Abstract:
A process for the preparation of N-debenzoylpaclitaxel (I) through esterification of 7-protected baccatin III with a carboxylic acid reactive derivative of general formula (II), and elimination of the ester-protecting groups in acid conditions and in a single step. In formula (II) R1 is aryl or heteroaryl. The compound of formula (I) can be conveniently used for the preparation of paclitaxel and analogues

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of international application PCT/EP03/03017 filed on 24 Mar. 2003, which designated the United States of America. International application PCT/EP03/03017 claimed priority to earlier Italian application MI2002 A 000782 filed on 12 Apr. 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process for the preparation of N-debenzoylpaclitaxel (I) 
     
       
                 
         
             
             
         
      
     
     which is a useful precursor for known molecules having antitumor activity. 
     SUMMARY OF THE INVENTION 
     According to present invention, the derivative of formula (I) is obtained by condensation of an oxazolidine of general formula (II) or of a reactive derivative thereof 
     
       
                 
         
             
             
         
      
     
     in which R1 is an aryl or heteroaryl group, with a baccatin derivative of general formula (III) 
     
       
                 
         
             
             
         
      
     
     in which R2 is a hydroxy-protecting group removable by acid-catalyzed solvolysis, to give a compound of general formula (IV) 
     
       
                 
         
             
             
         
      
     
     in which R1 and R2 are as defined above, which compound is subjected to controlled acidic conditions to afford in a single step the compound of formula (I), an useful intermediate for the preparation of known antitumor compounds. 
     The present invention is advantageous over the synthetic processes of the prior art, in that:
         the oxazolidine of general formula (II) is surprisingly enriched in one of the epimers at C 2;   all the nitrogen- and oxygen-protecting groups are simultaneously removed by simple solvolysis;   the reaction conditions minimize the formation of isomerization or degradation products.       

     R1 is preferably phenyl or phenyl substituted with one or more C 1 –C 3  alkoxy, halogen, C 1 –C 3  alkyl, halogen-C 1 –C 2  alkyl groups. More preferably, R1 is 2,4-dimethoxyphenyl. 
     R2 can be any hydroxy-protecting group which can be removed by acid-catalyzed solvolysis. Examples of suitable protective groups are acetals (particularly methoxypropyl), alkoxycarbonyls (such as t-butoxycarbonyl), sulfenyl derivatives (such as 2-nitrobenzenesulfenyl). Particularly preferred is the protection with the 2-nitrobenzenesulfenyl group. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to present invention, the baccatin derivative of general formula (III) is esterified with an acid, salt or reactive derivative of general formula (II) in the presence of a condensing agent, for example a carbodiimide such as cyclohexylcarbodiimide or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and an activating agent such as 4-dimethylaminopyridine or N-methylimidazole, in organic solvents selected from ethers (particularly tetrahydrofuran), hydrocarbons (such as toluene or hexane), halogenated hydrocarbons (particularly dichloromethane), or mixtures thereof at temperatures ranging from 0 to 90° C. It is particularly advantageous to carry out the reaction in toluene and dichloromethane at a temperature of about 70° C. 
     Among the acid derivatives of formula (II), particularly preferred is the use of an ammonium salt of formula (V) 
     
       
                 
         
             
             
         
      
     
     wherein R1 is as defined above and R3, R4 and R5, which can be the same or different, are a C 1 –C 6  alkyl group, in particular ethyl, aryl or arylalkyl, preferably benzyl. The use of the ammonium salt provides less drastic reaction conditions and better stability of the products involved. 
     In principle, any activated carboxylic acid derivative (III), such as mixed anhydrides, acyl halides, pentafluorophenyl ester, thioesters, can be used in the process of the invention according to known procedures. 
     The oxygen- and nitrogen-protecting groups are removed in a single step by acid-catalyzed solvolysis, preferably by treatment with methanol and p-toluenesulfonic acid, at a temperature ranging from −20 to 50° C. 
     The acid of formula (II) can be obtained by hydrolysis of an ester of formula (VI) 
     
       
                 
         
             
             
         
      
     
     to give a salt of formula (VII) 
     
       
                 
         
             
             
         
      
     
     wherein M is a metal having y charge ranging from 1 to 2 and n is an integer always equal to y. 
     The hydrolysis is usually carried out in alkali medium by means of inorganic bases, such as metal hydroxides or metal carbonates, in a water-alcoholic medium at a temperature ranging from 0 to 40° C. 
     The triethylammonium salts of formula (VIII) 
     
