Abstract:
Taught is a method for preparing S-fluoromethyl-6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carbothioate (fluticasone propionate). The present method is simple, convenient, and mild, yields highly pure product, and is suitable for use commercially on a large scale.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Patent Application No. PCT/CN2005/001339 with an international filing date of Aug. 29, 2005, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 200510028147.2, filed on Jul. 26, 2005. The contents of these specifications are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the field of pharmaceuticals, and specifically to a method for preparation of fluticasone propionate. 
         [0004]    2. Description of the Related Art 
         [0005]    A process for preparing fluticasone propionate, the compound of formula (1), has heretofore been described in U.S. Pat. No. 4,335,121. 
         [0006]    With reference to  FIG. 1 , the process starts with the reaction of compound (2) with N,N-dimethyl thiocarbamoyl chloride to obtain compound (3); refluxing it in the presence of diethylamine to obtain compound (4), which is then reacted with bromochloromethane, followed by displacement with sodium iodide and silver fluoride to obtain compound (1), fluticasone propionate. This process is complicated and the synthetic route is long. The yield is accordingly low, while the cost is high and the process uses an expensive reagent, silver fluoride. Therefore, this process isn&#39;t useful commercially. 
         [0007]    Similarly, in the Intl. Pat. Publ. No. WO 01/62722 (which corresponds to U.S. Pat. Appl. Publ. No. 2002/0133032), compound (2) is reacted with N,N-dimethyl thiocarbamoyl chloride in butanone in the presence of catalysts, sodium iodide and triethylamine to yield compound (3), which is then reacted with sodium hydrosulphide to yield compound (4), which is finally reacted with fluorochloromethane to yield compound (1). Since excess sodium hydrosulfide is turned into hydrogen sulfide and a large amount of chlorofluoromethane is employed, the process is environmentally unfriendly. Meanwhile, impurities resulting from using sodium hydrosulfide to yield sulfoacid are hard to remove from the reaction mixture, which makes it difficult to improve the quality of the final product. The cost is also high. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    This invention relates to a simple, efficient and economical method to produce fluticasone propionate on a commercial scale. 
         [0009]    This invention provides a method for preparing S-fluoromethyl-6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carbothioate (fluticasone propionate, compound (1)) comprising: 
         [0010]    (a) compound (2), 6α,9α-difluoro-11β-hydroxy-16α-methyl-17β-propionyloxy-3-oxoandrosta-1,4-diene-17β-carboxylic acid, is reacted with N,N-dimethyl thiocarbamoyl chloride in the presence of catalyst, 4-dimethylaminopyridine, in a ketonic or ethereal solvent, such as butanone or tetrahydrofuran, at between 0 and 40° C. to yield compound (3); 
         [0011]    (b) compound (3), 17β-((N,N-dimethyl-carbamyl)-thio)formoxyl-6α,9α-difluoro-11β-hydroxyl-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene is decarboxamidated in a reaction system comprising an alcohol and a phosphate under alkaline reaction conditions to yield an alkali salt of compound (4), i.e., compound (4a); the alkali salt is neutralized with acid to obtain compound (4); 
         [0012]    (c) fluorobromomethane is first reacted to form a complex with an organic base in a solvent to activate the fluoromethyl group and then reacted with 6α,9α-difluoro-11β-hydroxyl-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-thiocarboxylic acid (compound (4)) to yield compound (1) in a fluoromethylation reaction. 
         [0013]    In certain embodiments of the invention, the catalyst, sodium iodide or other iodides is not used for the preparation of compound (3) from compound (2). The acid absorbent, 4-dimethylaminopyridine, is used instead, and compound (2), 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carboxylic acid, is reacted with 4-dimethylamino-pyridine for 2-3 hours at between 0 and 40° C., and preferably at between 25 and 35° C., in a ketonic or ethereal solvent, such as butanone or tetrahydrofuran. Dimethyl acetamide and water are added, and then the temperature is cooled to between 0 and 5° C. to obtain compound (3) in a good quality. See  FIG. 2 . 
         [0014]    In certain embodiments of the invention, in the preparation of compound (4) from compound (3), compound (4a) is formed in a reaction system comprising an alcohol and an alkali metal phosphate, in which the metal (M) is potassium, sodium or lithium; and then compound (4) is obtained by acidification of compound (4a). 
         [0015]    In certain embodiments of the invention, in the reaction system comprising an alcohol and an alkali metal sulfate, the alkali metal phosphate is tripotassium phosphate, trisodium phosphate, lithium phosphate, etc.; and particularly tripotassium phosphate. 
         [0016]    In a class of this embodiment, the alcohol used in the reaction is methanol, ethanol or propanol; and particularly methanol. The reaction temperature is in the range of between 0 and 100° C.; and particularly between 20 and 40° C. TLC and HPLC can be used to monitor the progress of the reaction. 
         [0017]    In a class of this embodiment, a preferable choice of the reaction system is potassium phosphate and methanol at room temperature for 3-6 hours. 
