6-ALKYLIDENE PENEMS, THEIR PREPARATION AND USE, AND INTERMEDIATES

Abstract: Compounds of the general formula I: and their pharmaceutically acceptable salts and in vivo hydrolysable esters in which one of R¹ and R² denotes hydrogen, the other of R¹ and R² denotes an unsubstituted or substituted five-membered hetero-aromatic ring bonded through a carbon atom thereof and having one hetero-atom selectd from nitrogen, oxygen and sulphur and additionally having from one to three nitrogen atoms, and R³ denotes hydrogen or an organic group, are novel compounds with β-lactamase inhibitory and antibacterial properties.

Description: This invention relates to β-lactam compounds and in particular to a class of 6-alkylidene penems which have β-lactamase inhibitory and antibacterial properties. The compounds are therefore useful in the treatment of antibacterial infections in humans or animals, either alone or in combination with other antibiotics. European Patent Publication No. EP 0 041 768 A (Beecham; published 16 December 1981) discloses 6-alkylidene-2-penems of the general formula (A): in which each of R a and R b denotes hydrogen or an optionally substituted hydrocarbon or heterocyclic group, and R c denotes hydrogen or an organic group. Those compounds possess antibacterial activity and also inhibit β-lactamases and have a synergistic effect in combination with other β-lactam antibiotics. European Patent Publication No. EP 0 120 613 A (Beecham; published 3 October 1984) discloses a sub-group of compounds within the general formula (A) which have better activity than other compounds of the general formula (A). That sub-group consists of compounds of the general formula (B): in which R c denotes hydrogen or an organic group; one of R d and R e denotes hydrogen, and the other of R d and R e denotes a group of the sub-formula (C): in which R f denotes a substituent group; X denotes an oxygen atom, a sulphur atom or an =NR g group; R g denotes hydrogen, hydrocarbon or a nitrogen-protecting group; and n denotes 0, 1, 2 or 3. It has now been found that certain compounds of the general formula (A) exhibit improved β-lactamase inhibitory action and synergistic activity as compared with other compounds of that group. According to the present invention there is provided a compound of the general formula I: or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof in which one of R¹ and R² denotes hydrogen, the other of R¹ and R² denotes an unsubstituted or substituted five-membered hetero-aromatic ring bonded through a carbon atom thereof and having one hetero-atom selected from nitrogen, oxygen and sulphur and additionally having from one to three nitrogen atoms, and R³ denotes hydrogen or a group of formula -R⁴ or -SR⁴, where R⁴ denotes an unsubstituted or substituted (C₁₋₁₀) hydrocarbon or heterocyclyl group. The hetero-aromatic ring (which may also be referred to as a hetero-aryl ring) denoted by R¹ or R² contains five ring atoms, two, three or four of which may be hetero-atoms (that is to say, non-carbon atoms). The ring hetero-atoms may be solely nitrogen atoms, in which case there may be two, three or four ring nitrogen atoms, or the ring hetero-atoms may consist of one oxygen or sulphur atom plus one, two or three nitrogen atoms. The hetero-aromatic ring is bonded to the methylene carbon atom through a ring carbon atom. The hetero-aromatic ring may be unsubstituted or may be substituted by one or more substituents, each of which may be carried on a ring carbon atom or a ring nitrogen atom, provided of course that the aromaticity of the ring is not destroyed. Examples of suitable substituents which may be present in the hetero-aromatic ring R¹ or R² include (C₁₋₆)alkanoyl, (C₁₋₆)alkanoyloxy, heterocyclyl, amino, (C₁₋₆)alkanoylamino, (mono or di)-(C₁₋₆)alkylamino, hydroxy, (C₁₋₆)alkoxy, sulpho, mercapto, (C₁₋₆)alkylthio, (C₁₋₆)alkylsulphinyl, (C₁₋₆)alkylsulphonyl, heterocyclylthio, arylthio, sulphamoyl, carbamoyl, amidino, guanidino, nitro, halogen, carboxy, carboxy salts, carboxy esters, arylcarbonyl, and heterocyclylcarbonyl groups, and also unsubstituted or substituted (C₁₋₆)alkyl, (C₂₋₆)alkenyl, (C₂₋₆)alkynyl, aryl, and aryl(C₁₋₆)alkyl groups. Examples of suitable optional substituents for the above-mentioned (C₁₋₆)alkyl, (C₂₋₆)alkenyl, (C₂₋₆)alkynyl, aryl and aryl(C₁₋₆)alkyl substitutents include (C₁₋₆)alkanoyl, (C₁₋₆)alkanoyloxy, heterocyclyl, amino, (C₁₋₆)alkanoylamino, (mono or di)-(C₁₋₆)alkylamino, hydroxy, (C₁₋₆)alkylsulphinyl, (C₁₋₆)alkylsulphonyl, heterocyclylthio, arylthio, sulphamoyl, carbamoyl, amidino, guanidino, nitro, halogen, carboxy, carboxy salts, carboxy esters, arylcarbonyl and heterocyclylcarbonyl groups. When the hetero-aromatic ring R¹ or R² includes a carboxy salt or carboxy ester substituent, that substituent is suitably a pharmaceutically acceptable salt or pharmaceutically acceptable ester. The term 'heterocyclyl' as used herein includes aromatic and non-aromatic, single and fused, rings containing up to four hetero-atoms in each ring selected from oxygen, nitrogen and sulphur, which rings may be unsubstituted or substituted by up to three groups selected from halogen, (C₁₋₆)alkyl, (C₁₋₆)alkoxy, halo(C₁₋₆)alkyl, hydroxy, amino, carboxy, (C₁₋₆)alkoxycarbonyl, (C₁₋₆)alkoxycarbonyl(C₁₋₆)alkyl, aryl, (C₁₋₆)alkylthio, arylthio, mercapto and oxo groups. The term 'aryl' as used herein includes phenyl and naphthyl, which may be unsubstituted or substituted by up to five, preferably up to three, groups selected from halogen, (C₁₋₆)alkyl, phenyl, (C₁₋₆)alkoxy, halo(C₁₋₆)alkyl, hydroxy, amino, nitro, carboxy, (C₁₋₆)alkoxycarbonyl,(C₁₋₆)alkoxycarbonyl(C₁₋₆)alkyl, (C₁₋₆)alkylcarbonyloxy, (C₁₋₆)alkylcarbonyl (C₁₋₆)alkylthio, arylthio, and mercapto groups. The term 'hydrocarbon' as used herein includes groups having up to 18 carbon atoms, suitably up to 10 carbon atoms, conveniently up to 6 carbon atoms. Suitable hydrocarbon groups include (C₁₋₆)alkyl, (C₂₋₆)alkenyl, (C₂₋₆)alkynyl, (C₃₋₇)cycloalkyl, (C₃₋₇)cycloalkyl-(C₁₋₆)alkyl, aryl, and aryl(C₁₋₆)alkyl. Suitably, one of R¹ and R² denotes hydrogen and the other of R¹ and R² denotes a five-membered hetero-aromatic ring of the type defined above that is unsubstituted or is substituted by one or more (C₁₋₆)alkyl groups, for example methyl groups. Suitable five-membered hetero-aromatic rings R¹ or R² include pyrazoles, imidazoles, triazoles, tetrazoles, thiazoles, isothiazoles, oxazoles, isoxazoles, thiadiazoles, and oxadiazoles, each of which may be unsubstituted or substituted. (It is to be understood that, where appropriate, all isomeric forms of the above-mentioned hetero-aromatic rings are included). Particularly suitable hetero-aromatic rings R¹ or R² include oxazoles, isoxazoles, pyrazoles, and triazoles. Advantageously, the hetero-aromatic ring R¹ and R² includes at least two ring nitrogen atoms. Examples of individual hetero-aromatic groups R¹ or R² include isothiazolyl, isoxazolyl, methylthiazolyl, methyloxazolyl, dimethyloxazolyl, methyl-1,2,3-thiadiazolyl, methyl-1,2,4-oxadiazolyl, N-methylpyrazolyl, N-methylimidazolyl, N-methyl-1,2,3-triazolyl, N-methyl-1,2,4-triazolyl, and N-methyltetrazolyl groups. In general formula I, R³ denotes hydrogen; or a group of formula -R⁴ or -SR⁴, where R⁴ denotes an unsubstituted or substituted (C₁₋₁₀)hydrocarbon or heterocyclyl group. Preferably, R³ represents hydrogen, (C₁₋₁₀)alkyl or (C₁₋₁₀)alkylthio, or substituted (C₁₋₁₀)alkyl or substituted (C₁₋₁₀)-alkylthio, wherein the substituent may be hydroxy, (C₁₋₆)alkoxy, (C₁₋₆)alkanoyloxy, halogen, mercapto, (C₁₋₆)alkylthio, heterocyclylthio, amino, (mono or di)-(C₁₋₆)alkylamino, (C₁₋₆)alkanoylamino, carboxy, or (C₁₋₆)alkoxycarbonyl. Examples of suitable organic groups R³ include methyl, ethyl, propyl, methylthio, ethylthio, hydroxymethyl, methoxymethyl, ethoxymethyl, acetoxymethyl, (1 or 2)-acetoxyethyl, aminomethyl, 2-aminoethyl, acetamidomethyl, 2-acetamidoethyl, carboxymethyl, 2-hydroxyethylthio, methoxymethylthio, 2-methoxyethylthio, acetoxymethylthio, 2-aminoethylthio, acetamidomethylthio, 2-acetamidoethylthio, carboxymethylthio, 2-carboxyethylthio, aryl (especially phenyl), arylthio (especially phenylthio), pyridyl, pyrimidyl, isoxazolyl, pyrimidylthio, tetrazolylthio, and pyridylthio groups. In particular, R³ may be hydrogen. Pharmaceutically acceptable in vivo hydrolysable esters (also referred to as 'metabolisable esters') of the compounds of the general formula I are those esters which hydrolyse in the human body to produce the parent acid or its salt. Such esters may be identified by oral or intravenous administration to a test animal, and subsequent examination of the test animal's body fluids for the presence of the compound of the formula I or a salt thereof. In some cases, the in vivo hydrolysable ester moiety may constitute a link between two different active ingredient moieties, one of which is a compound according to the invention and the other of which may be another therapeutically active compound, such that on in vivo hydrolysis of the ester moiety, the ester link breaks to give the two separate active compounds. The linked entity may be referred to as a 'mutual pro-drug'. Suitable in vivo hydrolysable ester groups include those of part-formulae (a), (b) and (c): -CO₂CH₂-OA⁶ (c) in which A¹ denotes hydrogen, methyl, or phenyl; A² denotes (C₁₋₆)alkyl, (C₁₋₆)alkoxy or phenyl; or A¹ and A² together denote 1,2-phenylene, which may be unsubstituted or substituted by one or two methoxy groups; A³ denotes (C₁₋₆)alkylene, which may be unsubstituted or substituted by a methyl or ethyl group; each of A⁴ and A⁵ which may be identical or different, denotes (C₁₋₆)alkyl; and A⁶ denotes (C₁₋₆)alkyl. Examples of suitable in vivo hydrolysable ester groups include acetoxymethyl, pivaloyloxymethyl, α-acetoxyethyl, α-acetoxybenzyl, α-pivaloyloxyethyl, ethoxycarbonyloxymethyl, α-ethoxycarbonyloxyethyl, dimethylaminomethyl, diethylaminomethyl, phthalidyl and dimethoxyphthalidyl groups. Suitable pharmaceutically acceptable salts of the 3-carboxylic acid group of the compound of formula I include metal salts, e.g. aluminium salts, alkali metal salts (e.g. sodium or potassium salts), alkaline earth metal salts (e.g. calcium or magnesium salts), ammonium salts, and substituted ammonium salts, for example those with lower alkylamines (e.g.triethylamine), hydroxy-lower alkylamines (e.g. 2-hydroxyethylamine), di(2-hydroxyethyl)amine or tri(2-hydroxyethyl)amine), cycloalkylamines (e.g. dicyclohexylamine), or with procaine, and also dibenzylamine, N,N-dibenzylethylenediamine, 1-ephenamine, N-ethylpiperidine, N-benzyl-β-phenethylamine, dehydroabietylamine, N,N'-bishydroabietylethylene-diamine, bases of the pyridine type (e.g. pyridine, collidine and quinoline), and other amines which have been or can be used to form salts with penicillins. The compounds of the general formula I and also the salts and esters thereof may exist in two optically active forms and it is to be understood that both such forms as well as racemic mixtures thereof are embraced by the present invention. It is believed that the more active form is that of structure IA: in which R¹, R² and R³ are defined as above. Furthermore, in general formulae I and IA, it is thought to be advantageous that R¹ denotes the hetero-aromatic group and that R² denotes a hydrogen atom. Examples of individual compounds according to the invention include: (5RS) (Z)-6-(isothiazol-5-ylmethylene)penem-3-carboxylic acid; (5RS) (E)-6-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylpyrazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(2-methylthiazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-(isoxazol-3-ylmethylene)penem-3-carboxylic acid; (5RS) (Z)-6-[(2,4-dimethyloxazol-5-yl)methylene] penem-3-carboxylic acid; (5RS) (E)-6-[(4-methyl-1,2,3-thiadiazol-5-yl) methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(4-methyl-1,2,3-thiadiazol-5-yl) methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(2-methyloxazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (E)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5R) (Z)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(5-methyl-1,2,4-oxadiazol-3-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methyl-1,2,4-triazol-3-yl)methylene] penem-3-carboxylic acid; (5RS) 2-hydroxymethyl-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) 2-ethylthio-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) 2-(2-hydroxyethylthio)-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-(N-methyltetrazol-5-ylmethylene)penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylpyrazol-3-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylimidazol-4-yl)methylene] penem-3-carboxylic acid; and pharmaceutically acceptable salts and in-vivo hydrolysable esters thereof. A compound of the general formula I, or a salt or ester thereof, may be prepared by eliminating the elements of a compound of the general formula II: H-X II from a penem or penem intermediate of the general part-formula III: in which R¹ and R² are defined as above, and X denotes a hydroxy group or a leaving group, to give a compound of the general part-formula IV: in which R¹ and R² are defined as above, and, if the resulting compound of the general formula IV is a penem intermediate, converting it into a penem of the general formula I or a pharmeceutically acceptable salt or in vivo hydrolysable ester