Our International Patent Application No. WO 99/64417 describes a new class of antibacterial oxazolidinone compounds which are effective as anti-Gram positive bacterial agents, and certain processes for their preparation. These include compounds of formula a) : wherein HET is a C-linked 5-membered heteroaryl ring containing 2 to 4 heteroatoms independently selected from N, O and S, which ring is optionally substituted on an available carbon atom by 1 or 2 substituents independently selected from (1-4C) alkyl, amino, (1-4C) alkylamino, (1- 4C) alkoxy and halogen, and/or on an available nitrogen atom (provided that the ring is not thereby quaternised) by (1-4C) alkyl; R2 and R3 are independently hydrogen or fluoro; Rcp is of the formula Rl3Co- (wherein R13 is (1-10C) alkyl substituted by two or more hydroxy groups; 2 of which are in a 1,2-diol orientation, ie. there is a terminal primary alcohol with an adjacent secondary alcohol), or pharmaceutically-acceptable salts, or in-vivo- hydrolysable esters thereof.

It is to be understood that all terms used in the definition of formula (I) above are as defined in WO 99/64417.

Of the above compounds, those in which HET is (unsubstituted) isoxazol-3-yl, 1,2,4- oxadiazol-3-yl, isothiazol-3-yl or 1, 2,5-thiadiazol-3-yl are preferred.

Of the compounds of formula (I), those of formula (I-1) are the pharmaceutically active anti-bacterial enantiomer. The pure enantiomer depicted in (I-1), or mixtures of the 5R and 5S enantiomers, for example a racemic mixture are included in WO 99/64417. If a mixture of enantiomers is used, a larger amount (depending upon the ratio of the enantiomers) will be required to achieve the same effect as the same weight of the pharmaceutically active enantiomer. For the avoidance of doubt the enantiomer depicted below is the 5R enantiomer.

Furthermore, some compounds of the formula (I) and (1-1) may have other chiral centres, and such optical and diastereo-isomers, and racemic mixtures may possess antibacterial activity. It is well known in the art how to prepare optically-active forms (for example by resolution of the racemic form by recrystallisation techniques, by chiral synthesis, by enzymatic resolution, by biotransformation or by chromatographic separation) and how to determine antibacterial activity.

In-vivo hydrolysable esters include compounds of formula (I) and (I-1) in which any free hydroxy group independently forms a phosphoryl ester of the formula (PD3): where R19 and R20 are independently selected from hydrogen or a pharmaceutically acceptable cation, as described hereinafter, such as an alkaline metal ion such as sodium or potassium, to give a pharmaceutically acceptable salt.

Suitable pharmaceutically-acceptable salts include base salts such as an alkali metal salt for example sodium or potassium, an alkaline earth metal salt for example calcium or magnesium, an organic amine salt for example triethylamine, morpholine, N- methylpiperidine, N-ethylpiperidine, procaine, dibenzylamine, N, N-dibenzylethylamine, tris- (2-hydroxyethyl) amine, N-methyl deglucamine, piperazine, and amino acids such as lysine and arginine. There may be more than one cation or anion depending on the number of charged functions and the valency of the cations or anions. A preferred pharmaceutically- acceptable salt is the sodium salt. Another preferred pharmaceutically acceptable salt is the

potassium salt.

However, to facilitate isolation of the salt during preparation, salts which are less soluble in the chosen solvent may be preferred whether pharmaceutically-acceptable or not.

Of the above compounds of formula (I) and (1-1), 5 (R)-isoxazol-3-yloxymethyl-3- (4- (1- (2 (S)-hydroxy-3-phosphoryl-propanoyl)-1, 2,5,6-tetrahydropyrid-4-yl)-3,5- difluorophenyl) oxazolidin-2-one is especially preferred.

Various methods for preparing compounds of formula (I) and (1-1) are described in WO 99/64417. Yet further methods are described in copending International Patent Application No. PCT/GBOO/04527 (WO 01/40236) which specifically describes and claims a modified route to phosphate prodrugs of the compounds of formula (I) and the like.

