Pyridine derivatives

Abstract: The invention concerns novel, pharmaceutically useful 1,3-dioxane alkenoic acid derivatives of the formula I containing a pyridyl moiety at position 4 of the dioxane ring and in which the groups at positions 2, 4 and 5 have cis -relative stereochemistry, X is hydrogen, alkoxy or hydroxy, Y is vinylene, n is 1 or 2, A¹ is alkylene, R¹ is a variety of substituents defined hereinafter, and R² is hydroxy, a physiologically acceptable alcohol residue or alkanesulphonamido, and the pharmaceutically acceptable salts thereof. The invention also includes processes for the manufacture and use of the acid derivatives as well as pharmaceutical compositions for therapeutic use in one or more of a variety of diseases such as ischaemic heart disease, cerebrovascular disease, asthmatic disease and/or inflammatory disease.

Description: This invention concerns novel pyridine containing heterocyclic compounds and, more particularly, it concerns novel 1,3-dioxan-5-yl alkenoic acids containing a pyridyl moiety attached at position 4 of the 1,3-dioxane ring. The acids of the invention have valuable pharmaceutical properties and the invention includes pharmaceutical compositions containing the novel acids and processes for the manufacture and medical use of the novel acids. Also included in the invention is the use of a novel acid in the production of a medicament for use in treating warm blooded animals such as humans. It is known that the arachidonic acid metabolite thromboxane A₂ (hereinafter referred to as "TXA₂") is a powerful vasoconstrictor and a potent aggregator of blood platelets. TXA₂ is also a potent constrictor of bronchial and tracheal smooth muscle. TXA₂ may therefore be involved in a variety of disease conditions, for example ischaemic heart disease such as myocardial infarction, angina, cerebrovascular disease such as transient cerebral ischaemia, migraine and stroke, peripheral vascular disease such as atherosclerosis, microangiopathy, hypertension and blood clotting defects due to lipid imbalance. It is believed that TXA₂ exerts its physiological action through the thromboxane receptor, at which receptor various other prostanoid contractile substances derived from arachidonic acid, such as prostaglandins H₂, F₂ alpha and prostaglandin D₂, can exert contractile effects. There are two principal ways in which the effects of TXA₂ can be ameliorated. The first is by administering a pharmacological agent which preferentially occupies the thromboxane receptor, but yet does not produce the contractile effects which follow the binding of TXA₂ (or of prostaglandins H₂, F₂ alpha and/or D₂). Such an agent is said to possess TXA₂ antagonist properties. The second way is to administer a pharmacological agent which inhibits one or more of the enzymes involved in the production of TXA₂ and in particular which inhibits the enzyme known as thromboxane synthase (TXA₂ synthase). Such an agent is said to be a TXA₂ synthase inhibitor. Accordingly, it may be seen that agents which possess TXA₂ antagonist properties and which inhibit TXA₂ synthase may be expected to be of therapeutic value in the treatment of one or more of the above mentioned diseases or other diseases in which TXA₂ is involved. Also, agents which possess TXA₂ antagonist properties may be expected to be of value additionally in treating those diseases in which prostaglandins H₂,F₂ alpha and/or D₂ are involved, for example especially in asthmatic and inflammatory diseases. Although 1,3-dioxane TXA₂ antagonists are known (for example, in European patent, publication number 94239B1), as are certain TXA₂ synthase inhibitors (for example, in European patent application, publication number 98690A2), obtaining compounds which combine both properties to a useful extent is not straightforward. However, we have now discovered (and this is the basis for our invention) that certain 1,3-dioxan-5-yl alkenoic acids of the formula I (set out, together with the other chemical structures, at the end of this specification) containing a pyridyl moiety attached to position 4 of the 1,3-dioxane ring surprisingly are good inhibitors of TXA₂ synthase and also possess significant TXA₂ antagonist properties and are useful pharmaceutical agents. According to the invention there is provided a 1,3-dioxane alkenoic acid derivative of the formula I (set out hereinafter together with the other chemical formulae in Roman numerals) wherein: A¹ is (1-6C)alkylene; R¹ is (1-6C)alkyl, trifluoromethyl, (3-6C)cycloalkyl or (1-4C)alkoxy(1-4C)alkyl, or is a group of the formula R³.A²- in which R³ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano, and in which A² is (1-6C)alkylene, oxy(1-6C)alkylene, (2-6C)alkenylene or a direct bond to R³; R² is a hydroxy, a physiologically acceptable alcohol residue, or (1-4C)alkanesuphonamido; X is hydrogen, hydroxy or (1-4C)alkoxy; Y is vinylene; and n is the integer 1 or 2; or a pharmaceutically acceptable salt thereof. It will be appreciated that the compounds of formula I possess asymmetric carbon atoms and may exist and be isolated in racemic and optically active forms. The invention includes both the racemic forms and any optically active form (or mixtures thereof) which is capable of antagonising one or more of the actions of TXA₂ and inhibiting the synthesis of TXA₂, it being well known in the art how to prepare individual optical isomers (for example by synthesis from optically active starting materials or resolution of a racemic form) and how to determine the TXA₂ antagonist properties and TXA₂ synthase inhibitory properties using one or more of the standard tests referred to hereinafter. It will be understood that the groups at positions 2, 4 and 5 of the 1,3-dioxane moiety of formula I have cis -relative stereochemistry, as have the groups adjacent to the vinylene group Y (i.e. the latter compounds exist as the "Z" isomer). Further, although a particular configuration is shown in the chemical formulae attached hereto, this does not necessarily correspond to the absolute configuration. It is also to be understood that the generic term "alkylene" includes both straight chain and branched chain alkylene groups such as ethylene and ethylidene and other generic terms are to be construed similarly. However, when a specific term such as "butyl" is used, it is specific to the straight chain or "normal" butyl group, branched chain isomers such as " t -butyl" being referred to specifically when required. Particular values for R¹ when it is (1-6C)alkyl include, for example, methyl, ethyl, isopropyl and t -butyl, of which the latter two are preferred; when it is (3-6C)cycloalkyl include, for example, cyclopentyl and cyclohexyl; and when it is (1-4C)alkoxy(1-4C)alkyl include, for example, 1,1-dimethyl-2-methoxyethyl and 1-methyl-1-propoxyethyl. Particular values for R² when it is a physiologically acceptable alcohol residue are those which render the subsequent ester biodegradable and are chosen from, for example, (1-6C)alkyl optionally bearing a hydroxy of (1-4C)alkoxy substituent, such as methyl, ethyl, 2-hydroxyethyl, 2-methoxyethyl, propyl or 3-hydroxypropyl; phenyl; and benzyl; the latter two of which may optionally bear 1 or 2 optional substituents selected from halogeno (such as fluoro, chloro, bromo or iodo), (1-4C)alkyl (such as methyl or ethyl) and (1-4C)alkoxy (such as methoxy or ethoxy). Particular values for R² when it is (1-4C)alkanesulphonamido include, for example, methanesulphonamido, ethanesulphonamido and butanesulphonamido. Particular values for A¹ when it is (1-6C)alkylene include, for example: methylene, ethylene, trimethylene, tetramethylene, 1,1-dimethylethylene and 1,1-dimethyltrimethylene. Particular values for A² when it is (1-6C)alkylene include, for example, (1-4C)alkylene (such as methylene, ethylene, trimethylene, isopropylidene and 1,1-dimethylethylene) and 3,3-pentylidine; when it is (2-6C)alkenylene include, for example, vinylene, 1,3-propenylene and 1,4-buten-2-ylene; and when it is oxy(1-6C)alkylene include, for example, oxymethylene, oxytetramethylene (i.e. a group of the formula: -O.(CH₂)₄-), 1-oxy-1-methylethyl (i.e. a group of the formula: -O.C(CH₃)₂-) and 2-oxy-1,1-dimethylethyl (i.e. a group of the formula -O.CH₂.C(CH₃)₂-), it being understood that the oxy link is to the group R³ and not the 1,3-dioxane ring. A particular value for R³ when it is pyridyl is, for example, 3-pyridyl. A particular value for an optional halogeno substituent when R³ is halogenophenyl is, for example, fluoro, chloro or bromo. A particular value for X when it is (1-4C)alkoxy, is for example, methoxy or ethoxy. A preferred value for X is, for example, hydrogen. By way of example, a generally preferred value for n is 1, for Y is cis -vinylene and for A¹ is ethylene or trimethylene. A group of compounds of the invention of particular interest comprises compounds of formula II wherein: A³ is (1-4C)alkylene; R⁴ is trifluoromethyl, branched (3-6C)alkyl, or is a group of the formula R⁵.A⁴- in which R⁵ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano, and in which A⁴ is (1-4C)alkylene, oxy(1-4C)alkylene or a direct bond to R⁵; Y is vinylene; Q is a 3-pyridyl or 4-pyridyl moiety; and X, R² and n have any of the meanings defined above; together with the pharmaceutically acceptable salts thereof when R² is hydroxy or (1-4C)alkanesulphonamido. In the above group, a preferred value of Y is cis -vinylene, for R² is hydroxy, for n is the integer 1 and for X is hydrogen. A further group of compounds of the invention of particular interest comprises compounds of formula III wherein A3 is (1-4C)alkylene; R⁵ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano; and A⁴ is (1-4C)alkylene, oxy(1-4C)alkylene or a direct bond to R⁵; together with the pharmaceutically acceptable salts thereof. Particular values for A³ include, for example, those defined above for A¹ when it is (1-4C)alkylene, for example, ethylene, trimethylene and 1,1-dimethylethylene, of which values, ethylene is generally preferred. Particular values for A⁴ include, for example, those defined above for A² when it is a direct bond, (1-4C)alkylene or oxy(1-4C)alkylene, such as a direct bond, isopropylidene, 1,1-dimethylethylene and 1-oxy-1-methylethyl (i.e. a group of the formula: -O.C(CH₃)₂-); and for R⁵ include, for example, 3-pyridyl, phenyl, 4-halogenophenyl (such as 4-chloro- or 4-bromophenyl), 2-halogenophenyl (such as 2-fluoro- or 2-chloro-phenyl), dihalogenophenyl (such as 3,4-difluoro-, 3,4-dichloro- or 2,4-dichloro-phenyl), nitrophenyl (such as 2-nitro-, 3-nitro or 4-nitro-phenyl), 2-cyanophenyl, 4-cyanophenyl, 2-trifluoromethylphenyl and 4-trifluoromethylphenyl. Specific values for R¹ include, by way of example: trifluoromethyl, isopropyl, t -butyl, cyclohexyl, phenyl, 2-chlorophenyl, 3-chlorophenyl, 2-cyanophenyl, 4-cyanophenyl, 4-nitrophenyl, 2-chloro-5-nitrophenyl, 3,4-dichlorophenyl, 2,4-dichlorophenyl, 2-trifluoromethylphenyl, 4-trifluoromethylphenyl, 1-phenoxy-1-methylethyl (in which the phenoxy moiety may optionally bear 2-fluoro, 2-nitro, 2-trifluoromethyl, 3-fluoro, 3-bromo, 3-nitro, 4-fluoro, 4-bromo, 4-cyano, 4-nitro, 2,4-dichloro, 3,4-difluoro or 3,4-dichloro substituents), 3-pyridyl, 4-pyridyl, 1-methyl-1-(3-pyridyloxy)ethyl, 1-propoxy-1-methylethyl and 1,1-dimethyl-2-phenylethyl (in which the phenyl moiety may optionally bear 3-bromo, 3-nitro, 4-fluoro, 4-nitro, 4-trifluoromethyl, 3,4-difluoro or 3,4-dichloro substituents), styryl and 2-nitrostyryl. In the above compounds of the invention, a particularly preferred value for R² is, for example, hydroxy and for X is, for example, hydrogen. Particular novel compounds of the invention are described in the accompanying Examples and are provided, together with their pharmaceutically acceptable salts, as a further feature of the invention. The compounds of Examples 4, 8, 11 and 28 are particularly preferred. Although all of the formula I compounds can form salts with suitable acids, it will be appreciated that the compounds of formula I are amphoteric when R2 is hydroxy of alkanesulphonamido and can form salts with acids as well as bases. Particular pharmaceutically