FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
COMPLETE SPECIFICATION (See section 10)
PROCESS FOR PREPARING DIFLUPREDNATE
AJANTA PHARMA LTD.
A company incorporated under the laws of India having their office at
98, Ajanta house, Charkop, Kandivili (West)
Mumbai - 400067, Maharashtra, India
The following specification particularly describes the invention and the manner in which it is to be performed.

TECHNICAL FIELD OF THE INVENTION
The present invention relates to an improved, commercially viable and industrially advantageous process for the preparation of difluprednate of formula (I). The process provides difluprednate in higher yield and purity compared to the previously disclosed processes.

BACKGROUND
6α, 9α-difluoro-11β, 17,21- trihydroxypregna-l,4-diene-3,20-dione 21-acetate 17-butyrate, also known as difluprednate, is a potent gluco-corticoid anti inflammatory agent and used topically for the treatment of ocular inflammation and pain.
Difluprednate, as well as process for its preparation, are disclosed in U.S. Patent No. 3,780,177. Example 1 of this patent discloses preparation of difluprednate by following steps: (a) orthoesterification of 6a, 9a-difluoro prednisolone of formula (II) with methylorthobutyrate to give compound of formula (III), (b) followed by hydrolysis using oxalic acid to give compound of formula (IV) and (c) esterification of compound of formula (IV) with acetic anhydride to give difluprednate, as shown in following Scheme-1.


However, the above described process suffers with following drawbacks:
(i) Use of excessive amount of methyl orthobutyrate in the first stage, which is difficult to remove.
(ii) High reaction temperature and longer reaction time
(iii) Isolation of Intermediate of formula (III) done by distillation of DMF which is not suitable for large scale production. Also, additional purification of compound of formula is (III) required, which results in around 50% yield loss.
(iv) Use of oxalic acid for hydrolysis of cyclic ester of compound of formula (III) which results in more impurity formation. Additional purification of compound of formula (IV) is required to get desired purity which results in yield loss.

(v) Use of excessive amount of acetic anhydride for esterification of compound of formula (IV) which requires reaction under cooling.
Overall, the process for preparation of difluprednate described in the prior art is not suitable for large scale production due to use of excess of reagents, higher reaction temperature, longer reaction time, lower yield and additional purification steps. To date, there is no commercially viable process for preparation of difluprednate.
So, there is need for the process for preparation of difluprednate, which is applicable to industrial scale production and gives high yield with high purity of difluprednate and its intermediates. The present invention provides such an alternative process.
SUMMARY OF THE INVENTION
The present invention provides an improved process for preparation of difluprednate, represented by formula (I) or a pharmaceutically acceptable salt thereof

comprising
a) hydrolyzing compound of formula (III) with boric acid to give compound of formula (IV);


b) acetylating compound of formula (IV) to give compound of formula (I)
Compound of formula (III) can be prepared by orthoesterification of 6a, 9a-difluoroprednisolone, compound of formula (II),

with trimethylorthobutyrate in presence of organic solvent and an acid catalyst.
6α, 9α-difluoroprednisolone-17α, 21-methylorthobutyrate, compound of formula (III), can be recovered from the reaction mixture by quenching the reaction mixture using a base and adding water to the quenched reaction mixture. The precipitated 6α, 9α-difluoroprednisolone-17α, 21-methylorthobutyrate is isolated by filtration.
Compound of formula (III) can be hydrolyzed using boric acid to compound of formula (IV) which can be acetylated using an acetylating agent in presence of a base to give difluprednate.

DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an improved process for preparation of difluprednate and its intermediates in high purity and high yield, avoiding use of excess of reagent and additional purification, thus enabling cheaper and industrial scale suitable process.
The present invention provides a process for preparation of difluprednate, represented by formula (I) or a pharmaceutically acceptable salt thereof

comprising,

b) acetylating compound of formula (IV) to give compound of formula (I).
a) hydrolyzing compound of formula (III) with boric acid to give compound of formula (IV);

In one of the embodiment, compound of formula (III) is prepared by orthoesterification of 6α,9α-difluoroprednisolone of formula (II) using methyl orthobutyrate in presence of an acid catalyst in an organic solvent.

