GB2118552A - Thromboxane synthetase inhibitors - Google Patents

Abstract

A compound of the formula:- <IMAGE> or a pharmaceutically acceptable salt thereof, wherein "Het" is 1 -imidazolyl or 3- pyridyl; R<1> is C1-C4 alkyl; R<2>, which is in the 4-, 5-, 6-, or 7- position, is C1-C4 alkyl, C1-C4 alkoxy, halo, or hydroxy; R<3> is hydroxy, C1-C4 alkoxy or amino; X is O or S; and the group "Het-CH2-" is attached to the 4-, 5-, 6- or 7-position of the benzothiophene ring. The compounds selectively inhibit the action of the thromboxane synthetase enzyme and are thus useful in the treatment of, for example, thrombosis, ischaemic heart disease, stroke, etc.

Description

SPECIFICATION Thromboxane synthetase inhibitors This invention relates to certain benzothiophenes and benzofurans which are substituted by a carboxy, lower alkoxycarbonyl or carbamoyl group. Such compounds are able to selectively inhibit the action of the thromboxane synthetase enzyme without significantly inhibiting the action of the prostacyclin synthetase or cyclooxygenase enzymes. The compounds are thus useful as therapeutic agents, for example, in the treatment of thrombosis, ischaemic heart disease, stroke, transient ischaemic attack, migraine, cancer and vascular complications of diabetes.

Thus according to the invention, there are provided compounds of the general formula:

and their pharmaceutically acceptable salts, wherein "Het" is 1-imidazolyl or 3-pyridyl; R' is C1 -C4 alkyl; R2, which is in the 4-, 5-, 6- or 7-position, is C,-C4 alkyl, C,-C4 alkoxy, halo, or hydroxy; R3 is hydroxy, C1 -C4 alkoxy or amino; X is O or S; and the group " Het-CH2-" is attached to the 4-, 5-, 6- or 7-position of the benzothiophene ring.

Alkyl and alkoxy groups of 3 or 4 carbon atoms may be straight or branched chain.

Halo is F, Cl, Br or The preferred individual compounds have the formula:

The invention further provides a compound of the formula (I), or a pharmaceutically acceptable salt thereof, for use in treating an animal, including a human being, to inhibit the action of the thromboxane synthetase enzyme without significantly inhibiting the action of the prostacyclin synthetase or cyclo-oxygenase enzymes.

The invention also includes a pharmaceutical composition comprising a compound of the formula (I), or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable diluent or carrier.

The preferred salts are the pharmaceutically acceptable acid addition, and, when R3 is -OH, the pharmaceutically acceptable metal (e.g. sodium or potassium) or ammonium salts.

Pharmaceutically acceptable acid addition salts of the compounds of the invention are salts with acids containing pharmaceutically acceptable anions, e.g. the hydrochloride, hydrobromide, sulphate or bisulphate, phosphate or acid phosphate, acetate, maleate, fumarate, lactate, tartrate, citrate, gluconate, succinate and p-toluene sulphonate salts.

The compounds of the formula (I) can be prepared by a number of different routes, including the following: (1) The imidazolyl-containing esters where R2 is C1-C4 alkyl or C1-C4 alkoxy can be prepared as follows:

Q is a facile leaving group such as Cl, Br, C1 -C4 alkylsulphonyloxy or ArSO2O- where Ar is phenyl optionally substituted by 1 or 2 substituents selected from C,-C4 alkyl, C,-C4 alkoxy, Cl and Br. Q is preferably Cl or Br.

A preferred base/solvent combination is Na2CO3/acetone. Other suitable combinations are NaH/DMF and C2H5ONa/C2H5OH.

The reaction may proceed to completion at room temperature although heating at up to 1 20'C, e.g. under reflux, may be necessary to accelerate the reaction, which is generally complete in 24 hours or less. The product can be isolated and purified by conventional procedures.

The starting materials for this method are either known compounds or can be prepared by conventional methods, e.g. as follows

Alternatively, the 6-hydroxymethyl benzothiophene intermediates for this method can be prepared as follows:

(then as previous route) (2) The free acids (R3 = OH) can be prepared by the hydrolysis of the corresponding esters (R3 = C1-C4 alkyl). Alkaline hydrolysis by refluxing with sodium hydroxide in aqueous methanol for six hours or less, followed by acidification, is a typical method.

(3) The pyridyl-containing acids in which R2 is C1 -C4 alkyl or C,-C4 alkoxy can be prepared as follows:

In a typical procedure, rrbutyl lithium as a solution n hexane is added dropwise to a stirred mixture of di-isopropylamine and potassium t-butoxide in dry ether at - 78"C under dry nitrogen. After stirring for about an hour at this temperature the benzo-fuse heterocycle is added in e.g. dry ether, and the reaction mixture is again stirred at this temperature for about an hour.