acceptable salts for such compounds therefore include, for example, alkali metal and alkaline earth metal salts, ammonium salts, salts with organic amines and quaternary bases forming physiologically acceptable cations such as salts with methylamine, dimethylamine, trimethylamine, ethylenediamine, piperidine, morpholine, pyrrolidine, piperazine, ethanolamine, triethanolamine, N-methylglucamine, tetramethylammonium hydroxide and benzyltrimethylammonium hydroxide, as well as salts with acids affording physiologically acceptable anions, such as salts with mineral acids, for example with hydrogen halides (such as hydrogen chloride and hydrogen bromide), sulphuric and phosphoric acid, and with strong organic acids, for example with p -toluenesulphonic and methanesulphonic acids. The compounds of formula I may be manufactured by conventional procedures of organic chemistry well known in the art for the manufacture of structurally analogous compounds. Such procedures are provided as a further feature of the invention and are illustrated by the following representative procedures in which R¹, R², X, Y, A¹ and n have any of the meanings defined hereinbefore. (a) For those compounds of formula I in which R² is hydroxy, an aldehyde of the formula IV is reacted with a Wittig reagent of the formula: R₃P=CH.A¹.CO₂⁻ M⁺ wherein R is (1-6C)alkyl or aryl (especially phenyl, which is preferred) and M⁺ is a suitable metal cation, for example an alkali metal cation such as the lithium, sodium or potassium cation. The process in general produces the required compounds of formula I in which the substituents adjacent to the vinylene group Y have predominantly the preferred cis -relative stereochemistry i.e. in the "Z" isomeric form. However the process also produces generally small amounts of the analogous compounds having trans -relative stereochemistry (ie. the "E" isomeric form) which may be removed by a conventional procedure, such as chromatography or crystallisation. The process may be conveniently performed in a suitable solvent or diluent, for example an aromatic solvent such as benzene, toluene or chlorobenzene, an ether such as 1,2-dimethoxyethane, t -butyl methyl ether, dibutyl ether or tetrahydrofuran, in dimethyl sulphoxide or tetramethylene sulphone, or in a mixture of one or more such solvents or diluents. The process is generally performed at a temperature in the range, for example, -80°C to 40°C, but is conveniently carried out at or near room temperature, for example in the range 0 to 35°C. (b) For those compounds wherein X is hydroxy, a compound of the formula V wherein P is a protected hydroxy group, is deprotected by conventional means. Examples of particularly suitable protected hydroxy groups include, for example, (1-4C)alkoxy (such as methoxy), benzyloxy, allyloxy, tetrahydropyran-2-yloxy, (1-4C)alkanesulphonyloxy (especially methanesulphonyloxy) and trialkylsilyloxy of up to 10 carbon atoms. The deprotection conditions used will necessarily depend on the nature of the protected hydroxy groups. The removal of specific hydroxyl protecting groups is well documented in standard organic chemistry books and such conventional procedures well known in the art are included within the processes of the invention. Thus, for example, specific groups may be removed as follows:- (1), allyl or tetrahydropyran-2-yl: by treatment with strong acid such as trifluoroacetic acid, at e.g. 10 to 40°C; (2) trialkylsilyl (such as t -butyldimethylsilyl, which is preferred): by reaction with aqueous tetrabutylammonium fluoride or sodium fluoride conveniently in a suitable solvent or diluent, such as tetrahydrofuran, or t -butyl methyl ether, and generally at or near ambient temperature, e.g. in the range 10 to 35°C; (3) alkanesulphonyl: by hydrolysis in the presence of a base (such as sodium or potassium hydroxide) in a suitable aqueous solvent [such as an aqueous (1-4C)alkanol] and at e.g. 0 to 60°C; (4) alkyl: by treatment with an alkali metal thioalkoxide or diphenylphosphide (such as sodium thioethoxide in a solvent such as N,N-dimethylformamide at e.g. 50-160 °C, or lithium diphenylphosphide in a solvent such as methyl t -butyl ether or tetrahydrofuran at e.g. 0-60 °C); or (5) benzyl: by palladium catalysed hydrogenolysis in an alkanol such as ethanol at or near ambient temperature and pressure or by use of an alkali metal such as sodium in liquid ammonia. (c) A diol derivative of the formula VI wherein one of T¹ and T² is hydrogen and the other is hydrogen or a group of the formula -CRaRb.OH (wherein Ra and Rb are the same or different (1-4C)alkyl groups) is reacted with an aldehyde derivative of the formula R¹.CHO, or an acetal, hemiacetal or hydrate thereof. The latter aldehyde [or its hydrate, or its acetal or hemiacetal with a (1-4C)alkanol (such as methanol or ethanol)] may conveniently be present in an excess. The reaction is generally performed in the presence of an acid such as hydrogen chloride, hydrogen bromide, sulphuric acid, phosphoric acid, methanesulphonic acid or p -toluenesulphonic acid, conveniently in the presence of a suitable solvent or diluent, such as dichloromethane, toluene, xylene or an ether, for example tetrahydrofuran, dibutyl ether, methyl t -butyl ether or 1,2-dimethoxyethane, and at a temperature in the range, for example, 0 to 80°C. Those starting materials of formula VI wherein T¹ and T² are both hydrogen may be obtained, for example, by mild, acid catalysed, hydrolysis or alcoholysis of the dioxane ring of a compound of formula VII wherein one of Ra and Rb is hydrogen or (1-4C)alkyl (such as methyl or ethyl)and the other is (1-4C)alkyl, obtained by an analogous procedure to process (a) herein, for example, analogous to that described in European patent application, Publication No.94239. The hydrolysis or alcoholysis will normally be carried out at a temperature in range 10 to 80°C using an aqueous mineral acid such as hydrochloric acid in an alkanol such as ethanol or 2-propanol or an ether (such as tetrahydrofuran) as solvent. The starting materials of formula VI wherein one of T¹ and T² is hydrogen and the other is a group of the formula -CRaRb.OH are intermediates in the above-mentioned formation of the starting materials of formula VI wherein T¹ and T² are both hydrogen. However, said intermediates are not normally isolated or characterised. Accordingly, the invention also provides a preferred, modified procedure (d) of process (c) which comprises reacting a 1,3-dioxane of formula VII wherein one of Ra and Rb is hydrogen, methyl or ethyl and the other is methyl or ethyl with an excess of an aldehyde of the formula R¹.CHO (or a hydrate, acetal or hemiacetal thereof) in the presence of an acid (such as one of those given above), conveniently at a temperature in the range, for example, 10 to 80°C and, optionally in the presence of a suitable solvent or diluent (such as one of those given above). In some cases, it is necessary to modify procedures (c) and (d) where the aldehyde of formula R¹.CHO is not particularly reactive or tends to form an acyclic hemiacetal when reacted with the compound of the formula VI or VII, for example when 2,2,2-trifluoroacetaldehyde is used for the production of formula I compounds wherein R¹ is trifluoromethyl. Thus, a further procedure (e) of the invention comprises reacting a compound of the formula VI wherein T² is hydrogen and T¹ is alkanesulphonyl (especially methanesulphonyl) or arenesulphonyl (especially benzene- or toluene-sulphonyl) with an aldehyde of the formula R¹·CHO (or a hydrate, acetal or hemiacetal thereof) (for example with 2,2,2-trifluoroacetaldehyde or its hydrate) in the presence of a suitable acid and under the same general conditions as given above for procedure (c), followed by base-catalysed cyclisation of the acyclic intermediate obtained, for example using a suitable base (such as potassium carbonate or sodium hydride) in a suitable solvent of diluent (such as an ether described above) and at a temperature in the range, for example, 20-50°C. Such a procedure is illustrated in Example 34 hereinafter using a compound of formula VI in which R² is methoxy, which group is subsequently converted to hydroxy by hydrolysis after formation of the dioxane ring. The necessary starting alkanesulphonyl or arenesulphonyl esters of formula VI defined above may be conveniently obtained from the corresponding diol of formula VI (T¹ = T² = hydrogen) by reaction with one molecular equivalent of the appropriate alkanesulphonyl or arenesulphonyl halide (such as methanesulphonyl chloride or p -toluenesulphonyl chloride) in a suitable solvent or diluent (such as an ether or dichloromethane) at or near ambient temperature and in the presence of a suitable base (such as triethylamine or pyridine). (f) Decomposing an ester of the formula VIII wherein R⁶ is (1-6C)alkyl (especially methyl, ethyl, propyl or t -butyl), phenyl or benzyl the latter two optionally bearing 1 or 2 halogeno, (1-4C)alkyl or (1-4C)alkoxy substituents. The decomposition may be carried out using any one or more of the conventional reagents and conditions well known in the art for converting esters to acids. Thus, for example, the decomposition may conveniently be performed by base catalysed hydrolysis, for example by using an alkali metal hydroxide such as lithium, potassium or sodium hydroxide in an aqueous system conveniently in the presence of a suitable solvent or diluent such as tetrahydrofuran, methanol, ethanol or t -butyl methyl ether and a temperature in the general range, for example, 10 to 60°C, and, conveniently, at or near ambient temperature. Alternatively, when R⁶ is t -butyl, the decomposition may be carried out thermally by heating the compound of formula VIII at a temperature in the general range, for example, 80 to 150°C, alone or in the presence of a suitable diluent such as diphenylether or diphenylsulphone. The necessary starting materials for use in the above processes (a)-(f) may be obtained by general procedures well known for the production of structurally related compounds, for example using analogous procedures to those described in European patent no. 94239B1 and patent application no. 98690A2. The aldehydes of the formula IV may be obtained, for example, from the allyl compounds of formula XI as shown in Schemes 1 and 2 hereinafter and as illustrated in the Examples, it being recognised that when a particular stereoisomer is required, the sequence of selective reductions may need to be manipulated and any mixtures of isomers separated, for example using chromatography. Alternatively, when a particular stereoisomer is required, it may be obtained starting from a specific enantiomer of a 3-[2-(1-hydroxy-1-pyridylmethyl)pent-4-enyl]oxazolidin-2-one of the formula XI in which R⁷ is (1-4C)alkyl (especially isopropyl) itself obtained from aldol condensation of the corresponding 3-(4-pentenoyl)oxazolidin-2-one with pyridylcarboxaldehyde, as shown in Scheme 3 hereinafter. [This procedure is particularly suitable for obtaining individual enantiomers of the compounds of formula I]. The protected hydroxy derivatives of formula V may be obtained for example by carrying out process (c) or (d) above with a suitable compound analogous to the 1,3-dioxane of formula VII but in which X is a suitably protected hydroxy group, such a compound being itself readily obtainable using standard procedures analogous to those described above and to those set out in the accompanying Examples. The appropriate diols of formula VI for the production of dioxanes of formula I or VII wherein the pyridyl moiety bearing X and the alkenoic acid side-chain have cis -relative stereochemistry, may be obtained, for example, using an analogous procedure to that described in European patent application, publication no. 142323, starting from the appropriate pyridine-carboxaldehyde and succinic anhydride and a suitable base such as that used in Scheme 3 for the aldol condensation. The esters of formula VIII may be made, for example, by carrying out process (c) using the appropriate ester of the diol corresponding to formula VI. The necessary Wittig reagents may be