Process for the preparation of abiraterone or abiraterone acetate

Abstract

The present invention relates to a novel process for the synthesis of abiraterone and in particular abiraterone acetate, a compound of formula (I) reported below: having pharmacological activity suitable for slowing down the progression of advanced stage prostate cancer. The process is characterized by the fact that the intermediate triflation step is carried out on prasterone (DHEA) or its 3-acetate using ArN(OTf).sub.2 as the triflation reagent, but where Ar is not phenyl, and by the fact that the base used in this step is an alkali metal alcoholate. ##STR00001##

Claims

1. A process for preparing abiraterone or abiraterone acetate of formula I: ##STR00012## wherein X is H or CH.sub.3C(O), comprising: (i) reacting prasterone or prasterone acetate of formula Ill: ##STR00013## wherein X is as above, with an aromatic bis (trifluoromethane sulphonamide) of formula ArN(Tf).sub.2, wherein Ar is an aromatic radical other than phenyl, and N(Tf).sub.2 is: ##STR00014## in the presence of an alcoholate of an alkali metal, to form: ##STR00015## wherein X is as above, and (ii) reacting the compound of formula II with diethylpyridylborane in the presence of a palladium(II) catalyst, to form abiraterone or abiraterone acetate.

2. The process of claim 1, wherein said aromatic bis(trifluoromethanesulfonimide) has formula (IV) or (V): ##STR00016## wherein: R1, R2, R3, R4 and R5, independently, may be hydrogen, halo, NO.sub.2, a linear or branched alkyl radical, an amide radical RC(O)NH or an alkoxy radical RO, wherein R is a linear or branched alkyl group with the proviso that that R1, R2, R3, R4 and R5 cannot be all hydrogen atoms at the same time; and one of R6, R7, R8, R9 and R10 in formula (V) is -(Tf).sub.2, wherein the remaining radicals R6, R7, R8, R9, and R10, independently, have the same meanings as do R1, R2, R3, R4, and R5.

3. The process according to claim 2, wherein said bis(trifluoromethanesulfonimide) is N-(2-pyridyl)-bis(trifluoromethanesulfonimide).

4. The process according to claim 1, wherein said bis(trifluoromethanesulfonimide) is used in an amount ranging from 0.8 to 2 times the weight of prasterone or prasterone acetate.

5. The process according to claim 1, wherein said base is selected from the group consisting of sodium tert-butylate and potassium ten-butylate.

6. The process according to claim 1, wherein (i) is carried out at a reaction temperature between 80 C. and 30 C. for a reaction time of from 2 to 24 hours.

7. The process according to claim 1, wherein the palladium (II) catalyst is bis(triphenylphosphine)palladium(II)dichloride, Pd(PPh.sub.3).sub.2Cl.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a chromatogram, along with the peak listing thereof, of the product obtained according to the process of the invention;

(2) FIGS. 2, 3 and 4 show chromatograms, along with the peak listings thereof, of products obtained according to processes of the prior art.

DETAILED DESCRIPTION OF THE INVENTION

(3) The present invention relates to a novel process for the production of abiraterone or abiraterone acetate wherein is fundamental the reaction of the first step, the reaction of prasterone or prasterone acetate (III) with an aromatic bis(trifluoromethanesulfonimide) of general formula ArN(Tf).sub.2 in the presence of a base selected from alkali alcoholates to obtain the intermediate of formula (II); the bis(trifluoromethanesulfonimides) are commonly known in the art with the abbreviation triflimides. The thus obtained intermediate (II) is then further reacted in the second step of the process into the desired product.

(4) The aromatic radical of triflimide ArN(Tf).sub.2 used in the first step may be of any type, excluding the unsubstituted benzene ring; for example, it may be a mono- or polycyclic aromatic radical, of the hydrocarbon, heterocyclic or mixed type, made from a heterocyclic ring fused to a hydrocarbon ring, which in this latter case may also be saturated; the Ar radical may also be substituted. The preferred triflimides for the purposes of the invention are those corresponding to the general formulas (IV) or (V) reported below

(5) ##STR00010##
wherein: R1, R2, R3, R4 and R5, independently, may be hydrogen, halo, NO.sub.2, a linear or branched alkyl radical, an amide radical RC(O)NH or an alkoxy radical RO, wherein R is a linear or branched alkyl group, provided that R1, R2, R3, R4 and R5 cannot be all hydrogen atoms at the same time;
and one of R6, R7, R8, R9 and R10 is the radical N(Tf).sub.2, while the remaining radicals among R6 and R10, independently, have the same meanings reported hereinabove for radicals R1-R5.