thereof. The compound of the general part-formula III may suitably be a compound of the general part-formula IIIA: in which R¹, R² and X are defined as above. More especially it may be a compound of the general formula IIIB: in which R¹, R² and X are defined as above, R¹³ denotes (C₁₋₆)alkyl, aryl, aryl(C₁₋₆)alkyl, (C₁₋₆)alkylthio, arylthio, hetero-aromatic-thio, acyl (for example, (C₁₋₆)alkylcarbonyl, especially acetyl), (C₂₋₆)alkenyl (especially vinyl), or aryl(C₂₋₆)alkenyl, all of which may optionally be substituted, and R¹⁴ denotes hydrogen or an N-protecting group, or R¹³ and R¹⁴ together denote the remainder of a penem nucleus, which may be substituted and/or may optionally carry a protecting group. In the case where R¹³ and R¹⁴ in general formula IIIB together denote the remainder of a penem nucleus, they may suitably together denote the sub-formula V: in which R¹⁶ denotes the hydrogen atom or organic group R³ or a group convertible into R³ during the preparation of a penem of the general formula I or salt or ester thereof, and R X denotes hydrogen or a carboxyl-blocking group. In that case, the penem or penem intermediate of the general part-formula III is of the general formula IIID given below. A carboxyl-blocking group R x (also referred to as a carboxyl-protecting group) is suitably a group that can readily be removed at a later stage of the penem preparation process. Examples of suitable carboxyl-blocking derivatives that may form the group -CO₂R x include salt, ester, and anhydride derivatives of the carboxylic acid. The salts may be organic or inorganic and need not be pharmaceutically acceptable. Examples of suitable salt-forming groups R x include inorganic salts, for example alkali metal atoms (e.g. lithium and sodium), other metal atoms, tertiary amino groups (e.g. tri-lower-alkylamino, N-ethylpiperidino, and dimethylpiperazino groups). A preferred salt-forming group R x is the triethylamino group. An ester-forming group R x is advantageously one that can be removed under conventional conditions. Examples of suitable ester-forming groups R x include benzyl, p -methoxybenzyl, 2,4,6-trimethylbenzyl, 3,5-di-t-butyl-4-hydroxy-benzyl, benzoylmethyl, p -nitrobenzyl, 4-pyridylmethyl, 2,2,2-trichloroethyl, 2,2,2-tribromoethyl, allyl, acetonyl, t-butyl, t-amyl, diphenylmethyl, triphenylmethyl, adamantyl, 2-benzyloxyphenyl, 4-methylthiophenyl, tetrahydrofur-2-yl, tetrahydropyran-2-yl, pentachlorophenyl, p -toluene-sulphonylethyl, and methoxymethyl groups, and also silyl, stannyl and phosphorus-containing groups, and oxime radicals of formula -N=CHR o in which R o denotes aryl or heterocyclyl. Furthermore, the ester-forming group R x may be an in vivo hydrolysable ester group including, in particular, those listed above. The free carboxyl group may be regenerated from any of the above esters by usual methods appropriate to the particular R x group, for example, by acid-catalysed, base-catalysed or enzymically-catalysed hydrolysis, or by hydrogenation. The hydrolysis must of course be carried out under conditions in which the groups on the rest of the molecule are stable. When it is desired to produce a compound of the general formula I in the form of a free acid or in the form of a salt, by a process according to the invention, it is generally advantageous to use a compound in which R x denotes a carboxyl-blocking group. When it is desired to produce a compound of the general formula I in the form of a pharmaceutically acceptable ester, it is generally convenient to use a compound in which R x denotes the desired ester group. The process step according to the invention involves the elimination of the elements of a compound H-X from a penem or penem intermediate of the general part-formula III, in which X denotes a hydroxy group or a leaving group. In the case where X denotes a hydroxy group, the compound of the formula H-X being eliminated is water and the elimination reaction is a dehydration reaction, which may suitably be carried out by treating a compound of the general part-formula III with a compound of the general formula VI: R⁶O₂C-N=N-CO₂R⁷ VI in which each of R⁶ and R⁷, which may be identical or different, denotes aryl, (C₁₋₆)alkyl or aryl(C₁₋₆)alkyl, and a with compound of the general formula VII: in which each of a, b and c which may be identical or different, denotes 0 or 1, and each of R⁸, R⁹ and R¹⁰, which may be identical or different, denotes aryl, (C₁₋₆)alkyl or aryl(C₁₋₆)alkyl. In the compounds of the general formula VI, R⁶ and R⁷ are preferably selected from methyl, ethyl, propyl, butyl, phenyl, and benzyl, the ethyl and isopropyl groups being preferred. Advantageously, R⁶ and R⁷ may be identical. A preferred compound of the general formula VI is diethyl azodicarboxylate. Preferred compounds of the general formula VII include triarylphosphines and trialkylphosphites. Preferred groups R⁸, R⁹ and R¹⁰ include methyl, ethyl, n-propyl, n-butyl, benzyl, phenyl and methoxyphenyl. Advantageously, R⁸, R⁹ and R¹⁰ are all identical. A preferred compound of the general formula VII is triphenylphosphine. Advantageously, approximately two equivalents of each the compounds of the general formulae VI and VII are used per mole of the compound of the general part-formula III. The dehydration reaction may suitably be carried out at a non-extreme temperature, for example a temperature of from -20°C to +100°C. It may be convenient to begin the reaction at a depressed temperature, for example 0°C, and then to allow the temperature to rise to about room temperature. The reaction may suitably be carried out in an inert aprotic organic solvent. Suitable solvents include tetrahydrofuran, dioxane, ethyl acetate, benzene, and dichloromethane. In the cases where X, in general formula III, denotes a leaving group, which will hereinafter be referred to as X¹, it may suitably be a halogen atom or a group of one of the formulae -0-SO₂-(0) n -R¹¹ VIIIA -0-CO-(0) n -R¹¹ VIIIB or -0-PO-(OR¹²)₂ VIIIC in which n denotes 0 or 1, R¹¹ denotes (C₁₋₆)alkyl, aryl or aryl(C₁₋₆)alkyl, and R¹² denotes (C₁₋₆)alkyl or aryl. Preferred groups of formula VIIIB are those in which n denotes zero and R¹¹ denotes (C₁₋₆)alkyl, especially the acetoxy group. The elimination of the elements of a compound of the general formula II in which X denotes a leaving group X¹ from a compound of the general formula III,may suitably be effected by treating the compound of the general formula III with a base in an aprotic medium. Suitable bases for that purpose include, for example, powdered inorganic bases, for example alkali metal carbonates, bicarbonates, hydroxides, and hydrides (e.g. powdered potassium carbonate), and also organic bases of low nucleophilicity, for example 1,8-diazabicyclo[5.4.0]undec-7-ene. Suitable solvents for use as the aprotic medium in this reaction include, for example, dimethylformamide, hexamethylphosphoramide, dichloromethane, and tetrahydrofuran. The elimination may suitably be effected at a low temperature, for example a temperature of from -70°C to +70°C, advantageously from -40°C to 0°C. The compounds of the general formula III in which X denotes a leaving group X¹ may suitably be prepared from the corresponding compound in which X denotes a hydroxy group by replacing the hydroxy group by a leaving group X¹. Alternatively, in the case of the compounds of the general formula IIID below, the leaving group X¹ may be introduced into the molecule at an earlier stage in the synthesis of the penem nucleus. In particular, a group X¹ of the formula VIIIA or VIIIB may be introduced at the beginning of, or at any stage during, the synthesis of the penem. In each case, the group X¹ may suitably be introduced by replacing a hydroxyl group in known manner. The dehydration or other elimination reaction of the process according to the invention may be carried out at any suitable stage during the preparation of the penem of the general formula I or salt or ester thereof, suitably at an early stage or late stage in the manufacturing process. Further examples of suitable leaving groups will be apparent to those skilled in the art and include sulphoxide, selenoxide and xanthate groups, which can be eliminated by known methods (see W. Carruthers, 'Some modern methods of organic synthesis', Cambridge Univ. Press 1978 (2nd edition), pages 93-103). In particular, the dehydration or other elimination reaction may be carried out on a compound of the general formula IIIC: in which R¹, R² and X are defined as above, R¹⁷ denotes (C₁₋₆) alkyl, aryl, aryl(C₁₋₆)alkyl, (C₁₋₆)alkylthio, arylthio, hetero-aromatic-thio, acyl (for example, (C₁₋₆)alkylcarbonyl, especially acetyl), (C₂₋₆)alkenyl (especially vinyl), or aryl(C₂₋₆)alkenyl, all of which may optionally be substituted, and R¹⁸ denotes hydrogen or an N-protecting group, to give a compound of the general formula IVC: in which, R¹, R², R¹⁷ and R¹⁸ are defined as above, which may then subsequently be converted to a penem of the general formula I or salt or ester thereof in known manner, suitably by a conventional penem preparation method. Alternatively, the dehydration or other elimination reaction may be carried out on a compound of the general formula IIID: in which R¹, R², R¹⁶, R x and X are defined as above, to give a compound of the general formula IVD: in which R¹, R², R¹⁶ and R x are defined as above, and thereafter, if necessary or desired: (a) removing any carboxyl-blocking group R x , and/or (b) converting the group R¹⁶ into a group or atom R³, and/or (c) converting the product into the free acid or into a pharmaceutically acceptable salt or in vivo hydrolysable ester. The conversion of a compound of the general formula IVC to the desired penem may suitably proceed via a compound of the general formula IVD, according to known penem preparation methods. A compound of the general formula IIID, especially one in which X denotes a leaving group X¹, may conveniently be prepared from a compound of the general formula IIIC, especially one in which X denotes a leaving group X¹, according to known penem preparation methods. In a compound of the general formula IIIB or IIIC, R¹³ or R¹⁷, respectively, may suitably denote a triphenylmethyl group. Examples of suitable N-protecting groups, R¹⁴ or R¹⁸, include silyl groups, for example t-butyldimethylsilyl groups. In the general formula IIID and IVD, the group R¹⁶ may be a group convertible into R³ during the penem preparation process. One particular example of such a group, which may conveniently be used in the preparation of a group R³ of the formula -SR⁴ (in which R⁴ is defined as above), is the group of the formula IX: in which R¹⁵ denotes an organic radical different from the group R⁴. A sulphoxide compound of the general formula IIID or IVD in which R¹⁶ denotes a group of the formula IX may be reacted with a thiol of the general formula XI: R⁴-SH XI in which R⁴ is defined as above, or a reactive derivative thereof, to give a compound of the general formula IIID or IVD in which R¹⁶ denotes a group of the formula XII: -S-R⁴ XII in which R⁴ is defined as above. The reaction of the sulphoxide with the thiol may be carried out as described in European Patent Publication No. EP 0 046 363A. A sulphoxide compound of the general formula IIID or IVD in which R¹⁶ denotes a sulphoxide group of the formula IX above may be prepared by S-oxidation of a compound of the general formula IIID or IVD, respectively, in which R¹⁶ denotes a group of the formula -S-R¹⁵. The S-oxidation may be effected using a mild oxidising agent, for example a perbenzoic acid, hydrogen peroxide, selenium dioxide or sodium metaperiodate. Perbenzoic acids, for example m-chloroperbenzoic acid, are preferred. The present invention also provides a process for the preparation of a compound of the general formula I in which R³ denotes a group of the formula -SR⁴ (in which R⁴ is defined as above), or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof, which comprises reacting a compound of the general formula XIII: in which R¹, R², R¹⁵ and R x are defined as above, with a thiol of the general formula XI above, or a reactive derivative thereof; and thereafter if necessary or desired: (a) removing any carboxyl-blocking group R x , and/or (b) converting the product into the free acid or into a pharmaceutically acceptable salt or in vivo hydrolysable ester. The compounds of the general formulae IIIC and IVC are novel intermediates and also constitute subjects of the present invention. The compounds of the general formula IIID, in particular those in which R¹⁶ denotes R³ or a sulphoxide group of the formula IX, are also novel intermediates and form a further subject of the present invention. The compounds of the general formula IVD in which R¹⁶ denotes a sulphoxide group of the formula IX (i.e a compound of general formula XIII) are further novel intermediates and constitute a yet further subject of the present invention. The compounds according to the invention have β-lactamase inhibitory and antibacterial properties, and are useful for the treatment of infections in animals, especially mammals, including humans, in particular in humans and domesticated (including farm) animals. The compounds may be used, for example, for the treatment of infections of, inter alia , the respiratory tract, the urinary tract, and soft tissues, especially in humans. The compounds may be used for the treatment