INVENTION
The present invention relates to salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-1, 3-oxazolidin-2-one. The invention further relates to process of its preparation and its use in synthesis of Linezolid. Further (5S)-5-(arninomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one salt can be obtained in pure, stable form.
BACKGROUND OF THE INVENTION
Linezolid is an oxazolidinone marketed in the United States by Pfizer Inc. as tablet injection and oral suspension under the brand name ZYVOX®. It is mainly used to treat nosocomial pneumonia, skin and skin-structure infections, and vancomycin-resistant Enterococcus faecium infections. The chemical name for Linezolid is (S)-N-[[3-[3-Fluoro-4-(4-morpholinyl)phenyl]-2-oxo-5-oxazolidinyl] mefhyl]-acetamide and its chemical structure is represented below.

US5688792 and its European equivalent EP717738 disclose Linezolid specifically, process for its preparation and its pharmaceutical compositions. The example five particularly involves the process of preparation of Linezolid. The process involves mesytelation of (R)-N-[[3-(3-fluoro-4-morpholinyl] phenyl]-2-oxo-5- oxazolidinyl]-

methanol followed by its transformation to azide and then an amine intermediate, (S)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazoiidinyI)-methylamine (Formula-II).

The amine intermediate is also prepared by treating mesytel intermediate with potassium phthalimide followed by its deprotection reaction. Finally after the acetylation of the amine Linezolid is obtained.
US837870, US7307163, IN21306, US7351824, WO2010081404A1, WO2010084514A2, WO2009063505A2, WO2007116284A1, CN101638392, CN1673224 and US7429661 also disclose different process of preparing Linezolid.
US6444813 disclose process for preparation of polymorphic Form II of Linezolid. US7714128 disclose Polymorphic Form III of Linezolid and its process of preparation. US7732597, US7718800, US7718799, US7649096 and US20090062534 disclose different processes for preparation Polymorphic Form III of Linezolid.
WO2006091731 disclose that undesired reaction by-products are produced in the process for preparation of amine intermediate and Linezolid when it is prepared as per US5688792. Further chromatographic purification followed by trituration with ethyl acetate is necessary which leads to low yield of Linezolid as per process of US5688792. To overcome problem associated with yield, WO2006091731 disclose process for preparation of Linezolid by using solvent other than ethyl acetate. Though there are different process known in the art for the preparation of Linezolid, there remains a need to develop better process for preparation of Linezolid with high yield and purity by keeping impurity level low or below detectable limit. During the process for preparation of Linezolid we surprisingly found that when amine intermediate (Formula-II) is

converted to salt of compound of formula II is obtained in high yield and purity. In accordance with the present invention there is provided salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l; 3-oxazolidin-2-one. Process for preparation of (5S)-5-(aminomethyI)-3-(3-fluoro-4-morpholinylphenyl)-1, 3-oxazolidin-2-one salt and its use in Linezolid synthesis.
SUMMARY OF INVENTION
In one aspect of present invention there is provided salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).

Formula-II
In one aspect of present invention there is provided acetate salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).
In another aspect of present invention there is provided process for preparation of salt of
(5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one
(Formula-II) which comprises the steps of;
a) inducing hydrogenation of (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl) methylazide in presence of catalyst and organic solvent,
b) filtering catalyst from reaction mass
c) removing solvent from reaction mixture,
d) treating reaction mass with acid and

e) recovering salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyI)-l, 3-oxazolidin-2-one (Formula-II).
In yet another aspect of present invention there is provided process for preparation of salt of (5S)-5-(aminomethyl)-3-(3-fiuoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II) from (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl)-methylpthalimide which comprises the steps of;
a) treating (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl)-methylpthalimide with monomethylamine,
b) recovering reaction mass,
c) dissolving reaction mass in organic solvent,
d) treating reaction mass with acid and
e) recovering salt of (5S)-5-(aminomethyI)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).
In yet another aspect of present invention there is provided process for preparation of Linezolid from compound of Formula-II which comprises the steps of;
a) treating salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one acetate (Formula-II) with acetylating reagent in presence of organic solvent and optionally in presence of organic base and
b) recovering Linezolid from reaction mass.
In one aspect, acids employed to form salts include inorganic acids such as hydrochloric. hydrobromic, hydroiodic, sulfuric and phosphoric acid, as well as organic acids such as para-toluenesulfonic, methanesulfonic, oxalic, para-bromophenylsulfonic, carbonic, succinic, citric, benzoic and acetic acid, and related inorganic and organic acids. Pharmaceutically acceptable salts thus include sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, bromide, iodide, acetate, propionate, decanoate, caprylate, acrylate, formate, isobutyrate, caprate, heptanoate, propiolate. oxalate, malonate, succinate, suberate, sebacate, fumarate. maleate, butyne-l,4-dioate, hexyne-l,6-dioate,

