Process for preparing sildenafil

Abstract: A process for the preparation of a compound of formula (I): also known as sildenafil which comprises cyclisation of a compound of formula (II): wherein the cyclisation is conducted under neutral or basic conditions.

Description: The invention relates to a process for the preparation of the compound of formula (I): known as 5-[2-ethoxy-5-(4-methylpiperazin-1-ylsulphonyl)phenyl]-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin-7-one or sildenafil, and also to intermediates used therein. Sildenafil, which was originally disclosed in EP-A-0463756, has been found to be particularly useful in the treatment of, inter alia, male erectile dysfunction: see WO-A-94/28902. More specifically, the invention concerns a process for the preparation of sildenafil which is more efficient than that disclosed in EP-A-0463756 and which, surprisingly, can provide directly sildenafil of clinical quality standard, thus obviating the need for subsequent purification steps. In this context, sildenafil of clinical quality standard means material of sufficient purity for administration to humans. The key step in the over-all process involves the ring-closure of the immediate precursor to sildenafil, i.e. the bis-amide of formula (II): Thus the invention provides a process for the preparation of a compound of formula (I) which comprises cyclisation of a compound of formula (II). An analogous cydisation, which involves the use of polyphosphoric acid at 140°C, is disclosed in J. Med. Chem., 1987, 30 , 91. The compound of formula (I) may be isolated and purified by conventional techniques. For example, when (I) is produced in the form of a salt, by neutralisation of the optionally prediluted reaction mixture, followed by collection of the product by filtration/extraction and optional crystallisation thereof. Alternatively, the compound of formula (I) may be conveniently isolated and/or purified by standard chromatographic procedures. The compound of formula (II) required for the preparation of the compound of formula (I) may be obtained by the route depicted in the following reaction scheme using conventional procedures. Thus the compound of formula (IV) may be prepared by chlorosulphonylation of 2-ethoxybenzoic acid, i.e. the compound of formula (III). Typically, (III) is added to an ice-cooled mixture of about 1 mol. equiv. of thionyl chloride and about 4 mol. equivs. of chlorosulphonic acid, whilst maintaining the reaction temperature below 25°C; the reaction is then allowed to continue at room temperature until complete. Conversion of (IV) to the compound of formula (V) is achieved by N-sulphonylation of 1-methylpiperazine and may be conducted in a one-step or two-step procedure. In a one-step procedure, about 2.3 mol. equivs. of 1-methylpiperazine are added to an aqueous suspension of (IV) at about 10°C, whilst maintaining the reaction temperature below 20°C; the temperature of the resulting reaction mixture is then held at about 10°C. Alternatively, the quantity of 1-methylpiperazine can be reduced to about 1.1 mol. equiv. by employing about 1 mol. equiv. of sodium hydroxide as auxiliary base. In a two-step procedure, a solution of (IV) in a suitable solvent, e.g. acetone, is added to a mixture of about a 10% excess of 1-methylpiperazine and about a 10% excess of a suitable acid acceptor, e.g. a tertiary base such as triethylamine, whilst maintaining the reaction temperature below 20°C. When triethylamine is employed as auxiliary base, an intermediate hydrochloride-triethylamine double salt of (V), identified as the compound of formula (VA), is isolated. This salt may be transformed to (V) by treatment with water. A convenient alternative route to (V) is to employ a C 1 -C 4 alkyl 2-ethoxybenzoate (obtained by conventional esterification of (III)) as the chlorosulphonylation substrate, followed by treatment of the resulting sulphonyl chloride with 1-methylpiperazine as described above, then subsequent standard hydrolysis of the ester group. Other synthetic options for obtaining (V) from salicylic acid and its derivatives will be apparent to the person skilled in the art. Coupling of (V) with the compound