of infections caused by strains of, for example, Staphylococcus aureus , Klebsiella aerogenes , Escherichia coli , Proteus sp ., and Bacteroides fragilis . It is generally advantageous to use a compound according to the invention in admixture or conjunction with a penicillin, cephalosporin or other β-lactam antibiotic and that can often result in a synergistic effect, because of the β-lactamase inhibitory properties of the compounds according to the invention. In such cases, the compound according to the invention and the other β-lactam antibiotic can be administered separately or in the form of a single composition containing both active ingredients as discussed in more detail below. The compounds according to the invention are suitably provided in substantially pure form, for example at least 50% pure, advantageously at least 75% pure, preferably at least 95% pure, especially at least 98% pure, all percentages being calculated as weight/weight. An impure or less pure form of a compound according to the invention may, for example, be used in the preparation of a more pure form of the same compound or of a related compound (for example, a corresponding salt, ester or free acid) suitable for pharmaceutical use. Although the purity of any compound used as an intermediate may be less critical than that of a compound used as a final product, for example one used directly for pharmaceutical use (for example in a composition according to the invention as described below), nevertheless such an intermediate compound is advantageously provided in substantially pure form. It is generally advantageous to provide the compounds according to the invention in crystalline form. The present invention provides a pharmaceutical composition comprising a compound according to the invention that is to say, a compound of the general formula I or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof, and a pharmaceutically acceptable carrier. The present invention also provides a compound for use in the treatment of bacterial infections in admixture with a penicillin, cephalosporin or other β-lactam antibiotic. The compositions of the invention may be in a form adapted for oral, topical or parenteral use and may be used for the treatment of infection in animals especially mammals, including humans, in particular in humans and domesticated animals (including farm animals). The compositions of the invention may, for example, be made up in the form of tablets, capsules, creams, syrups, suspensions, solutions, reconstitutable powders, and sterile forms suitable for injection or infusion. Such compositions may contain conventional pharmaceutically acceptable materials, for example diluents, binders, colours, flavours, preservatives, and disintegrants, in accordance with conventional pharmaceutical practice in manner well understood by those skilled in the art of formulating antibiotics. It can be particularly advantageous for the compounds according to the invention to be administered to a patient by injection or infusion. That method of administration has the advantage of rapidly resulting in high blood levels of the active ingredient compound being administered. Accordingly, in one preferred form of the composition according to the invention, a compound according to the invention is present in sterile form, including in sterile crystalline form. A further preferred form of the composition according to the invention, is one in which the composition is in injectable or infusable form. One injectable or infusable form of the composition according to the invention is an injectable or infusable solution, which suitably comprises a solution of a compound according to the invention in a sterile pyrogen-free liquid, for example water or aqueous ethanol. A further injectable or infusable form of the composition according to the invention is an injectable or infusable suspension, in which case the compound according to the invention is advantageously present in finely particulate form. The suspension may be an aqueous suspension in, for example, sterile water or sterile saline, which may additionally include a suspending agent, for example polyvinylpyrrolidone. Alternatively, the suspension may be an oily suspension in a pharmaceutically acceptable oil suspending agent, for example arachis oil. A composition according to the invention may be in unit dosage form, for example unit dosage form for oral administration, topical administration, or parenteral administration (including administration by injection or infusion). A composition according to the invention may comprise a compound according to the invention as the sole active ingredient or therapeutic agent, or it may also comprise one or more additional active ingredients or therapeutic agents, for example a penicillin, cephalosporin or other β-lactam antibiotic, or pro-drug thereof. A composition comprising a compound according to the invention and another active ingredient or therapeutic agent, especially a penicillin, cephalosporin or other β-lactam antibiotic, or pro-drug thereof, can show enhanced effectiveness, and in particular can show a synergistic effect. Penicillins, cephalosporins and other β-lactam antibiotics suitable for co-administration with the compounds according to the invention - whether by separate administration or by inclusion in the compositions according to the invention - include both those known to show instability to or to be otherwise susceptible to β-lactamases and also those known to have a degree of resistance to β-lactamases. Examples of penicillins suitable for co-administration with the compounds according to the invention include benzylpenicillin, phenoxymethylpenicillin, carbenicillin, azidocillin, propicillin, ampicillin, amoxycillin, epicillin, ticarcillin, cyclacillin, pirbenicillin, azlocillin, mezlocillin, sulbenicillin, piperacillin, and other known penicillins. The penicillins may be used in the form of pro-drugs thereof, for example as in vivo hydrolysable esters,for example the acetoxymethyl, pivaloyloxymethyl, α-ethoxycarbonyloxyethyl and phthalidyl esters of ampicillin, benzylpenicillin and amoxycillin; as aldehyde or ketone adducts of penicillins containing a 6-α-aminoacetamido side chain (for example hetacillin, metampicillin and analogous derivatives of amoxycillin); and as α-esters of carbenicillin and ticarcillin, for example the phenyl and indanyl α-esters. Examples of cephalosporins that may be co-administered with the compounds according to the invention include, cefatrizine, cephaloridine, cephalothin, cefazolin, cephalexin, cephacetrile, cephapirin, cephamandole nafate, cephradine, 4-hydroxycephalexin, cephaloglycin, cefoperazone, cefsulodin, ceftazidime, cefuroxime, cefmetazole, cefotaxime, ceftriaxone, and other known cephalosporins, all of which may be used in the form of pro-drugs thereof. Examples of β-lactam antibiotics other than penicillins and cephalosporins that may be co-administered with the compounds according to the invention include aztreonam, latamoxef (Moxalactam - Trade Mark), and other known β-lactam antibiotics, all of which may be used in the form of pro-drugs thereof. In the compositions according to the invention, the compound according to the invention and the penicillin, cephalosporin or other β-lactam antibiotic may be linked by means of an in vivo hydrolysable ester group, in the form of a mutual pro-drug. Some penicillins and cephalosporins that may be included in the compositions according to the invention may not be suitable for oral administration, in which case the composition will be in a form suitable for parenteral or topical administration. Particularly suitable penicillins for co-administration with the compounds according to the invention include ampicillin, amoxycillin, carbenicillin, piperacillin, azlocillin, mezlocillin, and ticarcillin. Such penicillins may be used in the form of their pharmaceutically acceptable salts, for example their sodium salts. Alternatively, ampicillin or amoxycillin may be used in the form of fine particles of the zwitterionic form (generally as ampicillin trihydrate or amoxycillin trihydrate) for use in an injectable or infusable suspension, for example, in the manner hereinbefore described in relation to the compounds according to the invention. Amoxycillin, for example in the form of its sodium salt or the trihydrate, is particularly preferred for use in synergistic compositions according to the invention. Particularly suitable cephalosporins for co-administration with the compounds according to the invention include cephaloridine, cefoperazone and cefazolin, which may be used in the form of their pharmaceutically acceptable salts, for example their sodium salts. A compound according to the invention may be administered to the patient in an antibacterially effective amount or, when a compound according to the invention is being used in conjunction with a penicillin, cephalosporin, or other β-lactam antibiotic, it may be used in a synergistically effective amount. The compounds according to the invention may suitably be administered to the patient at a daily dosage of from 0.7 to 50 mg/kg of body weight. For an adult human (of approximately 70 kg body weight), from 50 to 3000 mg, preferably from 100 to 1000 mg, of a compound according to the invention may be adminstered daily, suitably in from 1 to 6, preferably from 2 to 4, separate doses. Higher or lower dosages may, however, be used in accordance with clinical practice. When the compositions according to the invention are presented in unit dosage form, each unit dose may suitably comprise from 25 to 1000 mg, preferably from 50 to 500 mg, of a compound according to the invention. Each unit dose may, for example, be 62.5, 100, 125, 150, 200 or 250 mg of a compound according to the invention. When the compounds according to the invention are co-administered with a penicillin, cephalosporin or other β-lactam antibiotic, the ratio of the amount of the compound according to the invention to the amount of the other β-lactam antibiotic may vary within a wide range. The said ratio may, for example, be from 100:1 to 1:100; more particularly, it may, for example, be from 2:1 to 1:30. The amount of penicillin or cephalosporin or other β-lactam antibiotic in a synergistic composition according to the invention will normally be approximately similar to the amount in which it is conventionally used per se , for example from about 50 mg, advantageously from about 62.5 mg, to about 3000 mg per unit dose, more usually about 125, 250, 500 or 1000 mg per unit dose. An example of a particularly suitable composition according to the invention is one comprising from 150 to 1000 mg, preferably from 200 to 700 mg, of amoxycillin or ampicillin or a pro-drug thereof, in admixture or conjunction with from 5 to 500 mg, preferably from 20 to 250 mg, of a compound according to the invention, per unit dose. In such a composition, the amoxycillin may suitably be in the form of its trihydrate or sodium salt; the ampicillin may suitably be in the form of ampicillin trihydrate, ampicillin anhydrate, sodium ampicillin, hetacillin, pivampicillin hydrochloride, bacampicillin hydrochloride or talampicillin hydrochloride; and the compound according to the invention may most suitably be in crystalline form. Such composition may be in a form suitable for oral or parenteral use, except when it comprises an in vivo hydrolysable ester of ampicillin or amoxycillin in which case the composition should not normally be intended for parenteral administration. A further example of a particularly suitable composition according to the invention is one comprising from 200 to 2000 mg of carbenicillin or ticarcillin or a pro-drug thereof, in admixture or conjunction with from 5 to 500 mg, preferably from 25 to 250 mg, of a compound according to the invention, per unit dose. In such a composition, the carbenicillin or ticarcillin may most suitably be in the form of its di-sodium salt, and the compound according to the invention may most suitably be in crystaline form. When the composition contains the carbenicillin or ticarcillin in the form of a di-salt, it is most suitably presented in a form suitable for parenteral administration. No toxicological effects are indicated when the compounds according to the invention are administered within the above-mentioned dosage ranges. The following examples illustrate the invention. Unless otherwise stated, all temperatures are given in degrees Celsius and all percentages are calculated by weight. The term 'Biogel' used in the examples is a Trade Mark. Preparation 1(a) 1-t-Butyldimethylsilyl-3-[hydroxy(5-isothiazolyl)methyl]-4-tritylthioazetidin-2-one A solution of n-butyl lithium (1.64M in hexane, 0.73 ml) was added to a solution of diisopropylamine (0.17 ml) in dry tetrahydrofuran (THF) (8 ml) at -30°C under dry argon. After stirring at -30°C for 10 minutes the mixture was cooled to -76°C and treated with a solution of 1-t-butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (459 mg) (Bristol-Myers Patent GB 2042515 A) in dry THF (4 ml). After a further 15 minutes at -76°C the stirred mixture was treated with a solution of 5-formylisothiazole (135 mg) (M.P.L. Caton et . al ., J . Chem . Soc ., 1964, 446) in dry THF (1 ml). After 20 minutes at -76°C the stirred mixture was treated with saturated ammonium chloride solution (5 ml) and allowed to attain 0°C. The mixture was diluted with ethyl acetate (25 ml) and washed with brine (5 ml), dilute sodium bisulphite solution (5 ml) and brine (3 x 5 ml). The dried (MgS0₄) organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give two fractions. The less polar fraction contained a 3:1 mixture of a trans (Isomer A) and a cis-isomer (Isomer B) of the title azetidinone (2) (112 mg), a solid, νmax (CHCl₃) 3600-3100, 1745 cm⁻¹; δppm (CDCl₃) 0.91 and 0.99 (9H, each s, ratio 3:1), 1.60-2.10 (1H, broad signal, exch. D₂0), 3.35-3.56 (1.25H, m), 4.06 (0.75H, d, J3Hz), 4.57 (0.75H, d, J2Hz), 4.88 (0.25H, d, J5Hz), 7.00-7.60 (16H, m), 8.29 (1H, d, J2Hz), both Me₃Si signals were obscured by the TMS signal. The more polar fraction contained a trans-isomer (Isomer C) of the title azetidinone (2) (144 mg), a solid, mp 183-184°C (prisms ex ethyl acetate/hexane); νmax (CHCl₃) 3600-3100, 1735 cm⁻¹; δ ppm (CDCl₃) 0.81 (9H, s), 2.70-3.10 (1H, broad signal, exch. D₂0), 3.70 (1H, dd, J7 and 2Hz), 4.10 (1H, d, J 2Hz), 4.30-4.50br (1H, m, collapses to d, J7Hz on exch. D₂0), 6.81br (1H, s), 7.20-7.50 (15H, m), 8.22 (1H, d, J2Hz), both Me₃Si signals were obscured by the TMS signal. (Found: C, 67.1; H, 6.3; N, 4.9; S, 11.2. C₃₂H₃₆N₂0₂S₂Si requires C, 67.1; H, 6.3; N, 4.9; S, 11.2%). Preparation 1(b) (3RS, 4SR) 3-[Hydroxy (5-isothiazolyl)methyl]-4-tritylthioazetidin-2-one The trans-azetidinone (2) (Isomer C) (86 mg) from Preparation 1(a) was dissolved in a mixture of methanol (1 ml) and dichloromethane (1 ml), cooled to -20°C, and treated with a solution of anhydrous potassium flouride (9 mg) in methanol (0.2 ml). After 1 hour at -20°C the stirred mixture was allowed to attain room temperature during 20 minutes. The mixture was diluted with ethyl acetate (10 ml) and washed with brine (3 x 2 ml). The dried (MgS0₄) organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give the title alcohol (3) (67 mg) as a solid, mp 197-198°C (needles ex. ethyl acetate/hexane); νmax (CHCl₃) 3600-3100, 1760 cm⁻¹; δppm (CDCl₃) 3.61 (1H, dd, J2.8 and 2.8Hz), 3.89br (1H, d, J4.4Hz, exch. D₂0), 4.22 (1H, s), 4.47 (1H, d, J2.8Hz), 5.55 br (1H, broad signal, collapses to d, J2.8Hz on exch. D₂0), 7.16-7.37 (16H, m), 8.60 (1H, d, J1.7Hz). (Found: C, 68.3; H, 4.9; N, 6.2; S, 14.1. C₂₆H₂₂N₂0₂S₂ requires C, 68.1; H, 4.8; N, 6.1; S, 14.0%). Preparation 1(c) (3RS, 4SR) 3-[Acetoxy(5-isothiazolyl)methyl]-4-tritylthioazetidin-2-one The alcohol (3) (56 mg) from Preparation 1(b) was dissolved in dry dichloromethane (2 ml), cooled in an ice bath, and treated with triethylamine (15 mg), 4-dimethylaminopyridine (1.5 mg) and acetic anhydride (15 mg). The mixture was allowed to attain room temperature and was stirred for a further 30 minutes. The mixture was diluted with ethyl acetate (10 ml) and washed with 5% citric acid (1 ml), brine (1 ml), saturated NaHC0₃ (1 ml) and brine (1 ml). The dried (MgS0₄) organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixture to give the title acetate (4) (40 mg) as a solid, mp 167-168°C (rhomboids ex ethyl acetate/hexane); νmax (CHCl₃) 3380, 1775 cm⁻¹; δppm (CDCl₃) 2.10 (3H, s), 3.66 (1H, dd, J5 and 3Hz, with further small coupling), 4.41 (d, J3Hz) and 4.15-4.65 (broad signal) together 2H, 5.54 (1H, d, J5Hz), 7.20-7.60 (16H, m), 8.51 (1H, d, J2Hz). (Found: C, 67.0; H, 4.8; N, 5.6; S, 12.4: C₂₈H₂₄N₂0₃S requires C, 67.2; H, 4.8; N, 5.6; S, 12.8%). Preparation 1(d) (3RS, 4SR) 3-[Acetoxy (5-isothiazolyl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthicazetidin-2-one The azetidinone (4) (4.63g) from Preparation 1(c) and p-nitrobenzyl glyoxylate monohydrate (2.32g) were heated in refluxing benzene (100 ml) with provision for azeotropic removal of water (Dean and Stark apparatus containing molecular sieves 4A) for 1 hour. The mixture was cooled to room temperature and treated with triethylamine (94 mg). After 30 minutes at room temperature the mixture was evaporated to give a crude hydroxyester (5) as an amorphous solid, νmax (CHCl₃) 3600-3100, 1770, 1750 cm⁻¹. A solution of the crude hydroxyester (5) in dry THF (100 ml) was cooled to -10°C and treated with 2,6-lutidine (1.62 ml) and thionyl chloride (1.01 ml). After stirring at -10°C for 10 minutes the mixture was filtered and evaporated. The residue was re-evaporated from dry toluene (2 x 10 ml) to give a crude chloroester (6) as an amorphous solid, νmax 1780 br. cm⁻¹. The crude chloroester (6) and triphenylphosphine (9.7g) were stirred in dry dioxan (20 ml) for 30 minutes. The resulting solution was evaporated to approximately half volume and treated with 2,6-lutidine (1.29 ml). The mixture was stirred at room temperature for 3½ days and then at 40°C for 24 hours. The mixture was diluted with ethyl acetate (300 ml) and washed with 5% citric acid (25 ml), brine (25 ml), saturated NaHC0₃ (25 ml), and brine (3 x 25 ml). The dried (MgS0₄) organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give the title phosphorane (7) (7.61g) as an amorphous solid, νmax (CHCl₃) 1755, 1615, 1605 sh. cm⁻¹. Preparation 1(e) (3RS, 4SR) Silver 3-[Acetoxy(5-isothiazolyl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl) azetidin-2-one-4-thiolate Silver nitrate (18.2 ml of a 0.15M solution in methanol) was added to a stirred mixture of the phosphorane (7) (2.0g) from Preparation 1(d) and pyridine (0.22 ml) in methanol (20 ml) and dichloromethane (20 ml) at room temperature. After 30 mins at room temperature the stirred mixture was cooled in an ice bath for 10 minutes and filtered. The residue was washed with cold methanol (5 ml) and dry ether (2 x 5 ml) and dried under vacuum to give the title silver thiolate (8) (1.17g) as an off-white amorphous solid, νmax (Nujol) 1750, 1600 cm⁻¹. The combined filtrates were evaporated to low volume, filtered, and the residue washed with methanol (2 ml) and dry ether (2 x 2ml) and dried under vacuum to give the title silver thiolate (8) (0.28g) as an off white amorphous solid. Preparation 1(f) (5RS, (SR) p-Nitrobenzyl 6-[Acetoxy(5-isothiazolyl)methyl]penem-3-carboxylate A stirred, ice bath cooled, suspension of the silver thiolate (8) (818 mg) from Preparation 1(e) in dry dichloromethane (10 ml) was treated with acetic-formic anhydride (0.80 ml), 4-dimethylaminopyridine (122 mg) and triethylamine hydrochloride (1.38 g). The ice bath was removed and the mixture was stirred for a further 10 minutes. The mixture was diluted with ethyl acetate (10 ml) and filtered through Kieselguhr the residue being washed with ethyl acetate (3 x 10 ml). The combined filtrates were washed with 5% citric acid (5 ml), brine (5 ml), saturated NaHC0₃ solution (5 ml), and brine (3 x 5 ml). The dried (MgS0₄) organic layer was heated at 50°C under argon for 40 minutes, evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give the title penem (9) (391 mg) as a solid, mp 154-156°C (plates ex ethyl acetate/hexane); λmax (CHCl₃/ EtOH) 307 (Em 9070) and 251 nm (16126); νmax (CHCl₃) 1800, 1750, 1720 cm⁻¹; δppm (CDCl₃) 2.17 (3H, s), 4.35 (1H, ddd, J5.4, 1.9 and 1.1Hz), 5.29 and 5.42 (2H, ABq, J13.5Hz), 5.85 (1H, d, J1.9Hz), 6.60 (1H, slightly broadened d, J5.4Hz), 7.25 (1H, dd, J1.8 and 0.7Hz), 7.33 (1H, d, J1.1Hz), 7.58 (2H, d, J8.8Hz), 8.24 (2H, d, J8.8Hz), 8.46 (1H, d, J1.8Hz). (Found: C, 49.5; H, 3:4; N, 9.0; S, 13.9; M-60 ⁺, 401.0135. C₁₉H₁₅N₃0₇S₂ requires C, 49.4; H, 3.3; N, 9.1; S, 13.9% M - CH₃C0₂H, 401.0141). Example 1(a) (5RS) p-Nitrobenzyl 6-(5-Isothiazolylmethylene)penem-3-carboxylate A solution of 1,8-diazabicyclo[5.4.0] undec -7-ene (49 mg) in dry dichloromethane (0.5 ml) was added, dropwise over 0.5 minute, to a stirred solution of the penem (9) (100 mg) from Preparation 1(f) in dry dichloromethane (3 ml) at -40°C. After 10 minutes at -40°C the stirred mixture was diluted with dichloromethane (10 ml) and washed with 5% citric acid (1 ml) , brine (1 ml), saturated NaHC0₃ solution (1 ml), and brine (3 x 1 ml). The dried (MgS0₄)organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/dichloromethane mixtures to give the title pens (10) (70 mg), a 4:1 mixture of (Z) and (E)-isomers, as a yellow solid, δppm (CDCl₃) 5.31 and 5.46 (2H, ABq, J13.6Hz), 6.47 (1H, d, J1.1Hz), 6.88 (0.2H, s), 7.34 (0.8H, d, J1.9Hz), 7.40 (0.8H, s), 7.42 (0.8H, s), 7.45 (0.2H, s), 7.62 (2H, d, J8.7Hz), 7.82 (0.2H, m), 8.25 (2H, d, J8.7Hz), 8.52 (0.2H, d, J2.1Hz), 8.58 (0.8H, d, J1.9Hz). The mixture was recrystallised twice from chloroform/hexane to give the (Z)- isomer as a microcrystalline solid (38 mg), mp 169-172°C; λmax (EtOH/CHCl₃) 288 nm (Em 28492); max (CHCl₃) 1790, 1720, 1675 cm⁻¹; δppm (CDCl₃) 5.31 and 5.46 (2H, ABq, J13.5Hz), 6.47 (1H, d, J1.1Hz), 7.34 (1H, d, J1.6Hz), 7.40 (1H, s), 7.42 (1H, slightly broadened s), 7.62 (2H, d, J8.7Hz), 8.25 (2H, d, J8.7Hz), 8.58 (1H, d, J1.6Hz). (Found: C, 50.7; H, 2.6; N, 10.1; S, 16.2. C₁₇H₁₁N₃0₅S₂ requires C, 50.9; H, 2.8; N, 10.5; S, 16.0%). Example 1(b) (5RS) Sodium (Z)-6-(5-Isothiazolylmethylene)penem-3-carboxylate The penem ester (10) (Z-isomer) (90 mg) from Example 1(a) was dissolved in a mixture of dioxan (64 ml) and water (16 ml) and hydrogenated over 5% palladium/charcoal catalyst (135 mg) at S.T.P. for 1 hour. A 1% sodium bicarbonate solution (1.88 ml) was added and the mixture filtered through Kieselguhr, the residue being washed with a little aqueous dioxan. The combined filtrates were evaporated and the residue chromatographed on Biogel P2 eluting with water. The appropriate fractions were freeze dried to give the title sodium salt (11) (34 mg) as an orange/brown amorphous solid, λmax (H₂0) 288 nm (Em 22924); νmax (KBr) 3850-2000, 1760, 1670 sh, 1600, 1555, 1505 sh. cm⁻¹; δppm (D₂0) 6.60 (1H, d, J0.9Hz), 7.08 (1H, s), 7.49 (1H, d, J1.9Hz), 7.53br(1H, s), 8.57 (1H, d, J1.9Hz). Preparation 2(a) 1-t-Butyldimethylsilyl-3-[hydroxy(1-methylpyrazol-4-yl)methyl]-4-tritylthioazetidin-2-one A solution of n-butyl lithium (1.06M. in hexane, 1.13 ml) was added to a solution of diisopropylamine (0.17 ml) in dry THF (8 ml) at -30°C under dry argon. After stirring at -30°C for 10 minutes the mixture was cooled to -76°C and treated with a solution of 1-t -butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (459 mg) in dry THF (4 ml). After a further 15 minutes at -76°C the stirred mixture was treated with a solution of 4- formyl-1-methylpyrazole (132 mg) (I.L. Finar and G.H. Lord, J. Chem. Soc. 1957, 3314) in dry THF (1 ml). After 20 minutes at -76°C the stirred mixture was treated with saturated ammonium chloride solution (5 ml) and worked up as for Preparation 1(a) to give two fractions. The less polar fraction contained a trans-isomer (Isomer A) of the title azetidinone (12) (165 mg), an amorphous solid, νmax (CHCl₃) 3600-3100, 1735 cm⁻¹; δppm (CDCl₃) 0.92 (9H, s), 1.1-1.8 (1H, br. signal, exch. D₂O), 3.32 (1H, dd, J2 and 2Hz), 3.50 (1H, d, J2Hz), 3.76 (3H, s), 4.46 (1H, d, J2Hz), 7.10-7.60 (17H, m), the Me₂Si signals were obscured by the TMS signal. [Found (diethylamine chemical ionisation): MH⁺, 570 and M+Et₂NH ⁺, 643]. The more polar fraction contained the other trans-isomer (Isomer B) of the title azetidinone (12) (185 mg), an amorphous solid, νmax (CHCl₃) 3600-3100, 1735 cm⁻¹; δppm (CDCl₃) 0.80 (9H, s), 1.7-2.5 (1H, br. signal, exch. D₂O), 3.59 (1H, dd, J6 and 2Hz), 3.70 (3H, s), 3.87 (2H, 2d, J2Hz and J6Hz), 6.91 (1H, s), 7.10-7.50 (16H, m), the Me₂Si signals were obscured by the TMS signal. [Found (diethylamine chemical ionisation): MH⁺, 570]. Preparation 2(b) (3RS, 4SR) 3-[Hydroxy(1-methylpyrazol-4-yl)methyl]-4-tritylthioazetidin-2-one A solution of anhydrous potassium fluoride (14 mg) in methanol (0.5 ml) was added to a stirred solution of the trans-azetidinone (12) (Isomer B) (126 mg) in methanol (1 ml) at -20°C. After 30 minutes at -20°C the mixture was allowed to attain room temperature and stirred for a further 1 hour. The mixture was diluted with ethyl acetate (10 ml) and was washed with brine (3 x 1 ml). The dried (MgSO₄) organic layer was evaporated and the residue triturated with ether to give the title alcohol (13) (80 mg) as a solid, mp 159-162°C (plates ex ethylacetate/hexane), νmax (CHCl₃) 3600-3100, 1750 cm⁻¹; δppm (CDCl₃) 2.3-2.9 (1H, br. signal, exch. D₂O), 3.43 (1H, dd, J5 and 3Hz), 3.90 (3H, s), 4.25-4.50 (br. signal) and 4.44 (d, J3Hz) (together 2H, the broad signal exch. D₂O), 5.08 (1H, d, J5Hz), 7.1-7.5 (17H, m). (Found: C, 71.1; H, 5.7; N, 9.2; S, 6.6. C₂₇H₂₅N₃O₂S requires C, 71.2; H, 5.5; N, 9.2; S, 7.0%). Preparation 2(c) (3RS, 4SR) 3-[Acetoxy (1-methylpyrazol-4-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (13) (46 mg) from Preparation 2 (b) was dissolved in dry dichloromethane (2 ml), cooled in an ice-bath, and treated with triethylamine (12 mg), 4-dimethylaminopyridine (1.2 mg), and acetic anhydride (12 mg). The ice-bath was removed and the mixture was stirred for 1 hour. The mixture was worked up as for Preparation 1(c) to give the title acetate (14) (35 mg) as a solid, mp 179-180°C (rhomboids ex ethyl acetate/hexane); νmax (CHCl₃) 3380 1770 cm⁻¹; δppm (CDCl₃) 2.06 (3H, s), 3.50-3.70 (1H, m), 3.85 (3H, s), 4.36 (1H, d, J3Hz), 4.51 (1H, brs), 6.10 (1H, d, J6Hz), 7.20-7.70 (17H, m). (Found: C, 70.0; H, 5.5; N, 8.3. C₂₉H₂₇N₃O₃S requires C, 70.0; H, 5.5; N, 8.4%). Preparation 2(d) (3RS, 4SR) 3-[Acetoxy (1-methylpyrazol-4-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The azetidinone (14) (200 mg) from Preparation 2(c) was converted to the hydroxyester (15), νmax (CHCl₃) 3600-3100, 