obtained by conventional procedures, for example by treating the corresponding phosphonium halides with a strong base such as sodium hydride, lithium diisopropylamide, potassium t -butoxide or butyllithium. They are generally formed in situ just prior to carrying out the condensation process (a) above. It will be understood that the compounds of formula I wherein R² is hydroxy may also be obtained by other conventional procedures well known in the art, for example by base catalysed hydrolysis of the corresponding amides or nitriles. In addition, those compounds of formula I wherein R² is other than hydroxy may be made by conventional esterification or sulphonamidation procedures from the compounds wherein R² is hydroxy (or a reactive derivative thereof) and the appropriate alcohol, phenol or (1-4C)alkanesulphonamide. Such procedures are also within the ambit of the invention. Whereafter, when a salt of a compound of formula I is required, it may be obtained by reaction with the appropriate base or acid affording a physiologically acceptable ion, or by any other conventional salt formation procedure. Further, when an optically active form of a compound of formula I is required, one of the aforesaid processes may be carried out using an optically active starting material (for example, one described in Scheme 3, as illustrated in Example 40 hereinafter). Alternatively, the racemic form of a compound of formula I may be resolved, for example by reaction with an optically active form of a suitable organic acid or base, for example, camphorsulphonic acid, ephedrine, N,N,N-trimethyl(1-phenylethyl)ammonium hydroxide or 1-phenylethylamine, followed by conventional separation of the diastereoisomeric mixture of salts thus obtained, for example by fractional crystallisation from a suitable solvent, for example a (1-4C)alkanol, whereafter the optically active form of said compound of formula I may be liberated by treatment with acid (or base) using a conventional procedure, for example using an aqueous mineral acid such as dilute hydrochloric acid (or aqueous alkali such as aqueous sodium hydroxide). In general, the enantiomeric form of the compound of formula I in which the groups on the dioxane ring have the 2 S ,4 S ,5 R configuration is preferred. Many of the intermediates defined herein are novel, for example those of formulae IV, V, VI, VII, VIII and IX and XI, and are provided as further, separate features of the invention. It should be noted that in addition certain of the compounds of formula VII (such as those in which Ra and Rb are both methyl or ethyl) possess useful thromboxane A₂ synthase inhibitory properties and may themselves be valuable as pharmaceuticals either per se or in the form of pharmaceutical compositions, which are also within the ambit of the invention. As stated earlier, the compounds of formula I possess significant TXA₂ antagonist properties and are inhibitors of TXA₂ synthase. The TXA₂ antagonism may be demonstrated in one or other of the following standard tests:- a) The rat aortic strip model analogous to that devised by Piper and Vane ( Nature , 1969, 223 , 29-35) using as agonist the TXA₂mimetic agent known as U46619 (described by R L Jones et alia in "Chemistry, Biochemistry and Pharmacological Activity of Prostanoids" edited by S M Roberts and F Scheinmann, at page 211; Pergamon Press, 1979); (b) a blood platelet aggregation test based on that described by Born ( Nature , 1962, 194, 927-929) and involving: (i) aggregating human, citrated, platelet-rich plasma by addition of the TXA₂ mimetic agent U46619 so that a dose-response curve is generated; (ii) generating a dose-response curve for U46619 stimulated platelet aggregation in the presence of increasing amounts of test compound (generally in the range 10⁻⁵M to 10⁻¹⁰M); and (iii) calculating a K B value indicating potency of TXA₂ antagonism for the test compound, averaged over several concentrations, from the calculated 50% response value for U46619 aggregation in the presence and absence of test compound; or (c) a bronchoconstriction test involving measuring the inhibition by a test compound of the bronchoconstriction induced in Konzett-Rossler, anaesthetised guinea-pig model (as modified by Collier and James, Brit . J . Pharmacol ., 1967, 30 , 283-307) by intravenous administration of the TXA₂ mimetic agent, U46619 and involving : (i) obtaining a cumulative dose-response curve to U46619 induced bronchoconstriction by intravenous administration of constant volumes of increasing concentrations of U46619 (0.2-4 µg/kg) in physiological saline solution and expressing bronchoconstriction as the maximum of that theoretically obtainable with no air flow to the test animal; (ii) generating a cumulative dose-response curve to U46619 induced bronchoconstriction at 30 minute intervals for 3 hours after oral dosing of test compound; and (iii) calculating a dose-ratio for the test compound (that is the ratio of concentration of U46619 required to cause 50% bronchoconstriction in the presence and absence of test compound) indicating the potency of TXA₂ antagonism. Test (b) may conveniently be modified to demonstrate the antagonism of the effects of TXA₂ in vivo by assessing the effects of a test compound on the aggregation of blood platelets obtained after administration of test compound to a laboratory animal, such as a rabbit, rat, guinea pig or dog. However, when the aggregation of dog platelets is being studied it is necessary to use a predetermined, threshold concentration of the platelet aggregation agent adenosine diphosphate (about 0.4-1.2 x 10⁻⁶M) together with the TXA₂ mimetic agent, U46619. The antagonism of the effects of TXA₂ on the vasculature may also be demonstrated, for example in rats in the following procedure: (d) Male rats (Alderley Park strain) are anaesthetised with sodium pentobarbital and blood pressure is monitored at the carotid artery. The TXA₂ mimetic agent U46619 is administered intravenously at 5 µg/kg via the jugular vein to produce 20-30 mm/Hg (2640-3970 pascal) increase in systolic blood pressure. The process is repeated twice to ensure adequacy of response. A test compound is then administered either intravenously (via the jugular vein) or orally (via a cannula) directly into the stomach and the animal challenged with U46619, five minutes after dosing with test compound and then successively every ten minutes until the hypertensive effect of U46619 is no longer blocked. The TXA₂ synthase inhibitory properties of a test compound may be demonstrated using the standard in vitro test procedure [test (e)] described by Howarth et alia ( Biochem . Soc . Transactions , 1982, 10 , 239 - 240) using a human platelet microsomal TXA₂ synthase preparation and using a quantitative thin layer radiochromatographic method to assess the conversion of [1-¹⁴C]arachidonic acid to the TXA₂ metabolite thromboxane B₂ (TXB₂). The TXA₂ synthase inhibitory properties of a test compound may also be demonstrated in a standard procedure [test (f)] involving obtaining blood samples from laboratory animals (typically rats, but also guinea pigs, rabbits or dogs) dosed with the test compound, generally by the oral route. The samples treated with anti-coagulant are first incubated at 37°C with collagen (at about 100 micro M), then mixed with the cyclooxygenase inhibitor indomethacin (at about 10⁻³ M), centrifuged and the level of the TXA₂ metabolite, TXB₂, determined by a standard radioimmunoassay technique. By comparison of the amount of TXB₂ present in the plasma from animals dosed with test compound with that in the plasma of a control group dosed with placebo, the TXA₂ synthase inhibitory properties may be assessed. In general, compounds of formula I wherein R² are hydroxy show effects in the following ranges in one or more of the above tests:- test (a): pA₂ of > 5.5 test (b): K B of < 1.5 x 10⁻⁶M test (c): dose ratio of > 5, 1 hour after dosing at 10 mg/kg test (d): significant inhibition of U46619 induced hypertension for at least 1 hour following oral dosing at 50 mg/kg or less test (e): IC₅₀ of < 1.0 x 10⁻⁶M test (f): significant inhibition of TXB₂ production 1 hour following a dose of 100 mg/kg or less. No overt toxic or other untoward effects have been observed with representative compounds of formula I having effects in in vivo tests (c), (d) or (f) at several multiples of the minimum effective dose. Compounds of the formula I wherein R² is other than hydroxy in general show lower activity in the above in vitro tests but show similar activity to the R² = hydroxy compounds of formula I in the in vivo tests. By way of illustration, the compound described in Example 2 hereinafter possesses both TXA₂ antagonist and TXA₂ synthase inhibitory properties as indicated by a K B of 6.5 x 10⁻⁷ in test (b), an IC₅₀ of 4.8 x 10⁻⁸M in test (e) and shows up essentially complete inhibition of TXB₂ production up to 5 hours following an oral dose of 25 mg/kg to rats in test (f) without any observable signs of toxicity to the test animals. As stated previously, by virtue of their combined TXA₂ antagonist and TXA₂ synthase inhibitory properties, the compounds of formula I may be used in the therapy or prevention of diseases or adverse conditions in warm-blooded animals in which TXA₂ (or prostaglandins H₂, D₂ and/or F₂ alpha) are involved. In general, a compound of formula I will be administered for this purpose by an oral, rectal, intravenous, subcutaneous, intramuscular or inhalation route, so that a dose in the range, for example 0.01-15 mg/kg body weight, will be given up to four times per day, varying with the route of administration, the severity of the condition and the size and age of the patient under treatment. The compounds of formula I will generally be used in the form of a pharmaceutical composition comprising a compound of formula I or, a pharmaceutically acceptable salt thereof as defined hereinabove, together with a pharmaceutically acceptable diluent or carrier. Such a composition is provided as a further feature of the invention and may be in a variety of dosage forms. For example, it may be in the form of tablets, capsules, solutions or suspensions for oral administration; in the form of a suppository for rectal administration; in the form of a sterile solution or suspension for administration by intravenous or intramuscular injection; in the form of an aerosol or a nebuliser solution or suspension, for administration by inhalation; and in the form of a powder, together with pharmaceutically acceptable inert solid diluents such as lactose, for administration by insufflation. The pharmaceutical compositions may be obtained by conventional procedures using pharmaceutically acceptable diluents and carriers well known in the art. Tablets and capsules for oral administration may conveniently be formed with an enteric coating, for example comprising cellulose acetate phthalate, to minimise contact of the active ingredient of formula I with stomach acids. The pharmaceutical compositions of the invention may also contain one or more agents known to be of value in diseases or conditions intended to be treated; for example a known platelet aggregation inhibitor, hypolipidemic agent, anti-hypertensive agent, thrombolytic agent, beta-adrenergic blocker or a vasodilator may usefully also be present in a pharmaceutical composition of the invention for use in treating a heart or vascular disease or condition. Similarly, by way of example, an anti-histamine, steroid (such as beclomethasone dipropionate), sodium cromoglycate, phosphodiesterase inhibitor or a beta-adrenergic stimulant may usefully also be present in a pharmaceutical composition of the invention for use in treating a pulmonary disease or condition. Still further, a known TXA₂ antagonist, such as a preferred compound described in European patent application, Publication No. 201354, or a known TXA₂ synthase inhibitor such as dazoxiben or furegrelate [U63557] may be present in addition to a compound of the formula I, or a pharmaceutically acceptable salt thereof, in a composition according to the invention in order to modify the overall balance of TXA₂ antagonist and TXA₂ synthase inhibitory effects for the required therapeutic effect in any of the aforesaid diseases or disease conditions. In addition to their aforesaid use in therapeutic medicine in humans, the compounds of formula I are also useful in the veterinary treatment of similar conditions affecting commercially valuable warm-blooded animals, such as dogs, cats, horses and cattle. In general for such treatment, the compounds of the formula I will generally be administered in an analogous amount and manner to those described above for administration to humans. The compounds of formula I are also of value as pharmacological tools in the development and standardisation of test systems for the evaluation of the effects of TXA₂ in laboratory animals such as cats, dogs, rabbits, monkeys, rats and mice, as part of the continuing search for new and improved therapeutic agents. The compounds of formula I may also be used because of their TXA₂ antagonist and synthase inhibitory properties in helping to maintain the viability of blood and blood vessels in warm-blooded animals (or parts thereof) under-going artificial extracorporeal circulation, for example during limb or organ transplants. When used for this purpose a compound of formula I, or a physiologically acceptable salt thereof, will generally be administered so that a steady state concentration in the range, for example, 0.1 to 10 mg. per litre is achieved in the blood. The invention will now be illustrated by the following non-limiting Examples in which, Examples 1 and 17 describe the production of a useful intermediate and, unless otherwise stated:- (i) concentrations and evaporations were carried out by rotary evaporation in vacuo ; (ii) operations were carried out at room temperature, that is in the range 18-26°C; (iii) flash column chromatography was performed on Fluka Kieselgel 60 (catalogue no. 60738) obtained from Fluka AG, Buchs, Switzerland CH-9470; (iv) yields, where given, are intended for the assistance of the reader only and are not necessarily the maximum attainable by diligent process development; (v) proton NMR spectra were normally determined at 90 or 200 MHz in CDCl₃ using tetramethylsilane (TMS) as an internal standard, and are expressed as chemical shifts (delta values) in parts per million relative to TMS using conventional abbreviations for designation of major peaks: s, singlet; m, multiplet; t, triplet; br, broad; d,doublet; (vi) all end-products were isolated as racemates and had satisfactory microanalyses; and (vii) for convenience, racemic end-products are named using " cis " or " trans " nomenclature to depict the relative configuration of substituents about the dioxane ring i.e. in such racemates, the substituents at positions 4 and 5 are referred to as (4,5- cis ) instead of the more precise (4 SR ,5 RS ) notation, which latter notation is used in naming the enantiomeric forms described in Example 40 hereinafter. Example 1 A solution of 2-[(4,5- cis )-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan-5-yl]acetaldehyde (D), (0.20g) in dry tetrahydrofuran (THF) (7ml) was added under argon to a stirred, ice-cooled solution of the ylid prepared from (3-carboxypropyl)triphenylphosphonium bromide (0.91g) and potassium t -butoxide (0.48g) in dry THF (30ml). The mixture was stirred for 2 hours and then treated with ice-cooled water (50ml). The solution was concentrated and more water was added (25ml). The pH was adjusted to 7 by addition of a few crystals of oxalic acid and the solution was extracted with ethyl acetate (3 x 40ml). The aqueous phase was then acidified to pH 4 with oxalic acid and extracted with ethyl acetate (3 x 50ml). These combined extracts were washed with saturated brine (50ml), dried (MgSO₄) and concentrated. The residue was purified by flash column chromatography, eluting with dichloromethane/methanol (95:5, v/v), to give 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid as an oil (0.19g); NMR: 1.55 (3H,s), 1.57 (3H,s), 1.5-2.6 (7H,m), 3.85 (1H,dd, J=12HZ, 1.5Hz), 4.15 (1H,dm, J=12Hz), 5.15-5.50 (3H,m), 7.3-7.4 (1H, m), 7.7-7.8 (1H,m), 8.1 (1H, brs) and 8.45-8.60 (2H, m). The necessary starting material was prepared as follows: (i) Methyl 2-(nicotinoyl)acetate (17.9g, prepared by the method of E Wenkert et al , J. Org. Chem ., 1983, 48 , 5006) was added under argon to a solution of sodium metal (2.3g) in methanol (200ml) and the resulting mixture was stirred at 25°C for 30 mins. Allyl bromide (12.0g) was then added and stirring was continued overnight. A further amount (about 2g) of allyl bromide was added, the mixture was stirred for 48 hours, and then concentrated. The residual oil was partitioned between water and ether and the aqueous layer was extracted three times with ether. The combined extracts were washed with saturated brine, dried (MgSO₄) and concentrated. The residue was purified by flash column chromatography, eluting with a mixture of petroleum ether (b.p. 60-80°C) and ethyl acetate (1:1, v/v) to give methyl-2-nicotinoyl-4-pentenoate (A) as a pale yellow oil (13.8g); NMR 2.6-2.9 (2H,m), 3.7 (3H,s), 4.4 (1H,m), 4.9-5.2 (2H,m), 5.5-6.0 (1H,m), 7.2-7.5 (1H,m), 8.1-8.3 (1H,m), 8.7-8.8 (1H,m) and 9.1-9.2 (1H,m). (ii) A solution of A (8.8g) in dry THF (40ml) was added to suspension of lithium aluminium hydride (1.8g) in dry THF (80ml) under argon at such a rate that the temperature did not exceed 10°C. After 2 hours the mixture was cooled in ice. Ethyl acetate (20ml) was then added to destroy excess reagent, followed by saturated aqueous ammonium chloride (50ml). The precipitate was removed by filtration and washed with ethyl acetate. The aqueous phase was separated and extracted with ethyl acetate (3 x 50ml). The combined organic fractions were washed with saturated brine, dried (MgSO₄) and concentrated. The residue was purified by flash column chromatography, eluting with a mixture of ethyl acetate and methanol, (95:5 v/v), to give 2-allyl-1-(3-pyridyl)-1,3-propanediol (B) (5.3g), as an oil (mixture of epimers); NMR: 1.8-2.2 (3H,m), 3.6-4.1 (4H,m), 4.7-5.2 (3H,m), 5.6-5.9 (1H,m), 7.2-7.4 (1H,m), 7.65-7.8 (1H,m) and 8.4-8.6 (2H,m). (iii) A mixture of B (5.2g), p -toluenesulphonic acid (5.2g), and 2,2-dimethoxypropane (50ml), was stirred overnight at room temperature. The pH was adjusted to 8-10 by addition of triethylamine and the solution was concentrated under reduced pressure. The residue was purified by flash column chromatography, eluting with a mixture of petroleum ether (b.p. 40-60°) and ethyl acetate (60:40 v/v) to give 5-allyl-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxane (C) (mixture of 4,5- cis and trans isomers) as an oil (4.6g); NMR: 1.4-1.6 (6H,m), 1.6-2.5 (3H,m), 3.65- 4.25 (2H,m), 4.5-5.7 (4H,m), 7.2-7.4 (1H,m), 7.6-7.8 (1H,m) and 8.45- 8.65 (2H,m). (iv) Ozone in oxygen was bubbled through a solution of C (3.4g), in ethyl acetate (130ml) at -70°C until a blue colour persisted throughout. Argon was then bubbled through the solution to discharge the excess ozone and a solution of triphenylphosphine (6g) in ethyl acetate (50ml) was added. The mixture was allowed to warm to room temperature and then stirred overnight. The solution was concentrated and ether (50ml) was added to precipitate triphenylphosphine oxide. The mixture was filtered and the filtrate was concentrated to give an oil which was purified by flash column chromatography, eluting with a mixture (60:40 v/v) of ethyl acetate and petroleum ether (b.p. 40-60°) to give initially 2-[(4,5- cis ]-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan-5-yl]acetaldehyde (D) as an oil (0.8g); NMR: 1.5 (3H,s), 1.55 (3H,s), 2.0-2.3 (1H,m), 2.3- 2.5(1H,m), 2.8-3.0 (1H,m), 3.8 (1H,dd, J=12Hz, 1.5Hz), 4.3 (1H,dm, J=12Hz), 5.25 (1H,d, J=3Hz), 7.25-7.35 (1H,m), 8.45-8.60 (2H,m) and 9.6 (1H,s); and then the corresponding 4,5- trans isomer; NMR: 1.47 (3H,s), 1.57 (3H,s), 2.0-2.6 (3H,m), 3.75-4.05 (2H,m), 4.68 (1H,d, J=10Hz), 7.25-7.40 (1H,m), 7.70-7.80 (1H,m), 8.50-8.65 (2H,m) and 9.5 (1H,br s); as an oil (0.7g). Example 2 A mixture of 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5yl]hexenoic acid (0.458g), 2-chlorobenzaldehyde, (0.84ml), and p -toluenesulphonic acid, (0.314g), was stirred at 25°C for 60 hours. The solution was made basic by addition of triethylamine and the entire reaction mixture was then purified by flash column chromatography, eluting first with dichloromethane to give unreacted aldehyde and second with dichloromethane/methanol (95:5, v/v) to give 4(Z)-6-[(2,4,5- cis )-2-(2-chlorophenyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid (0.16g), as an oil; NMR: 1.6-2.7 (7H,m), 4.1-4.4 (2H,m), 5.20-5.55 (3H,m), 6.05 (1H,s), 7.2-7.5 (5H,m), 7.65-7.95 (2H,m) and 8.4-8.6 (2H,m). Example 3 Using an analogous procedure to that described in Example 2 but starting from 5(Z)-7-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl)heptenoic acid (E) and 2-chlorobenzaldehyde, there was obtained 5(Z)-7-[(2,4,5- cis )-2-(2-chlorophenyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]heptenoic acid as an oil in 47% yield; NMR: 1.5-2.7 (9H,m), 4.1-4.4 (2H,m), 5.2-5.5 (3H,m), 6.05 (1H,s), 7.2-7.5 (5H,m), 7.7-7.9 (2H,m) and 8.45-8.65 (2H,m). The starting heptenoic acid (E) was obtained using an analogous procedure to that described in Example 1 for the corresponding hexenoic acid, except that (4-carboxybutyl)triphenylphosphonium bromide was used in place of (3-carboxypropyl)triphenylphosphonium bromide. The heptenoic acid (E) was obtained as an oil in 40% yield; NMR: 1.55 (3H,s), 1.57 (3H,s), 1.5- 2.6 (9H,m), 3.85 (1H,dd, J=12Hz; 1.5Hz), 4.15 (1H,dm J=12Hz), 5.15-5.50 (3H,m), 6.6 (1H, brs), 7.3-7.4 (1H,m), 7.7-7.8 (1H,m) and 8.45- 8.60 (2H,m). Example 4 Using a similar procedure to that described in Example 2, but starting from 2-phenoxy-2-methylpropanal instead of 2-chlorobenzaldehyde, there was obtained 4(Z)-6-[(2,4,5- cis )-2-(1-methyl-1-phenoxyethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid as a colourless oil (28% yield), which solidified on standing; NMR: 1.35 (3H,s), 1.40 (3H,s), 1.5-2.6 (7H,m), 3.9-4.3 (2H,m), 4.75 (1H,s), 5.1 (1H,d, J=2Hz), 5.15-5.55 (2H,m), 6.95-7.15 (3H,m), 7.2-7.4 (3H,m), 7.60-7.75 (1H,m) and 8.5-8.6 (2H,m). The starting aldehyde was prepared as described in European patent application, publication no. 201351 A2, Example 6. Example 5 3-Pyridinecarboxaldehyde (0.365ml) and p -toluenesulphonic acid (1.08g) were added to a solution of 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid (0.393g) in acetonitrile (8ml), under an atmosphere of argon. The mixture was heated at reflux for 4 hours and then allowed to cool. Ethyl acetate (10ml) was added and the mixture was extracted with 1M sodium hydroxide solution (50ml). The combined extracts were acidified to pH 4 with acetic acid and extracted with ethyl acetate (4x20ml). The combined organic extracts were dried (MgSO₄), and concentrated to give an oil, which was purified by flash column chromatography, eluting with methanol/dichloromethane (1:10 to 1:5 v/v), to give 4(Z)-6-[(2,4,5- cis )-2,4-bis-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid, (0.231g) as an oil; NMR: 1.6-1.9(2H,m), 2.3-2.7(5H,m), 4.1-4.35(2H,m), 5.2-5.55(3H,m), 5.8(1H,s), 7.3-7.4(2H,m), 7.9-8.0(1H,m) and 8.5-8.85(4H,m). Examples 6-16 Using an analogous procedure to that described in Example 5, but replacing 3-pyridinecarboxaldehyde by the appropriate aldehyde of the formula R⁴.CHO, the following acids of the formula III (A³=ethylene) were obtained as oils in yields of 14-86% :- The starting aldehyde for Example 7, 2-methyl-2-(3-pyridyloxy)propionaldehyde, was prepared as follows: (i) A solution of 3-hydroxypyridine (4.75g) in 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) (10ml) was added dropwise over 30 minutes to a stirred, ice-cooled, suspension of sodium hydride (50% w/w dispersion in mineral oil, 2.4g) in DMPU (40ml). The mixture was heated to 50°C. to give a clear solution and then cooled to 4°C. Ethyl 2-bromo-2-methylpropionate (4.38ml) and potassium iodide (100mg) were next added and the mixture stirred at ambient temperature for 16 hours. The mixture was poured into water (50ml) and extracted with ether (3x50ml). The combined extracts were washed with water (2x25ml), saturated brine (25ml), dried (MgSO₄) and evaporated. Purification by flash chromatography eluting with ether/hexane (1:1 v/v) gave ethyl 2-methyl-2-(3-pyridyloxy)propionate (A) as a clear oil (34%); NMR: 1.27 (3H,t,J=7Hz), 1.61 (6H,s), 4.25 (2H,q,J=7Hz), 7.19 (2H,m), 8.27 (2H,m). (ii) A 1.5M solution of diisobutylaluminium hydride in toluene (21ml) was added dropwise under argon to a stirred solution of A (2.09g) in toluene (75ml) at -70°C. Stirring was continued for 5 minutes after the addition was complete and then a 10% v/v solution of methanol in toluene (15ml) was added. The mixture obtained was added to water (300ml), vigorously stirred for 30 minutes and then filtered through kieselguhr. The organic phase was separated and the aqueous phase was saturated with sodium chloride and then extracted with ether (2x100ml). The combined organic phases were washed with saturated brine (3x100ml), then dried (MgSO₄) and evaporated. Purification of the residue by MPLC, eluting with ethyl acetate/hexane (1:1 v/v), gave 2-methyl-2-(3-pyridyloxy)propionaldehyde as a clear oil (56%); NMR: 1.46 (6H,s), 7.20 (2H,m), 8.31 (2H,m), 9.34 (1H,s). The starting aldehyde for Example 10, 2-(3-bromophenoxy)-2-methylpropanal, was prepared as follows: (i) A solution of methyl dichloroacetate (77.18g, 0.54mol) in anhydrous ether (50ml) was added to a stirred solution of methyl magnesium iodide [prepared from magnesium turnings (32.8g, 1.35mol) and methyl iodide (84.1ml, 1.35mol)] in anhydrous ether (750ml) at 0°C under an argon atmosphere, at such a rate that the temperature did not rise above 15°C. The mixture was stirred at 25°C for 30 minutes then cooled to 0°C. Water (100ml) was added and the mixture was acidified to pH4 with concentrated hydrochloric acid. The organic phase was separated and the aqueous phase extracted with ether (3x100ml). The combined organic phases were dried (MgSO₄) and concentrated. The residual oil was distilled under reduced pressure to give 1,1-dichloro-2-hydroxy2-methylpropane (A) (57.81g), as an oil; b.p. 48-50°C at 20mmHg; NMR: 1.45 (6H,s), 2.15 (1H,br s) and 5.65 (1H,s). (ii) Cetyltrimethyl ammonium bromide (0.28g, 0.77mmol) was added to a solution of m-bromophenol (6.66g, 38.5mmol) in a 3.85M aqueous sodium hydroxide solution (10ml), followed by a solution of A (1.37g, 9.6mmol) in ether (20ml). The mixture was stirred under an argon atmosphere for 18 hours then diluted with ether (50ml), and extracted with 2M aqueous sodium hydroxide solution (4x30ml), to remove unreacted phenol. The combined aqueous extracts were extracted with ether (50ml), and the organic phase was washed with 2M aqueous sodium hydroxide solution (20ml) followed by water (50ml). The combined organic phases were dried (MgSO₄), concentrated, and purified by flash column chromatography, eluting with ethyl acetate/hexane (1:10 v/v), to give 2-(3-bromophenoxy)-2-methylpropanal (0.89g), as an oil; NMR: 1.45 (6H,s), 6.75-7.20 (4H,m), 9.8 (1H,s). Using an analogous procedure to that described for the preparation of 2-(3-bromophenoxy)-2-methylpropanal, but starting from the appropriately substituted phenol, the following aldehydes used in Examples 11, 12, 13, and 15 were obtained: 2-(4-bromophenoxy)-2-methylpropanal; NMR: 1.4 (6H,s), 6.7-7.4 (4H,m), 9.8 (1H,s); 2-(4-fluorophenoxy)-2-methylpropanal; NMR: 1.4 (6H,s), 6.8-7.0 (4H,m), 9.8 (1H,s); 2-(3-fluorophenoxy)-2-methylpropanal; NMR: 1.45 (6H,s), 6.55-7.3 (4H,m), 9.8 (1H,s); and 2-(4-cyanophenoxy)-2-methylpropanal; NMR: 1.5 (6H,s), 6.85-7.6 (4H,m), 9.75 (1H,s). The starting aldehyde for Example 14 was prepared by the method described of H K Diefl and K C Brannock, Tetrahedron Letters, 1973, 14 , 1273. Example 17 Using an analogous procedure to that described in Example 1 but starting from 2-[(4,5- cis )-2,2-dimethyl-4-(4-pyridyl)-1,3-dioxan-5-yl]acetaldehyde, there was obtained 4(Z)-6-[2,2-dimethyl-4-(4-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid as an oil, which solidified on standing, in 7% yield; m.p. 167-169 °C (after recrystallisation from ethyl acetate/petroleum ether); NMR: 1.42(3H,s), 1.49(3H,s), 1.7-2.5(7H,m), 3.66(1H,d,J=12Hz), 4.12(1H,d,J=12Hz), 5.1-5.42(3H,m), 7.30(2H,d), and 8.52(2H,d). The above starting material was obtained as an oil, on 50% yield, using an analogous procedure to that described in Example 1; NMR: 1.5 (3H,s), 1.55 (3H,s), 2.0-2.3 (1H,m), 2.3-2.5 (1H,m), 2.8-3.0 (1H,m), 3.8 (1H,dd,J=12Hz,1.5Hz), 4.3 (1H, dm,J=12Hz), 5.2 (1H,d,J=3Hz), 7.25 (2H,d), 8.6 (2H,d) and 9.62 (1H,s); starting from methyl 3-(4-pyridyl)-3-oxo-propionate, which was prepared using a similar procedure to that of E.Wenkert et al , J . Org . Chem ., 1983, 48, 5006. The following intermediates analogous to those in Example 1 were obtained as oils and used without further purification:- (i) Methyl-2-isonicotinoyl-4-pentenoate, in 65% yield. (ii) 2-Allyl-1-(4-pyridyl)-1,3-propanediol, in 77% yield. (iii) 5-Allyl-2,2-dimethyl-4-(4-pyridyl)-1,3-dioxane (mixture of 4,5- cis and trans isomers), in 44% yield. Example 18 Using an analogous procedure to that described in Example 5 but starting from 4(Z)-6-[2,2-dimethyl-4-(4-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid and 2-chlorobenzaldehyde, there was obtained 4(Z)-6-[(2,4,5- cis )-2-(2-chlorophenyl)-4-(4-pyridyl)-1,3-dioxan-5-yl]hexenoic acid as an oil, in 21% yield; NMR: 1.6-2.7 (7H,m), 3.59 (1H,d,J=10.7Hz), 4.35 (1H,dd,J=10.7Hz,4.8Hz), 4.6 (1H,d,J=10.7Hz), 5.18-5.5 (2H,m), 5.98 (1H,s), 7.2-7.8 (6H,m) and 8.63 (2H,br s). Examples 19-29 Using an analogous procedure to that described in Example 5, but replacing 3-pyridinecarboxaldehyde by the appropriate aldehyde of the formula R⁴.CHO, the following acids of the formula III (A³=ethylene) were obtained. Using an analogous procedure to that described by R Subramanian, Chem . and Ind ., 1978, page 731, for the preparation of 2,2-dimethyl-3-phenylpropanal, but starting from the appropriately substituted benzyl halide, the following aldehydes used in Examples 23 and 25 were obtained: 3-(3-bromophenyl)-2,2-dimethylpropanal; NMR: 1.05(6H,s), 2.75(2H,s), 7.0-7.4(4H,m), 9.55(1H,s); 3-(4-fluorophenyl)-2,2-dimethylpropanal; NMR: 1.05(6H,s), 2.75(2H,s), 6.9-7.1(4H,m), 9.55(1H,s); Using an analogous procedure to that described for the preparation of 2-(3-bromophenoxy)-2-methylpropanal, but starting from the appropriately substituted phenol, the following aldehydes used in Examples 24, 27 and 28 were obtained: 2-(3,4-dichlorophenoxy)-2-methylpropanal; NMR: 1.45(6H,s), 6.7-7.35(3H,m), 9.75(1H,s); 2-(2-fluorophenoxy)-2-methylpropanal; NMR: 1.4(6H,s), 6.9-7.15(4H,m), 9.85(1H,s). 2-(3,4-difluorophenoxy)-2-methylpropanal; NMR: 1.4(6H,s), 6.55-7.1(3H,m), 9.8(1H,s). The starting aldehyde for Example 26, 2-(4-nitrophenoxy)-2-methylpropanal, was prepared using an analogous procedure to that described for the preparation of 2-methyl-2-(3-pyridyloxy)propionaldehyde, using 4-nitrophenol instead of 3-hydroxypyridine; NMR: 1.55(6H,s), 6.9(2H,d,J=7Hz), 8.15(2H,d,J=7Hz), 9.8(1H,s). Example 30 p -Toluenesulphonic acid (0.33g) was added to a solution of 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid (0.482 g) in acetonitrile (15 ml) and the mixture was stirred for 30 minutes. 2-phenoxyacetaldehyde diethyl acetal (1.04 g) was added and the mixture was heated at 90°C for 15 hours. The mixture was then allowed to cool and concentrated. The residual oil, which contained the required acid product together with its ethyl ester, was dissolved in methanol (6 ml). Aqueous 2M sodium hydroxide solution (3 ml) was added and the mixture was stirred for 1 hour. Ethyl acetate (25 ml) and water (25 ml) were added and the mixture was acidified with acetic acid and extracted with ethyl acetate (4 x 25 ml). The combined organic extracts were dried (MgSO₄), and concentrated. The resultant oil was purified by flash column chromatography, eluting with dichloromethane and increasing to methanol/dichloromethane (7:93 v/v), to give 4(Z)-6-[(2,4,5- cis )-2-phenoxymethyl-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid (0.146 g) as an oil; NMR: 1.65-1.85(2H,m), 2.2-2.6(5H,m), 4.0-4.25(4H,m), 5.1-5.45(4H,m), 6.9-7.45(6H,m), 7.75-7.8(1H,m) and 8.5-8.6(2H,m). The starting material, 2-phenoxyacetaldehyde diethyl acetal, was obtained as follows:- Sodium hydride (5.83 g of 55% dispersion in mineral oil) was added to a solution of phenol (12.56 g) in DMPU (25 ml) at 5°C and the mixture was stirred for 30 minutes. Bromoacetaldehyde diethyl acetal (10.05 ml) was added and the mixture was heated at 110°C for 5 hours, then allowed to cool. The mixture was partitioned between ethyl acetate (100 ml) and water (100 ml) and the organic phase was separated and washed sequentially with aqueous 2M sodium hydroxide solution (2 x 50 ml) and water (50 ml). The aqueous fractions were combined and re-extracted with ethyl acetate (100 ml). The combined organic extracts were dried (MgSO₄) and concentrated. The resultant oil was purified by flash column chromatography, eluting with ethyl acetate/hexane (1:10 v/v) to give 2-phenoxyacetaldehyde diethyl acetal (9.43 g) as an oil; NMR: 1.25(6H,t, J=7.0Hz), 3.6-3.85(4H,m), 4.05(2H,d, J=6.0Hz), 4.85 (1H,t, J=6.0Hz) and 6.9-7.35(5H,m). Example 31 p -Toluenesulphonic acid (0.358 g) was added to a solution of 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl[hexenoic acid (0.522 g) in acetonitrile (12 ml) and the mixture was stirred for 30 minutes. A solution of 2-(3,4-difluorophenoxy)-2-methylpropanal (1.02 g) in acetonitrile (5 ml) was added, followed by trimethyl orthoformate (0.21 ml), and the mixture was heated at reflux for 3 hours under an atmosphere of argon. To effect complete esterification, methanol (1 ml) was added and the solution was heated at reflux for a further 2 hours. The reaction mixture was allowed to cool and partitioned between aqueous 1M sodium hydroxide solution (2 ml) and ethyl acetate (25 ml). The organic phase was separated, dried (MgSO₄), and concentrated. The resultant oil was purified by flash column chromatography, eluting with methanol/dichloromethane (1:100 to 1:20, v/v), to give methyl 4(Z)-6-[(2,4,5- cis )-2-(1-methyl-1-(3,4-difluorophenoxy)ethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoate (0.333 g), as an oil; NMR: 1.37(3H,s), 1.40(3H,s), 1.5-1.8(2H,m), 2.2-2.6(5H,m), 3.65(3H,s), 3.9-4.3(2H,m), 4.75(1H,s), 5.05-5.5(3H,m), 6.7-7.7(5H,m) and 8.5-8.6(2H,m). Example 32 A mixture of 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid (0.500 g), 2-methyl-2-propoxypropionaldehyde (2.13 g) and p -toluenesulphonic acid monohydrate (0.342 g) was stirred for 18 hours. 