of formula (VII) may be achieved by any of the plethora of amide bond-forming reactions well known to those skilled in the art. For example, the carboxylic acid function of (V) is first of all activated using about a 5% excess of a reagent such as N,N'- carbonyldiimidazole in a suitable solvent, e.g. ethyl acetate, at from about room temperature to about 80°C, followed by reaction of the intermediate imidazolide with (VII) at from about 20 to about 60°C. The aminopyrazole (VII) is obtainable by conventional reduction of the corresponding nitropyrazole (VI), e.g. using palladium-catalysed hydrogenation in a suitable solvent such as ethyl acetate. The resulting solution of (VII) may be used directly, after filtration, in the coupling reaction with (V). The cyclisation reaction of (II) to provide the compound of formula (I) has been achieved in yields of up to 95%. Thus the over-all yield of (I) based on the benzoic acid derivative (III) as starting material, depending on whether the one-step or two-step sulphonylation procedure is used, can be as high as 51.7% or 47.8% respectively. This compares very favourably with the process disclosed in EP-A-0463756 in which the over-all yield of (I) from 2-ethoxybenzoyl chloride (and thus from (III) also, assuming that the acid chloride derivative can be generated quantitatively therefrom) is 27.6%. In an alternative comparison, the over-all yield of (I) based on the nitropyrazole (VI) can be as high as 85.2% in the presently disclosed process whilst, in the process disclosed in EP-A-0463756, the over-all yield of (I) from (VI) is 23.1%. Clearly then, the alternative process to (I) disclosed hereinbefore can be considerably more efficient and advantageous than that previously disclosed, and the intermediates of formulae (II), (V) and (VA) also form part of the invention. Alternatively, the cyclisation of a compound of formula (II) to a compound of formula (I) may be conducted under neutral or acidic conditions. Under neutral conditions, the compound of formula (II) is heated, optionally in the presence of a solvent and/or optionally in the presence of a dehydrating agent and/or mechanical water-removal system, e.g. a Dean-Stark apparatus. A suitable solvent may be selected from the group consisting of 1,2-dichlorobenzene, dimethyl sulphoxide, sulpholane, N-methylpyrrolidin-2-one and pyrrolidin-2-one, and mixtures thereof. Preferably the solvent is 1,2-dichlorobenzene, sulpholane or N-methylpyrrolidin-2-one. A suitable dehydrating agent may be selected from the group consisting of anhydrous potassium carbonate, anhydrous sodium carbonate, anhydrous magnesium sulphate, anhydrous sodium sulphate, phosphorus pentoxide and molecular sieves. Preferably the dehydrating agent is molecular sieves. Preferably also, the reaction is carried out at from 180 to 220°C for from 0.5 to 72 hours. Under acidic conditions, the cyclisation is carried out by reaction of a compound of formula (II) with a protic acid or Lewis acid, optionally in the presence of a solvent. A suitable protic acid may be selected from the group consisting of an inorganic acid, an organo-sulphonic acid, an organo-phosphonic acid and an organo-carboxylic acid. Preferably the protic acid is concentrated sulphuric acid, phosphoric acid or p -toluenesulphonic acid. A suitable Lewis acid may be selected from the group consisting of boron trifluoride, boron trichloride, boron tribromide, aluminium chloride, aluminium bromide, silicon tetrachloride, silicon tetrabromide, stannic chloride, stannic bromide, phosphorus pentachloride, phosphorus pentabromide, titanium tetrafluoride, titanium tetrachloride, titanium tetrabromide, ferric chloride, zinc fluoride, zinc chloride, zinc bromide, zinc iodide, mercuric chloride, mercuric bromide and mercuric iodide. Preferably the Lewis acid is boron trifluoride, aluminium chloride, silicon tetrachloride, stannic chloride, titanium tetrachloride, ferric chloride or zinc chloride. A suitable solvent may be selected from the group consisting of a C 5 -C 12 alkane, a C 5 -C 8 cycloalkane, a C 1 -C 12 alkanoic acid, a C 1 -C 4 alkanol, a C 3 -C 9 alkanone, a C 5 -C 12 alkyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, diglyme, tetrahydrofuran, 1,4-dioxan, benzene, toluene, xylene, chlorobenzene, dichlorobenzene, nitrobenzene, dichloromethane, dibromomethane, 1,2-dichloroethane, acetonitrile, dimethyl sulphoxide, sulpholane, dimethylformamide, dimethylacetamide, N-methylpyrrolidin-2-one and pyrrolidin-2-one, and mixtures thereof. Preferably the solvent is glacial acetic acid, tetrahydrofuran, 1,4-dioxan or chlorobenzene. Preferably also, the reaction is carried out at from 65 to 210°C for from 6 to 300 hours. The syntheses of the compound of formula (I) and the intermediates thereto are described in the following Examples and Preparations. In cases where the compound of formula (I) was not isolated and (if necessary) purified, the yields thereof were determined, and reaction mixtures analysed, by quantitative thin layer chromatography (TLC), using Merck silica gel 60 plates and toluene:methylated spirit:0880 aqueous ammonia mixtures as solvent systems, and/or high performance liquid chromatography (HPLC), using Gilson equipment with a 15 cm reverse phase C18 column and triethylamine:phosphoric acid buffer in aqueous acetonitrile:methanol mixtures as mobile phases. 1 H Nuclear magnetic resonance (NMR) spectra were recorded using a Varian Unity 300 spectrometer and were in all cases consistent with the proposed structures. Characteristic chemical shifts (δ) are given in parts-per-million downfield from tetramethylsilane using conventional abbreviations for designation of significant peaks: e.g. s, singlet; d, doublet; t, triplet; q, quartet; h, hextet; m, multiplet; br, broad. Room temperature means 20-25°C. TITLE COMPOUND 5-[2-Ethoxy-5-(4-methylpiperazin-1-ylsulphonyl)phenyl]-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin-7 -one EXAMPLE 37 The title compound of Preparation 4 (12.32 g, 0.025 mol) was heated at 215-220°C for 40 minutes and the resulting melt allowed to cool to room temperature. The tarry crude product was dissolved in dichloromethane (25 ml) and then purified by chromatography on silica gel, using increasingly polar mixtures of methanol in dichloromethane as eluant. Evaporation under vacuum of the appropriate single component fractions provided the pure (by 1 H nmr analysis) title compound (1.76 g, 14.8%), whilst a batch of less pure title compound (0.87 g, 7.3 %) was obtained from further fractions. Further chromatographic processing of the latter gave an additional quantity (0.48 g) of pure title compound, the total yield being 2.24 g, 18.8%. EXAMPLES 38-40 A stirred mixture of the title compound of Preparation 4 (12.32 g, 0.025 mol) and 1,2-dichlorobenzene (61 ml) was heated under reflux for 72 hours. The resulting dark brown reaction mixture was allowed to cool, diluted with dichloromethane (60 ml) and filtered. Evaporation under reduced pressure of the filtrate gave a solvent-containing dark brown oil (17.51 g), 28.2% of the solvent-excluded material of which was shown, by TLC and HPLC analyses, to be the title compound. Repetition using sulpholane as solvent at about 205°C for 5 hours provided a crude product (14% wt. yield) which, by TLC and HPLC analyses, contained an estimated yield of 12% of the title compound. Further repetition using N-methylpyrrolidin-2-one as solvent at 205-210°C for 3 hours provided a crude product (21.5% wt. yield) which, by TLC and HPLC analyses, contained an estimated yield of 6.5% of the title compound. EXAMPLE 41 Under reaction conditions similar to those described in Example 38, except that the reaction was conducted for 24 hours in the presence of 4Å molecular sieves, a solvent-containing product was obtained, 6.0% of the solvent-excluded material of which was shown, by HPLC analysis, to be the title compound. EXAMPLE 42 Concentrated sulphuric acid (1.0 ml, 1.84 g, 18.75 mmol) was added to a stirred suspension of the title compound of Preparation 4 (12.32 g, 0.025 mol) in chlorobenzene (61 ml) and then the resulting mixture heated until solvent began to distil out. When the distillate was no longer cloudy (after collection of ca . 