The routes used in the preparation of these compounds and particularly the phosphate prodrugs are relatively long and complex. Although the product obtained is generally acceptable, there is a possibility that impurities will be generated at some stage during the production process. As the compounds are intended for pharmaceutical use, extremely high levels of purity are required.

The applicants have found that certain novel techniques for effecting purification effectively.

According to the present invention, there is provided a process for purification of a compound of formula (H) or a salt thereof, wherein R8 is-OR9,-SR9,-NHRIO or-NRlRl2, where R9 is a C-linked 5-membered heteroaryl ring containing 2 to 4 heteroatoms independently selected from N, O and S, which ring is optionally substituted on an available carbon atom by 1 or 2 substituents independently selected from (1-4C) alkyl, amino, (1-4C) alkylamino, (1-4C) alkoxy and halogen, and/or on an available nitrogen atom (provided that the ring is not

thereby quaternised) by (1-4C) alkyl; or R9 is a C-linked 6-membered heteroaryl ring containing 1 or 2 nitrogen heteroatoms, which ring is optionally substituted on any available C atom (provided that when the N atom is adjacent to the link, there is no substitution on any C atom that is adjacent to this N atom) by 1,2 or 3 substituents independently selected from (1-4C) alkyl, amino, (1-4C) alkylamino, (1-4C) alkoxy, (1-4C) alkoxycarbonyl and halogen; Rlo is is a C-linked 5-membered heteroaryl ring containing 2 to 4 heteroatoms independently selected from N, O and S, which ring is optionally substituted on an available carbon atom by 1 or 2 substituents independently selected from (1-4C) alkyl, amino, (1-4C) alkylamino, (1- 4C) alkoxy, (1-4C) alkoxycarbonyl and halogen, and/or on an available nitrogen atom (provided that the ring is not thereby quaternised) by (1-4C) alkyl; or Rlo is a C-linked 6-membered heteroaryl ring containing 2 or 3 nitrogen heteroatoms, which ring is optionally substituted on any available C atom by 1,2 or 3 substituents independently selected from (1-4C) alkyl, amino, (1-4C) alkylamino, (1-4C) alkoxy, (1-4C) alkoxycarbonyl and halogen; R"and R 12 together with the nitrogen atom to which they are attached form a 5-membered heteroaryl ring, containing either (i) 1 to 3 further nitrogen heteroatoms or (ii) a further. heteroatom selected from O and S together with an optional further nitrogen heteroatom; which ring is optionally substituted on a C atom by an oxo or thioxo group; and/or the ring is optionally substituted on a C atom by 1 or 2 (1-4C) alkyl groups; and/or on an available nitrogen atom (provided that the ring is not thereby quaternised) by (1-4C) alkyl; or Ril and R12 together with the nitrogen atom to which they are attached form a 6-membered heteroaryl ring containing up to three nitrogen heteroatoms in total (including the linking heteroatom), which ring is substituted on a suitable C atom by oxo or thioxo and optionally substituted on any available C atom by 1 or 2 (1-4C) alkyl substituents; R2 and R3 are independently hydrogen or fluoro; R14 is a bond or a (1-8C) alkyl group which is optionally substituted by one or more hydroxy groups; which process comprises carrying out one or both of the following steps:

a) contacting the compound of formula (II) or salt thereof with an expanded brominated polystyrene resin, washing to remove inorganic contamination, and desorbing the product from the resin; and/or b) converting the compound of formula (II) to an ammonium salt thereof, removing impurities therefrom, and if desired, reconverting ammonium salt to a compound of formula (II) or to a pharmaceutically acceptable salt.

For the avoidance of doubt, it is to be understood that the current invention comprises carrying out only step a), only step b), step a) followed by step b), or step b) followed by step a).

Suitable pharmaceutically acceptable salts of compounds of formula (II) are alkali metal salts such as sodium or potassium and in particular are the disodium phosphate salts.

These will be of formula (IIA) where R2, R3, R8 and R14 are as defined above in relation to formula (In and R17 and Rl8 are a pharmaceutically acceptable cation, such as an alkaline metal ion such as sodium or potassium.