1770, 1755 sh. cm⁻¹; the chloroester (16), νmax (CHCl₃) 1775 cm⁻¹; and finally the title phosphorane (17) (256 mg), νmax (CHCl₃) 1745, 1605 cm⁻¹; using the methods described in Preparation 1(d). Preparation 2(e) (3RS, 4SR) Silver 3-[Acetoxy (1-methylpyrazol-4-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidin -2-one-4-thiolate The phosphorane (17) (250 mg) from Preparation 2(d) was treated with silver nitrate/pyridine as for Preparation 1(e) to give the title silver salt (18) (201 mg) as an amorphous solid, νmax (Nujol) 1740, 1600 cm⁻¹. Preparation 2(f) (5RS, 6SR) p-Nitrobenzyl 6-[Acetoxy (1-methylpyrazol-4-yl)methyl] penem-3-carboxylate The silver salt (18) (1.40g) from Preparation 2(e) was treated as for Preparation 1(f) to give the title penem (19) (675 mg) as a solid, m.p. 140-141°C (fine needles ex ethylacetate/ hexane); λmax (EtOH) 262 (Em 13069) and 315 nm (8588); νmax (CHCl₃) 1795, 1720 cm⁻¹; δppm (CDCl₃) 2.05 (3H, s), 3.84 (3H, s), 4.29 (1H, ddd, J7, 2 and approximately 1Hz), 5.21 and 5.41 (2H, ABq, J14Hz), 5.73 (1H, d, J2Hz), 6.14 (1H, d, J7Hz), 7.30-7.36 (1H, m), 7.41-7.49 (2H, m), 7.54 (2H, d, J9Hz), 8.19 (2H, d, J9Hz). (Found: C, 52.2; H, 4.0; N, 12.1; S, 6.7. C₂₀N₁₈N₄O₇S requires C, 52.4; H, 4.0; n, 12.2; S, 7.0%). Example 2(a) (5RS) p-Nitrobenzyl6-[(1-Methylpyrazol-4-yl)methylene]penem-3-carboxylate Treatment of the penem (19) (675 mg) from Preparation 2(f) with 1,8-diazabicyclo [5.4.0] undec-7-ene as for Example 1(a) gave two products. The less polar product, the (E)-isomer of the title penem (20) (45 mg) was obtained as a pale yellow solid, mp 173-175°C (needles ex chloroform/hexane); λmax (EtOH) 307 nm (Em 26375); νmax (CHCl₃) 1765, 1715, 1665 cm⁻¹; δppm (CDCl₃) 3.96 (3H, s), 5.31 and 5.45 (2H, ABq, J13.7Hz), 6.43 (1H, s), 6.56 (1H, s), 7.41 (1H, s), 7.62 (2H, d, J8.7Hz), 7.79 (1H, s), 8.25 (2H, d, J8.7Hz), 8.34 (1H, s). (Found: C, 54.5; H, 3.3; N, 13.8. C₁₈N₁₄N₄O₅S requires C, 54.3; H, 3.5; N, 14.1%). The more polar product, the (Z)-isomer of the title penem (20) (406 mg) was obtained as a yellow solid, m.p. 190-192°C (fine needles ex choroform/hexane); λmax (EtOH) 298nm (Em 34024); νmax (CHCl₃) 1780, 1720, 1680 cm⁻¹; δppm (CDCl₃) 3.96 (3H, s), 5.29 and 5.46 (2H, ABq, J13.6 Hz), 6.48 (1H, d, J0.7Hz), 7.10 (1H, slightly br. s), 7.34 (1H, s), 7.50-7.51 (2H, each s), 7.62 (2H, d, J8.7Hz), 8.24 (2H, d, J8.7Hz). (Found: C, 54.2; H, 3.6; n, 13.9; S, 8.1. C₁₈H₁₄N₄O₅S requires C, 54.3; H, 3.5; N, 14.1; S, 8.0%). Example 2(b) (5RS) Sodium (E-6-[(1-Methylpyrazol-4-yl)methylene]penem-3-carboxylate The (E)-penem ester (20) (36 mg) from example 2(a) was hydrogenated as for Example 1(b) to give the title sodium salt (21) (8.5 mg) as a yellow amorphous solid λmax (H₂O) 300 nm (Em 17375); δppm (D₂O) 3.88 (3H, s), 6.42 (1H, s), 6.73 (1H, s), 7.08 (1H, s), 7.96 (1H, s), 8.19 (1H, s). Example 2(c) (5RS) Sodium (Z)-6-[(1-Methylpyrazol-4-yl)methylene]penem-3-carboxylate The (Z)-penem ester (20) (200 mg) from Example 2(a) was hydrogenated as for Example 1(b) to give the title sodium salt (22) (58 mg) as a yellow amorphous solid, λmax (H₂O) 293 nm (Em 23152); νmax (KBr) 3700-2800, 1756, 1676, 1602, 1551 cm⁻¹; δppm (D₂O) 3.92 (3H, S), 6.59 (1H, s), 7.05 (1H, s), 7.18 (1H, s), 7.69 (1H, s), 7.86 (1H, s). Preparation 3(a) (3RS,4SR) 1-t-Butyldimethylsilyi-3-(2-methylthiazol-4-yl-carbonyl)-4-trityl-thioazetidin-2-one Di-isopropylamine (2.9 mls) was dissolved in dry THF (75 mls) at -70°C under argon n-Butyl-lithium in n-hexane (1.4M, 15.5 mls) was added dropwise with stirring and after 20 minutes at -70°C a solution of azetidinone (1) (8.25g) in THF (25 mls) was also added. The resulting pink solution was kept at -70°C for 30 minutes and then a solution of 2-methylthiazole-4-carboxylic acid ethyl ester (3.4g) in THF (7 mls) was added in one portion. After 5 minutes the tlc of a small sample showed no starting material in the reaction mixture. After 20 minutes the solution was diluted with saturated aqueous ammonium chloride and with brine. Extraction with ethyl acetate (200 ml X 2), washing of the extracts with brine, drying (MgSO₄) and evaporation gave a gum. Purification by silica gel chromatography, eluting with mixtures of ethyl acetate and n-hexane gave the title compound (23) as a buff coloured solid, 7.05g; νmax (CHCl₃) 1750, 1680 cm⁻¹; δ (CDCl₃) 0.95 (9H, s), 2.69 (3H, s); 4.59 (1H, d, J2Hz); 4.92 (1H, d, J2Hz); 6.9-7.5 (15H, m); 7.76 (1H, s), the Me₂Si signals were obscured by TMS. A sample crystallized from ethyl acetate-n-hexane had m.p. = 195-201°C. Preparation 3(b) (3RS,4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(2-methyl-4-thiazolyl)methyl]-4-tritylthio-azetidin-2-one The ketone (23) from Preparation 3(a) (6.85g) was dissolved in dioxane (100 ml) and pH 7 phosphate buffer (10 ml) added. Sodium borohydride (490 mg) was added in portions with stirring at ambient temperature. After 30 minutes tlc showed no change. More sodium borohydride (960 mg) was added in portions and the stirring continued for 2 hours; tlc showed no starting material. The solution was diluted with brine (ca. 100 ml) and extracted with ethyl acetate (200 mls, 100 mls). The combined extracts were washed with brine, dried (MgSO₄) and evaporated to a gum. Purification by column chromatography on silica eluting with ethyl acetate-n-hexane (1:2) gave two isomeric alcohols (24):-isomer I (25%), Rf 0.20; m.p. 162-163°C; νmax (CHCl₃) 1730cm⁻¹; δ (CDCl₃) 0.80 (9H, s); 2.55 (1H, d, J6Hz, exch. D₂O); 2.57 (3H, s); 3.54 (1H, dd, J 2.5, 1.8Hz); 3.90 (1H, ddd, J 6, 2.5 ca. 1Hz) collapses to dd with D₂O; 4.49 (1H, d, J 1.8Hz); 6.94 (1H, brs, J ca 1Hz); 7.1-7.55 (ca. 15H, m), the Me₂Si signal was obscured by TMS. isomer II (65%), Rf 0.15; νmax (CHCl₃) 1735 cm⁻¹; δ(CDCl₃) 0.78 (9H, s); 2.53 (3H, s); 2.75 (1H, d, J 9Hz, exch. D₂O); 3.68 (1H, dd, J 6, 1.8Hz); 4.13 (1H, dd, J 9, 6Hz) collapse to d with D₂O; 4.40 (1H, d, J 1.8Hz); 6.69 (1H, s); 7.1-7.5 (15H, m), the Me₂Si signal was obscured by TMS. Preparation 3(c) (3RS, 4SR) 3[Hydroxy(2-methyl-4-thiazolyl)methyl]-4-tritylyhioazetidin-2-one The azetidinone-alcohol (24) (isomer II), described in Preparation 3(b) (4.25g) was dissolved in CH₂Cl₂ (10 ml) and methanol (35 ml). The solution was cooled to -10°C and a solution of potassium fluoride (538 mg) in methanol (20 ml) was added dropwise with stirring. After 1 hour at -10°C the solution was diluted with brine and extracted with ethyl acetate (100 ml X 2). Washing of the extract with brine, drying (MgSO₄) and evaporation gave a white solid. Trituration with a small volume of ethyl acetate gave the title azetidinone (25) (2.84 g) white crystals, m.p. 227-229°C decomp. Evaporation of the mother liquors from the crystallization and chromatography of the residue gave a further 180 mg of product (25); νmax (CHCl₃) 3380 br, 1755 cm⁻¹; δ(CDCl₃) 2.61 (3H, s); 3.56 (1H, dd, J 3, 2Hz); 4.21 (1H, d); 4.8-4.95 (1H, br, m); 5.80 (1H, d, J 5Hz) exch. D₂O; 6.83 (1H, br, s) exch. D₂O; 7.0-7.4 (ca. 16H, m). Preparation 3(d) (3RS, 4SR) 3-[Acetoxy(2-methyl-4-thiazolyl)methyl]-4-tritylthioazetidin-2-one The azetidinone (25) from Preparation 3(c) was partially dissolved in dry dichloromethane (40 ml) and 4-dimethylaminopyridine (805 mg) was added. The mixture was cooled in an ice-bath and a solution of acetic anhydride (0.67 ml) in dichloromethane (5 ml) was added dropwise with stirring. The cooling bath was removed. After 30 minutes the clear solution was washed with INHCl, with aq. NaHCO₃ and with brine. Drying (MgSO₄) and evaporation gave a crude product which was purified by silica gel chromatography eluting with ethyl acetate-n-hexane (1:1). The title acetate (26) was obtained in quantitative yield, m.p. 157-158° (ex. ethyl acetate-n-hexane); νmax (CHCl₃) 3400, 1765 cm⁻¹; δ(CDCl₃) 2.07 (3H, s); 2.71 (3H, s); 3.80 (1H, ddd, J 6, 2,1Hz); 4.1 (ca 1H, brs, exch. D₂O); 4.53 (1H, d, J 2Hz); 6.18 (1H, d, J 6Hz); 7.01 (1H, s); 7.05-7.50 (15H, m). Preparation 3(e) (3RS, 4SR) 3-[Acetoxy(2-methyl-4-thiazolyl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The azetidinone (26) (2.73 g) from Preparation 3(d) was converted to the title phosphorane (27) using the same method as that described in Preparation 1(d). The conversion of the intermediate chloride to phosphorane (27) was almost complete after reaction at 40°C for about 15 hours. Phosphorane (27) was obtained as a yellow foam (3.7g); νmax (CHCl₃) 1740-1750, 1600-1620 cm⁻¹. Preparation 3(f) (3RS, 4SR) Silver 3-[Acetoxy(2-methyl-4-thiazolyl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-azetidin -2-one-4-thiolate Using the procedure described in Preparation 1(e) the S-trityl phosphorane (27) from Preparation 3(e) was converted to the corresponding silver thiolate (28) in 79% yield. Preparation 3(g) (5RS, 6SR) p-Nitrobenzyl 6[Acetoxy(2-methyl-4-thiazolyl)methyl]-penem-3-carboxylate The silver thiolate (28) (1.1g) from Preparation 3(f) was dissolved in dry dichloromethane (25 ml) and cooled to 0-5°C. 4-Dimethylaminopyridine (180 mg) was added followed by triethylamine hydrochloride (900 mg) and acetic-formic anhydride (1.05 ml). The mixture was stirred at 0 to 5°C for 30 minutes and the resulting grey precipitate was filtered off and washed well with ethyl acetate. The filtrate was washed with 1N HCl, with aq. NaHCO₃ and with brine. The organic solution was dried (MgS0₄) and diluted to ca. 200 ml by addition of more ethyl acetate and heated at 40°C under argon. After 1 hour the solution was evaporated to dryness giving a gum. Purification by column chromatography gave the title penem acetate (29) in 52% yield. When crystallized from ethyl acetate-n-hexane (29) had m.p. 143-145°c; λmax (EtOH) 317 (8,400), 261nm (13,000); νmax (CHCl₃) 1790, 1730 sh, 1720 cm⁻¹; δ(CDCl₃) 2.08 (3H, s); 2.64 (3H, s); 4.42 (1H, ddd, J4,2,1Hz); 5.20 (1H, d, J13Hz); 5.40 (1H, d, J13Hz): 6.09 (1H, d, J2Hz); 6.20 (1H, d, J4Hz); 7.18 (1H, s); 7.25 (1H, d, J1Hz); 7.5-7.6 and 8.12-8.22 (total 4H, m). Example 3(a) (5RS) p-Nitrobenzyl 6[(2-Methyl-4-thiazolyl)methylene]penem-3-carboxylate The penem acetate (29) (570 mg) prepared as described in Preparation 3(g) was dissolved in dry dichloromethane (10 ml) at -40°C under argon. 1,8-Diazabicyclo[5.4.0]undec-7-ene (241 mg) dissolved in dichloromethane (2 ml) was added dropwise with stirring; the solution darkened slightly. T.l.c. showed no starting material to be present and after 20 minutes the solution was diluted with 1N HCl and with a small volume of dichloromethane. Further washing of the organic phase with aq. NaHCO₃ and with brine followed by drying (MgSO₄) and evaporation gave a yellow solid. Chromatography on silica, eluting with 5% ethyl acetatedichloromethane gave two isomers (30):- isomer I, Z , Rf 0.5, 458 mg; λmax (EtOH)299 nm(Em 23,100); νmax (CHCl₃) 1775, 1710 1675 w. cm⁻¹; δ(CDCl₃) 2.73 (3H, s); 5.28 (1H, d, J13.6Hz); 5.46 (1H, d, J 13.6Hz), 6.64 (1H, d, J1.1Hz); 7.00 (1H, d, J1.1Hz); 7.37 (1H, s) 7.40 (1H, s); 7.61 and 7.65 (2H, arom.); 8.22 and 8.26 (2H, arom.). Isomer II, E , Rf 0.2, 12 mg; λmax (EtoH) 260 nm(Em 13,900); νmax (CHCl₃) 1760, 1720, 1670 w cm⁻¹; δ(CDCl₃) 2.73 (3H, s); 5.30 (1H, d, J 13.5Hz); 5.47 (1H, d, J 13.5Hz); 6.46 (1H, s); 6.87 (1H, s); 7.45 (1H, s); 7.60-7.64 (2H, m, arom.); 8.2-8.3 (2H, m, arom.). Example 3(b) (5RS) Sodium (Z)-6-[(2-methyl-4-thiazolyl)methylene]penem-3-carboxylate The Z-penem ester (30) (418 mg) from Example 3(a) was hydrogenated at 1 atmosphere and room temperature in 20% water-dioxan (40 ml) containing 5% Pd/C (505 mg). After 45 minutes tlc showed no ester and a solution of sodium hydrogencarbonate (1 equivalent) in water was added. Filtration and evaporation gave a yellow gum which was purified by passing through a column of Biogel P2 eluting with water. The title compound (31) was obtained as a yellow, freeze dried solid (27%), λmax (H₂O) 295 nm(Em 23,300); νmax (KBr) 1752, 1676w, 1601 cm⁻¹; δ(D₂O) 2.71 (3H, s); 6.65 (1H, s); 7.02 (1H, s); 7.13 (1H, s); 7.69 (1H, s). Preparation 4(a) (3RS,4SR) 1-t-Butyldimethylsilyl-3-(isoxazol-3-ylcarbonyl)-4-tritylthioazetidin-2-one A solution of diisopropylamine (0.18 ml, 1.24 mmol)in dry THF (8 ml) under argon at -30°C was treated dropwise with a solution of n -butyl lithium (1.66M, 0.75 ml, 1.24 mmol) in hexane. After 15 minutes the reaction mixture was cooled to -70°C and treated dropwise with a solution of the azetidinone (1) (459 mg, 1 mmol) in THF (8 ml). After a further 15 minutes the resulting pink solution was treated with a solution of 3-carboethoxyisoxazole (175 mg, 1.24 mmol) (R.G. Micetich, Can. J. Chem. 1970, 48 , 467) in THF (1 ml). After 10 minutes the reaction mixture was quenched with saturated ammonium chloride solution and worked-up as in Preparation 3(a) to give the title azetidinone (32), 426 mg, 77%. mp 152-4°C (plates ex hexane), νmax (CH₂Cl₂) 1750, 1695 cm⁻¹; δ(CDCl₃) (excluding Me ₂ Si-signals which were obscured by TMS) 0.98 (9H, s), 4.69 (1H, d, J 2.0Hz), 4.96 (1H, d, J 2.0Hz), 6.53 (1H, d, J 1.5Hz), 6.96-7.58 (15H, m), 8.38 (1H, d, J 1.5Hz). (Found C, 69.3; H, 6.4; N, 5.1; S, 5.6. C₃₂H₃₄N₂O₃SSi requires C, 69.3; H, 6.1; N, 5.1; 5, 5.8%). Preparation 4(b) (3RS,4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(isoxazol-3-yl) methyl]-4-tritylthioazetidin-2-one A solution of the ketone (32) from preparation 4(a) (3.24g, 5.86 mmol) in THF (30 ml) and EtOH (30 ml) at 0° under argon was treated portionwise with sodium borohydride (67 mg, 1.76 mmol). After 45 minutes