20 ml), the reaction mixture was allowed to cool to room temperature and a further quantity (20 ml) of chlorobenzene added before it was heated under reflux for 20 hours. The cool reaction mixture was treated with dichloromethane (100 ml) to form a solution, followed by water (100 ml). The pH of the resulting mixture was adjusted to 7 using 5M aqueous sodium hydroxide solution, then the organic phase separated, combined with a dichloromethane extract (50 ml) of the aqueous phase and evaporated under reduced pressure to furnish a solid (9.51 g), 5.5% of which was shown, by HPLC analysis, to be the title compound. EXAMPLE 43 Concentrated sulphuric acid (1.0 ml, 1.84g, 18.75 mmol) was added to a stirred solution of the title compound of Preparation 4 (6.16g, 12.5 mmol) in glacial acetic acid (31 ml) and the resulting mixture heated at 100°C for 115 hours. The solvent was removed by evaporation under reduced pressure, the residue "azeotroped" with toluene (2 x 50 ml) and the resulting oil (10.5 g) shaken with water (60 ml) to give a crystalline solid which was collected, washed with water (10 ml) and dried. This crop (2.03 g) was combined with a second crop (3.48g) obtained by neutralisation of the filtrate with 20% aqueous sodium hydroxide solution, followed by collection, washing and drying as before, to provide the crude product (5.51 g) which, by TLC and HPLC analyses, contained an estimated yield of 38% of the title compound. EXAMPLE 44 A stirred mixture of the title compound of Preparation 4 (6.16 g, 12.5 mmol) and glacial acetic acid (31 ml) was heated at 100°C for 7 hours and the resulting solution allowed to cool. TLC analysis of the reaction mixture showed that none of the title compound was present at this stage. 85% Aqueous phosphoric acid solution (0.5 ml) was added and the resulting mixture heated at 100°C, intermittently, for a total of 300 hours, then evaporated under reduced pressure. The residue was "azeotroped" with toluene and dissolved in water (50 ml), then the pH of the stirred aqueous solution adjusted to 7 with 20% aqueous sodium hydroxide solution. Stirring was continued for 2 hours, then the precipitate collected, washed with water (20 ml) and dried under vacuum at 50°C, to give the crude product (5.21 g) which, by TLC and HPLC analyses, contained an estimated yield of 9.1% of the title compound. EXAMPLE 45 A stirred mixture of p -toluenesulphonic acid monohydrate (5.71 g, 0.030 mol) and chlorobenzene (100 ml) was heated under reflux until all the water had been removed, using a Dean-Stark trap, and then allowed to cool to room temperature. The title compound of Preparation 4 (24.64 g, 0.050 mol) was added and the reaction mixture stirred under reflux for 24 hours, then allowed to cool. To the resulting mixture was added dichloromethane (200 ml) and water (200 ml), the pH was adjusted to 7 using 2M aqueous sodium hydroxide solution, and the organic phase separated and combined with a dichloromethane extract (100 ml) of the aqueous phase. The combined organic phases were washed with water (100 ml) and evaporated under reduced pressure to afford an off-white solid (24.86 g), 7.3% of which was shown, by TLC and HPLC analyses, to be the title compound. EXAMPLE 46 Titanium tetrachloride (3.3 ml, 5.69 g, 0.030 mol) was added to a stirred suspension of the title compound of Preparation 4 (12.32 g, 0.025 mol) in anhydrous 1,4-dioxan (61 ml), during which vigorous evolution of a gas was noted. The stirred reaction mixture was heated at about 70°C for 7.5 hours, allowed to cool to room temperature and then treated with water (200 ml) and concentrated hydrochloric acid (50 ml) to give a clear solution. The solution was washed with dichloromethane and its pH adjusted to 12 using 40% aqueous sodium hydroxide solution; it was then stirred for 10 minutes and its pH further adjusted to 7 using 5M hydrochloric acid. The precipitate was removed by filtration and washed with dichloromethane (2 x 200 ml), then the combined dichloromethane washings were used to extract the aqueous filtrate and evaporated under reduced pressure to give a solid (11.36 g), 33.7% of which was shown, by TLC and HPLC analyses, to be the title compound. EXAMPLES 47-52 Under reaction conditions similar to those described in Example 46, the variations of which are summarised in