FORM 2 THE PATENTS ACT 1970 (39 OF 1970) COMPLETE SPECIFICATION ( SECTION 10) " AN IMPROVED PROCESS FOR THE PREPARATION OF 1-CYCLOPROPYL—6,7 DIFLUORO -8- METHOXY -4-OXO- l,4-DIHYDROQUINOLINE-3-CARBOXYLIC ACID, AN INTERMEDIATE USEFUL FOR THE PREPARATION OF GATIFLOXACIN " UNICHEM LABORATORIES LIMITED, AN INDIAN COMPANY Registered under Indian Companies act 1956 having its registered office at MAHALAXMI CHAMBERS, 2"* FLOOR, 22, BHULABHAIDESAI ROAD, MUMBAI-400 026. MAHARASHTRA INDIA. The following specification particularly describes and ascertains the nature of the invention and the manner in which it is to be performed. Page 1 of30 FIELD OF Tfffi INVENTION. The present invention relates to an improved process for the preparation of l-cyclopropyl-6,7-difluoro-8-methoxy-4-oxo-l,4-dihydroquinoline-3-carboxylic acid useful for the preparation of Gatifloxacin. The invention particularly relates to an improved process for the preparation of 1-cyclopropyl-6,7-difluoro-8-metho5^-4-oxo-l,4-dihydroquinoline-3-carbo?Q^lic acid useful for the preparation of GatiQoxacin involving the use of 3-dimethyl amino acrylic acid methyl ester. BACKGROUND OF THE INVENTION Nalidixic acid was the first quinolone introduced to clinical practice, and is still in use for treatment of urinary tract infection caused by gram negative pathogens. Since the introduction of Nalidixic acid in the early 1960's, substantial improvements have been made in the antibacterial spectrum and potency of quinolones. Newer fluoroquinolones are well absorbed and distributed throughout the body making them very much useful for the treatment of a variety of systemic infections including those of the respiratory tract, gastrointestinal tract, the ear, nose and throat, sexually transmitted diseases and bone infections, (kirk-othmer, Encyclopedia of Chemical Technology, 4'*'Edition, 1991, Vol. 2, 855) Norfloxacin; 1-ethyl -€-fluoro-l,4-dihydro-4-oxo-7-(l-piperazinyl)-3-quinoline carboxylic acid is a fluorinated quinolone antibacterial agent. Its synthesis was reported by T. Irikura in U.S. Pat. 4146719 (1978 to kyorin). O O OH Page 2 of30 Ciprofloxacin; 1 -cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-( 1 -piperazinyl)-3-quinoline carboxylic acid is another fluoroquinolone antibacterial agent. Its synthesis was reported by K.Grohe et. al. in U.S.Pat. 4670444 (1983,1987 both to Bayer AG) O Ciprofloxacin Ofloxacin; (+) 9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-l-piperazinyl)-7-oxo-7H-pyrido-[l,2,3-de]-l,4-benzoxazine-6-carboxylic acid is broadspectrum, fluorinated quinolone antibacterial. Its preparation was reported by I.Hayakawa et.al. in U.S. Pat 4382892 (1982,1983 both to Daiichi). H,C O O Ofloxacin Lomefloxacin ; 1 -ethyl-6,8-difluoro-1,4-dihydro-7-(3 -methyl-1 -piperazinyl)-4-oxo-3 -quinoline carboxylic acid is another fluorinated quinolone antibacteriaLIts synthesis was reported by Y.Itoh et.al. in U.S.Pat. 4528287 (1985 to Hokuriku Pharm.) Page 3 of30 Q o Lomefloxacin Gatifloxacin sesquihydrate; (±)l-cyclopropyl-6-fluoro~8-methoxy-7-(3-methyl-l-piperazinyl)-4-oxo-l,4-dihydroquinoline-3-carboxylic acid, is highly effective broad spectrum fluorinated quinolone antibacterial agent. It is indicated for the treatment of community acquired respiratory tract infections for oral or intravenous administration. Its synthesis was reported by Masuzawa k. et.al in U.S. Pat. 4980470 (1990 to Kyorin Pharm, Co. Ltd.) O H3C Gatifloxacin Moxifloxacin hydrochloride; is another fluoroquinolone with antibacterial activity against Gram positive bacteria significantly better than those of Sparfloxacin and Ciprofloxacin. As compared to Ciprofloxacin, development of resistance was less pronounced in Moxifloxacin. It is (4a S-cis)-1 -cyclopropyl-6-fluoro-8-methoxy-7-(octahydro-6H-pyrrolo [3,4-b] pyridin-6-yl)-4-oxo-1,4-dihydro-3-quinoline carboj^lic acid hydrochloride. Its synthesis was reported by Petersen U. et.al. in