a further portion of sodium borohydride (34 mg) was added. After a further 15 minutes the reaction mixture was quenched with saturated ammonium chloride solution and worked up as for preparation 3(b) to give two fractions. The less polar fraction contained a trans isomer (Isomer A) of the title azetidinone (33) (769 mg, 23%) mp. 203° (plates ex ethyl acetate/ hexane); νmax (CH₂Cl₂) 3650-3450, 1745 cm⁻¹ δ(CDCl₃) (excluding Me ₂Si groups which were masked by TMS) 0.96 (9H, s), 2.20 (1H, bs, exch.), 3.36 (1H, dd, J 2, 3Hz), 3.84 (1H, bs, collapses to d, J 3Hz on D₂O exchange), 4.51 (1H; d, J 2Hz), 6.49 (1H, d, J 1.5Hz), 7.07-7.60 (15H, m), 8.19 (1H, d, J 1.5Hz). (Found C, 69.1; H, 6.7; N, 5.1; S, 5.7. C₃₂H₃₆N₂O₃SSi requires, C, 69.1; H, 6.5; N, 5.0; S, 5.8%). The more polar fraction contained the other trans isomer (Isomer B) of the title azetidinone (33), (2.10g, 65%), ν max (CH₂Cl₂) 3600-3300, 1755 cm⁻¹; δ(CDCl₃) (excluding Me ₂-Si-groups which were masked by TMS), 0.80 (9H, s), 2.60 (1H, bs, exch.), 3.60 (1H, dd, J 2, 5Hz), 3.94 (1H, bd, collapses to d, J 5Hz on D₂O exch.), 4.26 (1H, d, J 2Hz), 6.20 (1H, d, J 2Hz), 7.04-7.56 (15H, m), 8.16 (1H, d, J 2Hz). (Found [M+Et₂NH₂]⁺ 630). Preparation 4(c) (3RS, 4SR) 3-[Hydroxy (isoxazol-3-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (33) (isomer B) (2.08g, 3.74 mmol) from preparation 4(b) was treated as for preparation 2(b) to give the title azetidinone (34) (1.487g, 94%). mp. 205-6° (microplates ex ethyl acetate), νmaz (CH₂Cl₂) 3600-3300, 3400, 1765 cm⁻¹; δ(d₇-DMF), 3.71 (1H, dd, J 2.5, 3.3Hz), 4.40 (1H, d, J 2.5Hz), 5.15 (1H, bd, sharpens to d, J 3.3Hz, on D₂0 exch.), 6.35 (1H, bs, exch.), 6.51 (1H, d, J 1.5Hz) 7.13 (1H, bs, exch), 7.24-7.46 (15H, m), 8.97 (1H, d, J1.4 Hz). (Found C, 70.4; H, 5.0; N, 6.3; S, 6.9. C₂₆N₂₂N₂O₃S requires C, 70.6; H, 5.0; N, 6.3; S, 7.2%). Preparation 4(d) (3RS, 4SR) 3-[Acetoxy (isoxazol-3-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (34) (1.4g, 3.17 mmol) from preparation 4(c) was treated as for preparation (1c) to give the title acetate (35) (1.5g, 98%) mp. 144-5° (plates ex ethyl acetate/hexane) νmax (CH₂ Cl₂) 3400, 1780, 1755(sh) cm⁻¹; δ(CDCl₃) 2.07 (3H, s), 3.62-3.76 (1H, m), 4.21 (1H, bs), 4.49 (1H, dt J 3Hz), 6.22 (1H, d, J 5Hz), 6.35 (1H, d, J 2Hz), 7.15-7.51 (15H, m), 8.41 (1H, d, J 2Hz); (Found C, 69.2; H, 5.1; N, 5.9; S, 6.4. C₂₈H₂₄N₂O₄S requires C, 69.4; H, 5.0; N, 5.8; S, 6.6%). Preparation 4(e) (3RS, 4SR) 3-[Acetoxy(isoxazol-3-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (35) (484 mg, 1 mmol) from preparation 4(d) was converted to the hydroxyester (36) νmax (CH₂Cl₂), 3500, 1775, 1750 cm⁻¹; the chloroester (37) νmax (CH₂Cl₂), 1785, 1755(sh) cm⁻¹ and finally the title phosphorane (38) (596 mg, 64%), νmax (CH₂Cl₂) 1755, 1720, 1620 cm⁻¹ using the methods described in preparation 1(d). Preparation 4(f) (3RS, 4SR) Silver 3-[Acetoxy(isoxazol-3-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidin-2-one-4-thiolate. The phosphorane (38) (590 mg, 0.63 mmol) from Preparation 4(e) was treated with silver nitrate/pyridine as for preparation 1(e) to give the title silver salt (39) (447 mg, 89%) as an off-white solid. Preparation 4(g) (5RS, 6SR) p-Nitrobenzyl 6-[Acetoxy(isoxazol-3-yl)methyl]penem-3-carboxylate The silver salt (39) (447 mg) from preparation 4(f) was treated as for preparation 1(f) to give the title penem (40) (179 mg, 72%). λmax (EtOH) 262 nm (Em 12,890), 315 (8750); νmax (CH₂Cl₂) 1800, 1755, 1720 cm⁻¹; δ(CDCl₃) 2.16 (3H, s), 4.55-4.58 (1H, m), 5.28 and 5.42 (2H, ABq, J 13.6Hz), 5.97 (1H, d, J 2.0Hz), 6.33 (1H, d, J 3.5Hz), 6.45 (1H, d, J 1.7Hz), 7.35 (1H, d, J 0.9 Hz), 7.59 (2H, d, J 8.7Hz), 8.24 (2H, d, J 8.7Hz), 8.44 (1H, d, J 1.6Hz). Example 4(a) (5RS) p-Nitrobenzyl (Z)-6-(Isoxazol-3-ylmethylene)penem-3-carboxylate The penem (40) (135 mg) from preparation 4(g) was treated as for example 1(a) to give the title penem (42) (115 mg, 92%) λmax 256 nm (Em 24,740), 305 (10,620); νmax (CH₂Cl₂) 1790, 1720 cm⁻¹; δ(CDCl₃) 5.30 and 5.46 (2H, Abq, J 13.6Hz), 6.49 (1H, d, J 1.7Hz), 6.50 (1H, d, J 1.2Hz), 7.05 (1H, d, J 1.0Hz), 7.41 (1H, s), 7.62 (2H, d, J 8.9Hz), 8.25 (2H, d, J 8.8Hz), 8.53 (1H, d, J 1.9Hz). Example 4(b) (5RS) Sodium (Z)-6-(Isoxazol-3-ylmethylene)penem-3-carboxylate The penem ester (42) from example 4(a) was hydrogenated as for example 1(b) to give title sodium salt (43) (35 mg, 58%) as a yellow freeze-dried solid λmax (H₂O) 248nm (Em 14,470),290(infl), 375 (1310); δ(D₂O) 6.59 (1H, s), 6.67) (1H, s), 7.10 (1H, s), 7.22 (1H, s) 8.71 (1H, s). Preparation 5(a) 2,4-Dimethyloxazole-5-carboxaldehyde 2,4-Dimethyloxazole-5-carboxylic acid (1.54g) (J.W. Cornforth and R.H. Cornforth, JCS, 1953, 93), was suspended in thionyl chloride (20 ml) and heated at reflux temperature for one hour. The black solution was evaporated, toluene added and the process repeated to give the crude acid chloride (44) (1.15g) as a black gum; νmax (CH₂Cl₂) 1755 cm⁻¹; δ(CDCl₃), 2.50 (s), 2.55 (s). A solution of the crude acid chloride (44) (1.15g) in acetone (20 ml) at room temperature was sequentially treated with triphenyl -phosphine (4.2g) and bis(triphenylphosphine) copper (I) tetrahydroborate (5.31g). After one hour the reaction mixture was filtered and evaporated. Residue was stirred with ether for 10 minutes then filtered and evaporated. The residue was chromatographed on silica eluting with ethyl acetate/hexane mixtures to give the title aldehyde (45) (520 mg) as a low melting solid; νmax (CH₂Cl₂) 1685, 1670 cm⁻¹; δ(CDCl₃) 2.45 (3H, s), 2.54 (3H, s), 9.84 (1H, s). This material was of sufficient purity for further synthetic work. Preparation 5(b) 1-t-Butyldimethylsilyl-3-[(2,4-dimethyloxazol-5-yl)hydroxymethyl]-4-tritylthioazetidin-2-one 1-t-Butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (1.99g, 4.34 mmol) was reacted with 2,4-dimethyloxazole-5-carboxaldehyde (45) (520 mg, 4.16 mmol) using the method described in preparation 1(a) to give after work-up two fractions which were partially separable by chromatography. The less polar fraction (290 mg) contained a 4:1 mixture of a trans (isomer A) and a cis isomer (isomer B) of the title azetidinone (46); νmax (CH₂Cl₂) 3600-3300, 1745 cm⁻¹; δ(CDCl₃) 0.13 (4.8H, s), 0.35 (0.6H, s), 0.38 (0.6H, s), 0.89 (7.2H, s), 0.98 (1.8H, s) 1.59 (0.8H, d, J 5.6Hz), 1.95 (0.6H, s), 1.99 (2.4H, s), 2.35 (2.4H, s), 2.36 (0.6H, s), 2.90 (0.2H, d, J 5.3Hz), 3.53 (0.2H, dd, J 1.5, 4.6Hz), 3.67 (0.8H, dd, J 1.7, 3.2Hz), 3.76 (0.2H, bd, J 4.7Hz), 4.13 (0.8H, dd, J 3.2, 5.6Hz), 4.43 (0.8H d, J1.7Hz), 4.79 (0.2H, d, J 4.7Hz), 7.15-7.56 (15H m). (Found [MH]⁺ 585). The more polar fraction (610 mg) contained a trans isomer (isomer C) of the title azetidinone (46); νmax (CH₂Cl₂) 3600-3300, 1740 cm⁻¹; δ(CDCl₃), -0.02 (3H, s), 0.09 (3H, s), 0.87 (9H, s), 1.81 (3H, s), 2.31 (3H, s), 2.56 (1H, d, J 9.8Hz), 3.79 (1H, dd, J 1.9, 7.2Hz), 4.04 (1H, d, J 1.9Hz), 4.09 (1H, dd, J 7.3, 9.6Hz), 7.12-7.53 (15H, m). (Found [MH]⁺ 585). A 1:2 mixture of the less polar and more polar fractions (360 mg) was also isolated and this could be purified by further chromatography. Preparation 5(c) (3RS, 4SR) 3-[(2,4-Dimethyloxazol-5-yl)hydroxymethyl]-4-tritylthioazetidin-2-one The alcohol (46) (isomer C) (730 mg) from preparation 5(b) was treated as for preparation 1(b) to give the title azetidinone (47) (460 mg, 78%); νmax (CH₂Cl₂) 3600-3200, 1770 cm⁻¹; δ(CDCl₃), 2.14 (3H, s), 2.53 (3H, s), 2.60 (1H, bs, exch.), 3.45 (1H, dd, J 2.7, 4.0Hz), 4.18 (1H, s), 4.47 (1H, d, J 2.7Hz), 5.13 (1H, bd, sharpens to d, J 4.0Hz on D₂O exch.), 7.12-7.53 (15H, m). (Found [MH + diethylamine]⁺ 544). Preparation 5(d) (3RS, 4SR) 3-[Acetoxy(2,4-dimethyloxazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (47) (410 mg) from preparation 5(c) was treated as for preparation 1(c) to give title azetidinone (48) (430 mg, 96%); νmax (CH₂Cl₂) 3380, 1775, 1740 cm⁻¹; δ(CDCl₃), 2.07 (3H, s), 2.13 (3H, s), 2.46 (3H, s), 3.59 (1H, dd with further fine coupling J ∼3,∼7Hz), 4.22 (1H, bs), 4.63 (1H, d, J 2.7Hz), 6.14 (1H, d, J 6.7Hz), 7.22-7.73 (15H, m). Preparation 5(e) (3RS, 4SR) 3-[Acetoxy(2,4-dimethyloxazol-5-yl)methyl-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (48) (410 mg) from preparation 5(d) was converted via the hydroxyester (49), νmax (CH₂Cl₂) 3500, 1775, 1750 cm⁻¹; and the chloroester (50), νmax (CH₂Cl₂) 1785, 1750 cm⁻¹; to the title phosphorane (51) (443 mg, 57%) νmax (CH₂Cl₂) 1760, 1620 cm⁻¹; using the methods described in preparation 1(d). Preparation 5(f) (3RS, 4SR) Silver 3-[Acetoxy(2,4-dimethyloxazol-5-yl)methyl)1-(1-p-Nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl) azetidin-2-one-4-thiolate The phosphorane (51) (440 mg) from preparation 5(e) was treated as for preparation 1(e) to give the title silver salt (52) (345 mg, 91%) as an off-white solid. Preparation 5(g) (5RS, 6SR) p-Nitrobenzyl 6-[Acetoxy(2,4-dimethyloxazol-5-yl)methyl] penem-3-carboxylate The silver salt (52) (345 mg) from preparation 5(f) was treated as for preparation 1(f) to give the title penem (53) (130 mg, 66%); λmax (EtOH, 314 nm (Em 9350), 262 (12970); νmax (CH₂Cl₂), 1795, 1745, 1720 cm⁻¹; δ(CDCl₃) 2.09 (3H, s), 2.20 (3H, s), 2.42 (3H, s), 4.27 (1H, ddd, J 1.0, 1.8 4.4Hz), 5.28 and 5.42 (2H, ABq, J 13.5Hz), 6.08 (1H, d, J 2.0Hz), 6.2 (1H, d, J 4.5Hz), 7.32 (1H,d, J 1.0Hz), 7.59 (2H, d, J 8.7Hz), 8.29 (2H (2H, d, J 8.7Hz). Example 5(a) (5RS) p-Nitrobenzyl (Z)-6-[(2,4-dimethyloxazol-5-yl)methylene]penem -3-carboxylate. The penem (53) (99 mg) from preparation 5(g) was treated as for example 1(a) to give the title penem (54) (75mg, 82%) mp. 203-4° (yellow/orange microneedles ex CH₂Cl₂/Hexane); λmax (EtOH) 311 nm (Em 34,430); νmax (CH₂Cl₂), 1780, 1720, 1670 cm⁻¹; δ(CDCl₃) 2.27 (3H, s), 2.51 (3H, s), 5.30 and 5.46 (2H, ABq, J 13.6Hz), 6.55 (1H, d, J 0.8Hz), 6.96 (1H, d, J 0.7Hz), 7.37 (1H, s), 7.37 (1H, s), 7.62 (2H, d, J 8.7Hz), 8.25 (2H, d, J 8.7Hz). (Found M⁺ 413. 0678. C₁₉H₁₅ N₃O₆S requires M⁺ 413.0682). Example 5(b) (5RS) Sodium (Z)-6-[(2,4-dimethyloxazol-5-yl)methylene]penem-3-carboxylate The penem ester (54) from example 5(a) was hydrogenated as for example 1(b) to give the title sodium salt (55) (28mg, 77%) as a yellow freeze-dried solid; λmax (H₂O) 305 nm (Em 22,330); δ(D₂O) 2.22 (3H, s), 2.28 (3H, s), 6.61 (1H, s), 7.06 (1H, s); 7.11 (1H, s). Preparation 6(a) (3RS,4SR) 1-t-Butyldimethylsilyl-3-(4-methyl-1,2,3-thiadiazol-5-ylcarbonyl)-4-tritylthioazetidin-2-one 1- t -Butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (9.18g) was reacted with ethyl 4-methyl-1,2,3-thiadiazole-5-carboxylate (6.88g) (R. Raap and R.G. Micetich, Can. J Chem., 1968, 46 , 1057) using the method described in preparation 4(a). The crude product was chromatographed on silica eluting with dichloromethane/hexane mixtures to give, after trituration with ether, the title azetidinone (56). (7.19g, 61%); m.p. 163-5° (needles ex ethylacetate/hexane); νmax (CH₂Cl₂) 1750, 1680 cm⁻¹; δ(CDCl₃) 0.40 (3H, s), 0.41 (3H, s), 0.99 (9H, s) 2.84 (3H, s), 3.78 (1H, d, J 1.6Hz), 5.03 (1H, d, J 1.6Hz), 6.96-7.09 (3H, m), 7.12-7.28 (6H, m), 7.31-7.52 (6H, m). (Found: C, 6.5; H, 6.2; N, 7.3; S, 11.2. C₃₂H₃₅N₂O₂S₂ Si requires C, 65.6; H, 6.0; N, 7.2; S, 11.0%). Preparation 6(b) (3RS,4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(4-methyl-1,2,3-thiadiazol-5-yl)methyl]-4-tritylazetidin-2-one The ketone (56) (6.9g) from preparation 6(a) was treated as for preparation 4(b) to give two fractions. The less polar fraction contained a trans isomer (isomer A) of the title azetidinone (57) (1.98g, 28%) νmax (CHCl₃) 3600-3150, 1740 cm⁻¹; δ(CDCl₃), (excluding Me ₂Si groups which were obscured by TMS), 0.87 (9H, s), 2.01 (1H, d, J 6Hz), 2.43 (3H, s), 3.49 (1H, dd, J 2,4Hz), 4.23 (1H, dd, J 4,6Hz), 4.40 (1H, d, J 2Hz), 7.11-7.55 (15H, m). The more polar fraction contained the other trans isomer (isomer B) of the title azetidinone (57) (4.04g, 58%), m.p. 183-5° (needles ex ethyl acetate/hexane); νmax (CHCl₃) 3600-3150, 1730 cm⁻¹; δ(CDCl₃) (excluding Me ₂-Si groups which were masked by TMS), 0.83 (9H, s), 2.37 (3H, s), 2.76 (1H, bs), 3.65 (1H, dd, J 2,6Hz), 4.58 (1H, bd; J 6Hz), 7.18-7.51 (15H, m). (Found: C, 65.4; H, 6.4; N, 7.2; S, 10.6. C₃₂H₃₇N₂O₂S₂ requires C, 65.4; H, 6.3; N, 7.2; S, 10.9%). Preparation 6(c) (3RS, 4SR) 3- [Hydroxy(4-methyl-1,2,3-thiadiazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (57) (isomer B) (3.34g), from preparation 6(b) was treated as for preparation 2(b) to give the title azetidinone (58) (2.49g, 91%); m.p. 250-3°, (prisms ex chloroform/ether), νmax (KBr) 3640-3060, 1760 cm⁻¹, δ(d6-DMSO) 2.57 (3H, s), 3.61 (1H, dd, J 3,4 Hz), 4.01 (1H, d, J3Hz), 5.29 (1H, dd, J 4,4Hz, collapses to d, J 4Hz on D₂0 exch.); 6.74 (1H, d, J 4Hz, exch.), 7.04-7.38 (15H, m), 7.58 (1H, s, exch.). (Found: C, 66.3; H, 4.8; N, 8.8; S, 13.32. C₂₆H₂₃ N₃0₂S₂ requires C, 65.9; H, 4.9; N, 8.9; S, 13.5%). Preparation 6(d) (3RS, 4SR) 3-[Acetoxy(4-methyl-1,2,3-thiadiazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (58) (2.49g) from preparation 6(c) was treated as for preparation 1(c) to give the title acetate (59) (2.32g, 86%); m.p. 215° (dec) (plates ex ethyl acetate/hexane); νmax (Nujol) 3375, 1770, 1755 cm⁻¹ δ(CDCl₃), 2.07 (3H, s), 2.68 (3H, s), 3.53 (1H, dd, J 3,6Hz), 4.28 (1H, bs, exch.), 4.35 (1H, d, J 3Hz), 6.37 (1H, d, J 6Hz), 7.13-7.46 (15H, m). (Found: C, 65.5; H, 5.0; N, 8.0; S, 12.1. C₂₈S, 12.1. C₂₈H₂₅N₃O₃S₂ requires C, 65.2; H, 4.9; N, 8.1; S, 12.4%). Preparation 