Excess crushed solid CO2 is then added, and, after all the CO2 has evaporated, water is added and the mixture shaken. The aqueous layer is separated, washed with ether, acidified with e.g.

acetic acid, and the solid filtered off and dissolved in sodium hydroxide. The solution can be decolourised by warming with charcoal, filtered, acidified with e.g. acetic acid, and the resulting solid filtered off, washed with water, and dried to give the desired product.

(4) The compounds in which R2 is OH can be prepared as follows:

(5) Certain compounds in which R2 is OH can be prepared via the Mannich reaction as follows:

(6) Esters in which R2 is C1-C4 alkoxy can be prepared by alkylating the corresponding compounds in which R2 is -OH, e.g. by using an alkyl halide in the presence of a base, e.g.

NaH.

(7) The free acids can be converted to the esters by esterification using a C,-C4 alkanol.

Reaction of the ester with ammonia produces the am ides, which can also be prepared by the reaction of the corresponding acids with carbonyl di-imidazole and then with ammonia.

(8) Cyclisation can also be used to prepare certain of the benzofurans:

The reaction can be carried by refluxing the reactants together for up to 4 hours.

(9) Another route to certain pyridyl-containing acids is as follows:

Pharmaceutically acceptable salts can be prepared by conventional procedures, e.g. by reacting an organic solution of the compound with an organic solution of a suitable acid to obtain an acid addition salt either by precipitation, or by evaporation of the solution. In the case of ester starting materials, often the acid used will cause hydrolysis to the free acid in addition to salt formation.

The compounds of formula (I) and their pharmaceutically acceptable salts have been found to selectively inhibit the action of the thromboxane synthetase enzyme without significantly affecting the action of the prostacyclin synthetase or cyclooxygenase enzymes. Thus the compounds are of value in the treatment of a variety of clinical conditions which are characterised by an imbalance of prostacyclin/thomboxane A2. For the reasons given below these conditions may include thrombosis, ischaemic heart disease, stroke, transient ischaemic attack, migraine, cancer and the vascular complications of diabetes.

Research work has established that in most tissues the major product of the arachidonic acid metabolism is either of two unstable substances, thromboxane A2 (TxA2) or prostacyclin (PG 12).

(Proc. Nat. Acad. Sci. U.S.A., 1975, 72, 2994, Nature, 1976, 263, 663. Prostaglandins, 1976, 12, 897). In most cases the prostaglandins PGE2, PGF2a and PGD2 are comparatively minor by-products in this bio-synthetic pathway. The discovery of thromboxane A2 and prostacyclin has significantly increased our understanding of vascular homeostasis, prostacyclin for instance is a powerful vasodilator and inhibitor of platelet aggregation, and in this last respect is the most potent endogenous substance so far discovered. The prostacyclin synthetase enzyme is located in the endothelial layer of the vasculature, and may be fed by endoperoxides released by blood platelets coming into contact with the vessel wall.The prostacyclin thus produced is important for prevention of platelet deposition on vessel walls. (Prostaglandins, 1976, 12 685, Science, 1976, 17, Nature, 1978, 273, 765).

Thromboxane A2 is synthesised by the thromboxane synthetase enzyme which is located in, for example, the blood platelets. Thromboxane A2 is a powerful vasoconstrictor and proaggregatory substance. As such its actions are in direct opposition to those of prostacyclin. If, for any reason, prostacyclin formation by the vasculature is impaired, then the endoperoxides produced by platelets coming into contact with the vessel wall are converted into thromboxane, but are not converted effectively into prostacyclin (Lancet, 1977, 18, Prostaglandins, 1978, 13 3). Alteration of the prostacyclin/thromboxane balance in favour of the latter substance could result in platelet aggregation, vasospasm (Lancet, 1977, 479, Science, 1976, 1135, Amer. J.

Cardiology, 1978, 41, 787) and an increased susceptibility to atherothrombosis (Lancet (i) 1977, 1216). It is also known that in experimental atherosclerosis prostacyclin generation is suppressed and thromboxane A2 production is enhanced (Prostaglandins, 1977, 141025 and 1035). Thus thromboxane A2 has been implicated as the causative agent in variant angina, myocardial infarction, sudden cardiac death and stroke (Thromb. Haemostasis, 1977, 38, 132).