       
                 
         
             
             
         
      
     
     can be obtained by treating the salts of formula (VII) with a triethylammonium chloride methanolic solution in a wide range of temperatures. 
     The ester (VI) in which R1 is 2,4-dimethoxyphenyl can be obtained by reacting 2,4-dimethoxybenzaldehyde dimethylacetal (X) with N-(2-nitrobenzenesulfenyl)-3-phenylisoserine of formula (IX) 
     
       
                 
         
             
             
         
      
     
     in an inert organic solvent, or in mixtures of inert organic solvents, in the presence of a mild acid catalyst such as pyridinium p-toluenesulfonate at a temperature ranging from 0° C. to the boiling temperature of the mixture. Suitable solvents are aromatic hydrocarbons. 
     The compound of formula (IX) can be prepared by reacting 3-phenylisoserine hydrochloride methyl ester with 2-nitrobenzenesulfenyl chloride in a diphasic mixture consisting of a water-immiscibile inert organic solvent (preferably ethyl acetate or dichloromethane) and an aqueous basic buffer (such as a sodium bicarbonate saturated solution) at temperatures ranging from 4 to 50° C. 
     7-(2-Nitrobenzenesulfenyl)-baccatin III can be easily prepared by reacting baccatin III with 2-nitrobenzenesulfenyl chloride in inert solvents, particularly ethers or halogenated hydrocarbons, in the presence of an organic or inorganic base, at temperatures ranging from −10 to 40° C. 
     The compounds:
     7-(2-nitrobenzenesulfenyl)-baccatin III;   13-[N-(2-nitrobenzenesulfenyl)-N,O-(2,4-dimethoxybenzylidene)-3-phenylisoserinoyl]-7-(2-nitrobenzenesulfenyl)-baccatin III;   2-(2,4-dimethoxyphenyl)-3-(2-nitrobenzenesulfenyl)-4-phenyl-5-oxazo-lidinecarboxylic acid and the salts and C1–C3 alkyl esters thereof, in particular the sodium and triethylammonium salts and the methyl ester;   N-(2-nitrobenzenesulfenyl)-3-phenylisoserine, are novel, useful intermediates and are a further object of the invention.   