         [0018]    In certain embodiments of the invention, in the preparation of compound (1) from compound (4) or compound (4a), fluorobromomethane first forms a complex with an organic base in a solvent so as to activate the fluoromethyl, and then reacts with compound (4) to yield compound (1) by fluoromethylation. 
         [0019]    In a class of this embodiment, the organic base is triethylamine, diethylamine, pyridine, 4-dimethylamino pyridine, α-methylpyridine, etc, all of which have been proven to obtain a satisfying result. The solvent used is a commercially available solvent. In a class of this embodiment, it is a ketone, an alcohol, an ester, an amide, or a halohydrocarbon. The reaction temperature is in a range of between 0 and 30° C. In a class of this embodiment, fluorobromomethane is directly added to the reaction system comprising compound (4a) to yield compound (1). 
         [0020]    Compound (2), 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carboxylic acid, is obtained by oxidation of flumethasone by periodic acid, and then acylation of the resultant intermediate by propionyl chloride. The synthetic route is shown in  FIG. 3 . 
         [0021]    The methods of this invention are carried out under mild reaction conditions to obtain a high quality product. The methods of this invention are suitable for commercial exploitation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  illustrates a prior art process for preparing fluticasone propionate; 
           [0023]      FIG. 2  illustrates a method for preparing fluticasone propionate according to one embodiment of the invention; and 
           [0024]      FIG. 3  illustrates a method for preparing 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carboxylic acid according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The following examples are intended to illustrate the embodiments of the invention and are not intended to limit the scope of the present invention. 
       EXAMPLES 
     Example 1 
     Preparation of 6α,9α-difluoro-11β,17α-dihydroxy-16α-methyl-3-oxoandrosta-1,4-diene-17β-carboxylic acid 
       [0026]    The mixture of 100 g of flumethasone and 500 mL of tetrahydrofuran was cooled to 0° C. A solution of periodic acid (85 g of periodic acid in 250 mL of water) was slowly added and the reaction mixture was allowed to stir for 3 hours. Then, 4 L of water was added, and the system was stirred until a solid had precipitated. The reaction mixture was filtered and the solid was dried to obtain the title compound with a purity higher than 99%. 
       Example 2 
     Preparation of 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-4-diene-17β-carboxylic acid (Compound (2)) 
       [0027]    A mixture of 50 g 6α,9α-difluoro-11β,17α-dihydroxy-16α-methyl-3-oxoandrosta-1,4-diene-17β-carboxylic acid obtained in Example 1, 30 g of triethylamine and 800 mL of acetone was stirred and cooled to between −5 and 0° C. 28 g of propionyl chloride was slowly added and stirred for 1 hour, and then 40 mL of diethylamine was added and stirred for 1 hour. The mixture was poured into an acid solution. A solid had precipitated out. The reaction mixture was filtered and the solid was dried to obtain the title compound with a purity of higher than 98%. 
       Example 3 
     Preparation of 17β-((N,N-dimethyl-carbamyl)-thio)formoxyl-6α,9α-difluoro-11β-hydroxyl-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene (Compound (3)) 
       [0028]    A mixture of 45.5 g of 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carboxylic acid, 1 L of butanone, 30.5 g of 4-dimethylamino-pyridine and 24.7 g of N,N-dimethyl thiocarbamoyl chloride was stirred at room temperature for 3 hours and cooled to between 10 and 15° C. 300 mL of DMA and 2 L of water were added. The mixture was cooled to between 0 and 5° C., filtered and washed with water. The resultant solid was dried to obtain the title compound with purity of more than 98.5%. 
       Example 4 
     Preparation of 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-thiocarboxylic acid (Compound (4)) 
       [0029]    A mixture of 20 g of 17β-((N,N-dimethyl-carbamyl)-thio)formoxyl-6α,9α-difluoro-11β-hydroxyl-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene, 12.7 g of tripotassium phosphate and 200 mL of methanol was stirred at room temperature for 3 hours under an atmosphere of nitrogen. 200 mL of water were added. The pH value was adjusted to between 1 and 2 with 2N aqueous hydrochloric acid at a temperature controlled below 20° C. The reaction mixture was filtered, and washed with water until the pH was normal. The resultant solid was dried to obtain the title compound with a purity of 98.6%. 
       Example 5 
     Preparation of S-fluoromethyl-6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-carbothioate (Compound (1), Fluticasone propionate) 
       [0030]    A mixture of 100 mL of acetone and 12.2 g of 4-dimethylamino-pyridine was cooled to between 0 and 5° C. 5.65 g of fluorobromomethane were added and the reaction mixture was stirred for 1 hour. 20 g of 6α,9α-difluoro-11β-hydroxy-16α-methyl-17α-propionyloxy-3-oxoandrosta-1,4-diene-17β-thiocarboxylic acid were added and the reaction mixture was stirred for 2 hours. The mixture was then poured into an acid solution, filtered and washed with water until its pH was neutral. The resultant precipitated solid (fluticasone propionate) was dried. The product was recrystallized from ethyl acetate and a high purity product was obtained. The purity of the product as determined by HPLC was 99.6%.