Studies in rabbits have shown that ECG changes typical of these conditions were produced when freshly prepared thromboxane A2 was injected directly into the animal's heart (Biochem.

aspects of Prostaglandins and Thromboxanes, Editors, N. Kharasch and J. Fried, Academic Press 1 977 page 189). This technique is considered to represent a unique animal model of the heart attacks of coronary patients and has been used to show that administration of a compound believed to antagonise the effects of thromboxane A2 protects the rabbits from the adverse consequences of thromboxane A2 injection.

Another area where a PGI2/TxA2 imbalance is considered to be a contributory factor is that of migraine.

The migraine headache is associated with changes in intra and extracerebral blood flow, in particular a pre-headache reduction of cerebral blood flow followed by dilatation in both vascular areas during the headache phase.

Prior to the development of the headache, blood levels of 5-hydroxytryptamine are elevated, and this suggests the occurrence of in vivo aggregation and release of the amine from the platelet stores. It is known that the blood platelets of migraine patients are more prone to aggregate than are those of normal individuals (J. Clin. Pathol., 1971, 24, 250, J. Headache, 1977, 17, 101). Furthermore, it has now been postulated that not only is an abnormality of platelet function a major factor in the pathogenesis of migraine attacks but it is in fact their prime cause (Lancet (i), 1978, 501). Thus a drug that selectively modifies platelet function to inhibit thromboxane A2 formation could be of considerable benefit in migraine therapy.

Abnormalities of platelet behaviour have been reported in patients with diabetes mellitus (Metabolism, 1 979, 28, 394, Lancet, 1 978 (i) 235). Diabetic patients are known to be particulariy susceptible to microvascular complications, atherosclerosis and thrombosis and platelet hyper-reactivity has been suggested as the cause of such angiopathy. Diabetic platelets produce elevated amounts of TxB2 and malondialdehyde (Symposium "Diabetes and Thrombosis-lmplications for Therapy", Leeds U.K., April 1979). Also it has been shown that in rats with experimental diabetes vascular prostacyclin production is impaired and TxA2 synthesis from the platelets is elevated (IV International Prostaglandin Conference, Washington, D.C., May 1979).

Thus the imbalance between prostacyclin and TxA2 is considered to be responsible for the microvascular complications of diabetes. A TxA2-synthetase inhibitor could therefore find clinical utility in preventing these vascular complications.

Aspirin and most other non-steroidal anti-inflamlmatory drugs inhibit the cyclo-oxygenase enzyme. The effect of this is to shut down the production of the PGG2/H2 endoperoxiees and by so doing to reduce both the prostacyclin and thromboxane A2 levels. Aspirin and aspirin-like drugs have been evaluated clinically for prevention of stroke and heart attack (New England and J. Med. 1978, 299,53, B.M.J., 1978, 1188, Stroke, 1977, 8, 301.

Although some encouraging results have been obtained with these drugs, a compound which specifically inhibits thromboxane A2 formation leaving the biosynthesis of prostacyclin unimpaired would be more valuable in these clinical conditions (Lancet (ii), 1 978, 780).

The ability of primary neoplasms to metastasize is a principal cause of failure to cure human cancers. It has been suggested the metastatic tumour cells can alter the critical PGI2-TxA2 balance in favour of thrombosis (Science, 1981, 212, 1270). Prostacyclin has recently been shown to be a powerful anti-metastatic agent by virtue of its platelet antiaggregatory action. This result indicates that a TxA2-synthetase inhibitor may function as an antimetastatic agent in vivo (J. Cell. Biol. 1980, 87 64).

The effect of the compounds of the formula (I) on the thromboxane synthetase enzyme, and the prostacyclin synthetase and cyclooxygenase enzymes has been measured by the following in vitro enzyme assays:1. Cyclo-oxygenase Ram seminal vesicle microsomes (Biochemistry, 1971, 10, 2372) are incubated with arachidonic acid (1 00,iM: 1 min.: 22 ) to produce PGH2 and aliquots of the reaction mixture injected into a stream of Krebs-bicarbonate at 37"C containing a mixture of antagonists (Nature, 1978, 218, 1135)and indomethacin (Brit. J. Pharmacol., 1972, 45,451) which is superfus- ing a spirally-cut rabbit aorta strip (Nature, 1969, 223, 29).

The ability of a compound to inhibit the enzyme is measured by comparing the increases in isometric tension produced by PGH2 in the absence of the test compound, and following preincubation of the enzyme with the test compound for 5 minutes (Agents and Actions, 1981, 11, 274).