0.2M Sodium hydroxide solution (20 ml) was added and the mixture was washed with ether (2 x 10 ml), acidified to pH 5 with acetic acid, and then extracted with ether (3 x 25 ml). The combined ether extracts were washed with water (2 x 10 ml), saturated sodium chloride solution (10 ml), and dried (MgSO₄). The organic extracts were concentrated to give a brown oil, which was purified by medium pressure liquid chromatography (MPLC), eluting with ethyl acetate/hexane/acetic acid (80:20:1 v/v), to give a clear oil which on trituration with ether gave 4(Z)-6-[(2,4,5- cis )-2-(1-methyl-1-propoxyethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid, 0.25 hydrate (0.053 g) as a solid, m.p. 116-118°C; NMR (200 MHz, d₆ DMSO): 0.83(3H,t, J=7Hz), 1.18(3H,s), 1.20(3H,s), 1.42(3H,m), 1.84(1H,m), 2.14(4H,m), 2.35(1H,m), 3.40(2H,t, J=6Hz), 3.94(2H,m), 4.65(1H,s), 5.15(1H,d, J=2Hz), 5.18(1H,m), 5.34(1H,m), 7.38(1H,m), 7.68(1H,dm, J=7Hz), 8.49(2H,m); microanalysis, found: C,65.8; H,8.3; N,3.7%, C₂₁H₃₁NO₅, 0.25 H₂O requires: C, 66.0; H,8.3; N,3.7%. The starting aldehyde was prepared as described in European patent application, publication no. 201351 A2, Example 7. Example 33 Using an analogous procedure to that described in Example 5, but replacing 3-pyridinecarboxaldehyde with cyclohexanecarboxaldehyde, and carrying out the reaction at ambient temperature in the presence of only 1.1 equivalents of p -toluenesulphonic acid monohydrate, there was obtained 4(Z)-6-[(2,4,5- cis )-2-cyclohexyl-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid hydrate as a white solid (47% yield), m.p. 121-125°C; NMR (200 MHz, CDCl₃): 1.22 (5H,m), 1.74(8H,m), 2.29(4H,m), 2.44(1H,m), 3.89(1H,d, J=11Hz), 4.12 (1H,d, J=11Hz), 4.51(1H,d, J=4Hz), 5.00 (1H,d, J=1.5Hz), 5.22(1H,m), 5.38(1H,m), 7.33(1H,m), 7.72(1H,d, J=7Hz),8.53(2H,m); microanalysis, found: C,67.2; H,8.1; N,3.7%; C₂₁H₂₉NO₄, 1H₂O requires: C,66.8; H,8.2; N,3.7%. Example 34 1M Sodium hydroxide solution (6.28 ml) was added to a stirred solution of methyl 4(Z)-6-[(2,4,5- cis )-4-(3-pyridyl)-2-trifluoromethyl-1,3-dioxan-5-yl]hexenoate (A) (563 mg) in methanol (10 ml). After 2 hours, water (40 ml) was added and the mixture was washed with ether (2 x 20 ml), acidified to pH 5 with acetic acid, and then extracted with ethyl acetate (3 x 30 ml). The combined organic extracts were washed with water (20 ml), saturated sodium chloride solution (2 x 20 ml), and then dried (MgSO₄). The solvent was removed by evaporation to give an oil which was purified by MPLC, eluting with ethyl acetate/methanol/acetic acid (95:5:1 v/v) to give 4(Z)-6-[(2,4,5- cis )-4-(3-pyridyl)-2-trifluoromethyl-1,3-dioxan-5-yl]hexenoic acid monoacetate adduct as an oil (587 mg); NMR (200 MHz, CDCl₃): 1.71(1H,m), 1.83(1H,m), 2.10(3H,s), 2.30(4H,m), 2.51(1H,m), 4.05(1H,dm, J=11Hz), 4.30(1H,d, J=11Hz), 5.12(1H,q, J=3Hz), 5.20(1H,d, J=2Hz), 5.22(1H,m), 5.46(1H,m), 7.42(1H,m), 7.80(1H,d, J=7Hz), 8.59(2H,b); microanalysis, found: C,53.3; H,5.6; N,3.4%; C₁₆H₁₈NO₄F₃, 1CH₃COOH requires: C,53.5; H,5.4; N,3.5%. The necessary starting material A was prepared as follows: (i) 1M Hydrochloric acid (10 ml) was added to a solution of 4(Z)-6-[2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan- cis -5-yl]hexenoic acid (1.42 g) in THF (15 ml) and the mixture was stirred for 2 hours. Water (40 ml) was added and the pH adjusted to 12 with 2M sodium hydroxide solution. The mixture was washed with ethyl acetate (2 x 25 ml), acidified to pH 5 with acetic acid, and then saturated with solid sodium chloride. The aqueous mixture was then extracted with ethyl acetate (12 x 50 ml) and the combined extracts were dried (MgSO₄). The solvent was removed by evaporation to give 4(Z)- erythro -8-hydroxy-7-hydroxymethyl-8-(3-pyridyl)-4-octenoic acid (B) as a brown oil (1.114 g), which was used without further purification. For the purposes of characterisation, a sample was purified by flash chromatography eluting with methanol/dichloromethane (1:5 v/v); NMR (200 MHz, CDCl₃): 1.91(3H,m), 2.23(5H,m), 3.59(2H,m), 5.02(1H,m), 5.35(3H,m), 7.30(1H,m), 7.76(1H,m), 8.46(1H,dd,J=4 and 1Hz), 8.60(1H,d,J=2Hz). (ii) p -Toluenesulphonic acid monohydrate (1.06 g) was added to a solution of B (1.114 g) in methanol (25 ml) and the mixture was stirred for 3 hours. Triethylamine (0.83 ml) was added and the mixture was concentrated to a small volume. Saturated sodium chloride solution (20 ml) was added and the mixture was extracted with ethyl acetate (4 x 25 ml). The combined organic extracts were washed with saturated sodium chloride solution (10 ml), dried (MgSO₄) and the solvent was removed by evaporation. The resultant oil was purified by MPLC, eluting with methanol/dichloromethane (1:12 v/v) to give methyl 4(Z)- erythro -8-hydroxy-7-hydroxymethyl-8-(3-pyridyl)-4-octenoate (C) as an oil (1.044 g); NMR (250 MHz, CDCl₃): 1.82(2H,m), 2.16(1H,m), 2.44(4H,m), 4.91(2H,b), 3.67(3H,s), 3.81(2H,d, J=3Hz), 5.20(1H,d, J=2Hz), 5.30(2H,m), 7.33(1H,m), 7.79(1H,m), 8.51(1H,m), 8.61(1H,m). (iii) A solution of methanesulphonyl chloride (0.32 ml) in dichloromethane (2.0 ml) was added during ten minutes to a stirred solution of C (995 mg) and triethylamine (0.59 ml) in dichloromethane (20 ml). The mixture was stirred for a further 1 hour and then diluted with ethyl acetate (50 ml). The subsequent mixture was washed with water (2 x 15 ml), saturated sodium chloride solution (15 ml), and dried (MgSO₄). The solvent was removed by evaporation to give an oil which was purified by MPLC, eluting with methanol/dichloromethane (1:32 v/v), to give methyl 4(Z)- erythro -8-hydroxy-7-(methylsulphonyloxymethyl)-8-(3-pyridyl)-4-octenoate (D) as a colourless oil (886 mg); NMR (250 MHz, CDCl₃): 2.24(8H, m), 3.01(3H,s), 3.68(3H,s), 4.10(1H,m), 4.31(1H,m), 5.02(1H,d, J=2H), 5.38(2H,m), 7.34(1H,m), 7.77(1H,d, J=7Hz), 8.57(2H,m). (iv) Anhydrous potassium carbonate (994 mg) and trifluoroacetaldehyde hydrate (1.13 g) were added to a solution of D (857 mg) in dry THF (10 ml). The mixture was stirred for 15 minutes at ambient temperature and then at 60°C for 5 hours. The mixture was then diluted with ethyl acetate (75 ml) and washed with water (25 ml), followed by saturated sodium chloride solution (25 ml). The organic phase was dried (MgSO₄) and the solvent was removed by evaporation. The resultant oil was purified by MPLC, eluting with ethyl acetate/hexane (7:3 v/v) to give, firstly, methyl 4(Z)-6-[2,4- trans , 4,5- cis )-4-(3-pyridyl)-2-trifluoromethyl-1,3-dioxan-5-yl]hexenoate as a colourless oil (137 mg); NMR (250 MHz, CDCl₃): 1.66 (1H,m), 2.02(1H,m), 2.29(4H,m), 2.43(1H,m), 3.66(3H,s), 3.96(1H,dd, J=11 and 2Hz), 4.36(1H,dm, J=11Hz), 5.20(1H,m), 5.30(1H,q, J=6Hz), 5.42(1H,m), 5.48(1H,d, J=2Hz), 7.35(1H,m), 7.71(1H,m), 8.59(2H,m), and then, methyl 4(Z)-6-[(2,4,5- cis )-4-(3-pyridyl)-2-trifluoromethyl-1,3-dioxan-5-yl]hexenoate (A) as a colourless oil (578 mg); NMR (250 MHz, CDCl₃): 1.60(1H,m), 1.81(1H,m), 2.30(4H,m), 2.55(1H,m), 3.66(3H,s), 4.04(1H,dm,J=11Hz), 4.29(1H,d,J=11Hz), 5.12(1H,q, J=3Hz), 5.19(1H,d, J=2Hz), 5.22(1H,m), 5.45(1H,m), 7.38(1H,m), 7.74(1H,m), 8.58(2H,m). Examples 35-39 Using an analogous procedure to that described in Example 5, but replacing 3-pyridinecarboxaldehyde by the appropriate aldehyde of the formula R⁴.CHO, the following acids of the formula III (A³=ethylene) were obtained. Example R⁴ ¹H NMR (ppm) 35 3NO₂-PhO.C(CH₃)₂- 1.45(3H,s), 1.47(3H,s), 1.55-1.8(2H,m), 2.2-2.55(5H,m), 3.95-4.3(2H,m), 4.8(1H,s), 5.1-5.55(3H,m), 7.3-8.0(6H,m), 8.5-8.6(2H,m). 36 2NO₂-PhO.C(CH₃)₂- 1.45(6H,s), 1.5-1.8(2H,m), 2.15-2.5(5H,m), 3.9-4.2(2H,m), 4.85(1H,s), 5.1-5.5(3H,m), 7.1-7.8(6H,m), 8.4-8.6(2H,m). 37 2,4-Cl₂-PhO.C(CH₃)₂- 1.46(3H,s), 1.48(3H,s), 1.55-1.8(2H,m), 2.15-2.55(5H,m), 3.95-4.25(2H,m), 4.95(1H,s), 5.1-5.55(3H,m), 7.1-7.65(5H,m), 8.45-8.65(2H,m). 38 4NO₂-PhCH₂C(CH₃)₂- 1.0(3H,s), 1.02(3H,s), 1.5-1.75(2H,m), 2.25-2.6(5H,m), 2.85(2H,s), 3.8-4.2(2H,m), 4.3(1H,s), 5.0-5.55(3H,m), 7.25-8.1(6H,m), 8.5-8.65(2H,m). 39 3NO₂-PhCH₂C(CH₃)₂- 1.02(3H,s), 1.04(3H,s), 1.5-1.75(2H,m), 2.25-2.6(5H,m), 2.85(2H,s), 3.85-4.25(2H,m), 4.3(1H,s), 5.0-5.55(3H,m), 7.35-8.1(6H,m), 8.5-8.65(2H,m). Using an analogous procedure to that described for the preparation of 2-methyl-2-(3-pyridyloxy)propionaldehyde, but starting from the appropriately substituted phenol, the following aldehydes used in Examples 35, 36 and 37 were obtained: 2-(3-nitrophenoxy)-2-methylpropanal; NMR: 1.5(6H,s), 7.15-7.95(4H,m), 9.85(1H,s): 2-(2-nitrophenoxy)-2-methylpropanal; NMR: 1.5(6H,s), 6.9-7.8(4H,m), 9.85(1H,s); 2-(2,4-dichlorophenoxy)-2-methylpropanal; NMR: 1.45(6H,s), 6.8-7.4(3H,m), 9.85(1H,s). Using an analogous procedure to that described by R Subramanian, Chem . and Ind ., 1978, page 731, for the preparation of 2,2-dimethyl-3-phenylpropanal, but starting from the appropriately substituted benzyl halide, the following aldehydes used in Examples 38 and 39 were obtained: 3-(4-nitrophenyl)-2,2-dimethylpropanal; NMR: 1.1(6H,s), 2.9(2H,s), 7.3(2H,d,J=8Hz), 8.15(2H,d,J=8Hz), 9.55(1H,s); 3-(3-nitrophenyl)-2,2-dimethylpropanal; NMR: 1.1(6H,s), 2.9(2H,s), 7.45-8.15(4H,m), 9.6(1H,s). Example 40 Using an analogous procedure to that described in Example 5, but starting from 4( Z )-6-[( 4S ,5 R )-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid and 2-(4-bromophenoxy)-2-methylpropanal, there was obtained 4( Z )-6-[(2 S ,4 S ,5 R )-2-[1-(4-bromophenoxy)-1-methylethyl]-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid as an oil with ²⁵[α] D -98.5° (EtOH, c 0.48) and NMR essentially identical with that of the racemic material described in Example 11. The starting 2,2-dimethyl-1,3-dioxane derivative was obtained as follows:- (i) A 1.53M solution of butyllithium in hexane (23.9 ml) was added to a solution of 4S-(-)-isopropyl-2-oxazolidinone (4.68 g) in dry THF (75 ml), cooled to -78°C under argon. The mixture was allowed to warm to -50°C and then stirred for 30 minutes. The mixture was then recooled to -78°C and a solution of 4-pentenoyl chloride (4.33 g) in dry THF (10 ml) was added dropwise. After the addition, the mixture was stirred at -78°C for 30 minutes, and then allowed to warm to -20°C. Saturated aqueous ammonium chloride solution (20 ml) was added and the mixture was extracted with ethyl acetate (3 x 100 ml). The combined organic phases were dried (MgSO₄) and concentrated. The residue was purified by flash column chromatography, eluting with ethyl acetate/hexane (20:80 v/v) to give (4 S )-4-isopropyl-3-(4-pentenoyl)oxazolidin-2-one (A) (6.34 g), as an oil; NMR: 0.85-0.95(6H,m), 2.3-2.5(3H,m), 2.9-3.2(2H,m), 4.15-4.5(3H,m), 4.95-5.15(2H,m), 5.75-6.0(1H,m). (ii) A 1M solution of dibutylboron triflate in dichloromethane (32.7 ml) was added to a solution of A (6.28 g) in dry dichloromethane (110 ml), cooled to 5°C under argon, followed by diisopropylethylamine (6.25 ml). The reaction mixture was stirred at 5°C for 30 minutes and then cooled to -78°C. 3-Pyridinecarboxaldehyde (3.1 ml) was added dropwise. The mixture was stirred for 30 minutes at -78°C, and then allowed to warm to -50°C over 30 minutes. The cooling bath was removed and the reaction mixture was stirred at room temperature for 2 hours. The mixture was then cooled to 5°C and hydrogen peroxide (11.5 ml, 30% w/v aqueous solution) was added. The mixture was stirred for 30 minutes and then poured into water (50 ml) and extracted with dichloromethane (3 x 100 ml). The combined extracts were dried (MgSO₄) and evaporated. The residue was purified by flash column chromatography, eluting with ethyl acetate/hexane (1:1 v/v, gradually increasing to 100% ethyl acetate), to give (4 S )-(3-[(2 S )-2-[(1 S -1-hydroxy-1-(3-pyridyl)methyl]pent-4-enoyl)-4-isopropyloxazolidin-2-one (B), as a solid, m.p. 112-113°C (after recrystallisation from toluene); ²⁵[α] D = +136.0 (EtOH, c 0.311); NMR: 0.85(6H,dd,J=7Hz), 2.15-2.7(4H,m), 4.0-4.2(2H,m), 4.3-4.55(2H,m), 4.95-5.1(3H,m), 5.65-5.9(1H,m), 7.25-7.35(1H,m), 7.75-7.85(1H,m), 8.5-8.65(2H,m). (iii) A 30 wt. % solution of sodium methoxide in methanol (3.65 ml) was added to a solution of B (5.76 g) in methanol (40 ml), cooled to 50°C. The mixture was stirred for 15 minutes and then saturated aqueous ammonium chloride solution (10 ml) and ether (50 ml) were added. Sufficient water was added to dissolve any precipitated inorganics and the mixture was then extracted with ether (3 x 50 ml). The combined extracts were dried (MgSO₄) and evaporated. The residue was purified by flash column chromatography, eluting with ethyl acetate to give methyl (2 S )-2-[(1 S )-1-hydroxy-1-(3-pyridyl)methyl]pent-4-enoate (C) (3.245 g) as an oil; NMR: 2.3-2.6(2H,m), 2.8-2.9(1H,m), 3.6(3H,s), 4.95-5.1(3H,m), 5.65-5.85(1H,m), 7.25-7.35(1H,m), 7.7-7.75(1H,m), 8.45-8.6(2H,m). (iv) A solution of C (3.88 g) in THF (10 ml) was added dropwise to a cooled suspension of lithium aluminium hydride (767 mg) in THF (50 ml) at such a rate to maintain the temperature below 10°C. After the addition was complete, the mixture was stirred at 5°C for 4 hours. Ethyl acetate (20 ml) was added, followed by saturated aqueous ammonium chloride solution (10 ml) and water (10 ml). The mixture was extracted with ethyl acetate (3 x 50 ml). The combined extracts were dried (MgSO₄) and evaporated. The residue was purified by flash column chromatography, eluting with ethyl acetate, gradually increasing to methanol/ethyl acetate (1:9 v/v), to give (1 S ,2 R )-2-allyl-1-(3-pyridyl)-1,3-propanediol (D) (2.69 g), as an oil; NMR: 1.65-1.8(1H,m), 1.95-2.15(2H,m), 3.15-3.45(2H,m), 4.4-4.5(1H,m), 4.75-5.0(3H,m), 5.25(1H,d,J=7Hz), 5.6-5.85(1H,m), 7.3-7.4(1H,m), 7.65-7.7(1H,m), 8.4-8.5(2H,m). (v) p -Toluenesulphonic acid monohydrate (2.91 g) was added to a solution of D (2.68 g) in 2,2-dimethoxypropane (15 ml) and the mixture was stirred for 18 hours. Triethylamine (10 ml) was added and the mixture was partitioned between ether (50 ml) and water (20 ml). The organic layer was dried (MgSO₄) and evaporated. The residue was purified by flash column chromatography, eluting with ethyl acetate/hexane (1:1 v/v) to give (4 S ,5 R )-5-allyl-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxane (E) (2.39 g), as an oil; NMR: 1.53(3H,s), 1.55(3H,s), 1.6-1.75(1H,m), 1.9-2.0(1H,m), 2.3-2.5(1H,m), 3.85-4.2(2H,m), 4.9-5.0(2H,m), 5.27(1H,d,J=3Hz), 5.45-5.7(1H,m), 7.25-7.35(1H,m), 7.65-7.7(1H,m), (8.5-8.6(2H,m). (vi) Ozone was passed through a solution of the allyl compound (E) (530 mg) in methanol (30 ml) cooled to -78°C, until a blue colouration was formed. The mixture was purged with argon before methyl sulphide (1.6 ml) was added. The mixture was then stirred at room temperature for 18 hours, before being concentrated in vacuo and partitioned between ether (50 ml) and water (20 ml). The organic layer was dried (MgSO₄) and evaporated. The residue was purified by flash column chromatography, eluting with a mixture of methanol and methylene chloride (5:95 v/v) to give 2-[4 S ,5 R )-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan-5-yl]acetaldehyde (F), as an oil; NMR: 1.53(3H,s), 1.55(3H,s), 2.15-2.4(2H,m), 2.85-2.95(1H,m), 3.8-3.85(1H,m), 4.25-4.35(1H,m), 5.28(1H,d,J=3Hz), 7.25-7.7(2H,m), 8.5-8.6(2H,m), 9.6(1H,s). [Note: The optical purity was assessed as >99% by proton NMR by addition of ( R )-(-)2,2,2-trifluoro-1-(9-anthryl)ethanol and observing the region 2.7-2.9 (delta), which shows 4 doublets centred at 2.77, 2.71, 2.82 and 2.85 (1H,.C H -CHO)]. (vii) The acetaldehyde (F) is then converted to 4( Z )-6-[(4 S ,5 R )-2,2-dimethyl-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid, having ²⁵[α] D -113.3 (EtOH, c 0.465) and NMR essentially identical with that of the racemic material described in Example 1, using an analogous procedure to that described in the first part of Example 1. Example 41 Illustrative pharmaceutical dosage forms include the following tablet, capsule, injection and aerosol formulations, which may be obtained by conventional procedures well known in the art of pharmacy and are suitable for therapeutic or prophylactic use in humans:- (a) Tablet I mg/tablet Compound Z 1.0 Lactose Ph. Eur. 93.25 Croscarmellose sodium 4.0 Maize starch paste (5% w/v aqueous paste) 0.75 Magnesium stearate 1.0 (b) Tablet II mg/tablet Compound Z 50 Lactose Ph. Eur 223.75 Croscarmellose sodium 6.0 Maize starch 15.0 Polyvinylpyrrolidone (5% w/v aqueous paste) 2.25 Magnesium stearate 3.0 (c) Tablet III mg/tablet Compound Z 100 Lactose Ph. Eur. 182.75 Croscarmellose sodium 12.0 Maize starch paste (5% w/v aqueous paste) 2.25 Magnesium stearate 3.0 (d) Capsule mg/capsule Compound Z 10 mg Lactose Ph.Eur. 488.5 Magnesium stearate 1.5 (e) Injection I (50 mg/ml) Compound Z (free acid form) 5.0% w/v 1M Sodium hydroxide solution 15.0% v/v 0.1M Hydrochloric acid (to adjust pH to 7.6) Polyethylene glycol 400 4.5% w/v Water for injection to 100% (f) Injection II (10 mg/ml) Compound Z (free acid form) 1.0% w/v Sodium phosphate EP 3.6% w/v 0.1M Sodium hydroxide solution 15.0% v/v Water for injection to 100% (g) Injection III (1mg/ml, buffered to pH 6) Compound Z (free acid form) 0.1% w/v Sodium phosphate BP 2.26% w/v Citric acid 0.38% w/v Polyethylene glycol 400 3.5% w/v Water for injection to 100% (h) Aerosol I mg/ml Compound Z 10.0 Sorbitan trioleate 13.5 Trichlorofluoromethane 910.0 Dichlorodifluoromethane 490.0 (i) Aerosol II mg/ml Compound Z 0.2 Sorbitan trioleate 0.27 Trichlorofluoromethane 70.0 Dichlorodifluoromethane 280.0 Dichlorotetrafluoroethane 1094.0 (j) Aerosol III mg/ml Compound Z 2.5 Sorbitan trioleate 3.38 Trichlorofluoromethane 67.5 Dichlorodifluoromethane 1086.0 Dichlorotetrafluoroethane 191.6 (k) Aerosol IV mg/ml Compound Z 2.5 Soya lecithin 2.7 Trichlorofluoromethane 67.5 Dichlorodifluoromethane 1086.0 Dichlorotetrafluoroethane 191.6 The tablet compositions (a)-(c) may be enteric coated by conventional means, for example to provide a coating of cellulose acetate phthalate. The aerosol compositions (h)-(k) may be used in conjunction with standard, metered dose aerosol dispensers, and the suspending agents sorbitan trioleate and soya lecithin may be replaced by an alternative suspending agent such as sorbitan monooleate, sorbitan sesquioleate, polysorbate 80, polyglycerol oleate or oleic acid.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GB, IT, LI, LU, NL, SE A 1,3-dioxane alkenoic acid derivative of the formula I: wherein: A¹ is (1-6C)alkylene; R¹ is (1-6C)alkyl, trifluoromethyl, (3-6C)cycloalkyl or (1-4C)alkoxy(1-4C)alkyl, or is a group of the formula R³.A²- in which R³ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano, and in which A² is (1-6C)alkylene, oxy(1-6C)alkylene, (2-6C)alkenylene or a direct bond to R³; R² is hydroxy, a physiologically acceptable alcohol residue, or (1-4C)alkanesulphonamido; X is hydrogen, hydroxy or (1-4C)alkoxy; Y is vinylene; and n is the integer 1 or 2; or a pharmaceutically acceptable salt thereof. A compound as claimed in claim 1 wherein: A¹ is methylene, ethylene, trimethylene, tetramethylene, 1,1-dimethylethylene or 1,1-dimethyltrimethylene; R¹ is methyl, ethyl, isopropyl, t -butyl, trifluoromethyl, cyclopentyl, cyclohexyl, 1,1-dimethyl-2-methoxyethyl, 1-methyl-1-propoxyethyl, or is a group of the formula R³.A²- in which R³ is 3-pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from fluoro, chloro, bromo, trifluoromethyl, nitro and cyano, and in which A² is methylene, ethylene, trimethylene, isopropylidene, 1,1-dimethylethylene, 3.3-pentylidine, oxymethylene, oxytetramethylene, 1-oxy-1-methylethyl, 2-oxy-1,1-dimethylethyl, vinylene, 1,3-propenylene, 1,4-buten-2-ylene, or a direct bond to R³; R² is hydroxy, (1-6C)alkoxy optionally bearing a hydroxy or (1-4C)alkoxy substituent, or R² is methanesulphonamido, ethanesulphonamido, butanesulphonamido, phenoxy or benzyloxy, the latter two of which may optionally bear 1 or 2 optional substituents selected from halogeno, (1-4C)alkyl and (1-4C)alkoxy; and X is hydrogen, hydroxy, methoxy or ethoxy. A compound as claimed in claim 1 or 2 wherein R¹ is selected from trifluoromethyl, isopropyl, t -butyl, cyclohexyl, phenyl, 2-chlorophenyl, 3-chlorophenyl, 2-cyanophenyl, 4-cyanophenyl, 4-nitrophenyl, 2-chloro-5-nitrophenyl, 3,4-dichlorophenyl, 2,4-dichlorophenyl, 2-trifluoromethylphenyl, 4-trifluoromethylphenyl, 1-phenoxy-1-methylethyl (in which the phenoxy moiety may optionally bear 2-fluoro, 2-nitro, 2-trifluoromethyl, 3-fluoro, 3-bromo, 3-nitro, 4-fluoro, 4-bromo, 4-cyano, 4-nitro, 2,4-dichloro, 3,4-difluoro or 3,4-dichloro substituents), 3-pyridyl, 4-pyridyl, 1-methyl-1-(3-pyridyloxy)ethyl, 1-propoxy-1-methylethyl and 1,1-dimethyl-2-phenylethyl (in which the phenyl moiety may optionally bear 3-bromo, 3-nitro, 4-fluoro, 4-nitro, 4-trifluoromethyl, 3,4-difluoro or 3,4-dichloro substituents), styryl and 2-nitrostyryl. A compound as claimed in claim 1, 2 or 3 wherein A¹ is ethylene or trimethylene; Y is cis -vinylene; n is the integer 1; X is hydrogen; and R² is hydroxy. A compound of the formula III: wherein A³ is (1-4C)alkylene; R⁵ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano; and A⁴ is (1-4C)alkylene, oxy(1-4C)alkylene or a direct bond to R⁵; together with the pharmaceutically acceptable salts thereof. A compound as claimed in claim 5 wherein A³ is ethylene, trimethylene or 1,1-dimethylethylene; A⁴ is a direct bond, isopropylidene, 1,1-dimethylethylene or 1-oxy-1-methylethyl; and R⁵ is 3-pyridyl, phenyl, 4-halogenophenyl, 2-halogenophenyl, dihalogenophenyl, nitrophenyl, 2-cyanophenyl, 4-cyanophenyl, 2-trifluoromethylphenyl or 4-trifluoromethylphenyl. A compound selected from: 4(Z)-6-[(2,4,5- cis )-2-(1-methyl-1-phenoxyethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(4-cyanophenyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(1-[4-bromophenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(1-[3,4-difluorophenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; and the pharmaceutically acceptable salts thereof. A salt of a compound as claimed in any one preceding claim which is an alkali metal or alkaline earth metal salt, an ammonium salt, a salt with an organic amine or quaternary base forming a physiologically acceptable cation, or a salt with an acid affording a physiologically acceptable anion. A pharmaceutical composition which comprises a compound of the formula I or a pharmaceutically acceptable salt thereof, as claimed in claim 1, together with a pharmaceutically acceptable diluent or carrier. A process for the manufacture of a compound of the formula I or a pharmaceutically acceptable salt thereof as claimed in claim 1 which is characterised in that:- (a) for those compounds of formula I in which R² is hydroxy, an aldehyde of the formula IV: is reacted with a Wittig reagent of the formula: R₃P=CH.A¹.CO₂⁻ M⁺ wherein R is (1-6C)alkyl or aryl and M⁺ is a suitable metal cation; (b) for those compounds wherein X is hydroxy, a compound of the formula V: wherein P is a protected hydroxy group, is deprotected by conventional means; (c) a diol derivative of the formula VI: wherein one of T¹ and T² is hydrogen and the other is hydrogen or a group of the formula -CRaRb.OH (wherein Ra and Rb are the same or different (1-4C)alkyl groups) is reacted with an aldehyde derivative of the formula R¹.CHO , or with an acetal, hemiacetal or hydrate thereof; (d) a 1,3-dioxane of the formula VII: wherein one of Ra and Rb is hydrogen, methyl or ethyl and the other is methyl or ethyl is reacted with an excess of an aldehyde of the formula R¹.CHO , or with a hydrate, acetal or hemiacetal thereof, in the presence of an acid; (e) for those compounds wherein R¹ is trifluoromethyl, a compound of the formula VI: wherein T² is hydrogen and T¹ is alkanesulphonyl or arenesulphonyl is reacted with an aldehyde of the formula R¹·CHO , or with a hydrate, acetal or hemiacetal thereof, in the presence of an acid; followed by base-catalysed cyclisation of the acyclic intermediate thereby obtained using a suitable base; or (f) decomposing an ester of the formula VIII: wherein R⁶ is (1-6C)alkyl, phenyl or benzyl, the latter two optionally bearing 1 or 2 halogeno, (1-4C)alkyl or (1-4C)alkoxy substituents; whereafter, when a salt of a compound of formula I is required, it may be obtained by reaction with the appropriate base or acid affording a physiologically acceptable ion, or by any other conventional salt formation procedure; and further, when an optically active form of a compound of formula I is required, one of the aforesaid processes may be carried out using an optically active starting material or the racemic form of a compound of formula I may be resolved by conventional means; and wherein R¹, R², A¹, X, Y and n have any of the meanings defined in claims 1-6. A 2,2-disubstituted 1,3-dioxane of the formula VII: wherein one of Ra and Rb is hydrogen, methyl or ethyl and the other is methyl or ethyl, and wherein n, R², A¹, Y and X have any of the meanings defined in any one of claims 1-4. A 4-pyridyl-5-allyl-1,3-dioxane of the formula XI: wherein R¹ and X have any of the meanings defined in any one of claims 1-4. Claims for the following Contracting State : ES A process for the manufacture of a 1,3-dioxane alkenoic acid derivative of the formula I:- wherein: A¹ is (1-6C)alkylene; R¹ is (1-6C)alkyl, trifluoromethyl, (3-6C)cycloalkyl or (1-4C)alkoxy(1-4C)alkyl, or is a group of the formula R³.A²- in which R³ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano, and in which A² is (1-6C)alkylene, oxy(1-6C)alkylene, (2-6C)alkenylene or a direct bond to R³; R² is hydroxy, a physiologically acceptable alcohol residue, or (1-4C)alkanesulphonamido; X is hydrogen, hydroxy or (1-4C)alkoxy; Y is vinylene; and n is the integer 1 or 2; or a pharmaceutically acceptable salt thereof; characterized in that:- (a) for those compounds of formula I in which R² is hydroxy, an aldehyde of the formula IV: is reacted with a Wittig