Table 3, alternative Lewis acids provided the corrected yields of title compound shown. EXAMPLE LEWIS ACID SOLVENT REACTION TIME (HOURS) % YIELD 47 BF 3 tetrahydrofuran 72 7.0 48 AlCl 3 1,4-dioxan 30 7.8 49 FeCl 3 tetrahydrofuran 24 6.3 50 ZnCl 2 tetrahydrofuran 72 2.8 51 SiCl 4 1,4-dioxan 44 20.5 52 SnCl 4 1,4-dioxan 48 30.8 PREPARATION 1 5-Chlorosulphonyl-2-ethoxybenzoic acid Molten 2-ethoxybenzoic acid (25.0 g, 0.150 mol) was added to a stirred, ice-cooled mixture of thionyl chloride (11 ml, 0.151 mol) and chlorosulphonic acid (41.3 ml, 0.621 mol), whilst maintaining the temperature of the reaction mixture below 25°C. The resulting mixture was stirred at room temperature for 18 hours and then poured into a stirred mixture of ice (270 g) and water (60 ml) to give an off-white precipitate. Stirring was continued for 1 hour, then the product was collected by filtration, washed with water and dried under vacuum to provide the title compound (36.08 g). A reference sample, m.p. 115-116°C, was obtained by crystallisation from hexane:toluene. Found: C,41.02; H,3.27. C 9 H 9 ClO 5 S requires C,40.84; H,3.43%. δ (CDCl 3 ): 1.64(3H,t), 4.45(2H,q), 7.26(1H,d), 8.20(1H,dd), 8.80(1H,d). PREPARATION 2 2-Ethoxy-5-(4-methylpiperazin-1-ylsulphonyl)benzoic acid (a): one-step procedure 1-Methylpiperazine (33.6 ml, 0.303 mol) was added to a stirred suspension of the title compound of Preparation 1 (34.4 g, 0.130 mol) in water (124 ml) at about 10°C, whilst maintaining the temperature of the reaction mixture below 20°C. The resulting solution was cooled to about 10°C and, after 5 minutes, crystallisation of a solid commenced. After a further 2 hours, the solid was collected by filtration, washed with ice-water and dried under vacuum to furnish the crude product (36.7 g). A sample (15.0 g) was purified by stirring it in refluxing acetone for 1 hour; the resulting suspension was allowed to cool to room temperature and the crystalline solid collected by filtration and dried under vacuum to afford the title compound (11.7 g), m.p. 198-199°C, whose 1 H nmr spectrum is identical with that obtained for the product of procedure (b) below. (b): two-step procedure A solution of the title compound of Preparation 1 (50.0 g, 0.189 mol) in acetone (150 ml) was added dropwise to a stirred mixture of 1-methylpiperazine (20.81 g, 0.208 mol) and triethylamine (28.9 ml, 0.207 mol), whilst maintaining the temperature of the reaction mixture below 20°C. A white crystalline solid formed during the addition and stirring was continued for a further 1.5 hours. Filtration, followed by washing with acetone and drying under vacuum of the product, provided the hydrochloride-triethylamine double salt of the title compound (78.97 g), m.p. 166-169°C. Found: C,51.33; H,8.14; N,9.06; Cl,8.02. C 14 H 20 N 2 O 5 S; C 6 H 15 N; HCI requires C,51.55; H,7.79; N,9.02; Cl, 7.61%. δ (CD 3 SOCD 3 ): 1.17(9H,t), 1.32(3H,t), 2.15(3H,s), 2.47(6H,br s), 2.86(2H, br s), 3.02(6H,q), 4.18(2H,q), 7.32(1H,d), 7.78(1H,dd), 7.85(1H,d). The double salt (30.0 g) was stirred in water (120 ml) to produce an almost clear solution, from which crystallisation of a solid rapidly occurred. After 2 hours, the solid was collected by filtration, washed with water and dried under vacuum to give the title compound (14.61 g) as a white solid. A reference sample, m.p. 201°C, was obtained by recrystallisation from aqueous ethanol. Found: C,51.09; H,6.16; N,8.43. C 14 H 20 N 2 O 5 S requires C,51.21; H,6.14; N,8.53%. δ (CD 3 SOCD 3 ): 1.31(3H,t), 2.12(3H,s), 2.34(4H,br s), 2.84(4H, br s), 4.20(2H,q), 7.32(1H,d), 7.80(1H,dd), 7.86(1H,d). PREPARATION 3 4-Amino-1-methyl-3-n-propylpyrazole-5-carboxamide A stirred suspension of 1-methyl-4-nitro-3-n-propylpyrazole-5-carboxamide (EP-A-0463756; 237.7 g, 1.12 mol) and 5% palladium on charcoal (47.5 g) in ethyl acetate (2.02 I) was hydrogenated at 344.7 kPa (50 psi) and 50°C for 4 hours, when the uptake of hydrogen was complete. The cool reaction mixture was filtered, then the filter pad washed with ethyl acetate, the combined filtrate and washings thus furnishing an ethyl acetate solution of the title compound (EP-A-0463756) which was of sufficient purity to use directly in the