(6) Preferably ArN(Tf).sub.2 is N-(2-pyridyl)-bis(trifluoromethanesulfonimide).

(7) The amount of triflimide is between 0.8 and 2 times the weight of the starting prasterone acetate.

(8) The reaction solvent is selected from toluene, xylene, diethyl ether, methyl tertbutyl ether, tetrahydrofuran (THF), methyl tetrahydrofuran, chloroform, dichloromethane and 1,2-dichloroethane. Preferred solvents are ethers, for example tetrahydrofuran.

(9) The base that may be used is an alcoholate of an alkali metal, preferably sodium tert-butylate or potassium tert-butylate.

(10) The reaction temperature is between 80 C. and 30 C., while the reaction time is between 2 and 24 hours.

(11) Once the intermediate of formula (II) is obtained, this may be transformed into abiraterone or abiraterone acetate (I) in the second step of the process, reacting the mixture resulting from the first step described hereinabove with diethylpyridylborane in the presence of a palladium (II) catalyst such as, for example, bis(triphenylphosphine)palladium(II)dichloride, Pd(PPh.sub.3).sub.2Cl.sub.2.

(12) The formed abiraterone acetate is separated from the reaction mixture by means of salification with an acid; then, the abiraterone acetate is recovered by treatment with an aqueous base, and the product obtained is purified by known methods, such as solvent-based crystallisation or silica gel chromatography.

(13) The invention is further illustrated by the following examples, reported as illustrating and non-limiting examples of the present invention.

(14) The reactants used in the examples are of common commercial availability and are employed without the need of further purifications.

(15) For the analytic controls by means of thin-layer chromatography (TLC) are used silica gel TLC plates 60 F.sub.254 on aluminium sheet or silica gel HPTLC 60 F.sub.254 with a concentration zone, from Merck.

(16) The HPLC chromatograms are recorded with an Agilent 1200 model chromatograph by elution with a gradient of methanol/water from 85/15 to 100/0 on C18 analytical column; 150 mm4.6 mm; 2.7 m. The detector is UV (=220 nm). The RRT values reported in the examples for some impurities indicate the relative retention times and represent the retention time of the peak corresponding to the specific impurity divided by the retention time of the peak corresponding to the desired product.

(17) The HPLC-MS chromatograms are recorded with an Agilent 1100 model chromatograph coupled to an API 2000 model mass spectrometer from Applied Biosystem. The ionisation of the sample is obtained by acidification of the mobile phase with formic acid (chemical ionisation).

(18) All the concentrations of solutions indicated with a percentage value shall be considered as weight values, unless otherwise stated.

EXAMPLE 1

(19) This example illustrates a preparation of abiraterone acetate (I) starting from prasterone acetate (III) according to the invention.

(20) A solution of 25.3 g of potassium tert-butylate and THF (1 l) is stirred for 30 minutes keeping the temperature at below 70 C. Under stirring prasterone acetate (50 g) is added. Then, N-(2-pyridyl)-bis(trifluoromethanesulfonimide) (65 g) is added portion-wise and is kept under stirring at a temperature between 70 and 80 C. for 2 hours. The cold solution is poured onto a biphasic solution consisting of isopropyl acetate (1 l) and a 25% ammonium chloride aqueous solution (800 ml).

(21) The phases are separated and the organic phase is washed with 800 ml of 25% sodium acetate aqueous solution and then with an aqueous solution of sodium chloride (800 ml). A part of the solvent is distilled off at reduced pressure obtaining the precipitation of a white solid which is eliminated by filtration. The residual solution is concentrated to oil still at reduced pressure. The oil thus obtained is crystallized from the mixture methanol/triethylamine. The obtained sample (49 g) verified in HPLC (=220 nm) against authentic sample is intermediate (II) with 98.7% purity.