benzoate. chlorobenzoate, methylbenzoate, dinitrobenzoate, hydroxybenzoate, methoxybenzoate, phthalate, terephathalate, sulfonate, xylenesulfonate, phenylacetate, phenylpropionate, phenylbutyrate, citrate, lactate, p-hydroxybutyrate, glycollate, maleate, tartrate, methanesulfonate, propanesulfonates, naphthalene-1-sulfonate, naphthalene-2-sulfonate, mandelate and the like salts. Preferred pharmaceutically acceptable acid addition salts include those formed with mineral acids such as hydrochloric acid and hydrobromic acid, and especially those formed with organic acids such oxalic acid and maleic acid. Acid used for salt formation is preferably organic acid, more preferably acetic acid.
In yet another aspect of the present invention the process for the preparation of salt of compound of Formula-II, preferably acetate salt of compound of Formula-II is cost effective, commercially viable and industrially feasible. Further salt of compound of Formula-II, preferably acetate salt of compound of Formula-II and Linezolid obtained by above process is having higher yield and purity.
DESCRIPTION OF THE DRAWINGS:
Figure 1: An X-ray powder diffraction pattern of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-1, 3-oxazolidin-2-one acetate.
Figure 2: An X-ray powder diffraction pattern of Linezolid obtained by process described in example 3
Figure 3: An X-ray powder diffraction pattern of Linezolid obtained by process described in example 4.
Figure 4: An X-ray powder diffraction pattern of Linezolid obtained by process described in example 5.

DETAILED DESCRIPTION OF THE INVENTION
Unless defined 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 materials similar 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,
The present invention relates to salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).
In one aspect of present invention there is provided acetate salt of (5S)-5-(aminornethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).
In another aspect of present invention there is provided process for preparation of salt of
(5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one
(Formula-II) which comprises the steps of;
a) inducing hydrogenation of (R)-(N-3-(3-fluoro-4-morphoIinylphenyl)-2-oxo-5-oxazolidinyl) methylazide in presence of catalyst and organic solvent,
b) filtering catalyst from reaction mass
c) removing solvent from reaction mixture,
d) treating reaction mass with acid and
e) recovering salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l. 3-oxazolidin-2-one (Formula-II).
In yet another aspect of present invention there is provided process for preparation of salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II) from (R)-(N-3-(3-fIuoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl)-methylpthalimide which comprises the steps of;
a) treating (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl)-
methylpthalimide with monomethylamine,

b) recovering reaction mass,
c) dissolving reaction mass in organic solvent,
d) treating reaction mass with acid and
e) recovering salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).
The Reaction mixture is flushed with gas before hydrogenation. The gas includes Argon. Nitrogen, Hydrogen and the like preferably Nitrogen.
The catalyst used for hydrogenation of (i?)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl) methylazide is selected from noble metal catalyst such as platinum, palladium and the like. The noble metal catalyst may be provided on inert support such as carbon, activated carbon or alumina. The catalyst used for hydrogenation reaction is in catalytic amount ranging from 2- 30 % to that of (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl) methylazide and preferable 5- 20 %.
The Organic solvent used in reaction is selected from C1-C4 alkyl esters, C1-C8 linear or branched-chain aliphatic alcohols, C6-C12 aromatic hydrocarbons, mono-,di-, or tri-C1-C4 alkyl substituted or unsubstituted benzenes and chlorinated hydrocarbons and the like.
Examples of C1-C4 alkyl esters used include methyl acetate, ethyl acetate, butyl acetate, isobutyl acetate, preferably ethyl acetate. C1-C8 linear or branched-chain aliphatic alcohols include methanol, ethanol, propanol, isopropanol, butanol, isobutanol and the like. Mono-, di-, or tri-C1-C4 alkyl substituted or unsubstituted benzenes include benzene, toluene, xylene and the like. Chlorinated hydrocarbons include dichloromethane, chloromethane, carbon tetrachloride, ethylene dichloride and the like.
The Hydrogen source used for hydrogenation of (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl) methylazide is Hydrogen gas. The reaction is