When the purification is carried out using step (a) above, the polystyrene resin is suitably pre-swollen with water. A particularly suitable polystyrene resin is SP207 available from Mitsubishi, Japan. This resin maybe mixed with water, which is absorbed and so swells the resin. Preferably also an alcohol such as ethanol or methanol, is mixed with the water used to swell the resin.

Thus in the process of step (b), the compound of formula (I) is first adsorbed onto the resin, preferably at a loading of about 40 to 70 mg/lg resin. Once adsorbed, it may be subject to procedures such as water or acid such as HCl washes, to remove contaminants. After washing, the product is suitably desorbed from the resin by eluting it with an appropriate organic solvent such as a (1-6C) alcohol, for example methanol or ethanol.

When purification is effected using step (b) above, the applicants have found that the ammonium salts of compounds of formula (In and in particular the diammonium phosphate salts of formula (hui)

where R2, R3, R14 and R8 are as defined in relation to formula (H) ; have a good crystallinity. They can be prepared for example by addition of ammonia or ammonia solutions to the free acid, and are amenable to washing with organic solvents such as (1-6C) alcohols such as methanol, to remove contaminants. This can be done using conventional methods.

Ammonium salts of compounds of formula (II) and in particular compounds of formula (E) are novel and form a further aspect of the invention.

In particular, purification is carried out using a combination of steps (a) and (b) above.

In one embodiment, the compound is subjected to a preliminary purification using the method of step (a), then subjected to step (b), and finally, to a further step (a) purification.

Preferred compounds of formula (II) and methods for preparing them are as described in WO 99/64417.

Suitably, in the compound of formula (I1), R8 is-OR9.

Examples of compounds of formula (II) where R8 is a group-NRllRiz, and methods for preparing them, are described in copending International Patent Application no.

PCT/GBO1/01815 (WO 01/81350). Particular examples of group R8 in this case is N-linked tetrazole or triazole.

Examples of compounds of formula (II) where R8 is a group-NHR10 and methods of preparation are shown in WOOO/21960. Particular examples of Rlo are isoxazol-3-yl, isoxazol-5-yl, 1, 2,4-oxadiazol-3-yl, isothiazol-3-yl, 1, 2,4-thiadiazol-3-yl or 1, 2,5-thiadiazol-3- yl.

In addition, it may be preferred that in the compound of formula (In, R9 is a C-linked 5-membered heteroaryl ring containing 2 to 4 heteroatoms independently selected from N, O and S, which ring is optionally substituted on an available carbon atom by 1 or 2 substituents independently selected from (1-4C) alkyl, amino, (1-4C) alkylamino, (1-4C) alkoxy and halogen, and/or on an available nitrogen atom (provided that the ring is not thereby quaternised) by (1-4C) alkyl.

Suitably the compound of formula (If) is obtained in the form of a pharmaceutically acceptable salt as hereinbefore described, such as an alkali metal salt such as sodium or potassium and in particular the disodium phosphate salts, or is converted to such a salt after recovery using conventional methods.

Particular examples of compounds of formula (n) are compounds of formula (IV) where R2, R3, R9 and R14 are as defined in relation to formula (II).

Most preferably, in the above compounds R14 is a direct bond.

Suitably R2 and R3 are fluorine.

A particular example of the R9 or Rlo where these are present are isoxazolyl, and particularly isoxazol-3-yl.

Thus a particularly preferred compound of formula (II) is 5 (R)-Isoxazol-3-yloxymethyl-3- (4- (1- (2 (S)-hydroxy-3-phoshoryl-propanoyl)-1, 2,5,6- -tetrahydropyridy-4-yl)-3, 5-difluorophenyl) oxazolidin-2-one and a preferred salt is the disodium salt.

It follows that a preferred compound of formula (ici) is of formula (V)

The invention will now be illustrated but not limited by the following Examples.