reagent of the formula: R₃P=CH.A¹.CO₂⁻ M⁺ wherein R is (1-6C)alkyl or aryl and M⁺ is an suitable metal cation; (b) for those compounds wherein X is hydroxy, a compound of the formula V: wherein P is a protected hydroxy group is deprotected by conventional means; (c) a diol derivative of the formula VI: wherein one of T¹ and T² is hydrogen and the other is hydrogen or a group of the formula -CRaRb.OH (wherein Ra and Rb are the same or different (1-4C)alkyl groups) is reacted with an aldehyde derivative of the formula R¹.CHO , or with an acetal, hemiacetal or hydrate thereof; (d) a 1,3-dioxane of the formula VII: wherein one of Ra and Rb is hydrogen, methyl or ethyl and the other is methyl or ethyl is reacted with an excess of an aldehyde of the formula R¹.CHO , or with a hydrate, acetal or hemiacetal thereof, in the presence of an acid; (e) for those compounds wherein R¹ is trifluoromethyl, a compound of the formula VI: wherein T² is hydrogen and T¹ is alkanesulphonyl or arenesulphonyl is reacted with an aldehyde of the formula R¹ ·CHO , or with a hydrate, acetal or hemiacetal thereof, in the presence of an acid; followed by base-catalyzed cyclisation of the acyclic intermediate thereby obtained using a suitable base; or (f) decomposing an ester of the formula VIII: wherein R⁶ is (1-6C)alkyl, phenyl or benzyl, the latter two optionally bearing 1 or 2 halogeno, (1-4C)alkyl or (1-4C)alkoxy substituents; whereafter, when a salt of a compound of formula I is required, it may be obtained by reaction with the appropriate base or acid affording a physiologically acceptable ion, or by any other conventional salt formation procedure; and further, when an optically active form of a compound of formula I is required, one of the aforesaid processes may be carried out using an optically active starting material or the racemic form of a compound of formula I may be resolved by conventional means; and wherein R¹, R², A¹, X, Y and n have any of the meanings defined above. A process according to part (a) of claim 1 characterised in that R is phenyl, M⁺ is a lithium, sodium or potassium cation, and the reaction is carried out at a temperature in the range -80 to 40°C and in a suitable solvent or diluent, such as benzene, toluene, chlorobenzene, 1,2-dimethoxyethane, t -butyl methyl ether, dibutyl ether, tetrahydrofuran, dimethyl sulphoxide, or tetramethylenesulphone. A process according to part (b) of claim 1 characterised in that P is a trialkylsilyloxy group of up to 10 carbon atoms such as t-butyldimethylsilyl and the deprotection is performed using aqueous tetrabutylammonium fluoride or sodium fluoride in a suitable solvent or diluent such as tetrahydrofuran or tetrahydrofuran and at a temperature in the range 10 to 35°C. A process according to part (c) or (d) of claim 1 characterised in that it is performed in the presence of an acid such as hydrogen chloride, hydrogen bromide, sulphuric acid, phosphoric acid, methanesulphonic acid or p -toluenesulphonic acid in the presence of a suitable solvent or diluent such as dichloromethane, toluene, xylene, tetrahydrofuran, dibutyl ether, t -butyl methyl ether or 1,2-dimethoxyethane and at a temperature in the range 0 to 80°C. A process according to part (e) of claim 1 characterised in that T¹ is methanesulphonyl or p -toluenesulphonyl; the reaction with the excess aldehyde of the formula R¹.CHO is carried out in the presence of an acid and in a solvent or diluent and at a temperature, as defined in claim 4; and the acyclic intermediate is cyclised by reaction with a base such as potassium carbonate or sodium hydride in a suitable solvent such as tetrahydrofuran, dibutyl ether, t -butyl methyl ether or 1,2-dimethoxyethane and at a temperature in the range 0 to 80°C. A process a ccording to part (f) of claim 1 characterised in that R⁶ is methyl, ethyl, propyl or t -butyl, and the decomposition is carried out by base catalysed hydrolysis using an aqueous alkali metal hydroxide in a suitable solvent or diluent such as tetrahydrofuran, methanol, ethanol or t -butyl methyl ether and at a temperature in the range 10 to 60°C. A process according to any one preceding claim wherein in the starting materials A¹ is ethylene or trimethylene; Y is cis -vinylene; n is the integer 1; X is hydrogen; and R² is hydroxy. A process according to any one of claims 1-4 or 6 for the manufacture of a compound selected from: 4(Z)-6-[(2,4,5- cis )-2-(1-methyl-1-phenoxyethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(4-cyanophenyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(1-[4-bromophenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(1-[3,4-difluorophenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; and the pharmaceutically acceptable salts thereof. A process for the manufacture of a pharmaceutical composition comprising a compound of the formula I or a pharmaceutically acceptable salt thereof, as defined in claim 1, together with a pharmaceutically acceptable diluent or carrier, which is characterised by performing a process as claimed in any one preceding claim and subsequently adding the compound of formula I, or the pharmaceutically acceptable salt thereof, thereby obtained, to said diluent or carrier using a conventional pharmaceutical procedure. Claims for the following Contracting State : GR A process for the manufacture of a 1,3-dioxane alkenoic acid derivative of the formula I: wherein: A¹ is (1-6C)alkylene; R¹ is (1-6C)alkyl, trifluoromethyl, (3-6C)cycloalkyl or (1-4C)alkoxy(1-4C)alkyl, or is a group of the formula R³.A²- in which R³ is pyridyl, phenyl or phenyl bearing 1 or 2 substituents selected from halogeno, trifluoromethyl, nitro and cyano, and in which A² is (1-6C)alkylene, oxy(1-6C)alkylene, (2-6C)alkenylene or a direct bond to R³; R² is hydroxy, a physiologically acceptable alcohol residue, or (1-4C)alkanesulphonamido; X is hydrogen, hydroxy or (1-4C)alkoxy; Y is vinylene; and n is the integer 1 or 2; or a pharmaceutically acceptable salt thereof; characterised in that:- (a) for those compounds of formula I in which R² is hydroxy, an aldehyde of the formula IV: is reacted with a Wittig reagent of the formula: R₃P=CH.A¹.CO₂⁻ M⁺ wherein R is (1-6C)alkyl or aryl and M⁺ is an suitable metal cation; (b) for those compounds wherein X is hydroxy, a compound of the formula V: wherein P is a protected hydroxy group is deprotected by conventional means; (c) a diol derivative of the formula VI: wherein one of T¹ and T² is hydrogen and the other is hydrogen or a group of the formula -CRaRb.OH (wherein Ra and Rb are the same or different (1-4C)alkyl groups) is reacted with an aldehyde derivative of the formula R¹.CHO , or with an acetal, hemiacetal or hydrate thereof; (d) a 1,3-dioxane of the formula VII: wherein one of Ra and Rb is hydrogen, methyl or ethyl and the other is methyl or ethyl is reacted with an excess of an aldehyde of the formula R¹.CHO , or with a hydrate, acetal or hemiacetal thereof, in the presence of an acid; (e) for those compounds wherein R¹ is trifluoromethyl, a compound of the formula VI: wherein T² is hydrogen and T¹ is alkanesulphonyl or arenesulphonyl is reacted with an aldehyde of the formula R¹·CHO , or with a hydrate, acetal or hemiacetal thereof, in the presence of an acid; followed by base-catalysed cyclisation of the acyclic intermediate thereby obtained using a suitable base; or (f) decomposing an ester of the formula VIII: wherein R⁶ is (1-6C)alkyl, phenyl or benzyl, the latter two optionally bearing 1 or 2 halogeno, (1-4C)alkyl or (1-4C)alkoxy substituents; whereafter, when a salt of a compound of formula I is required, it may be obtained by reaction with the appropriate base or acid affording a physiologically acceptable ion, or by any other conventional salt formation procedure; and further, when an optically active form of a compound of formula I is required, one of the aforesaid processes may be carried out using an optically active starting material or the racemic form of a compound of formula I may be resolved by conventional means; and wherein R¹, R², A¹, X, Y and n have any of the meanings defined above. A process according to part (a) of claim 1 characterised in that R is phenyl, M⁺ is a lithium, sodium or potassium cation, and the reaction is carried out at a temperature in the range -80 to 40°C and in a suitable solvent or diluent, such as benzene, toluene, chlorobenzene, 1,2-dimethoxyethane, t -butyl methyl ether, dibutyl ether, tetrahydrofuran, dimethyl sulphoxide, or tetramethylenesulphone. A process according to part (b) of claim 1 characterised in that P is a trialkylsilyloxy group of up to 10 carbon atoms such as t-butyldimethylsilyl and the deprotection is performed using aqueous tetrabutylammonium fluoride or sodium fluoride in a suitable solvent or diluent such as tetrahydrofuran or tetrahydrofuran and at a temperature in the range 10 to 35°C. A process according to part (c) or (d) of claim 1 characterised in that it is performed in the presence of an acid such as hydrogen chloride, hydrogen bromide, sulphuric acid, phosphoric acid, methanesulphonic acid or p -toluenesulphonic acid in the presence of a suitable solvent or diluent such as dichloromethane, toluene, xylene, tetrahydrofuran, dibutyl ether, t -butyl methyl ether or 1,2-dimethoxyethane and at a temperature in the range 0 to 80°C. A process according to part (e) of claim 1 characterised in that T¹ is methanesulphonyl or p -toluenesulphonyl; the reaction with the excess aldehyde of the formula R¹.CHO is carried out in the presence of an acid and in a solvent or diluent and at a temperature, as defined in claim 4; and the acyclic intermediate is cyclised by reaction with a base such as potassium carbonate or sodium hydride in a suitable solvent such as tetrahydrofuran, dibutyl ether, t -butyl methyl ether or 1,2-dimethoxyethane and at a temperature in the range 0 to 80°C. A process a ccording to part (f) of claim 1 characterised in that R⁶ is methyl, ethyl, propyl or t -butyl, and the decomposition is carried out by base catalysed hydrolysis using an aqueous alkali metal hydroxide in a suitable solvent or diluent such as tetrahydrofuran, methanol, ethanol or t -butyl methyl ether and at a temperature in the range 10 to 60°C. A process according to any one preceding claim wherein in the starting materials A¹ is ethylene or trimethylene; Y is cis -vinylene; n is the integer 1; X is hydrogen; and R² is hydroxy. A process according to any one of claims 1-4 or 6 for the manufacture of a compound selected from: 4(Z)-6-[(2,4,5- cis )-2-(1-methyl-1-phenoxyethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(4-cyanophenyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(1-[4-bromophenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; 4(Z)-6-[(2,4,5- cis )-2-(1-[3,4-difluorophenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hexenoic acid; and the pharmaceutically acceptable salts thereof. A process for the manufacture of a pharmaceutical composition comprising a compound of the formula I or a pharmaceutically acceptable salt thereof, as defined in claim 1, together with a pharmaceutically acceptable diluent or carrier, which is characterised by performing a process as claimed in any one preceding claim and subsequently adding the compound of formula I, or the pharmaceutically acceptable salt thereof, thereby obtained, to said diluent or carrier using a conventional pharmaceutical procedure. A 2,2-disubstituted 1,3-dioxane of the formula VII: wherein one of Ra and Rb is hydrogen, methyl or ethyl and the other is methyl or ethyl, and wherein n, R², A¹, Y and X have any of the meanings defined claim 1. A 4-pyridyl-5-allyl-1,3-dioxane of the formula XI: wherein R¹ and X have any of the meanings defined in any one of claims 1-4.

Inventors: BREWSTER ANDREW GEORGE, BROWN GEORGE ROBERT, FAULL ALAN WELLINGTON, JESSUP REGINALD, SMITHERS MICHAEL JAMES, BREWSTER, ANDREW GEORGE, BROWN, GEORGE ROBERT, FAULL, ALAN WELLINGTON, JESSUP, REGINALD, SMITHERS, MICHAEL JAMES
Applicants: ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1989
Main classification: C07D405
IPC codes: C07D405, A61P7, A61P11, A61P9, A61K31, A61P43, A61P29