carried out at Hydrogen gas pressure ranging from 1,5-10 kg/cm2 and is stirred forl- 10 hours.
The hydrogenation reaction is carried out at 15-70 °C preferably at 20- 60 °C and more preferable 25- 40 °C.
(5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-1, 3-oxazolidin-2-one
(Formula-II) is recovered from reaction mass preferably by filtering and more preferably filtering through celite filter. The solvent removed by distillation to 30- 80 % preferably 40- 60 % of its original volume by avoiding solid formation or complete distillation to get solid formation. The amine solution may be prepared by dissolution of amine (Formula-II) in organic solvent.
Acid is added slowly in amine (Formula-II) solution by maintaining reaction temperature at about 0- 40 °C preferably 5- 30 °C and more preferably at 10- 20 °C. Reaction mixture is stirred for 1-5 hours preferably 1.5- 4 hours and more preferably 2-3 hours. The stirring is carried out at temperature from 0- 40 °C preferably 5- 30 °C and more preferably at 10- 20 °C.
Acids employed to form salts include inorganic acids such as hydrochloric, hydrobromic, hydroiodic, sulfuric and phosphoric acid, as well as organic acids such as para-toluenesulfonic, methanesulfonic, oxalic, para-bromophenylsulfonic, carbonic, succinic, citric, benzoic and acetic acid, and related inorganic and organic acids. Pharmaceutically acceptable salts thus include sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate. chloride, bromide, iodide, acetate, propionate, decanoate, caprylate, acrylate, formate, isobutyrate, caprate, heptanoate, propiolate. oxalate, malonate, succinate, suberate, sebacate, fumarate, maleate, butyne-l,4-dioate, hexyne-l,6-dioate, benzoate, chlorobenzoate, methyl benzoate, dinitro benzoate, hydroxybenzoate, methoxy benzoate, phthalate, terephathalate, sulfonate, xylene sulfonate, phenylacetate, phenylpropionate, phenylbutyrate, citrate, lactate, P-hydroxybutyrate, glycollate, maleate, tartrate.

methane sulfonate, propanesulfonates, naphthalene-1-sulfonate, naphthalene-2-sulfonate, mandelate and the like salts. Preferred pharmaceutically acceptable acid addition salts include those formed with mineral acids such as hydrochloric acid and hydrobromic acid, and especially those formed with organic acids such oxalic acid and maleic acid. Acid used for salt formation is preferably organic acid, more preferably acetic acid.
The solid formed in the reaction is recovered to obtain salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-1, 3-oxazolidin-2-one (Formula-II), preferably acetate salt. The recovery of product can be carried out by filtration, solvent distillation and the like. Product is dried by conventional method such as heating, drying at room temperature under vacuum, heating under vacuum and the like.
The monomethylamine used in the reaction is having concentration of 20- 60 % preferable 30- 50 % and more preferably 40 %.
Salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II) may be inorganic acid salts such as hydrochloric, hydrobromic, hydroiodic, sulfuric and phosphoric acid, as well as organic acid salts such as para-toluenesulfonic, methanesulfonic, oxalic, para-bromophenylsulfonic, carbonic, succinic, citric, benzoic and acetic acid, and related inorganic and organic acids. Pharmaceutically acceptable salts thus include sulfate, pyrosulfate, bisulfate, sulfite, bisulfite, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, bromide, iodide, acetate, propionate, decanoate, caprylate, acrylate, formate, isobutyrate, caprate, heptanoate, propiolate, oxalate, malonate, succinate, suberate, sebacate, fumarate, maleate. butyne-l,4-dioate, hexyne-l,6-dioate, benzoate, chlorobenzoate, methylbenzoate, dinitrobenzoate, hydroxybenzoate, methoxybenzoate, phthalate, terephathalate, sulfonate, xylene sulfonate, phenyl acetate, phenylpropionate, phenylbutyrate, citrate, lactate, β-hydroxybutyrate, glycollate, maleate, tartrate, methanesulfonate, propanesulfonates, naphthalene-1-sulfonate, naphtha!ene-2-sulfonate, mandelate and the like salts. Preferred pharmaceutically acceptable acid addition salts include those formed with mineral acids such as hydrochloric acid and hydrobromic acid,