     The following examples illustrate the invention in greater detail. 
     EXAMPLES 
     Example I 
     N-(2-nitrobenzenesulfenyl)-3-phenylisoserine 
     5 g of phenylisoserine methyl ester dissolved in 100 ml of ethyl acetate and 130 ml of a saturated NaHCO 3  solution are mixed in a 500-ml round-bottom flask. The diphasic mixture is kept under vigorous stirring and 5 g of 2-nitrobenzenesulfenyl chloride are added thereto in 30 minutes. The mixture is left under stirring for 30 min, then the organic phase is separated, dried over sodium sulfate and evaporated under reduced pressure. The residual yellow oil is purified by chromatography (silica,hexane-ethyl acetate, gradient 25 to 50% ethyl acetate) to give the desired product in 74% yield. 
     Example II 
     2-(2,4-dimethoxyphenyl)-3-(2-nitrobenzenesulfenyl)-4-phenyl-5-oxazolidinecarboxylic acid methyl ester 
     A hot solution of 6.6 g N-(2-nitrobenzenesulfenyl)-3-phenylisoserine in 100 ml of dry benzene is added with 0.5 g of pyridinium p-toluenesulfonate and 5.3 g of 2,4-dimethoxybenzaldehyde dimethylacetal. The solution is refluxed for 4 hours, then left to cool to room temperature. After that, 10 ml of a NaHCO 3  saturated solution are added and the phases are separated. The aqueous phase is extracted with ethyl acetate and the combined organic phases are dried over Na 2 SO 4  and evaporated under reduced pressure. The residual yellow oil is purified by chromatography (silica,hexane-ethyl acetate 5:1 with 2% triethylamine) to give the desired product in 74% yield. 
     Example III 
     Sodium 2-(2,4-dimethoxyphenyl)-3-(2-nitrobenzene-sulfenyl)-4-phenyl-5-oxazolidinecarboxylate 
     A solution of 5 g of 2-(2,4-dimethoxyphenyl)-3-(2-nitrobenzenesulfenyl)-4-phenyl-5-oxazolidinecarboxylic acid methyl ester in 150 ml of methanol are added with 22 ml of 2% sodium hydroxide. The mixture is refluxed for 1 hour. The solvent is distilled off and the residue is dried at 40° C. under vacuum overnight. 
     Example IV 
     Triethylammonium 2-(2,4-dimethoxvphenyl)-3-(2-nitrobenzenesulfenyl)-4-phenyl-5-oxazolidinecarboxylate 
     A solution of 13 mmol of the salt described above in 20 ml of dry methanol is added with 1.83 g of triethylammonium chloride. The mixture is kept under stirring for 3 hours, then diluted with 150 ml of toluene. The resulting suspension is filtered with suction and the mother liquors are evaporated to give the desired product in almost quantitative yield. The product is used without further purification. 
     Example V 
     7-(2-nitrobenzenesulfenyl)-baccatin III 
     8.8 g of baccatin III and 3.13 g of 2-nitrobenzenesulfenyl chloride are dissolved in 100 ml of dry methylene chloride in a 500-ml round-bottom flask. After cooling the solution at 0° C., 5 ml of pyridine are dropped therein at such a rate as to keep temperature below 5° C. The mixture is then kept under stirring at 0° C. for 30 min, then diluted with 50 ml of methylene chloride and washed with 5% NaHCO 3  and then with brine. After drying over magnesium sulfate, the organic phase is evaporated under reduced pressure. The resulting crude is purified by chromatography (silica,hexane-ethyl acetate 6:4) to give 5.4 g of the desired product. 
     Example VI 
     13-[N-(2-nitrobenzenesulfenyl)-N,O-(2,4-dimethoxy-benzylidene)-3-phenylisoserinoyl]-7-(2-nitrobenzenesulfenyl)-baccatin III 
     A mixture of 2.9 g of 7-(2-nitrobenzenesulfenyl)-baccatin III, 2.9 g of triethylammonium 2-(2,4-dimethoxyphenyl)-3-(2-nitrobenzenesulfenyl)-4-phenyl-5-oxazolidinecarboxylate in 15 ml of methylene chloride and 30 ml of dry toluene, is added with 1.5 g of dicyclohexylcarbodiimide and 0.24 g of 4-dimethylaminopyridine. The reaction mixture is refluxed for 2 hours, then left under stirring at room temperature overnight. The organic phase is filtered with suction, then washed with 30 ml of saturated sodium hydrogen carbonate, then with brine and evaporated under reduced pressure. The residue is purified by chromatography (silica,hexane-ethyl acetate 7:3) to give the desired product in 75% yield. 
     Example VII 
     N-debenzoylpaclitaxel 
     4.4 g of 13-[N-(2-nitrobenzenesulfenyl)-N,O-(2,4-dimethoxybenzylidene)-3-phenylisoserinoyl]-7-(2-nitrobenzenesulfenyl)-baccatin III and 1.4 g of p-toluenesulfonic acid are dissolved in 15 ml of dry methanol at 0° C. The solution is left under stirring at 0° C. for 3-8 hours. The reaction is monitored by TLC. 15 ml of a sodium bicarbonate saturated solution are then added, the solvent is evaporated off and the residue is dissolved in ethyl acetate. The organic layer is washed with water and dried over Na 2 SO 4 . Evaporation of the solvent and purification on a column (methylene chloride-methanol 95:5) give the desired product in 80% yield. 
     Example VIII 
     N-debenzoylplaclitaxel 
     4.4 g of 13-[N-2-nitrobenzenesulfenyl)-N,O-(2,4-dimethoxybenzylidene)-3 -phenylisoserinoyl]-7-2-nitrobenzenesulfenyl)-baccatin III was dissolved in a mixture of CH 2 Cl 2  (16 mL), MeOH (8 ml) and Pyridine (4 ml). To the solution was added dropwise 12 N HCl (2 ml) and the reaction was left under stirring for an hour. After an hour, additional 3.4 ml of 12 N HCl were added dropwise and the reaction mixture was kept under stirring at 35° C. for another hour. CH 2 Cl 2  (25 ml) was added to the solution and a washing of the organic solution with HCl 1 M (1×20 ml) was performed. 
     The organic layer was evaporated to dryness and the residue, redissolved in CH 2 Cl 2  (30 ml), was precipitated by pouring into 500 ml of toluene. After filtration and dryness under vacuum at 50° C., 2.3 g of N-debenzoyl placitaxel as hydrochloride salt were obtained (yeld: 80%).