next stage of the reaction sequence (see Preparation 4). PREPARATION 4 4-[2-Ethoxy-5-(4-methylpiperazin-1-ylsulphonyl)benzamido]-1-methyl-3 -n-propylpyrazole-5-carboxamide N,N'-Carbonyldiimidazole (210.8 g, 1.30 mol) was washed into a stirred suspension of the title compound of Preparation 2 (408.6 g, 1.24 mol) in ethyl acetate (1.50 l) using ethyl acetate (1.36 l) and the resulting mixture heated at 55°C for 0.5 hour and then under reflux for a further 2 hours before being allowed to cool to room temperature. An ethyl acetate solution of the title compound of Preparation 3 (2.185 Kg of solution containing 204 g, 1.12 mol of amine) was added and the reaction mixture stirred at room temperature for 72 hours to afford a crystalline solid which was collected by filtration and dried under vacuum. The title compound (425 g), m.p. 204-206°C, thus obtained was combined with a further crop (70 g) which was recovered by concentration of the mother liquor. A reference sample, m.p. 206-208°C, was obtained by recrystallisation from aqueous methanol. Found: C,53.65; H,6.54; N,17.07. C 22 H 32 N 6 O 5 S requires C,53.64; H,6.55; N,17.06%. δ (CDCl 3 ): 0.96(3H,t), 1.58(3H,t), 1.66(2H,m), 2.27(3H,s), 2.45(4H,m), 2.52(2H,t), 3.05(4H,br s), 4.05(3H,s), 4.40(2H,q), 5.61(1H, br s), 7.61(1H,d), 7.65(1H, br s), 7.90(1H,dd), 8.62(1H,d), 9.25(1H,br s). PREPARATION 5 Methyl 2-ethoxybenzoate Concentrated sulphuric acid (0.5 ml) was added to a solution of 2-ethoxybenzoic acid (50 g, 0.301 mol) in methanol (500 ml) and the resulting mixture heated under reflux for 70 hours, then evaporated under reduced pressure to give an oil which was dissolved in dichloromethane (300 ml). This solution was washed successively with water (150 ml), aqueous sodium bicarbonate solution (150 ml) and water (150 ml), then evaporated under reduced pressure to give the title compound (49.7 g) as an oil. δ (CDCl 3 ): 1.44 (3H,t), 3.90 (3H,s), 4.12 (2H,q), 6.95 (2H,m), 7.44 (1H,t), 7.78 (1H,d). PREPARATION 6 Methyl 5-chlorosulphonyl-2-ethoxybenzoate The title compound of Preparation 5 (36.04 g, 0.20 mol) was added dropwise over 10 minutes to stirred, ice-cooled chlorosulphonic acid (59.8 ml, 0.90 mol), whilst maintaining the temperature of the reaction mixture below 22°C. The reaction mixture was stirred at room temperature for 18 hours, then thionyl chloride (14.6 ml, 0.20 mol) added and the resulting solution stirred at room temperature for 6 hours, then poured into a stirred mixture of ice (530 g) and water (120 ml). The quenched mixture was extracted with dichloromethane (2 x 200 ml) and the combined extracts evaporated under reduced pressure to give the crude title compound (44.87 g) as a white solid. A reference sample, m.p. 99-100°C, was obtained by crystallisation from toluene. δ (CDCl 3 ): 1.52 (3H,t), 3.93 (3H,s), 4.25 (2H,q), 4.25 (2H,q), 7.12 (1H,d), 8.12 (1H,dd), 8.46 (1H,d). PREPARATION 7 Methyl 2-ethoxy-5-(4-methylpiperazin-1-ylsulphonyl)benzoate A solution of the crude title compound of Preparation 6 (27.87 g) in acetone (140 ml) was added dropwise over 10 minutes to a stirred, ice-cooled solution of 1-methylpiperazine (11.02 g, 0.11 mol) and triethylamine (15.3 ml, 0.11 mol) in acetone (140 ml), whilst maintaining the temperature of the reaction mixture below 20°C. A white precipitate formed during the addition and stirring was continued for a further 4 hours. The resulting mixture was filtered, the filtrate evaporated under reduced pressure and the residue azeotroped with toluene to provide a pale brown gum (41.9 g). This crude product was granulated by stirring with water (100 ml) for 2 hours and the resulting material collected by filtration, washed with water (2 x 50 ml) and dried under vacuum at 50°C to furnish the title compound, m.p. 110-111°C. δ (CDCl 3 ): 1.48 (3H,t), 2.27 (3H,s), 2.47 (4H,t), 3.03 (4H,t), 3.90 (3H,s), 4.18 (2H,q), 7.04 (1H,d), 7.81 (1H,dd), 8.15 (1H,d). The compound obtained as above was shown to be identical with that produced by conventional methyl esterification of the title compound of Preparation 2. Furthermore, conventional base hydrolysis of the compound obtained as above afforded a product identical with that of Preparation 2.

