Background of the invention The present invention relates to a novel method for preparing 3,4-difluoroaniline. This compound is useful in the preparation of quinolone antibacterials as described in United States Patent 4,833,270.

United States Patent 4,145,364 refers to the prepara¬ tion of monofluoroanilines via a Bamberger rearrangement or an arylhydroxylamine route. The patent indicates that the arylhydroxylamine route "suffers from the disadvantage of concomitant formation of corresponding unfluorinated aniline, as well as the symmetrical azo and azoxy com¬ pounds" and that "considerable tar is formed making product isolation difficult." The patent also refers to the preparation of monofluoroanilines and difluoroanilines by treating aromatic azides with anhydrous hydrogen fluoride. T.J. Broxton et al., J. Org. Chem., 42, 643-649 (1977) refer to the effect of oxygen and nitrogen atmospheres on thermolysis of arenediazonium salts. The process of the present invention is advantageous in that it uses hydroxylamines which are relatively stable compared to aromatic azides. The latter are used as starting materials in the process of U.S. Patent 4,145,364.

Azides, on the other hand, are not stable and tend to decompose when warmed or when left for a long period of time at room temperature. Furthermore, hydroxylamines are, in most cases, crystalline and can be conveniently purified should the need arise.

The process of the present invention does not result in the formation of a significant amount of unfluorinated material or tar. I have found that the use of anhydrous conditions and the exclusion of oxygen avoid such undesir¬ able results.

Summarv of the Invention The present invention relates to a process for pre¬ paring 3,4-fluoroaniline comprising reacting 1-hydroxyl- amine-3-fluorobenzene with anhydrous hydrogen fluoride in the absence of oxygen. Preferably, oxygen is excluded by conducting the reaction under an inert atmosphere such as nitrogen or argon. The solvent for the foregoing reaction is preferably pyridine.

In one embodiment of the invention, l-hydroxylamine-3- fluorobenzene is prepared by hydrogenating 3-fluoro-1-nitro- benzene. The hydrogenation is preferably accomplished by reacting the latter compound with hydrazine hydrate in the presence of platinum on carbon in an alcoholic solvent such as ethanol. Detailed Description of the Invention

The present invention also relates to the novel compound 3-fluoro-1-hydroxylamine.

The reaction of l-hydroxylamine-3-fluorobenzene with hydrogen fluoride should be conducted in an inert solvent. Suitable solvents include acetonitrile and pyridine. Pyridine is a preferred solvent. The reaction will generally be conducted at a temperature from about 0°C to about 50°C, preferably from about 20°C to 25°C. The temperature is most preferably 25°C. The hydrogenation of 3-fluoro-l-nitrobenzene is preferably accomplished by reacting the compound with hydrazine in the presence of platinum on carbon. Other sources of hydrogen such as ammonium formate and other hydrogenation catalysts such as Raney Nickel may also be used. Suitable solvents for the hydrogenation reaction include most alcohols. The preferred solvent is ethanol. The reaction temperature will generally be from about -20°C to about 50°C, preferably from about 0°C to about 5°C. The temperature is most preferably 0°C. The pressures of the foregoing reactions are not critical. The reactions will generally be conducted at a

pressure of about 0.5 to about 2 atmospheres, preferably at ambient pressure (generally about one atmosphere) .

3,4-Difluoroaniline may be reacted as described in United States Patent 4,833,270 to provide 1-cyclopropyl- 6,7-difluoro-l,4-dihydro-4-oxo-3-quinolinecarboxylic acid, which may be used to prepare various quinolone antibiotics, including danofloxacin. As discussed in that patent, an aqueous solution of 3,4-difluoroaniline and hydroxylamine hydrochloride containing hydrochloric acid is reacted with an aqueous solution of chloral hydrate and sodium sulfate at the reflux temperature, then filtered while hot, giving N-(3,4-difluorophenyl)-2-(hydroxyimino)-aceta ide. The latter compound is reacted with concentrated sulfuric acid with heat, then added to cracked ice, giving 5,6-difluoro- lH-indole-2,3-dione. A basic aqueous solution of the dione is treated with hydrogen peroxide and heat, and then cooled and acidified, giving 2-amino-4,5-difluorobenzoic acid. The 2-amino-4,5-difluorobenzoic acid is added to a mixture of anhydrous copper at 0-5°C, and then added to a dilute mineral acid, giving 2-chloro-4,5-difluorobenzoic acid.