The acid catalyst employed is selected from p-toluenesulphonic acid, acetic acid, sulfuric acid, boron trifluoride. A preferred acid catalyst is p-toluenesulfonic acid.
Preferably, the quantity in weight percentage of the acid catalyst to 6a, 9a-difluoroprednisolone is in the range from 1 to 10%, preferably in the range from 3.5 to 4.5%.
Preferably the molar ratio of methyl orthobutyrate to 6α, 9α-difluoroprednisolone is in the range of 1 to 10, more preferably in the range of 3 to 5, most preferably in the range of 2.8 to 3.2.
Preferably the organic solvent is selected from dimethylformamide, dimethylacetamide, dimethyl suphoxide or N-methylpyrrolidone or mixture thereof. More preferably the inert solvent is dimethylformamide.
Preferably the reaction temperature for the above step is about 20° C to about 70° C, more preferably 25° C to about 30° C and the reaction is completed within 3 to 4 hours.
In another embodiment, compound of formula (III) is isolated by filtration after quenching the reaction mixture with a base and precipitating it by adding water to the

quenched reaction mixture to get compound of formula (III) having high purity with yield of 75 to 80 % without any further purification. Preferably purity of the compound of formula (III) is greater than 90%, more preferably greater than 95% and most preferably greater than 98%.
Preferably base used for quenching the reaction mixture is selected pyridine, triethyl amine, ammonia, sodium carbonate, sodium bicarbonate. More preferably, the base is pyridine.
In another embodiment, compound of formula (III) is hydrolyzed to 6a, 9a-difluoroprednisolone-17-butyrate of formula (IV) using boric acid in polar solvent Instead of boric acid other acids selected from oxalic acid, hydrochloric acid, p-toluene sulphonic acid, acetic acid, citric acid can be used. It has been surprisingly found out that compound of formula (IV) can be obtained in higher purity (greater than 97%) when boric acid is used in the above reaction as compared to the other acids like oxalic acid and hydrochloric acid, which when used for the above reaction gives compound of formula (IV) having purity around 80%.
Preferably the molar ratio of boric acid to the compound (III) is in the range of 7 to 9. More preferably the ratio is in the range of 7.5 to 8. Preferably the polar solvent for hydrolysis is selected from methanol, ethanol, isopropyl alcohol, acetone, water or mixture thereof. More preferably solvent for hydrolysis is methanol and water mixture.
In one of the embodiment, compound (IV) is isolated by adding water to the reaction mixture followed by filtration of precipitated compound (IV) and washing the cake with water. Preferably purity of the compound of formula (IV) obtained after washing is greater than 90%, more preferably greater than 95% and most preferably greater than 97% with yield of 88 to 90%.


In another embodiment, compound of formula IV is acetylated in presence of a base to give crude difluprednate. Preferably acetylating agent for acetylation is selected from acetic anhydride, acetyl chloride. More preferably acetylating agent is acetic anhydride. Preferably, the molar ratio of acetic anhydride to compound (IV) is in the range of 2 to 3, more preferably in the range of 2.3 to 2.6. Preferable base used for acetylation is selected from pyridine, triethylamine, Lutidine. More preferably, the base is pyridine. Preferably solvent used for acetylation is selected from pyridine, tetrahydrofuran, dichloromethane, acetone, dimethylformamide. More preferably, the solvent is pyridine.
In another embodiment, crude difluprednate is purified by recrystallization using mixture of inert solvent and water. Preferably the inert solvent is dimethylformamide.
The following examples are intended to further illustrate the present invention and are not intended to limit the scope of this invention.
Example-1: Preparation of 6α, 9α-difluoroprednisolone-17-a, 21-methylorthobutyrate
A mixture of 6α,9α-difluoroprednisolone (1.0 Kg, 2.52 mol), DMF (5 L) and trimethyl orthobutyrate (1.12 kg 7.57 mol) was stirred in 4-neck round bottom flask under nitrogen at 25°C to 30°C for 5 to 10 min. To the above mixture was added p-toluenesulphonic acid (29 g, 0.15 mol) under nitrogen environment and stirred at 25°C to 30°C for 2.0 h. A second lot of p-toluenesulphonic acid (6 g, 0.032 mol) was added under nitrogen and stirred at 25°C to 30°C for 1.0 h. After completion of reaction, the reaction

