Process for preparing intermediates to florfenicol

FIELD OF THE INVENTION

0001The present invention relates to a novel process for preparing florfenicol.

BACKGROUND OF THE INVENTION

0002Florfenicol, also known as [R-(R*, S*)]-2,2-Dichloro-N-[1-(fluoromethyl)-2-hydroxy-2-[4-(methylsulfonyl)phenyl]ethyl]acetamide, is a broad spectrum antibacterial agent useful in the treatment of gram positive, gram negative and rickettsial infections as disclosed in <patcit id="pcit0001" dnum="US4361557A"><text>U. S. Patent 4,361,557</text></patcit> which is hereby incorporated by reference. The present invention relates to intermediates to florfenicol and to a novel process for preparing them. The intermediates described in the present specification can be used to prepare florfenicol as can be seen, for example, in <patcit id="pcit0002" dnum="US4876352A"><text>US Patent 4,876,352</text></patcit>, which is hereby incorporated by reference.

SUMMARY

0003The present invention is directed toward a process for producing florfenicol ([R-(R*, S*)]-2,2,-dichloro-N-[1-(fluoromethyl)-2-hydroxy-2-[4-methylsulfonyl)phenyl]ethyl]acetamide), which comprises: <ul id="ul0001" list-style="none"><li>preparing a compound of formula (I): <chemistry id="chem0001" num="0001"><img file="EP0922040B2_D0001.tif" /></chemistry></li><li>wherein R is CH₃SO₂,and R" is aryl, halo aryl, benzyl, substituted benzyl, C₁ to C₆ alkyl, C₃ to C₇ cycloalkyl, and haloalkyl, and the configuration of the oxazoline ring is 4R trans:</li><li>which comprises contacting a compound of formula II: <chemistry id="chem0002" num="0002"><img file="EP0922040B2_D0002.tif" /></chemistry></li><li>wherein R is as described above, and R' is H, C₁ to C₆ alkyl, C₃ to C₇ cycloalkyl, benzyl, substituted benzyl or aryl; with a reducing agent such as potassium borohydride, in a protic solvent in a suitable reaction vessel, to obtain a compound of formula III: <chemistry id="chem0003" num="0003"><img file="EP0922040B2_D0003.tif" /></chemistry></li><li>wherein R is as described above, then in the same reaction vessel reacting a compound of formula III, with a compound of the formula IV:         R"―C≡N     IV wherein R" is as described above so as to obtain compound of the formula I, and converting said compoud of formula (I) to florfenicol.</li></ul>

0004The present invention has the advantage of being an efficient and economical process for preparing florfenicol and oxazoline intermediates thereto.

DETAILED DESCRIPTION OF THE EMDODIMENTS

0005When utilized in the present specification and in the appended claims, the terms listed hereinbelow, unless otherwise indicated, are defined as follows:

0006The term "protic solvent" is intended to mean hydrogen-bonding solvent, as defined in <nplcit id="ncit0001" npl-type="b"><text>James B. Hendrickson, Cram, Donald J., and Hammond, George S., Organic Chemistry, Mcgraw Hill Book Company, New York, New York, (1970), 1279 pp</text></nplcit>. The solvent should preferably, but not necessarily, be capable of precipitating oxazoline (I) out of solution. Such solvents include, but are not limited to, water, C₁ to C₁₀ alkanoic acids such as formic acid, acetic acid and the like, C₁ to C₁₀ alcohols such as methanol and ethanol and mixtures thereof, C₂ to C₁₀ dialcohols such as ethylene glycol and C₁ to C₁₀ trialcohols such as glycerin. Alternatively, the protic solvent can be admixed with any suitable cosolvent in order to effect precipitation of oxazoline compound (I). Such cosolvents can include other protic solvents which are miscible with the protic solvent such as C₄ to C₁₀ alkanes, aromatic solvents such as benzene, toluene, xylenes, halobenzenes such as chlorobenzene, and ethers such as diethylether, tert-butytmethylether, isopropylether and tetrahydrofuran, or mixtures of any of the above solvents or cosolvents.

0007The term "alkyl" means a straight or branched alkyl such as methyl, ethyl, propyl, or sec-butyl. Alternatively, the number of carbons in alkyl may be specified. For example, C₁ to C₆ alkyl means an alkyl as described above containing 1 to 6 carbon atoms. "Haloalkyl" means an "alkyl" as described above wherein one or more hydrogens are replaced by halo.

0008The term "aryl" means phenyl, or phenyl substituted by C₁ to C₆ alkyl or halo.

0009Substituted benzyl means benzyl substituted by C₁ to C₆ alkyl, or halo.

0010The term "halo" means fluoro, chloro, bromo, or iodo.