A solution of 2-chloro-4,5-difluorobenzoic acid in acetonitrile containing a catalytic amount of dimethyl- formamide is reacted under an inert atmosphere with the dropwise addition of oxalyl chloride, giving 2-chloro- 4,5-difluorobenzoic acid chloride, which is dissolved in diethyl ether and slowly added to a cold solution of magnesium diethylmalonate, which is then added to ice water and acidified to pH 2.5, giving (2-chloro-4,5-difluoro- benzoyl)propanedioic acid diethyl ester. A solution of the diethyl ester in p-dioxane and water is heated at reflux, and then evaporated and distilled, giving 2-chloro-4,5- difluoro-β-oxobenzenepropanoic acid ethyl ester. A solution of the ethyl ester and triethyl orthoformate in acetic anhydride is heated at 150°C for 2 hours, giving 2-chloro-α-(ethoxymethylene)-4,5-difluoro-β-oxobenzenepro- panoic acid ethyl ester. Cyclopropylamine is added to a

solution of the latter ethyl ester in ethanol, giving 2-chloro-α-[ (cyclopropylamino)methyleneJ-4,5-difluoro-β- oxobenzenepropanoic acid ethyl ester, which is reacted with sodium hydride in dry dimethylformamide under an inert atmosphere with heat, giving l-cyclopropyl-6 ,7-difluoro- l,4-dihydro-4-oxo-3-quinolinecarboxylic acid ethyl ester, which is then refluxed with acid, giving 1-cyclopropyl- 6 ,7-difluoro-1 ,4-dihydro-4-oxo-3-quinolinecarboxylic acid. Danofloxacin may be prepared by reacting the acid thus prepared with (S,S)-2-methyl-2,5-diazabicyclo[2.2.1]heptane and an amine base.

Quinolone antibacterials such as danofloxacin (dis¬ closed in U.S. Patent 4,861,779) and the pharmaceutically acceptable acid addition salts thereof are useful in the treatment of bacterial infections of broad spectrum, particularly the treatment of gram-positive bacterial strains.

The quinolone antibacterials of U.S. Patent 4,861,779 may be administered alone, but will generally be ad inis- tered in admixture with a pharmaceutical carrier selected with regard to the intended route of administration and standard pharmaceutical practice. For example, they can be administered orally or in the form of tablets containing such excipients as starch or lactose, or in capsules either alone or in admixture with excipients, or in the form of elixirs or suspensions containing flavoring or coloring agents. In the case of animals, they are advantageously contained in an animal feed or drinking water in a concen¬ tration of 5-5000 ppm, preferably 25-500 ppm. They can be injected parenterally, for example, intramuscularly, intravenously or subcutaneously. For parenteral adminis¬ tration, they ar best used in the form of a sterile aqueous solution which can contain other solutes, for example, enough salt or glucose to made the solution isotonic. In the case of animals, compounds can be administered intra¬ muscularly or subcutaneously at dosage levels of about

0.1-50 mg/kg/day, advantageously 0.2-10 mg/kg/day given in a single daily dose or up to 3 divided doses.

The quinolone antibacterials can be administered to humans for the treatment of bacterial diseases by either the oral or parenteral routes, and may be administered orally at dosage levels of about 0.1 to 500 mg/kg/day, advantageously 0.5-50 mg/kg/day given in a single dose or up to 3 divided doses. For intramuscular or intravenous administration, dosage levels are about 0.1-200 mg/kg/day, advantageously 0.5-50 mg/kg/day. While intramuscular administration may be a single dose or up to 3 divided doses, intravenous administration can include a continuous drip. Variations will necessarily occur depending on the weight and condition of the subject being treated and the particular route of administration chosen as will be known to those skilled in the art.

Example 1 l-Hvdroxylamine-3-fluorobenzene 3-Fluoro-l-nitrobenzene (20 g, 0.14 mol) was dissolved in 400 ml of ethanol and 5% platinum on carbon (2.0 g, 10 weight %) was added. The mixture was then cooled with an ice bath to 0°C and hydrazine hydrate (13.8 ml, 0.28 mmol) was added dropwise over a period of 45 minutes. After an additional stirring period of 30 minutes, the reaction mixture was filtered through diatomaceous earth (Celite (trademark)) and the solvent was evaporated under vacuo. The residual oil was suspended in 400 ml of water and extracted with 3x500 ml of methylene chloride. The com¬ bined organic solvents were dried (MgS0 ₄ ) and evaporated to produce 12.84 g of an off white solid in 71% yield, .p. 58-59°C. Anal, calc'd for C^R^F: C, 56.69; H, 4.76; N, 11.02; Found: C, 56.73; H, 4.80; N, 11.14.

Example 2 3.4-Difluoroaniline A plastic bottle was charged with 75 ml of HF - pyridine under an inert atmosphere at 0°C and to that

1-hydroxylamine-3-fluorobenzene (5.0 g, 39.4 mmol) was added carefully. The reaction mixture was allowed to warm to room temperature and was stirred for 48 hours. The reaction mixture was carefully quenched with 2 1 of sat- urated aqueous NaHC0 ₃ solution and extracted with 5 x 500 ml portions of methylene chloride. The combined organic solvents were washed with saturated CuS0 ₄ solution (3x) , dried (MgS0 ₄ ) and evaporated to produce 4.6 g of an oil in 90% yield. NMR(CDC1 ₃ ) : S 6.95 (q, IH) , 6.55 (m, IH) , 6.34 (m, IH) , 3.6 (broad, 2H) .