2. Prostacyclin (PGI2) Synthetase Pig aorta microsomes (Nature, 1976, 263, 663) are incubated (30 sec.; 22"C) with PGH2 produced as in 1 and the reaction terminated with 5 volumes of ethanol. PGl2 production is assessed by measuring its stable breakdown product, 6-keto PGF1a, using a specific radioimmunoassay. PGl2 production can be completely inhibited by pre-incubation of the enzyme with the selective PGl2 synthetase inhibitor, 1 5-hydroperoxy-arachidonic acid (Prostaglandins, 1976, 12, 715). The test compound is pre-incubated with the enzyme for 5 minutes and its ability to prevent the production of PGl2 (6-keto PGFa) is measured.

3. Thromboxane As (TxA2) Synthetase Indomethacin pretreated human platelet microsomes (Science 1976, 193, 163) are incubated (2 min.: O"C) with PGH2 (produced as in 1) and the reaction terminated with 5 volumes of ethanol. TxA2 production is assessed by measuring its stable metabolite TxB2, using a specific radioimmunoassay.

The test compound is pre-incubated with enzyme for 5 minutes, and its ability to inhibit the thromboxane synthetase enzyme is measured as reduction of the TxA2 (TxB2) production.

Compounds of the formula (i) tested in this way have been shown to be capable of selectively inhibiting the thromboxane syntehtase enzyme.

In addition to the above an in vitro assay for measuring the inhibition of human blood platelet aggregation has been described and this may be predictive of anti-thrombotic efficacy clinically (Lancet (ii), 1974, 1223, J. Exp. Med., 1967, 126, 171). Both clinically effective agents aspirin and sulphinpyrazone show inhibitory activity in vitro agains a variety of aggregating agents in this test.

A number of in vivo tests in animals have also been described for evaluating potential antithrombotic drugs.

The method of Patrono et al is adapted to study the generation of Tx2 in whole blood samples removed from animals prior to and following drug treatment. Briefly, blood samples are taken into glass tubes and allowed to clot at 37"C. Serum is separated by centrifugation and the samples stored at - 40"C until assayed for TxB2, when appropriate dilutions of ethanol deproteinised samples are analysed by RIA. This technique is used in experiments with the test compounds to determine intravenous potency in anaesthetised rabbits: Anaesthetised Rabbits Male New Zealand white rabbits (2.6-5.6 kg) are anaesthetised with sodium pentobarbitone (30 mg/kg i.v.) followed by urethane (500 mg/kg i.p.). After cannulation of the trachea, a carotid artery is catheterised for collection of blood samples.The catheter is kept patent by slow infusion (0.2 ml/minute) of sterile saline. Control carotid arterial blood samples are taken 30 and 5 minutes prior to administration of the test compound or vehicle (0.9% w/v NaCI, 0.2 ml/kg) via a marginal ear vein. Three groups of rabbits are used. The first group receive 0.03 mg/kg of the test compound followed, one hour later, by 0.1 mg/kg. Similarly, the second group receive 0.3 mg/kg, followed by 1 mg/kg. The third group receive vehicle, followed one hour later by a further vehicle injection. Carotid arterial blood samples are taken 1 5 and 45 minutes after all doses. At each time point, a 1 ml blood sample is taken into a glass test tube, without anticoagulant, for TxB2 determination.For the latter, the blood sample is allowed to clot during a two hour incubation at 37"C (which preliminary experiments had shown to give maximum TxB2 production) and the serum obtained by centrifugation. Serum samples are then processed through the TxB2 RIA after deproteinisation with ethanol and dilution with Isogel Tris buffer.

Intravenous injection of arachidonic acid causes death in rabbits by causing platelet clumping and embolisation in the lungs. Again both the clinically effective aspirin (Agents and Actions, 1977, 1, 481) and sulphinpyrazone (Pharmacology, 1976, 14, 522) protect the rabbit from the lethal effect of the injection. Sulphinpyrazone has also been shown to prevent the aggregation of platelets in an extra corporeal loop of the abdominal aorta of rats in vivo (Thromb. Diathes.

Haem., 1973, 30, 138).

The compounds may be administered orally in the form of tablets or capsules containing a unit dose of the compound together with such excipients as maize starch, calcium carbonate, dicalcium phosphate, alginic acid, lactose, magnesium stearate, "Primogel" (Trade Mark) or talc. The tablets are typically prepared by granulating the ingredients together and compressing the resulting mixture to give tablets of the desired size. Capsules are typically prepared by granulating the ingredients together and filling them into hard gellatine capsules of the appropriate size to contain the desired dosage.