and especially those formed with organic acids such oxalic acid and maleic acid. Preferred organic acid salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II) is formed with acetic acid i.e. acetate salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II).
The present invention further provides a process of preparing Linezolid from salt of compound of Formula-II which comprises the steps of;
a) treating salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one (Formula-II) with acetylating reagent in presence of organic solvent and optionally in presence of organic base and
b) recovering Linezolid from reaction mass.
The acetylating agent selected from acetyl halide or acetic anhydride and the like. Acetyl halide is selected from acetyl chloride, acetyl bromide and the like.
The organic base can be selected from NR1 R2 R3, wherein R1, R2, R3 are independently H or C1-4 straight or branched alkyl, morpholine, dimethyl- aniline, pyridine, piperidine, N-methyl pyrrolidine. N-methyl pyrrolidone and the like or mixtures thereof. Examples of NR1 R2 R3 include but are not limited to dimethylamine, triethylamine, diethylamine, tert-butylamine and the like.
The organic solvent used in reaction may be same or different to that of solvent used for hydrogenation of (R)-(N-3-(3-fluoro-4-morpholinyIphenyl)-2-oxo-5-oxazolidinyl) methylazide.
Linezolid recovered by conventional method such as filtration, solvent distillation and the
like.
Salts of compound of Formula-II and Linezolid obtained by above process are having higher yield and purity.

Acetate salt of compound of Formula-II and Linezolid obtained by above process are having higher yield and purity.
Linezolid obtained by above process is highly stable.
Certain specific aspects and embodiments of this invention are described in further detail by the examples below, which are provided only for the purpose of illustration and are not intended to limit the scope of the invention in any manner.
Example 1:
A solution of (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl) methylazide (30 gm) in ethyl acetate (400 ml) was added to the solution of palladium (6 gm) in ethyl acetate (400 ml) under Nitrogen atmosphere in autoclave, The reaction mixture was flushed with Nitrogen gas twice (1 kg/cm2) and Hydrogen gas twice (0.5 kg/cm2 ). The reaction mixture was stirred under Hydrogen pressure (2.5 ±0.5 kg/cm ) till completion of reaction (about 2 hours) at room temperature and then filtered through celite under Nitrogen atmosphere and washed the bed with hot ethyl acetate (200 ml). Solvent was distilled out up to 400 ml under vacuum at 47 ±5 °C from reaction mass and then it is cooled to 10- 15 °C. Acetic acid (6.7 ml) was slowly added in reaction mixture under stirring by maintaining temperature at 10- 15 °C. The reaction mixture was stirred for 2 hours at 10- 15 °C then filtered and washed with chilled ethyl acetate (2 X 60 ml). The solid obtained was dried under vacuum at 45 ±5 °C for 8-10 hours to get 27 gm (81.8 %) of product with 99.53 % purity.
Example 2:
A mixture of (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl)-methylpthalimide (40 gm) and 40 % monomethylamine (400 ml) heated at 73±2 °C for three to four hours. Reaction mixture was cooled to room temperature and water (400 ml) was added to it followed by addition of dichloromethane (400 ml). Reaction mixture was