DE 4208789 (1996 to BAYER AG) Page 4 of30 H H3C Moxifloxacin Norfloxacin, Ciprofloxacin, Ofloxacin, Lomefloxacin, Gatifloxacin & Moxifloxacin are approved oral antibacterials. As the importance of fluoroquinolones as highly effective, broad spectrum antibacterial chemotherapeutic agent, started increasing, interest in the various cost effective routes for synthesis of fluoroquinolones and its intermediates also gained importance. The present invention discloses a safe, cost effective, facile route for the synthesis of l-cyclopropyl-6,7-difluoro-8-methoxy-4-oxo-l,4-dihydroquinoline-3-carboxylic acid which is used as an intermediate for the manufacture of Gatifloxacin sesquihydrate and Moxifloxacin hydrochloride. DESCRIPTION OF THE PRIOR ART l-99.5%) under stirring and cooling, keeping temperature <45 C. R.M. Page 23 of 30 gradually cooled to R.T. charged 320 gms (1.035 moles) of methyl-1-cyclopropyl-6,7-difluoro-l,4-dihydro-8-methoxy-4-oxo-3-quinoline carboxylate of the formula XXI prepared according to the step (v) described above . Charged 1680 gm of glacial acetic acid at R.T. r.m. o temperature raised to reflux temperature (100 C), and refluxed for one hour. Gradually cooled o o to 70 C and then added into 21 litres of ice cold water under vigorous stirring at 10-15 C. r.m. o stirred at 15 C for one hour. Solid material which has separated out, is filtered. Washed with 2 o liter distilled water. Dried at 55 C under vacuum for 20 hours to get 291 gms of crude acid. It is chemically purified as given below. 291 gm of crude dry acid is suspended in 2.9 litres of distilled water. To it imder stirring added 10% Sodium hydroxide solution till pH = 12.2 to 12.5 (460 ml of 10% NaOH solution is required). Added 15gm activated charcoal. Stirred at R.T. for half an hour, solution filtered off on hyflo bed and then through sintered glass fiinnel, pH of the filtrate is adjusted to 6.8 to 7.2 (78 ml of 50% sulphuric acid is required). Added 2 litre distilled water, so that efficient stirring is possible. Solid product filtered, washed with 3 X 1.5 litre dist. water, dried at 55 to o 60 C under vacuum to get 284 gm (92 - 96 %) of l-cyclopropyl-6,7-difluoro-8-methoxy-4- o oxo-l,4-dihydroquinolitie-3-carboxylic acid.of the formula I M.P. = 188 -190 C, Purity by HPLC = 99.70%. Advantages of the inventioii 1. The process is simple and cost effective 2. The process is relatively safe employing relatively safe starting material 3. The process has shorter reaction steps and hence economical Page 24 of 30 WE CLAIM: 1) A process for the preparation of 1 -cyclopropyl-6, 7-difluoro-8-methoxy-4-oxo-1, 4-dihydroquinoline-3-carboxylic acid of formula (I). which comprises (I) (i) methylating 3- hydroxy-2,4,5 - triflxjrobenzoic acid of the formula XII to get methyl-3 methoxy -2,4,5-trifluoro benzoate of the formula XIII , at temperature range from 50 - 85 ° C (XIII) (ii) hydrolysing the methyl-3 methoxy -2,4,5-trifluoro benzoate of the formula XIII obtained in step(i) to produce 3-methoxy-2,4,5- trifluorobenzoic acid of the formula (V) Page 25 of 30 (V) (iii) Converting the 3- methoxy -2,4,5-trifluoro benzoic acid of the formula V obtained in step (ii) to the corresponding acid chloride viz. 3 - methoxy 2,4,5 trifluoro benzoyl chloride of the formula XIX (iv) Reacting the 3 - methoxy- 2,4,5 trifluoro benzoyl chloride of the formula XIX with an amine such as pyridine, picoline, triethyl amine preferably triethyl amine, to get the corresponding condensation product. (v) Reacting the condensation product obtained in step (iv) with cyclopropyl amine to yield methyl-2-(3-methoxy-2,4,5- trifluoro benzoyl)-3-cyclopropyl amino acrylate of the formula (XX). OCH3 (XX) (vi) Cyclising the methyl-2- (3-methoxy-2, 4,5- trifluoro benzoyl)-3- cyclopropyl Page 26 of 30 amino acrylate of the formula (XX) obtained in step(v) to yield methyl-1-cyclopropyl-6,7-difluro-1,4-dihydro-8-methoxy-4-oxo-3-quinoline carboxylate of the formula (XXI) and (XXI) (vii) Hydrolysing the methyl-1-cyclopropyl-6,7-difluro-1,4-dihydro-8-methoxy-4-oxo-3-quinoline carboxylate of the formula (XXI) obtained in step (vi) to yield l-cyclopropyl-6,7-difluoro-8-methoxy-^10X0-1,4- dihydroquinoline - 3 -carboxylic acid of the formula I 2. A process as claimed in claim 1 wherein the methylation in step (i) is effected by using methylating agent such as dimethyl sulphate, and a base. 3. A process as claimed in claim 2 wherein the base used is anhydrous potassium carbonate, anhydrous sodium carbonate. 