(22) 48 g of intermediate (II) is dissolved in THF (980 ml), then bis(triphenylphosphinepalladium(II)dichloride Pd(PPh.sub.3).sub.2Cl.sub.2 (1.46 g), diethyl(pyridyl)borane (23 g) and an aqueous solution of sodium carbonate (240 ml, 43.5 g sodium carbonate) are added under stirring at 20-25 C. Reflux is kept for 2 hours, further 370 mg of bis(triphenylphosphinepalladium(II)dichloride Pd(PPh.sub.3).sub.2Cl.sub.2 is added and the reflux is continued for further 45 minutes. Transformation is complete (TLC check).

(23) Cooling down to 20-25 C. is performed and isopropyl acetate (1 l) and water (850 ml) are added.

(24) The phases are separated, the organic phase is concentrated at reduced P after filtration and washing with water. A dark oil (62.4 g) is obtained which is then dissolved in methanol (180 ml). The solid obtained is filtered which turns out to be unreacted diethyl(pyridyl)borane.

(25) The methanolic solution is concentrated at reduced P and the residue re-dissolved in isopropyl acetate (315 ml). The solution, pre-cooled down at a temperature between 0 and 5 C., is then treated with oxalic acid dihydrate (26 g). Stirring is performed at a temperature between 0 and 5 C. for 1 hour then the solid is filtered and washed with isopropyl acetate.

(26) The abiraterone acetate oxalate thus obtained is stirred at a temperature between 0 and 5 C. with methylene chloride (300 ml) and an aqueous solution of sodium bicarbonate (500 ml, 40 g) obtaining a complete solution.

(27) The phases are separated and the organic phase is concentrated to dry product at reduced P.

(28) The solid obtained is crystallized from methanol obtaining, after drying, 20.5 g of abiraterone acetate (RT=7.021, 99.85% HPLC purity, =220 nm).

(29) All the impurities detectable by the HPLC chromatogram reported in FIG. 1 have a % area lower than 0.05. The presence of abiraterone, which is considered an impurity of abiraterone acetate by the European Pharmacopoeia, is not detectable.

EXAMPLE 2

(30) This example illustrates a preparation of abiraterone acetate (I) on an industrial scale starting from prasterone acetate (III) according to the invention.

(31) 51 kg of potassium tert-butylate in THF (760 kg) and prasterone acetate (100 kg) are stirred at a temperature below 70 C. Then N-(2-pyridyl)-bis(trifluoromethanesulfonimide) (133 kg) is added maintaining the stirring for 2 hours at a temperature between 70 and 80 C.

(32) The cold solution is poured onto a biphasic solution consisting of isopropyl acetate (360 kg) and a 10% ammonium chloride aqueous solution.

(33) The phases are separated and the organic phase is first washed with an aqueous solution of ammonium chloride, then with 450 kg 10% sodium acetate aqueous solution, and eventually with an aqueous solution of sodium chloride.

(34) A part of the solvent is distilled off at reduced pressure obtaining the precipitation of a solid which is eliminated by filtration. The residual solution is then distilled until obtaining an oil which is crystallized from the mixture methanol/triethylamine.

(35) The sample obtained after drying (112 kg) verified in HPLC (=220 nm) against an authentic sample is intermediate (II) with 98.14% titer.

(36) 112 kg of intermediate (II) is dissolved in THF (1079 kg), then bis(triphenylphosphinepalladium(II)dichloride Pd(PPh.sub.3).sub.2Cl.sub.2 (3.2 kg), diethyl(pyridyl)borane (129.3 kg) and an aqueous solution of sodium carbonate are added under stirring at 20-25 C.

(37) Reflux is kept for 1 hour (TLC check), further 400 g of bis(triphenylphosphinepalladium(II)dichloride Pd(PPh.sub.3).sub.2Cl.sub.2 is added, obtaining complete transformation (TLC check) after further 30 minutes of reaction.

(38) Cooling down to 20-25 C. is performed, the phases are separated by washing the organic phase with an aqueous solution of sodium chloride.

(39) The organic phase is then distilled until obtaining a dark oil which is then solubilized with methanol, recovering by fractional crystallization the excess diethyl(pyridyl)borane.

(40) Methanol is eliminated by distillation, the residue is dissolved in isopropyl acetate, then the solution is filtered after treatment with decolorising carbon and silica gel.

(41) The solution, adjusted at T=205 C., is then treated with oxalic acid dihydrate (60 kg).