Example 1 Purification using polystyrene resin Polystyrene resin, sold as Resin SP207B by Misubishi (500g) was placed in a reaction vessel and ethanol in the form of Industrial Methylated Spirit 74op (IMS) (400ml) added, followed by water (600ml). The mixture was stirred and then allowed to stand for 2 hours.

This process swelled the resin and removed any bromine trapped within the pores of the resin.

The swollen resin was placed in a filter column and the liquor allowed to run off. It was then washed with water (4x 1000ml).

A disodium salt of 5 (R)-isoxazol-3-yloxymethyl-3- (4- (l- (2 (S)-hydroxy-3-phosphoryl- propanoyl)-1, 2,5,6-tetrahydropyrid-4-yl)-3,5-difluorophenyl) oxazolidin-2-one, prepared as described for example in WO 99/64417 (50g) was dissolved in water (1000ml) and added to the pre-swollen resin. The aqueous liquor was allowed to run off slowly with minimal disruption of the resin cake, which was then washed with water (3x 500ml) to remove inorganic contamination. The product was then desorbed from the resin by eluting with aqueous IMS. The elution was carried out three times, first with a mixture of 200ml IMS and 800ml water, twice more with a mixture of 400ml IMS and 600ml water. The combined eluates were concentrated by evaporation under reduced pressure, keeping the temperatures at less than 50°C, to give an aqueous concentrate (105mol) of the product. The pH may be adjusted to pH 8.0 by addition of 1M sodium hydroxide.

The desired disodium salt product product may then be precipitated from the concentrate by adding the concentrate to ethanol, as described for Example 2. (Yield 82.5 %).

Example 2 Purification using polystyrene resin by way of ammonium salt Polystyrene resin sold as Resin SP207B (200g) was pre-swollen as described in Example 1.

A disodium salt of 5 (R)-isoxazol-3-yloxymethy-3- (4- (1- (2 (S)-hydroxy-3-phosphoryl- propanoyl)-1, 2,5,6-tetrahydropyrid-4-yl)-3,5-difluorophenyl) oxazolidin-2-one, prepared as described for example in WO 99/64417 (12g) was dissolved in water (480ml) and added to the pre-swollen resin on a column. This was washed with chilled aqueous 1M hydrochloric acid (120ml) and then with water (3 xl20ml) to remove inorganic salts. The product was desorbed from the resin by eluting with aqueous IMS as described in Example 1. The combined eluents were evaporated under reduced pressure, maintaining the temperature at less than 50°C, to give a sticky gum. The gum was dissolved into methanol (120ml), cooled to 20 °C and ammonia (20.36ml 2M in methanol) added dropwise, whilst maintaining the temperature at 20°C.

A crystalline precipitate formed (the di-ammonium phosphate salt) which was isolated by filtration and washed with chilled methanol (2 x 12ml). The structure was confirmed by X ray powder diffraction.

The diammonium phosphate salt was dissolved in water (480ml), added to fresh pre- swollen resin SP207B (200g before swelling) on a column, washed with aqueous 1M hydrochloric acid (120ml), and then with water (3 x 120ml) to remove inorganic salts. The product was again desorbed from the resin by eluting with aqueous IMS as described above, concentrated by evaporation as described above (to 24ml). The temperature was adjusted to 20°C, and the pH was adjusted to pH 8.0 with 2M sodium hydroxide.

The solution was diluted by addition of an equal volume of water, then added dropwise to a mixture of IMS (200ml) and water (20ml) over at least one hour, with vigorous stirring, maintaining the temperature at approximately 40°C. The resulting mixture was stirred for at least 30 minutes at 40°C. The precipitated solid was filtered, washed with IMS and sucked dry, then dried in vacuum oven below 40°C to give desired di-sodium salt (71.6%). i. r. (cm~l) 1,744 (C=O) carbamate, 1637 (C=O) tertiary amide.

3C n. m. r. (125MHz) ppm. 172.2,172.1,162.3,160.4 (2C), 156. 3, 138.4,127.1,125.8,114.3, 102.6 (2C), 96.6,72.3,70.4,69.6,66.1,47,45.5,40.3,29.5.