mass was cooled up to 15°C to 20°C, pyridine (0.1 w/v) was added to it and stirred for 10 min. Water (5 L) was added in 20 min at 15°C to 20°C and the mixture was maintained at the same temperature for 30 min. The precipitated solid product was filtered and washed with water to get 6a, 9a-difluoroprednisolone-17a,21-methylortho butyrate (yield: 1.15 kg, HPLC purity 98.74%).
Example-2: Preparation of 6a, 9a -difluoroprednisolone-17a-butyrate
A mixture of 6α,9α-difluoroprednisolone-17α,21-rnethylortho butyrate (1.15 kg, 2.39 mol), methanol (12 L), boric acid (1.15 kg , 18.61 mol) was dissolved in water (4 L) and stirred in 4-neck round bottom flask to get a thick slurry mass. The mixture was heated to 60°C to 65°C for 60 min The reaction mixture was cooled to 25°C to 30°C, water (25 L) was added within 30 min at 25°C to 30°C and the mixture was maintained at 25°C to 30°C for 60 min. The precipitated solid product was filtered and washed with water to get 6a,9a-difluoroprednisolone-17a-butyrate (yield: 1.09 kg, HPLC purity: 97.63%).
Example-3: Preparation of crude difluprednate
a) A mixture of 6a, 9a -difluoroprednisolone-17α-butyrate (1.09 kg, 2.34 mol)
and pyridine (5 L) stirred in 4-neck round bottom flask under nitrogen atmosphere at
25°C to 30°C. Acetic anhydride (0.59 kg, 5.78 mol) was added under nitrogen at 25°C to
30°C in 40 min. The reaction mass was stirred at 25°C to 30°C for 1 hour. After
completion of reaction, reaction mass was cooled to 15°C to 20°C, water (10 L) was
added in 30 min and mass was stirred for 60 min. The precipitated product was filtered
and washed with water to get difluprednate (yield: 1.02 kg, HPLC purity: 98.70%),
compound of formula (I).
b) A mixture of 6a,9a-difluoroprednisolone-17a-butyrate (1.09 kg, 2.34 mol) and
pyridine (5 L) was stirred in 4-neck round bottom flask under nitrogen atmosphere at
25°C to 30°C. Acetic anhydride (0.59 kg, 5.78 mol) was added under nitrogen at 25°C to
30°C in 40 min. The resulting mixture was stirred at 25°C to 30°C for 1 h. The reaction
mass was cooled to 15°C to 20°C, water (9.0 w/v) was added and diflupredenate was
extracted from the aqueous layer using ethyl acetate (10 L). Combined ethyl acetate

layer was washed with 2N hydrochloric acid and ethyl acetate was distilled off under vacuum to get difluprednate (yield: 1.0 kg, HPLC purity 99.20%), compound of formula
(I).
Example-4: Preparation of pure difluprednate
Difluprednate crude (1.0 kg) was dissolved in DMF (2 L) at 25°C to 30°C in 4-neck round bottom flask to get clear solution. To the above solution activated charcoal (5% w/w) was added and stirred for 15 min. The resulting mixture was filtered through celite and to the filtrate water was added drop wise to get white precipitated solid. The solid product was filtered off and washed with dimethylformamide and water mixture (1:1) to get difluprednate pure (yield: 0.7 kg, HPLC purity 99.30%).

We claim:
1. A process for preparation of compound of formula (I) or a pharmaceutically acceptable salt thereof comprising,

c) hydrolyzing compound of formula (III) with boric acid to give compound of formula (IV);

d) acetylating compound of formula (IV) to give compound of formula (I).
2. A process according to claim 1, wherein the ratio of boric acid to compound of formula (III) is in the range of 7 to 9.
3. A process according to claim 1, wherein compound of formula (IV) is isolated from the reaction mixture by adding water to the reaction mixture and filtering the precipitated product.
4. A process according to claim I, wherein compound of formula (III) is prepared by orthoesterification of compound of formula (II) using trimethylorthoformate in presence of acid catalyst


5. A process according to claim 4, wherein orthoesterification is carried out at 25-30 °C.
6. A process according to claim 4, wherein the acid catalyst is p-toluene sulphonic acid.
7. A process according to claim 4, wherein compound of formula III is isolated from the reaction mixture by quenching the reaction mixture with a base, precipitating compound of formula (III) by adding water to reaction mixture and filtering the precipitates.
8. A process according to claim 1, wherein compound of formula (IV) is converted to compound by acetylation using acetic anhydride or acetyl chloride in presence of a base.
9. A process according to claim 8, wherein the base is pyridine.
10. A process according to claim 8, wherein compound of formula (I) obtained is further purified by recrystallization using mixture of DMF and water.