4. A process as claimed in claims 2 and 3 wherein the methylation is carried out in the presence of solvent like aliphatic ketone such as acetone, methyl ethyl ketone. 5. A process as claimed in claims 2 to 4 wherein the ratio of the 3- hydroxy-2, 4,5 - trifluorobenzoic acid of the formula XII, the methylating agent and the base employed is 1:2.2:2.0 6 A process as claimed in claim 1 wherein the methylation is effected at a temperature in the range of 50 to 80° C, preferably at 56 to 58° C. Page 27 of 30 7. A process as claimed in claim 1 wherein the methylation reaction is effected for a period ranging from 2 to 5 hours, preferably for 2 hours. 8. A process as claimed in claim 1 wherein the hydrolysis of the methyl-3 methoxy -2,4,5-trifluoro benzoate of the formula XIII is carried out using a base such as sodium hydroxide, potassium hydroxide, preferably sodium hydroxide. 9. A process as claimed in claim 1 wherein the hydrolysis is effected in the presence of solvent such as water, aqueous methanol, aqueous ethanol, aqueous Isopropyl alcohol, preferably water. 10. A process as claimed in claim 1 wherein the hydrolysis is effected at a temperature in the range of 50 to 100° C. 11. A process as claimed in claim 10 wherein the hydrolysis is effected at a pH in the range of 2.0 to 2.2, preferably at pH 2.0 12. A process as claimed in claim 11 wherein the pH is adjusted using an itiorganici acid like aqueous hydrochloric acid, sulphuric acid, preferably hydrochloric acid of 36% strength. 13. A process as claimed in claim 1, step iii), wherein the conversion of 3-methoxy -2,4,5-trifluoro benzoic acid of the formula V is effected using chlorinating agents like thionyl chloride, phosphorus oxychloride. 14. A process as claimed in claim 10 wherein the reaction is effected at a temperature in the range of 78 to 90° C, preferably at 85° C. Page 28 of 30 15. A process as claimed in above claims wherein the reaction is effected in the presence of hydrocarbon solvent such as beiizene,toluene,xylene etc, preferably toluene. 16. A process as claimed in claim 1 wherein the reaction of the condensation product obtained at step (vii), is effected in the presence of hydrocarbon solvent such as benzene, toluene, xylene. 17. A process as claimed in claim 16 wherein the reaction is effected at a temperature in the range of 0 to + 15° C, preferably at +10° C. 18. A process as claimed in claim 1 wherein the cyclisation reaction is effected employing inorganic base such as anhydrous potassium carbonate, anhydrous sodium carbonate. 19. A process as claimed in claim 18 wherein the cyclisation reaction is effected in the presence of solvent like toluene, dimethyl formamide. 20. A process as claimed in claims 18 and 19 wherein the cyclisation reaction is carried out at a temperature in the range of 110 to 114° C,preferably at 114° C. 21. A process as claimed in claim 1 wherein the hydrolysis of the methyl-1-cyclopropyl-6, 7-dtfluro-l, 4-dihydro-8-methoxy-4-oxo-3-quinoline carboxylate of the formula (XXIV) is effected by using mixture of dilute sulphuric acid and acetic acid. Page 29 of 30 22. A process for the preparation of l-cyclopropyl-6,7-difluoro-8 methoxy-4-oxo 1,4- dihydroquinoline-3-carboxylic acid of the formula I substantially as herein described with reference to the Example 1. Page 30 of 30 Dated this 15* day of May 2002