The compounds may also be administered parenterally, for example by intramuscular, intravenous or subcutaneous injection. For parenteral administration, they are best used in the form of a sterile aqueous solution which may contain other solutes such as tonic and pH adjusters. The compounds may be added to distilled water and the pH adjusted to 3-6 using an acid such as citric, lactic or hydrochloric acid. Sufficient solutes such as dextrose or saline may be added to render the solution isotonic. The resulting solution may then be sterilised and filled into sterile glass vials of an appropriate size to contain the desired volume of solution. The compounds of the invention may also be administered by the infusion of a parenteral formulation as described above into a vein.

For oral administration to human patients, it is expected that the daily dosage level of a compound of the formula (I) will be from 0.1 to 20 mg/kg per day for a typical adult patient (70 kg). For parenteral administration, it is expected that the daily dosage level of a compound of the formula (I) will be from 0.01-0.5 mg/kg. per day, for a typical adult patient. Thus tablets or capsules can generally be expected to contain from 5 to 1 50 mg of the active compound for administration orally up to 3 times a day. Dosage units for parenteral administration can be expected to contain from 0.5-35 mg of the active compound. A typical vial could be a 10 ml vial containing 5 mg of the active compound in 6-10 ml of solution.

It should of course be appreciated that in any event the physician will determine the actual dosge which will be most suitable for the individual and it will vary with the age, weight and response of the patient.

The above dosages are exemplary of the average patient, there may of course be individual cases where higher or lower dosage ranges are merited.

The preparation of the novel compounds of the formula (I) is illustrated by the following Examples. All temperatures are in "C Example I 6-( 1 -lmidazolylmethyl)-3, 7-dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester (a) 2-Bromo-3, 7-dimeth ylbenzofbjthiophene Bromine (48.0 g) was added dropwise over 1 5 minutes to a stirred solution of 3,7 dimethylbenzob]thiophene (48.6 g) in chloroform (500 ml). The solution was stirred at room temperature for three hours and then washed successively with water, dilute potassium hydroxide solution, water and then dried (Na2SO4). The chloroform was evaporated and the residue was fractionally distilled.The fraction of b.p. 93-97"/0.05 mm was crystallised from ethanol to give 2-bromo-3,7-dimethylbenzobJthiophene (28.1 g), m.p. 49-51"C.

Analysis %: Found: C,49.87; H,3.78.

Required for C,OHgBrS: C,49.80; H,3.76.

A fraction of b.p. 135-140"/0.05 mm was crystallised from acetone to give 2,6-dibromo3,7-dimethylbenzo[b]thiophene (8.9 g), m.p. 103-104"C.

Analysis %: Found: C,37.31; H,2.43. C,OH8Br2S Requires: C,37.52; H,2.52.

(b) 6-Acetyl-2-bromo-3, 7-dimethylbenzo[b]thiophene Aluminium chloride (14.80 g) was added portionwise over one hour to a stirred solution of 2 bromo-3,7-dimethylbenzo[b]thiophene (24.1 g) and acetyl chloride (8.72 g) in nitrobenzene (69 ml) at - 4"C. The mixture was stirred at O"C for 5 hours and then poured onto a mixture of ice and concentrated hydrochloric acid with stirring. The resulting mixture was steam-distilled to remove nitrobenzene and the cooled residue was extracted several times with ethyl acetate. The combined extracts were washed with water, dried (Na2SO4) and evaporated.He residue was crystallised from ethyl acetate/petrol (b.p. 60-80") to give 6-acetyl-2-bromo-3,7-dimethylben- zotb]thiophene (13.0 g), m.p. 147-148"C.

Analysis %: Found: C,48.16; H,3.77 C12H,,BrOS Requires: C,48.17; H,3.71.

(c) 2-Bromo-3, 7-dimethylbenzofbjthiophene-6-carboxylic acid A solution of potassium hydroxide (18.0 g) and potassium carbonate (63.0 g) in water (180 ml) was added to a mixture of calcium hypochlorite (90 g) in warm water (360 ml) and the mixture was shaken vigorously for a few minutes. It was then filtered and the residue was sucked as dry as possible. The filtrate was added to dioxan (140 ml) and the solution was heated to 50"C. 6-AcetyI-2-brnmo-3,7-dimethyIbenzo[bhiophene (12.4 g) was added portionwise with stirring and the resulting mixture was stirred at 80"C for six hours. It was then cooled, washed with ethyl acetate to remove unreacted starting material and then acidified with concentrated hydrochloric acid.The solid was filtered off, washed with water and crystallised from acetic acid to give 2-bromo-3,7-dimethylbenzo[b]thiophene-6-carboxylic acid (5.70 g), m.p. 258-259"C.