stirred for 15-20 minutes. Organic layer was separated and aqueous layer was extracted by dichloromethane (2 X 200 ml). Organic layers were combined and washed with water (3 X 200 ml). Solvent was evaporated from reaction mass residue was dissolved in ethyl acetate (220 ml) at 45-50 °C. The reaction mixture was cooled to 15- 20 °C and acetic acid (6.7 ml) was slowly added to it at this temperature. Reaction mass was stirred at 15-20 °C for one hour. Solid obtained in this reaction was filtered and washed with ethyl acetate (2 X 20 ml). After drying under vacuum at 50±3 °C for eight to ten hours 22 gm (65.84 %) of product was obtained,
Example 3:
Triethyl amine (4.27 gm) was added in the solution of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one acetate (5 gm) in ethyl acetate (50 ml) at room temperature. The reaction mixture was cooled to 10- 15 °C and Acetic anhydride (4.31 gm) was slowly added in it by maintaining temperature at 10- 15 °C, The reaction mixture was stirred till completion of reaction (about one hour) and then at 5- 10 °C for one hour. The reaction mass was filtered and washed with chilled ethyl acetate (10 ml). The solid obtained was dried under vacuum at 50 °C for 10-12 hours to get 3.5 gm (74.44 %) of product with 99.85% purity.
Example 4:
To a solution of Linezolid (2 gm) and methanol (20 ml), methylene dichloride (6 ml) was added. Mixture was stirred at room temperature to obtain clear solution. Activated charcoal (0.1 gm) was added to reaction mixture and stirred the reaction mixture for 15-20 minutes. Reaction mixture was filtered through celite and washed the celite bed with methanol (6 ml). Toluene (60 ml) was added in filtrate and solvent was removed from reaction mass under vacuum at room temperature till solid formation observed. Reaction mass was stirred for one hour at room temperature. Solid product was filtered and washed with Toluene (6 ml). Solid was dried under vacuum at 50°C for 10- 12 hrs to obtain pure 1.6 gm Linezolid.

Example 5:
To a solution of Linezolid (2 gm) and methylene chloride (16 ml), ethyl acetate (12 ml) was added. Mixture was stirred at room temperature to obtain clear solution. Activated charcoal (0.1 gm) was added to reaction mixture and stirred the reaction mixture for 15-20 minutes. Reaction mixture was filtered through celite and washed the celite bed with methylene dichloride (4 ml) and ethyl acetate (4 ml). Ethyl acetate (20 ml) was added in filtrate and solvent was removed from reaction mass under vacuum at room temperature till solid formation observed. Reaction mass was stirred for one hour at room temperature. Solid product was filtered and washed with ethyl acetate (5 ml). Solid was dried under vacuum at 50°C for 10- 12 hrs to obtain pure 1.6 gm Linezolid.

We claim:
1. A process for preparation of (5S)-5-(aminomethyI)-3-(3-fluoro-4-morpholinylphenyl)-1, 3-oxazolidin-2-one (Formula-II)

or salt thereof
which comprises the steps of
a) inducing hydrogenation of (R)-(N-3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl) methylazide in presence of catalyst and organic solvent,
b) filtering catalyst from reaction mass,
c) distilling solvent from reaction mixture,
d) treating reaction mass with and
e) isolating salt of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinyJphenyl)-1, 3-oxazolidin-2-one.

2. The process as claimed in claim 1, wherein the catalyst used for hydrogenation is noble metal catalyst selected from the group consisting of platinum, palladium or noble metal catalyst on inert support such as carbon, activated carbon or alumina.
3. The process as claimed in claim 1, wherein organic solvent used in reaction is selected from the group consisting of C1-C4 alkyl esters, C1-C8 linear or branched-chain aliphatic alcohols, C6-C12 aromatic hydrocarbons, mono-,di-, or tfi-C1-C4 alkyl substituted or unsubstituted benzenes and chlorinated aromatic hydrocarbons.
4. The process as claimed in claim 3, wherein organic solvent used in reaction is C1 C4 alkyl esters, preferably ethyl acetate.
5. The process as claimed in claim 1, wherein salt is acetate of (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one.

6. The process as claimed in claim 1 and 5, wherein (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one acetate comprises HPLC purity more than 99.5 %.
7. An improved process for preparation of Linezolid from (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l. 3-oxazolidin-2-one acetate which comprises the steps of

a) treating (5S)-5-(aminomethyl)-3-(3-fluoro-4-morpholinylphenyl)-l, 3-oxazolidin-2-one acetate (Formula-Ill) with acetylating reagent in presence of organic base and organic solvent and
b) isolating Linezolid from reaction mass.

8. The process as claimed in claim 7, wherein acetylating reagent is acetyl halide or acetic anhydride
9. The process as claimed in claim 7, wherein organic base is selected from group consisting of NR1 R2 R3, wherein R1 , R2 , R3 are independently H or C1-4 straight or branched alkyl, morpholine, dimethyl- aniline, pyridine, piperidine, N-methyl pyrrolidine, N-methyl pyrrolidone and organic solvent is C1-C4 alkyl esters.
10. The process as claimed in claim 7, wherein Linezolid comprises HPLC purity more than 99.8 %.