0011The term "halo aryl" means phenyl substituted by halo.

0012In the present specification, <chemistry id="chem0004" num="0004"><img file="EP0922040B2_D0004.tif" /></chemistry>    which is an aminodiol sulfone, is referred to as ADS.

0013The procedure for preparing florfenicol includes the following steps : <chemistry id="chem0005" num="0005"><img file="EP0922040B2_D0005.tif" /></chemistry>    wherein R, R' and R" are as described herein.

0014With reference to the formula scheme above, a compound of formula I may be prepared as follows. A compound of formula II <chemistry id="chem0006" num="0006"><img file="EP0922040B2_D0006.tif" /></chemistry>    wherein R and R' are as described above is treated with a reducing agent such as NaBH₄, Ca(BH₄)₂, LiBH₄ or more preferably KBH₄ in a protic solvent such as ethanol, ethylene glycol, or more preferably methanol, at a temperature in the range of about 0 °C to about 30 °C more preferably room temperature for a period of about 2 to about 8 hours, more preferably about 4 hours to obtain a compound of formula III: <chemistry id="chem0007" num="0007"><img file="EP0922040B2_D0007.tif" /></chemistry>    wherein R is as described above.

0015If, for example, in the reaction described just above, methanol is employed as the solvent in the reduction, it may be recovered by distillation for reuse in subsequent reactions. Removing methanol can also improve the yield of the compound of formula I.

0016In the same reaction vessel, the compound of formula III is contacted with a compound of formula IV,         R^a-C≡N     IV    wherein R" is as described above, in the amount of about 1.1 to about 2.5 equivalents, preferably about 1.7 equivalents as compared to the compound of formula III.

0017Compounds of formula IV can be, for example, benzonitrile or dichloroacetonitrile. The reaction is run at a temperature between 25°C and 115°C depending upon the nitrile and the solvent employed. The reaction is run from about 6 hours to about 30 hours, preferably 18 hours. The reaction mixture is then cooled, for example, by the addition of cold water and worked up by conventional means such as filtration and washing to afford a compound of formula I. Preferably this step of the reaction is run at a pH in a range of about 6 to about 7.

0018An advantage of the present process is that it eliminates the need to isolate ADS, since both reaction steps are run in the same vessel.

0019Formation of the dichloromethyl oxazoline is preferred and the formation of the phenyl oxazoline is most preferred.

0020The serine ethyl ester, as shown below, is the preferred starting material of formula II. <chemistry id="chem0008" num="0008"><img file="EP0922040B2_D0008.tif" /></chemistry>

0021The following examples illustrate the present invention in a manner by which it can be practiced but, as such, should not be construed as limitations on the scope of the invention.

EXAMPLE 1

0022<chemistry id="chem0009" num="0009"><img file="EP0922040B2_D0009.tif" /></chemistry>

0023KBH₄ (1g) is placed in about 40 mL of methanol. D-Threo p-methylsulfonyl phenyl serine ethyl ester (5 g) is added with stirring. The reduction to ADS is complete in several hours and can be monitored by HPLC. When the reaction is complete, 20 mL of glycerin is added to destroy any excess reducing agent and methanol is removed by distillation. After the methanol has been removed, the resulting mixture is heated to 105 °C and benzonitrile (3.1 mL) is added while continuing heating for about 18 hours. Formation of the desired oxazoline can be monitored by HPLC. The reaction is cooled to room temperature and worked up by addition of cold water, filtration of the resulting solids, washing of the solids with methanol then drying under vacuum. The yield is about 4.7 g (81 %) of material that is identical to an authentic sample of the phenyl oxazoline.

EXAMPLE 2

0024<chemistry id="chem0010" num="0010"><img file="EP0922040B2_D0010.tif" /></chemistry>

0025KBH₄ (1 g) is placed in about 40 mL of methanol. D-Threo p-methylsulfonylphenyl serine ethyl ester (5 g) is added with stirring. The reduction to ADS is complete in several hours and can be monitored by HPLC. When the reaction is complete, 20 mL of glycerin is added to destroy any excess reducing agent and methanol is removed by distillation. After the methanol has been removed, the resulting mixture is acidified to a pH of about 6 to 7 with H₂SO₄ and dichloroacetonitrile (2.4 g) is added. The reaction is stirred at 50°C for about 18 hours. Formation of the desired oxazoline can be monitored by HPLC. The reaction is cooled to room temperature and worked up by filtration of the solids, washing of the solids with isopropanol and 2 % NaHCO₃ then drying under vacuum. The yield is about 3.8 g (65 %) of material that is identical to an authentic sample of the dichloro oxazoline.

THE STARTING MATERIALS

0026The starting materials of formula (II) and (IV) are known to those skilled in the art.