Analysis %: Found: C,46.54; H,3.16 C11H9BrO2S requires: C,46.33; H,3.18 (d) 2-Bromo-6-hydroxymethyl-3, 7-dimethylbenzo[bgthiophene A 1 M solution of borane-tetrahydrofuran complex (100 ml) was added dropwise to a stirred solution of 2-bromo-3,7-dimethylbenzo[b]thiophene-6-carboxylic acid (5.0 9) in dry tetrahydrofuran (100 ml) and the resulting solution was stirred at O"C for two hours and then allowed to stand for 1 8 hours. The excess borane complex was destroyed by the cautious addition of methanol and the solution was evaporated to small volume. Ethyl acetate was added and the mixture was washed with water and dried (Na2SO4).Evaporation of the solvent gave a solid which was crystallised from ethyl acetate to give 2-bromo-6-hydroxymethyl-3,7-dimethylbenzo[- b]thiophene (3.60 g), m.p. 175-176"C.

Analysis %: Found: C,48.85; H,4.05 C"H"BrOS requires: C,48.71; H,4.09.

(e) 6-Hydroxymethyl-3, 7-dimethylbenzofbjthiophene-2-carboxylic acid A solution of 2-bromo-6-hydroxymethyl-3,7-dimethylbenzo[b]thiophene (2.60 g) in dry tetrahydrofuran (150 ml) was added dropwise to a stirred solution of n-butyl-lithium (26 ml of 1.6 M solution in hexane) in dry tetrahydrofuran (50 ml) at - 70"C and the mixture was stirred at this temperature for two hours followed by 30 minutes at 0 C. It was then poured onto a mixture of freshly crushed solid carbon dioxide and ether. When all the carbon dioxide had evaporated the mixture was extracted several times with water. The combined aqueous extracts were washed with ether and acidified with concentrated hydrochloric acid.The solid was filtered off, washed with water and crystallised from ethanol to give 6-hydroxymethyl-3,7-dimethylben- zo[b]thiophene-2-carboxylic acid (1.48 9), m.p. 264-265"C.

Analysis %: Found: C,60.99; H,5.23 C,2H,203S requires: C,60.99; H,5.12 (f) 6-Hydroxymethyl-3, 7-dimethylbenzofbjthiophene-2-carboxylic acid methyl ester A solution of 6-hydroxymethyl-3,7-dimethylbenzo[b]thiophene-2-carboxylic acid (1.16 g) in methanol (80 ml) at 0" was treated with an excess of an ethereal solution of diazomethane. The solution was allowed to stand for two hours and then acetic acid was added dropwise until effervescence ceased. The solution was evaporated to dryness and the residue was dissolved in ethyl acetate. The solution was washed with diluted sodium bicarbonate solution and dried (Na2SO4).Evaporation of the solvent gave a quantitative yield of 6-hydroxymethyl-3,7-dimethyl- benzo(b]thiophene-2-carboxylic acid methyl ester, m.p. 149-150 C (from chloroform/petrol).

Analysis %: Found: C,62.20; H,5.50 C13H,403S requires: C,62.37; H,5.64.

(9) 6-Chloromethyl-3, 7-dimeth ylbenzofbjthiophene-2-carboxylic acid methyl ester Thionyl chloride (0.5 ml) was added dropwise to a stirred solution of 6-hydroxymethyl-3,7dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester (0.5 g) and pyridine (3 drops) in chloroform (10 ml) and the mixture was stirred at room temperature for 30 minutes. It was then washed successively with water, sodium bicarbonate solution, water and dried (Na2SO4).

Evaporation of the solvent gave 6-chloromethyl-3,7-dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester (0.5 9), m.p. 150-151 C, whch was used directly in the next stage.

(h) 6-( 1 -lmidazolylmethyl)-3, 7-dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester A mixture of 6-chloromethyl-3,7-dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester (0.2 g), imidazole (0.51 g), sodium bicarbonate (67 mg) and dry acetone (20 ml) was heated under reflux with stirring for 14 hours and then evaporated. The residue was dissolved in ethyl acetate and the solution was washed with water, dried (Na2SO4) and the solution was evaporated. The residue was chromatographed on silica gel. Elution with chloroform first gave some impurity followed by pure product.The product-containing fractions were evaporated and the residue was crystallised from ethyl acetate/petrol (b.p. 40-60") to give 6-(1-imidazolylme thyl)-3,7-dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester (0.159), m.p. 183-184"C.

Analysis %: Found: C,64.07; H,5.68; N,9.30 C16H'6N202S requires: C,63.97; H,5.37; N,9.33.