Claims: A process for the preparation of a compound of formula (I): which comprises cyclisation of a compound of formula (II): wherein the cyclisation is conducted under neutral or acidic conditions by heating a compound of formula (II), optionally in the presence of a solvent and/or optionally in the presence of a dehydrating agent and/or mechanical water-removal system. A process according to claim 1 wherein the solvent is selected from the group consisting of 1,2-dichlorobenzene, dimethyl sulphoxide, sulpholane, N-methylpyrrolidin-2-one and pyrrolidin-2-one, and mixtures thereof; and the dehydrating agent is selected from the group consisting of anhydrous potassium carbonate, anhydrous sodium carbonate, anhydrous magnesium sulphate, anhydrous sodium sulphate, phosphorus pentoxide and molecular sieves. A process according to claim 2 wherein the solvent is 1,2-dichlorobenzene, sulpholane or N-methylpyrrolidin-2-one; the dehydrating agent is molecular sieves; and the reaction is carried out at from 180 to 220°C for from 0.5 to 72 hours. A process according to claim 1 wherein the cyclisation is carried out in the presence of a protic acid or Lewis acid . A process according to claim 4 wherein the protic acid is selected from the group consisting of an inorganic acid, an organo-sulphonic acid, an organo-phosphonic acid and an organo-carboxylic add; the Lewis acid is selected from the group consisting of boron trifluoride, boron trichloride, boron tribromide, aluminium chloride, aluminium bromide, silicon tetrachloride, silicon tetrabromide, stannic chloride, stannic bromide, phosphorus pentachloride, phosphorus pentabromide, titanium tetrafluoride, titanium tetrachloride, titanium tetrabromide, ferric chloride, zinc fluoride, zinc chloride, zinc bromide, zinc iodide, mercuric chloride, mercuric bromide and mercuric iodide; and the solvent is selected from the group consisting of a C 5 -C 12 alkane, a C 5 -C 8 cycloalkane, a C 1 -C 12 alkanoic acid, a C 1 -C 4 alkanol, a C 3 -C 9 alkanone, a C 5 -C 12 alkyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, diglyme, tetrahydrofuran, 1,4-dioxan, benzene, toluene, xylene, chlorobenzene, dichlorobenzene, nitrobenzene, dichloromethane, dibromomethane, 1,2-dichloroethane, acetonitrile, dimethyl sulphoxide, sulpholane, dimethylformamide, dimethylacetamide, N-methylpyrrolidin-2-one and pyrrolidin-2-one, and mixtures thereof. A process according to claim 5 wherein the protic acid is concentrated sulphuric acid, phosphoric add or p -toluenesulphonic acid; the Lewis acid is boron trifluoride, aluminium chloride, silicon tetrachloride, stannic chloride, titanium tetrachloride, ferric chloride or zinc chloride; the solvent is glacial acetic acid, tetrahydrofuran, 1,4-dioxan or chlorobenzene; and the reaction is carried out at from 65 to 210°C for from 6 to 300 hours.

Inventors: DUNN PETER JAMES, WOOD ALBERT SHAW, DUNN, PETER JAMES, WOOD, ALBERT SHAW, Dunn, Peter James Pfizer Central Research, Wood, Albert Shaw Pfizer Central Research
Applicants: PFIZER LTD, PFIZER RES & DEV, PFIZER LIMITED, PFIZER RESEARCH AND DEVELOPMENT COMPANY, N.V./S.A.
Application year: 1997
Main classification: C07D487
IPC codes: C07D295, C07D231, C07D487