(42) Stirring is performed at T=205 C. for 8 hours and then the solid is filtered and washed with isopropyl acetate.

(43) The abiraterone acetate oxalate obtained is stirred at a temperature between 0 and 5 C. with methylene chloride (880 kg) and an aqueous solution of sodium bicarbonate.

(44) The phases are separated and the organic phase is distilled.

(45) The solid obtained is dissolved in isopropyl acetate, then treated with Quadrasil (registered trademark of Johnson Matthey Finland Oy) for 6 hours, for the removal of the catalyst; the Quadrasil scavengers, sold by Sigma-Aldrich, consist of porous silica beads having defined pore size, wherein the silica surface is functionalised with metal binders, and allow a quick and effective removal of traces of metals from aqueous or organic solutions.

(46) After filtration, a part of the solvent is distilled off, cooling down to 05 C. is performed obtaining the crystallisation of the product.

(47) The abiraterone acetate obtained after drying (65 kg) meets the specifications reported in the corresponding chapter of the European Pharmacopoeia.

EXAMPLE 3 (Comparative)

(48) This example illustrates the preparation of abiraterone acetate from prasterone acetate, wherein the triflation reaction is carried out according to procedures derived from the article Steroids, 75 (2010) 936-943.

(49) A solution obtained dissolving 50 g of prasterone acetate and 64.9 g of N-phenyl-bis(trifluoromethanesulfonimide) (Ph-N(Tf).sub.2) in 750 ml tetrahydrofuran is cooled down to 78 C. under stirring. 303 ml of a 0.5 M solution of potassium hexamethyldisilazane in toluene are slowly added and maintained under stirring for 2 hours at a temperature between 80 and 70 C. The reaction mixture is then brought to a temperature between 0 and 5 C. and maintained as such for additional 30 minutes.

(50) 750 ml isopropyl acetate and 1152 ml aqueous solution saturated with ammonium chloride are added. The phases are separated and the organic phase is washed with 1600 ml 1M HCl aqueous solution and with 1600 ml aqueous solution saturated with NaCl.

(51) The solvent is eliminated at reduced pressure obtaining a dark oil (132.7 g).

(52) The raw oil obtained is then crystallized from ethanol (312 ml) obtaining, after drying, 64.8 g of solid product.

(53) 60 g of such product, dissolved in 650 ml tetrahydrofuran, are added, under stirring and at a temperature between 20 and 25 C., with bis(triphenylphosphine)palladium(II)dichloride (Pd(PPh.sub.3).sub.2Cl.sub.2, 637 mg), diethyl(pyridyl)borane (20 g) and 179 ml 2 M aqueous solution of sodium carbonate. Reflux is kept at about 70 C. for 2 hours. Cooling is performed down to a temperature between 20 and 25 C. and isopropyl acetate (660 ml) and water (660 ml) are added.

(54) The phases are separated, the organic phase is washed with 20% NaCl aqueous solution, filtered with decolorising carbon and concentrated at reduced pressure.

(55) The product obtained is then dissolved in methanol at 45 C. (200 ml) eliminating by filtration the reprecipitated solid which turns out to be unreacted diethyl(pyridyl)borane; at a HPLC control, the product shows a purity of 99.1% (chromatogram recorded at 220 nm). The methanolic solution is concentrated at reduced pressure.

(56) The residue, controlled by HPLC, shows, inter alia, the presence of an impurity with RRT=1.257; this residue is solubilized again in isopropyl acetate at 45 C. (164 ml) eliminating by filtration the undissolved part.

(57) The solution is concentrated at a final volume of 120 ml, and is cooled down to 0<T<5 C. 12.6 g of phosphoric acid is added by cold stirring for 3 hours. The solid thus formed is filtered and solubilized with 350 ml of basic aqueous solution (5% NaHCO.sub.3) and 400 ml of methylene chloride (DCM). Stirring at a temperature between 0 and 5 C. is performed for 2 hours, then it is checked that the pH of the aqueous phase is basic (pH of about 8). The phases are separated and the organic phase, after washing with water, is anhydrified and concentrated to dry product at reduced pressure.

(58) The obtained solid (26.4 g) is repeatedly crystallized from methyl isobutyl ketone obtaining, after drying, 9 g of abiraterone acetate.