Example 2 6-( 1 -lmidazolylmethyl)-3, 7-dimethylbenzofbjthiophene-2-carboxylic acid A mixture of 6-( 1 -imidazolylmethyl)-3, 7-dimethylbenzo[b]thiophene-2-carboxylic acid methyl ester (0.149), sodium hydroxide (0.1 9), methanol (1 ml) and water (10 ml) was heated under reflux for four hours and then evaporated. The residue was dissolved in water and the solution was acidified with acetic acid. The solid was filtered off and purified by redissolving in dilute sodium hydroxide solution, filtering the solution and acidifying the filtrate with acetic acid. The solid was filtered off, washed with water and dried to give 6-(1-imidazolylmethyl)-3,7-dimethylbenzo[b]thiophene-2-carboxylic acid (0.1 9), m p. > 3O0'C.

Analysis %: Found: C,63.03; H,4.99; N,9.81 C1 > H14N202S requires: C,62.91; H,4.93; N,9.79.

Example 3 3, 4-Dimethyl-5-(3-pyridylmethyl)benzofbjthiophene-2-carboxylic acid (a) 6, 7-Dihydro-3-meth yl-4-trimeth ylsilyloxybenzo[b] thiophene Trimethylsilyl triflate (3.0ml) was added dropwise to a stirred solution of 6,7-dihydro-3methylbenzo[b]thiophene-4(5H)-one (1.66g.) and triethylamine (1.209.) in dry methylene chloride (40ml.) at room temperature. The mixture was stirred at room temperature for 1 hr. and then allowed to stand for 1 8 hrs. The solvent was evaporated and the residue was triturated with petrol (b.p. 40-60"). The mixture was filtered and the filtrate was evaporated to give an oil which was distilled.The fraction of b.p. 142-146"/10m.m. was collected to give 6,7-dihydro 3-methyl-4-trimethylsilyloxybenzo[bl thiophene (1.939.).

(b) 6, 7-Dihydro-3-methyl-5-(3-pyridylmeth ylene)benzofbjthiophene-4-(5H)-one A mixture of 6,7-dihydro-3-methyl-4-trimethylsilyloxybenzo[b]thiophene (6.859.) and pyridine3-carboxaldehyde (3.409.) in dry tetrahydrofuran (10my.) was added to a stirred solution of anhydrous tetrabutylammonium fluoride (0.759.) in dry tetrahydrofuran (60ml.) at - 78'. The mixture was stirred at - 78" for 2 hr., then allowed to warm up to 0" and then stirred at this temperature for 30 min. The solvent was evaporated and the residue was dissolved in ether. The solution was washed with water and evaporated.The residue was dissolved in ethanol (50ml.) and concentrated hydrochloric acid (30ml.) and the solution was heated under reflux for 2 hrs.

The solution was evaporated to around half volume, diluted with water (ca. 50ml.) and washed with ether. It was then made alkaline with dilute aqueous sodium hydroxide solution and the mixture was extracted several times with ethyl acetate. The combined extracts were washed with water, dried (Na2SO4), and evaporated to give an oil. The oil was dissolved in ether and the solution was filtered through "Hyflo" (Trademark) to remove tarry material. The filtrate was evaporated and the residue was chromatrographed on silica gel. Elution with chloroform first gave some impurity followed by pure product. The product-containing fractions were combined and evaporated to give an oil which solidified on scratching.The solid was crystallised from ethyl acetate/petrol (b.p. 60-80") to give 6,7-dihydro-3-methyl-5-(3-pyridylmethylene) benzo[b]thiophene-4(5H)-one (3.349.), n.p. 95-98".

Analysis %: Found: C,70.33; H,4.97; N,5.56 C,5H1SNOS requires: C,70.56; H,5.13; N,5.49.

(c) 6, 7-Dihydro-3-methyl-5-(3-pyridylmethyl)benzo[b]thiophene-4(5H)-one A solution of 6, 7-dihydro-3-methyl-5-(3-pyridylmethylene) benzo[b]thiophene-4(5H)-one (4.569.) in ethanol (100ml.) was hydrogenated at 50 and 4 atm. pressure in the presence of 5% palladium/charcoal (1.0g.). The catalyst was filtered off and the solution was evaporated.

The residue was chromatografted on silica gel. Elution with chloroform gave 6,7-dihydro-3 methyl-5-(3-pyridylmethyl)benzo[b]thiophene-4(5H)-one (2.80g.) as an oil.

A portion of the product was dissolved in ether and treated with an excess of ethereal hydrogen chloride to give the hydrochloride salt, m.p. 215-217" (from isopropanol).

Analysis %: Found: C,61.28; H,5.54; N,4.77 C,5H,5NOS.H9 requires: C,61.23; H,5.49; N,4.77.