6(e) (3RS, 4SR) 3-[Acetoxy(4-methyl-1,2,3-thiadiazol-5-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritythioazetidin-2-one The acetate (59) (2.03g) from preparation 6(d) was converted via the hydroxy esters (60), νmax (CHCl₃) 3500, 1775, 1750 cm⁻¹; and the chloro ester (61), to the title phosphorane (62) (2.62g, 69%) m.p. 237-9° (plates ex CH₂Cl₂/MeOH) νmax (CHCl₃), 1725, 1615 cm⁻¹. (Found, C, 68.3; H, 4.5; N, 5.6; S, 6.8; P, 3.3. C₅₅H₄₅N₄S₂ O₇P requires C, 68.2; H, 4.7; N, 5.8; S, 6.6; P, 3.2%), using the methods described in preparation 1(d). Preparation 6(f) (3RS, 4SR) Silver 3-[Acetoxy(4-methyl-1,2,3-thiadiazol-5-yl)methyl-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidin-2-one-4-thiolate The phosphorane (62) (2.00g) from preparation 6(e) was treated as for preparation 1(e) to give the title silver salt (63) (1.70g) νmax (Nujol) 1750, 1595 cm⁻¹. Preparation 6(g) (5RS, 6SR) p-Nitrobenzyl 6-[Acetoxy (4-methyl-1,2,3-thiadiazol-5-yl)methyl]penem-3-carboxylate The silver salt (63) (350 mg) from preparation 6(f) was treated as for preparation 1(f) to give the title penem (64) (100 mg, 50%); m.p. 151-4°, (light yellow needles ex ethyl acetate/hexane), λmax (2% CHCl₃/EtOH) 312 nm (Em 7500), 261 (15610); νmax (CH₂Cl₂, 1795, 1750, 1720 cm⁻¹; δ(CDCl₃) 2.08 (3H, s), 2.75 (3H, s), 4.19 (1H, d, with further fine coupling, J 6Hz), 5.21 and 5.40 (2H, ABq, J 14Hz), 5.79 (1H, d, J 2Hz), 6.47 (1H, d, J 6Hz), 7.26 (1H, s), 7.51 (2H, d, J 9Hz), 8.18 (2H, d, J9Hz). (Found C, 47.7; H, 3.5; N, 11.8; S, 13.3 C₁₉H₁₆N₄S₂O₇ requires C, 47.9; H, 3.4; N, 11.7; S, 13.5%). Example 6(a) (5RS) p-Nitrobenzyl 6-[(4-methyl-1,2,3-thiadiazol-5-yl)methylene] penem-3-carboxylate The penem (64) (315 mg) from preparation 6(g) was treated as for example 1(a) to give two products. The less polar material contained the Z isomer of the title penem (65) (129 mg, 47%) m.p. 174-7° (orange plates ex chloroform/hexane); νmax (2% CHCl₃/EtOH) 308 nm (Em 25200), 264 (21630); νmax (CHCl₃), 1785, 1715 cm⁻¹; δ (CDCl₃) 2.84 (3H, s), 5.31 and 5.46 (2H, ABq, J 13.4Hz), 6.38 (1H, d, J 1.0Hz), 7.37 (1H, d, J 0.8Hz), 7.39 (1H, s), 7.61 (2H, d, J 8.7 Hz), 8.25 (2H, d, J 8.7Hz); (Found: C, 49.1; H, 2.9; N, 13.2; S, 15.6. C₁₇H₁₂N₄S₂0₅ requires C, 49.1; H, 2.9; N, 13.5; S, 15.4%). The more polar material contained the E -isomer of the title penem (65) (41 mg, 15%), m.p. 205-8° (orange microcrystalline solid ex CHCl₃/CH₂Cl₂/hexane), νmax (2% CHCl₃/Et0H), 309 nm (Em 22450), 263 (17590); νmax (KBr), 1780, 1720 cm⁻¹; δ(CDCl₃), 2.81 (3H, s), 5.33 and 5.44 (2H, ABq, J 13.5Hz), 6.49 (1H, s), 6.82 (1H, d, J 0.5Hz), 7.45 (1H, s), 7.61 (2H, d, J 8.9Hz), 8.26 (2H, d, J 8.8Hz). (Found M⁺ 416.025. C₁₇H₁₂N₄0₅S₂ requires M 416.025). Example 6(b) (5RS) Sodium (E)-6-[(4-methyl-1,2,3-thiadiazol-5-yl)methylene] penem-3-carboxylate The ( E )-penem ester (65) (62 mg) from example 6(a) was hydrogenated as for example 1(b) to give the title sodium salt (66) (11 mg, 27%) as an orange freeze-dried solid, λmax (H₂O) 280 nm (Em 5729), 306 (8,290); δ(D₂O) 2.72 (3H, s), 6.52 (1H, s), 7.09 (1H, s), 7.15 (1H, s). Example 6(c) (5RS) Sodium (Z)-6-[(4-methyl-1,2,3-thiadiazol-5-yl)methylene] penem-3-carboxylate The ( Z )-penem ester (65) (90 mg) from example 6(a) was hydrogenated as for example 1(b) to give the title sodium salt (67) (23 mg, 38%) as an orange freeze-dried solid; λmax (H₂O) 265 nm (Em 9740), 306 (13970); δ(D₂0) 2.76 (3H, s), 6.53 (1H, s), 7.09 (1H, s), 7.51 (1H, s). Preparation 7(a) (3RS,4SR) 1-t-Butyldimethylsilyl-3-(2-methyloxazol-4-ylcarbonyl)-4-tritylthioazetidin-2-one 1-t-Butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (9.18g) was reacted with ethyl 2-methyloxazole-4-carboxylate (3.56g) (J.W. Cornforth and R.H. Cornforth, JCS, 1947, 96) using the method described in preparation 4(a) to give the title azetidinone (68) (6.89g, 60%); m.p. 163-6° (plates ex ethyl acetate/hexane), νmax (CH₂Cl₂) 1725, 1680 cm⁻¹; δ(CDCl₃) (excluding Me ₂-Si groups which were obscured by TMS). 2.47 (3H, s), 4.23 (1H, d, J 3Hz), 4.89 (1H, d, J3Hz), 6.99-7.67 (15H, m), 8.00 (1H, s); (Found: C, 69.4; H, 6.6; N, 4.9; S, 5.5. C₃₃H₃₆N₂O₃SSi requires C, 69.7; H, 6.4; N, 4.9; S, 5.6%). Preparation 7(b) (3RS,4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(2-methyloxazol-4-yl) methyl]-4-tritylthioazetidin-2-one The ketone (68) (6.79g) from preparation 7(a) was treated as for preparation 4(b) to give two fractions. The less polar fraction contained a trans isomer (isomer A) of the title azetidinone (69) (1.31g, 19%), m.p. 200-3° (plates ex ethylacetate/ hexane); νmax (Nujol) 3250, 1725 cm⁻¹; δ(d6-DMSO) (excluding Me ₂-Si groups which were obscured by TMS) 2.36 (3H, s); 3.61 (1H, bs); 3.73-3.80 (1H, m) collapses to 3.75 (bs) on D₂O exch.), 4.38 (1H, d, J 5Hz), 4.56 (1H, d, J 2Hz), 7.22-7.56 (16H, m). (Found MH⁺ 571). The more polar fraction contained the other trans isomer (isomer B) of the title azetidinone (69) (4.27g, 62%) - this material was contaminated with about 5% of the azetidinone (69) (isomer A), but was used in further synthetic transformations. Recrystallization from ethyl acetate/hexane afforded pure (isomer B) m.p. 196-8°; ν max (CH₂Cl₂) 1720 cm⁻¹; δ(d6-acetone), (excluding Me ₂Si signals which were obscured by TMS) 2.31 (3H, s), 3.73 (1H, bs, collapses to dd, J 1.6, 2.9Hz on D₂O exch.) 4.08 (2H, bs collapses to 1H, d, J1.6Hz on D₂O exch.) 4.55 (1H, d, J 1.8Hz), 7.22-7.56 (16H, m); (Found, C, 69.1; H, 6.6; N, 4.9; S, 5.7. C₃₃H₃₈N₂O₃SSi requires C, 69.4; H, 6.7; N, 4.9; S, 5.6%). Preparation 7(c) (3RS, 4SR) 3-[Hydroxy(2-methyloxazol-4-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (69) (isomer B) (4.1g) from preparation 7(b) was treated as for preparation 2(b) to give the title azetidinone (70) (2.84g, 86%). νmax (CH₂Cl₂), 3400, 1765 cm⁻¹. δ(CDCl₃) 2.55 (3H, s), 3.20 (1H, bs, exch.), 3.63 (1H, dd, J 2.7, 3.5Hz), 4.23 (1H, bs), 4.54 (1H, d, J 2.7Hz), 5.08 (1H, d, J 3.6Hz), 7.19-7.40 (15H, m), 7.62 (1H, d, J 1.0Hz). Preparation 7(d) (3RS, 4SR) 3-[Acetoxy(2-methyloxazol-4-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (70) (2.84g) from preparation 7(c) was treated as for preparation 1(c) to give the title acetate (71) (3.09g) m.p. 147-150 (microcrystalline ex ethyl acetate/hexane); νmax (CHCl₃) 3380, 1770 cm⁻¹; δ(CDCl₃) 2.07 (3H, s), 2.48 (3H, s), 3.65-3.87 (1H, m), 4.28 (1H, bs), 4.58 (1H, d, J 3Hz), 6.07 (1H, d, J 10Hz), 7.10-7.51 (16H, m). (Found, C, 69.7; H, 5.1; N, 5.6; S, 6.4. C₂₉ H₂₆N₂O₄S requires C, 69.9; H, 5.2; N, 5.6; S, 6.4%). Preparation 7(e) (3RS, 4SR) 3-[Acetoxy(2-methyloxazol-4-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (71) (2.63g) from preparation 7(d) was converted via the hydroxy esters (72), νmax (CHCl₃), 3510, 1760 cm⁻¹; and the chloroesters (73), νmax (CHCl₃) 1775 cm⁻¹; to the title phosphorane (74), (3.14g, 63%), νmax (CHCl₃) 1725, 1605 cm ⁻¹, using the methods described in preparation 1(d). Preparation 7(f) (3RS, 4SR) Silver 3-[Acetoxy(2-methyloxazol-4-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl) azetidin-2-one-4-thiolate The phosphorane (74) (3.14g) from preparation 7(e) was treated as for preparation 1(e) to give the title silver salt (75) (1.92g, 95%) as a buff solid, νmax (Nujol) 1720, 1600 cm⁻¹. Preparation 7(g) (5RS, 6SR) p-Nitrobenzyl 6-[Acetoxy(2-methyloxazol-4-yl)methyl] penem-3-carboxylate The silver salt (75) (1.92g) from preparation 7(f) was treated as for preparation 1(f) to give two fractions. The less polar fration contained the title penem (76) (isomer B) (554 mg, 51%) m.p. 137-9° (microcrystalline ex ethyl acetate/hexane, λmax (2% CHCl₃/EtOH), 261 nm (Em 16345), 316 (9160); νmax (CHCl₃) 1795, 1720 cm⁻¹; δ(CDCl₃), 2.09 (3H, s), 2.43 (3H, s), 4.36 (1H, dddd, J 1.1, 1.9, 3.8Hz), 5.27 and 5.42 (2H, ABq, J 13.6Hz), 6.10-6.13 (2H, m), 7.33 (1H, d, J 0.9Hz), 7.59 (2H, d, J 9.4Hz), 7.61 (1H, s), 8.23 (2H, d, J 8.8Hz). (Found: C, 52.2; H, 3.7; N, 9.1; S, 7.0. C₂₀H₁₇ N₃SO₈ requires C, 52.3; H, 3.7; N, 9.1; S, 7.0%). This material was derived from the alcohol (69) (isomer B). The more polar fraction contained an impure sample of the title penem (76) (isomer A) (100 mg), νmax (CHCl₃) 1790, 1720 cm⁻¹; δ(CDCl₃) 2.15 (3H, s), 2.45 (3H, s), 2.45 (3H, s), 4.45-4.49 (1H, m), 5.25 and 5.40 (2H, ABq, J 13.5 Hz), 5.78 (1H, d, J 2.0Hz), 6.20 (1H, d, J 5.9Hz), 7.31 (1H, d, J 1.0 Hz), 7.57 (2H, d, J 8.7Hz), 7.64 (1H, d, J 0.4Hz), 8.23 (2H, d, J 8.7 Hz). Example 7(a) (5RS) p-Nitrobenzyl 6-[(2-methyoxazol-4-yl)methylene]penem-3-carboxylate The penem (76) (isomer B) (610 mg) from preparation 7(g) was treated as for example 1(a) to give two products. The less polar material contained the (Z)-isomer of title penem (77) (518 mg, 97%) m.p. 199-201° (orange needles ex CHCl₃/CH₂Cl₂/hexane); λmax (2% CHCl₃/EtOH) 287 nm (Em 27260); νmax (KBr) 1785, 1710 cm⁻¹; δ(CDCl₃) 2.48 (3H, s), 5.28 and 5.45 (2H, ABq, J 13.6Hz), 6.63 (1H, d, J 1.1 Hz), 6.93 (1H, d, J 0.8Hz), 7.37 (1H, s), 7.62 (2H, d, J 8.7Hz), 7.78 (1H, s), 8.24 (2H, d, J 8.7Hz); (Found; C, 54.1; H, 3.2; N, 10.6; S, 7.6. C₁₈H₁₃N₃SO₆ requires C, 54.1; H, 3.3; N, 10.5; S, 8.0%). The more polar fraction contained the (E) isomer of the title penem (77) (17mg, 3%). νmax (KBr) 1777, 1710 cm⁻¹; δ(CDCl₃) 2.49 (3H, s), 5.30 and 5.45 (2H, ABq, J 13.5Hz), 6.45 (1H, s), 6.57 (1H, s), 7.42 (1H, S), 7.61 (2H, d, J 8.6Hz), 8.25 (2H, d, J 8.7Hz), 8.68 (1H, d, J 0.6Hz). Example 7(b) (5RS) p-Nitrobenzyl 6-[(2-methyloxazol-4-yl)methylene]penem-3-carboxylate The penem (76) (isomer A) (85 mg) from preparation 7(g) was treated as for example 1(a) to give the (Z) isomer (27 mg) and the (E) isomer (26 mg) of the title penem (77). These materials were identical to those obtained from example 7(a). Example 7(c) (5RS) Sodium (Z)-6-[(2-methyloxazol-4-yl)methylene]penem-3-carboxylate The (Z)-penem ester (77) (100 mg) from example 7(a) was hydrogenated as for example 1(b) to give the title sodium salt (78) (49 mg, 69%) as a yellow freeze dried solid; λmax (H₂O) 286 nm (Em 20,000); δ(D₂O) 2.47 (3H, s), 6.64 (1H, s), 7.05 (1H, s), 7.07 (1H, s), 8.06 (1H, s). Preparation 8(a) (3RS,4SR)1-t-Butyldimethylsilyl-3-(1-methyl-1,2,3-triazol-4-yl-carbonyl)-4-tritylthioazetidin-2-one A solution of diisopropylamine (1.8 ml, 12.4 mmol) in dry THF (80 ml) under argon at -30° was treated dropwise with a solution of n -butyl lithium (1.7M, 7.3 ml, 12.4 mmol) in hexane. After 15 minutes the reaction mixture was cooled to -70° and treated dropwise with a solution of the azetidinone (1) (4.59 g, 10 mmol) in THF (80 ml). After a further 15 minutes the resulting pink solution was removed from the cooling bath and immediately treated in one portion with a hot solution of methyl 1-methyl-1,2,3-triazole-4-carboxylate (1.75 g, 12.4 mmol) (prepared by treating methyl propiolate with excess methyl azide; mp 161 lit. m.p. 159-161° [T.C. Thurber and L.D. Townsend, J. Org. Chem., 1976, 41 , 1041] in THF (100 ml). After a few minutes the reaction mixture was returned to the cooling bath. After a further 10 minutes the reaction mixture was quenched with saturated ammonium chloride and worked-up as in Preparation 3(a) to give the title azetidinone (79) 4.7 g, 83% m.p. 170-1 (plates ex ethyl acetate/hexane); ν max (CH₂Cl₂) 1745, 1680 cm⁻¹; δ(CDCl₃) (excluding Me ₂Si signals) which were masked by TMS) 0.98 (9H, s), 4.04 (3H, s), 4.80 (1H, d, J1.5Hz), 5.01 (1H, d, J1.5Hz), 7.02-7.65 (15H, m), 7.89 (1H, s). (Found C, 67.5; H, 6.3; N, 9.7; S, 5.5. C₃₂N₃₆N₄0₂SSi requires C, 67.6; H, 6.3; N, 9.9; S, 5.6%). Preparation 8(b) (3RS,4SR)1-t-Butyldimethylsilyl-3-[hydroxy(1-methyl-1, 2,3-triazol-4-yl)methyl]-4-tritylthioazetidin-2-one The ketone (79) (4.8 g, 8.45 mmol) from Preparation 8(a) was treated with sodium borohydride (642 mg, 16.90 mmol) as for Preparation 4(b) to give two fractions. The less polar fraction contained a trans isomer (isomer A) of the title azetidinone (80) (1.12 g, 23%), m.p. 221-2 o (rods ex ethyl acetate), ν max (CH₂Cl₂) 3600-3500, 1740 cm⁻¹ δ(CDCl₃) 0.30 (6H, s), 0.96 (9H, s), 1.94 (1H, d, J 4.1Hz), 3.34 (1H, bs) 3.84 (1H, bs, collapses to d, J1.7Hz on irradiation a δ1.94) 4.01 (3H, s), 4.54 (1H, d, J1.7Hz), 7.18-7.49 (15H, m), 7.90 (1H, s). (Found C, 67.4; H, 6.8; N, 9.6; S, 5.4. C₃₂H₃₈N₄0₂SSi requires C, 67.4; H, 6.7; N, 9.8; S, 5.6%). The more polar fraction contained the other trans isomer (isomer B) of the title azetidinone (80) (3.01 g, 62%) ν max (CH₂Cl₂) 3600-3400, 1735 cm⁻¹; δ(CDCl₃) -0.04 (3H, s), 0.05 (3H, s), 0.78 (9H, s), 2.57 (1H, d, J 8.5Hz), 3.62 (1H, dd, J 1.6, 5.9Hz), 3.97 (3H, s), 4.10 (1H, dd, J 6.0, 8.5Hz), 4.46 (1H, d, J 1.7Hz), 7.15-7.58 (16H, m). Preparation 8(c) (3RS, 4SR) 3-[Hydroxy(1-methyl-1,2,3-triazol-4-yl) methyl]-4-tritylthioazetidin-2-one The alcohol (80) (isomer B) (3.01 g) from Preparation 8(b) was treated as for preparation 2(b) to give the title azetidinone (81) (2.27 g, 94%); ν max (CH₂Cl₂) 3600-3400, 1765 cm⁻¹; δ(CDCl₃), 3.47 (1H, d, J 4.9Hz), 3.63 (1H, ddd, J 1.0, 2.7, 5.2Hz), 4.16 (3H, s), 4.31 (1H, bs), 4.60 (1H, d, J 2.7Hz), 5.25 (1H, dd, J 5.1, 5.1Hz), 7.15-7.46 (15H, m), 7.63 (1H, s). Preparation 8(d) (3RS, 4SR) 3-[Acetoxy(1-methyl-1,2,3-triazol-4-yl) methyl]-4-tritylthioazetidin-2-one The alcohol (81) (2.17 g) from Preparation 8(c) was treated as for Preparation 1(c) to give the title acetate (82) (2.31 g, 97%); ν max (CH₂Cl₂) 3400, 1775, 1740 (sh) cm⁻¹; δ(CDCl₃) 2.11 (3H, s), 3.83 (1H, ddd, J 1.1, 2.7, 6.5Hz), 4.11 (3H, s), 4.17 (1H, bs), 4.69 (1H, d, J 2.7Hz), 6.17 (1H, d, J 6.4Hz), 