(59) The product controlled by means of HPLC shows a purity of 99.1% (chromatogram shown in FIG. 2, recorded at 220 nm), and as the main impurity a peak at RRT=1.257 having area 0.375%.

(60) The HPLC-Mass analysis indicates that such impurity has a molecular weight of 405 amu, in accordance with the following structure:

(61) ##STR00011##

(62) In order to try and eliminate the impurity shown hereinabove, the sample of abiraterone acetate is further crystallized from methanol. The product obtained, controlled by means of HPLC, shows a purity of 99.4% (chromatogram shown in FIG. 3, recorded at 220 nm).

(63) The impurity with RRT=1.257 retains a % area of 0.363, almost unchanged with respect to the value before purification.

EXAMPLE 4 (Comparative)

(64) This example illustrates a complete preparation of abiraterone acetate starting from prasterone acetate using as a base potassium hexamethyldisilazane according to the description of Steroids, 75 (2010) 936-943.

(65) A solution of 18.11 g potassium hexamethyldisilazane and THF (400 ml) is stirred for 30 minutes keeping the temperature at below 70 C. Under stirring prasterone acetate (20 g) is added. Then, N-(2-pyridyl)-bis(trifluoromethanesulfonimide) (23.85 g) is added portion-wise and is kept under stirring at a temperature between 70 and 80 C. for about 2 hours. The cold solution is poured onto a biphasic solution consisting of isopropyl acetate (800 ml) and a 25% ammonium chloride aqueous solution (800 ml).

(66) The phases are separated and the organic phase is washed with 800 ml of 25% sodium acetate aqueous solution and then with an aqueous solution of sodium chloride (800 ml).

(67) The organic solution is concentrated to oil still at reduced pressure. The oil thus obtained is treated with 84 ml heptane obtaining the precipitation of a solid which is eliminated by filtration. The organic part is then concentrated at reduced pressure and the residue is crystallized from the mixture ethanol/triethylamine.

(68) The sample obtained (21 g) verified by HPLC (=220 nm) against authentic sample is intermediate (II).

(69) 20.7 g of such intermediate (II) are dissolved in THF (230 ml), then bis(triphenylphosphinepalladium(II)dichloride (Pd(PPh.sub.3).sub.2Cl.sub.2, 283 mg), diethyl(pyridyl)borane (8.9 g) and an aqueous solution of sodium carbonate (80 ml, 16.8 g of sodium carbonate) are added under stirring at 20-25 C. After 2 hours, further 35 mg of Pd(PPh.sub.3).sub.2Cl.sub.2 are added.

(70) Reflux is kept until complete transformation (TLC check). Cooling down to 20-25 C. is performed and isopropyl acetate (500 ml) and water (250 ml) are added.

(71) The phases are separated, the organic phase is concentrated at reduced P after filtration and washing with water. A dark oil (20.5 g) is obtained which is then dissolved in methanol. The solid obtained is filtered which turns out to be unreacted diethyl(pyridyl)borane.

(72) The methanolic solution is concentrated at reduced P and the residue re-dissolved in methyl isobutyl ketone (80 ml). The solution, pre-cooled down at a temperature between 0 and 5 C., is then treated with oxalic acid dihydrate (7 g).

(73) Stirring is performed at a temperature between 0 and 5 C. for 1 hour and then the solid is filtered and washed with methyl isobutyl ketone.

(74) The obtained abiraterone acetate oxalate is stirred at a temperature between 0 and 5 C. with methylene chloride (150 ml) and an aqueous solution of sodium bicarbonate (150 ml, 20 g) obtaining a complete solution. The phases are separated and the organic phase is concentrated to dry product at reduced P. The solid obtained is crystallized several times from methyl isobutyl ketone obtaining, after drying, 7.5 g abiraterone acetate (98.6% HPLC purity recorded at =220 nm).

(75) The HPLC chromatographic profile (chromatogram shown in FIG. 4) shows a number of impurities with % area greater than 0.10, among which the highest (RRT=2.98) having % area of 0.48.

(76) Comment on Results

(77) As shown by the test results, operating according to the method of the invention, abiraterone acetate having high purity is obtained which is suitable for the envisaged pharmaceutical usage. Vice versa, operating according to the procedures which may be derived from the cited article, Steroids, 75 (2010) 936-943, such result is not achieved.