(d) 4-Hydroxy-3, 4-dimeth yl-5-(3-pyridylmeth yl)-4, 5,6, 74etrah ydrobenzorb]thioph en e Methyl magnesium bromide (12 ml. of 3M solution in ether) was added dropwise with stirring and cooling to a solution of 6,7-dihydro-3-methyl-5-(3-pyridylmethyl)benzo[bjthiophene-4(5H) one (2.0g.) in dry tetrahydrofuran (30ml.). The mixture was stirred at room temperature for 1 hr., heated under reflux for 5 hrs. and then allowed to stand at room temperature for 1 8 hrs. An excess of a saturated aqueous solution of ammonium chloride was added cautiously with stirring. The mixture was extracted with ethyl acetate and the organic layer was saturated and dried (Na2SO4).Evaporation of the solvent gave an oil which was chromatographed on silica gel.

Elution with chloroform first gave some impurity followed by product. The product-containing fractions were combined and evaporated to give 4-hydroxy-3, 4-dimethyl-5-(3-pyridylmethyl) 4,5,6,7-tetrnhydrnbenzobJthiophene as a gum (2.09.), which was used directly in the next stage.

(e) 3, 4-Dimeth yl-5-(3-pyridylmeth yl)benzo[b]thiophen e A mixture of 4-hydroxy-3,4-dimethyl-5-(3-pyridylmethyl)-4, 5,6, 7-tetrnhydrnbenzo[bhiophene (1.83g.) and triphenylmethanol (2.819.) in trifluoroacetic acid (40ml.) was heated under reflux for 6 hrs. The excess of trifluoroacetic acid was evaporated and the residue was treated with dilute sodium hydroxide solution. The mixture was extracted several times with ethyl acetate and the combined extracts were washed with water and dried (Na2SO4). Evaporation of the solvent gave an oil which was chromatographed on silica gel. Elution with chloroform first gave triphenylmethane and triphenylmethanol followed by pure product.The product-containing fractions were combined and evaporated to give 3,4-dimethyl-5-(3-pyridylmethyl)benzo[b]thio- phene (1.04g.), m.p. 97-98" (from petrol b.p. 60-80").

Analysis %: Found: C,75.76; H,5.99; N,5.46 C,6H15NS requires: C,75.85; H,5.97; N,5.53.

(f) 3,4-Dimethyl-5-(3-pyridylmethyl)benzo[b]thiophene-2-carboxylic acid n-Butyl-lithium (1.40ml. of 1.55M solution in hexane) was added to a stirred mixture of potassium t-butoxide (0.25g.) and di-isopropylamine (0.22g.) in dry ether (50ml.) at - 78"C and the mixture was stireed at this temperature for 45 minutes. A solution of 3,4-dimethyl-5-(3pyridylmethyl)benzo[b]thiophene (0.50g.) in dry ether (5ml.) was added and the resulting mixture was stirred at - 78" for 1 hr. and then an excess of crushed solid carbon dioxide was added. When all the carbon dioxide had evaporated the mixture was shaken with water and the layers were separated. The aqueous layer was acidified with acetic acid and the solid was filtered off and washed with water. The product was purified by dissolving in dilute aqueous ammonia solution, filtering the solution and acidification of the filtrate with acetic acid to give 3,4-dimethyl-5-(3-pyridylmethyl)benzo[b]thiophene-2-carboxylic acid (0.1 7g.), m. p. 282-285".

Analysis %: Found: C,68.29; H,5.04; N,4.86 C,7H,5NO2S requires: C,68.66; H,5.08; N,4.71.

Claims (4)

1. A compound of the formula:

or a pharmaceutically accetable salt thereof, wherein "Het" is 1-imidazolyl or 3-pyridyl; R' is C,-C4 alkyl; R2, which is in the 4-, 5-, 6- or 7-position, is C1-C4 alkyl, C1-C4 alkoxy, halo, or hydroxy; R3 is hydroxy, C1-C4 alkoxy or amino; X is O or S; and the group " Het-CH2-" is attached to the 4-, 5-, 6- or 7-position of the benzothiophene ring.

2. A compound as claim 1, which has the formula:

3. A pharmaceutical composition comprising a compound of formula (I) as claimed in claim 1 or 2, or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable diluent or carrier.

4. A compound of the formula (I) as claimed in claim 1 or 2, or a pharmaceutically acceptable salt thereof, for use in treating a human being to inhibit the action of the thromboxane synthetase enzyme without significantly inhibiting the action of the prostacyclin synthetase or cyclo-oxygenase enzymes.