7.14-7.62 (16H, m). Preparation 8(e) (3RS, 4SR) 3-[Acetoxy(1-methyl-1,2,3-triazol-4-yl) methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (82) (2.2g) from Preparation 8(d) was converted via the hydroxy esters (83), ν max (CH₂Cl₂) 3500, 1775, 1750 cm⁻¹; and the chloro esters, ν max (CH₂Cl₂) 1780, 1750 cm⁻¹ to the title phosphorane (84) (2.98g, 71%), ν max (CH₂Cl₂) 1750, 1620 cm⁻¹, using the methods described in Preparation 1(d). Preparation 8(f) (3RS, 4SR) Silver 3-[Acetoxy(1-methyl-1,2,3-triazol-4-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidin-2-one-4-thiolate The phosphorane (84) (951 mg, 1 mmol) in dichloromethane (1 ml) and methanol (10 ml) under argon at room temperature was treated with pyridine (0.105 ml, 1.3 mmol) and a solution of silver nitrate (0.15M, 8.7 ml, 1.3 mmol) in methanol. After 30 minutes the reaction mixture was evaporated to give the crude silver salt (85). This material was of sufficient purity for further synthetic transformations. Preparation 8(g) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy(1-methyl-1,2,3-triazol-4-yl) methyl]penem-3-carboxylate The crude silver salt (85) from Preparation 8(f) was treated as for Preparation 1(f) to give the title penem (86) (227 mg, 52%), ν max (CH₂Cl₂) 1795, 1740, 1720, 1605 cm⁻¹; δ(CDCl₃) 2.09 (3H, s), 4.09 (3H, s,), 4.56 (1H, bs), 5.28 and 5.43 (2H, ABq, J 13.5Hz), 7.20 (1H, d, J 3.4Hz), 7.23 (1H. d, J 2.0Hz), 7.36 (1H, d, J 0.7Hz), 7.60 (2H. d, J 8.6Hz), 7.67 (1H. s), 8.24 (1H. d, J 8.7Hz). Example 8(a) (5RS) p-Nitrobenzyl 6-(1-methyl-1,2,3-triazol-4-yl methylene)penem-3-carboxylate The penem (86) (217 mg) from Preparation 8(g) was treated as for Example 1(a) to give the (Z)-isomer of the title penem (87a) (160 mg; 85%), ν max (CH₂Cl₂) 1780, 1715 cm⁻¹; δ(CDCl₃) 4.15 (3H, s), 5.29 and 5.46 (2H, ABq, J 13.6Hz), 6.68 (1H, d, J 0.8Hz), 7.06 (1H, d, J 1.0Hz), 7.39 (1H, s), 7.63 (2H, d, J 8.6Hz), 7.72 (1H, s), 8.24 (2H, d, J 8.8Hz). (Found M⁺ 399.0637. C₁₇H₁₃N₅0₅S requires M 399.0638). Also obtained was the (E)-isomer of the title penem (87b)(5%), ν max (KBr) 1750, 1710, 1600 cm⁻¹; δ ppm (CDCl₃) 4.17 (3H, s), 5.31 and 5.43 (2H, ABq, J 13.6Hz), 6.48 (1H, d, J 0.6Hz), 6.94 (1H,s), 7.45 (1H, s), 7.62 (2H, d, J 8.8Hz), 8.26 (2H, d, J 8.8Hz), 8.74 (1H, s) (Found: M+, 399.0632. C₁₇H₁₇N₅O₅S requires M, 399.0638). Example 8(b) (5RS) Sodium (Z)-6-(1-methyl-1,2,3-triazol-4-yl-methylene)penem-3-carboxylate The (Z)-penem ester (87a) (150 mg) from Example 8(a) was hydrogenated as for Example 1(b) to give the title sodium salt (88a) (80 mg, 74%) as a yellow freeze-dried solid; λ max (H₂0) 282 nm (Em 19,880); δ(D₂0) 4.13 (3H, s), 6.62 (1H, s), 7.07 (1H, s), 7.21 (1H, s), 8.16 (1H, s). Example 8(c) (5RS) Sodium (E)-6-(1-methyl-1,2,3-triazol-4-yl methylene)penem-3-carboxylate The (E)-penem ester (87b) (45mg) from Example 8(a) was hydrogenated as for Example 1(b) to give the title sodium salt (88b) (17mg, 53%) as a yellow freeze dried solid; λ max (H₂0) 285 nm (12,582);ν max (KBr) 1751, 1604br, 1554 br. cm⁻¹; δppm (D₂0) 4.13 (3H,s), 6.47 (1H,s), 6.90 (1H,s), 7.13 (1H,s), 8.69 (1H,s). Preparation 9(a) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-(5-methyl-1,2,4-oxadiazol-3-yl-carbonyl)-4-tritylthioazetidin-2-one 1- t -Butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (9.18g) was reacted with ethyl 5-methyl-1,2,4-oxadiazole-3-carboxylate (3.76 g) (H.Hillman, H. Piechota and W. Schwiersh, Chem. Ber. 1961, 94 , 757) using the method described in preparation 4(a) to give the title compound (89) (5.24 g, 47%), m.p. 156-8 (plates ex ethyl acetate/hexane) ν max 'CHCl₃) 1725, 1705 cm⁻¹; δ(CDCl₃) (excluding Me ₂Si groups which are obscured by T.M.S.) 0.98 (9H, s); 2.61 (3H, s); 4.57 (1H, d, J 2Hz), 4.98 (1H, d, J 2Hz), 7.02-7.69 (15H, m). (Found C, 67.1; H, 6.4; N, 7.4; S, 5.6; C₃₂H₃₅N₃0₃SSi required C, 67.4; H, 6.2; N, 7.3; S, 5.6%). Preparation 9(b) (3RS, 4SR)1-t-Butyldimethylsily-3-[hydroxy (5-methyl-1,2,4-oxadiazol-3-yl)methyl]-4-tritylthioazetidin-2-one The ketone (89) (5.26 g) from Preparation 9(a) was treated as for Preparation (4b) to give two fractions. The less polar fraction contained a trans isomer (isomer A) of the title azetidinone (90) (2.23 g, 42%) ν max (CH₂Cl₂) 3650-3140; 1720 cm⁻¹; δ(CDCl₃) (excluding MeSi groups obscured by T.M.S.) 0.90 (9H, s); 2.17-2.45 (1H, b. exch); 2.48 (3H, s); 3.5-3.83 (2H, m. sharpens on D₂0 exch); 4.51 (1H, d, J 3Hz) 7.01-7.55 (15H, m) (Found MH⁺ 572).] The more polar fraction contained the other trans isomer (isomer B) of the title azetidinone (90) (1.95 g, 37%); m.p. 108-110° (plates ex ethyl acetate/hexane) ν max (CH₂Cl₂) 3600-3250; 1740 cm⁻¹ δ(CDCl₃) (excluding Me₂Si groups obscured by T.M.S.) 0.84 (9H, s); 2.45 (3H, s); 2.50-2.80 (1H, b. exch); 3.73 (1H, dd, J6H₃, 3Hz) 3.94-4.15 (1H, b sharpens to d, J 6Hz on D₂0 exch.) 4.19 (1H, d, J 3Hz); 6.98-7.55 (15H, m). Preparation 9(c) (3RS, 4SR) 3-[Hydroxy(5-methyl-1,2,4-oxadiazol-3-yl) methyl]-4-tritylthioazetidin-2-one The alcohol (90) (isomer B) (1.83g) from Preparation 9(b) was treated as for Preparation 2(b) to give the title azetidinone (91) (1.46 g, 100%) m.p. 196-9° (needles ex chloroform/hexane); ν max (KBr) 3201, 3084, 1758 cm⁻¹ δ(CDCl₃) 2.71 (3H, s), 3.53 (1H, d, J 5Hz, exch.), 3.66 (1H, dd, J 6,3Hz) 4.24 (1H, bs), 4.68 (1H, d, J 3Hz); 5.26 (1H, dd, J 5, 3Hz, collapses to d, J 3Hz on D₂0 exch.) 7.19-7.41 (15H, m). (Found MH⁺ 458). Preparation 9(d) (3RS, 4SR) 3-[Acetoxy(5-methyl-1,2,4-oxadiazol-3-yl) methyl]-4-tritylthioazetidin-2-one The alcohol (91) (1.00 g) from Preparation 9(c) was treated as for Preparaton 1(c) to give the title acetate (92) (690 mg, 63%) m.p. 152-3° (needles ex Toluene/Hexane) ν max (CH₂Cl₂) 3400, 1780 cm⁻¹ δ(CDCl₃) 2.12 (3H, s); 2.67 (3H, s); 3.79 (1H, dd, J7, 4Hz); 4.34 (1H, bs); 4.70 (1H, d, J4Hz); 6.27 (1H, d, J 8Hz); 7.26-7.58 (15H, m) (Found C, 67.5; H, 5.1; N, 8.6; S, 6.4; C₂₈H₂₄N₃0₄S required C, 67.5; H, 4.9; N, 8.6; S, 6.4%). Preparation 9(e) (3RS, 4SR) 3-[Acetoxy(5-methyl-1,2,4-oxadiazol-3-yl) methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (92) (730 mg) from Preparation 9(d) was converted via the hydroxy eaters (93), ν max (CHCl₃) 3505, 1760 cm⁻¹; and the chloroesters (94), ν max (CHCl₃) 1790 cm⁻¹; to the title phosphorane (95) (570 mg, 53%) ν max (CHCl₃) 1725, 1605 cm⁻¹, using the methods described in Preparation 1(d). Preparation 9(f) (3RS, 4SR) Silver 3-[acetoxy(5-methyl-1,2,4-oxadiazol-3-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one-4-thiolate The phosphorane (95) (570 mg) from Preparaton 9(e) was treated as for Preparation 1(e) to give the title silver salt (96) (511 mg, 91%). Preparation 9(g) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy(5-methyl-1,2,4-oxadiazol-3-yl)methyl]penem-3-carboxylate The silver salt (96) (511 mg) from Preparation 9(f) was treated as for Preparation 1(f) to give the title penem (97) (53 mg, 66%) m.p. 147-9° (plates ex ethyl acetate/hexane λ max (2% CHCl₃/EtOH) 314 nm (Em 7646), 260 nm (Em 13128). ν max (KBr) 1789, 1753, 1715 cm⁻¹; δ(CDCl₃) 2.1 (3H, s); 2.61 (3H, s); 4.46 (1H, d,d,d, J 3.8Hz, 2.2 Hz, 0.8Hz); 5.27 and 5.41 (2H, ABq, J 13.5); 5.98 (1H, d, J 1.9Hz); 6.37 (1H, d, J 3.7); 7.52 (1H, d, J0.8); 7.58 (2H, d, J 8.8Hz); 8.23 (2H, d, J 8.7). (Found C, 49.5; H, 3.4; N, 12.1; S, 6.7; C₁₉H₂₆N₄0₈S required C, 49.6; H, 3.5; N, 12.2; S, 7.0). Example 9(a) (5RS) p-Nitrobenzyl (Z)-6-(5-methyl-1,2,4-oxadiazol-3-ylmethylene)penem-3-carboxylate The penem (97) (115 mg) from Preparation 9(g) was treated as for Example 1(a) to give the title penem (98) (99 mg, 100%) m.p. 165-8° (microneedles ex ethyl acetate/hexane). λ max (2% CHCl₃/Et0H) 303 nm (Em 10915), 243 nm (Em 22954) δ(CDCl₃) 2.64 (3H, s); 5.29 and 5.44 (2H, Abq, J13.5Hz)+ 6.58 (1H, d, J 6.9Hz); 7.10 (1H, d, J 1.1Hz); 7.40 (1H, s); 7.61 (2H, d, J 8.6Hz); 8.23 (2H, d, J 8.8Hz). (Found C, 51.0; H, 3.1; N, 13.8; S, 7.8. C₁₇H₁₃N₄0₆S requires C, 50.9; H, 3.3; N, 14.0; S, 8.0%). Example 9(b) (5RS) Sodium (Z)-6-(5-methyl-1,2,4-oxadiazol-3-yl-methylene)penem-3-carboxylate. The penem ester (98) (30 mg) from Example 9(a) was hydrogenated as for Example 1(b) to give the title sodium salt (99) (10 mg, 48%) as an orange freeze-dried solid, λ max (H₂0) 290 nm (Em 5,300), 240 (14,400); δ (D₂0) 2.63 (3H, s); 6.62 (1H, d, J0.6Hz); 7.11 (1H, s); 7.14 (1H, d, J0.7Hz). Preparation 10(a) Ethyl 1,2,4-triazole-3-carboxylate hydrochloride 1,2,4-Triazole-3-carboxylate (16 g) [G.I. Chipen and V.Ya. Grinshtein, Chem. Heterocyclic Compd . (U.S.S.R), 1,420 (1965)] dissolved in ethanol (400 ml) was saturated with HCl gas. Ice-bath cooling was applied to avoid overheating during saturation and the reaction was left stirring for three days. The product which had precipitated out was filtered off and washed with cold ethanol and then diethyl ether before being dried under vacuum giving 12 g of the product, 48% yield. ν max (Nujol Mull) 1754 cm⁻¹. Preparation 10(b) Ethyl 1-methyl-1,2,4-triazole-3-carboxylate An ethanolic (30 ml) solution of the hydrochloride from Preparation 10(a) (10 g) was added portion-wise to an ethanolic solution of sodium metal (2.6 g). The reaction was stirred for 1 hour after which methyl iodide was added (16 g). The reaction was left stirring for 24 hours after which it was evaporated and taken up into warm ethanol and 50% distilled water added before it was passed down an Amberlite IRA-410 (C1) column. The fractions were combined and evaporated. The residue was extracted with hot ethyl acetate and filtered. The desired product crytallised out from ethyl acetate giving 4.3 g, 49% yield, m.p. 115°C δ (CDCl₃) 1.43 (3H, t, J 7Hz), 4.03 (3H, s), 4.43 (2H, q, J 7Hz) and 8.19 (1H, s). Preparation 10(c) 1-t-Butyldimethylsilyl-3-[(1-methyl-1,2,4-triazol-3-yl) carbonyl]-4-tritylthioazetidin-2-one 1- t -Butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (10.74 g) was reacted with ethyl-1-methyl-1,2,4-triazole-3-carboxylate (2.79 g) in hot tetrahydrofuran ( circa . 20 ml) using the method described in Preparation 4(a) to give the title compound (99a) 8.77 g, 66% yield, m.p. 180 - 182°C, ν max (Nujol Mull) 1751 and 1693 cm⁻¹; δ (CDCl₃) 0.33 (6H, s), 0.98 (9H, s), 3.81 (3H, s), 4.62 (1H, d, J2 Hz), 4.87 (1H, d, J2Hz), 6.7-7.4 (15H, m), and 7.92 (1H, s). Preparation 10(d) 1-t-Butyldimehtylsilyl-3[(1-methyl-1,2,4-triazol-3-yl)hydroxymethyl]-4-tritylthioazetidin-2-one The ketone (99a) (8.8 g) from Preparation 10(c) was dissolved in aqueous dioxane and treated portionwise with NaBH₄ (2 g) over 2 hours. After 3 hours the reaction was diluted with ethyl acetate and washed with dilute acetic acid, brine, and saturated sodium hydrogen carbonate solution. Purification as for Example 3(b) gave the two trans isomers. The more polar isomer (Isomer A) of the title azetidinone (100) was isolated in 28% yield, 2.48 g, m.p. 152 - 154°C. ν max (KBr) 3,431, 1747 and 1725 cm⁻¹, δ (CDCl₃) SiMe₃ peaks obscured by locking signal, 0.78 (9H, s), 1.6-2.5 (1H, br.), 3.71-3.87 (4H, m), 4.14 (1H, br.d., J 6Hz), 4.32 (1H, d, J2 Hz), 7.1-7.71 (15H, m) and 7.75 (1H, s). [Addition of D₂0 caused the br.s. at δ1.6-2.5 to exchange and brd at δ 4.4 to sharpen]. Preparation 10(e) (3RS,4SR)-3[(1-methyl-1,2,4-triazol-3-yl)hydroxymethyl]-4-tritylthioazetidin-2-one The alcohol (100) (Isomer A) (2.23 g) from Preparation 10(d) was treated as for Preparation 2(b) to give the title azetidinone (101) 1.55 g, 87% yeild, m.p. 251° (decomp.) ν max (KBr) 3,390 and 1761 cm⁻¹, δ(DMS0, d6/D₂0) 3.55 (1H, dd, J3 and 4Hz), 3.86 (3H, s), 4.41 (1H, d, J 3Hz), 4.82 (1H, d, J 4Hz), 7.06-7.46 (15H, br s) and 8.40 (1H, s). Preparation 10(f) (3RS,4SR)-3-[(1-methyl-1,2,4-triazol-3-yl)acetoxymethyl]-4-tritylthioazetidin-2-one The alcohol (101) (1.53 g) from Preparation 10(e) was suspended in dichloromethane and treated as for Preparation 1(c) with the exception that the reaction was allowed to proceed at ambient temperature for 2.75 hours and was chromatographed using methyl acetate as eluent. Thus the title azetidinone (102) was obtained as a crystalline solid 1.52 g, 91% yield, m.p. 172 - 175°C. ν max (Nujol Mull) 1781, 1760 and 1733 cm⁻¹, δ(CDCl₃) 2.09 (3H, s), 3.63-3.97 (4H, m), 4.1 (1H, br s), 4.6 (1H, d, J 3Hz), 6.16 (1H, d, J 5Hz), 6.9-7.5 (15H, m) and 7.94 (1H, s). Preparation 10(g) (3RS,4SR)-3-[Acetoxy(1-methyl-1,2,4-triazol-3-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (102) (1.42 g) from Preparation 10(f) was converted via the hydroxy esters (103), ν max (CHCl₃) 1774 and 1750 sh cm⁻¹; and the chloroesters (104), to the title phosphorane (105) (2.1 g, 77% yield), ν max (CHCl₃) 1750 and 1610 cm⁻¹ using the methods described in Preparation 1(d). Preparation 10(h) (3RS,4SR) Silver 3-[(1-methyl-1,2,4-triazol-3-yl)acetoxymethyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The phosphorane (105) (2.34 g) from Preparation 10(g) was treated as for Preparation 1(e) to give the title silver salt (106) 1.69 g, 84% yield. Prepration 10(i) (5RS,6SR) p-Nitrobenzyl 6-[(1-methyl-1,2,4-triazol-3-yl)acetoxymethyl]penem-3-carboxylate The silver salt (106) (1.69 g) from Preparation 10(h) was treated as for Preparation 1(f) to give the title penem (107) 0.78 g, 82% yield. m.p. 150°C (decomp). λ max (EtOH) 262 (Em 14,068) and 316 nm (Em 9,317), ν max (Nujol Mull) 1793, 1739 and 1705 cm⁻¹, δ (CDCl₃) 2.13 (3H, s) 3.88 (3H, s), 4.32 - 4.55 (1H, m), 5.20 and 5.47 (2H, ABq, J 16Hz), 6.03 (1H, br.s), 6.36 (1H, d, J 4Hz), 7.29 (1H, s) 7.3 - 8.16 (aromatics) and 8.23 (1H, s). Example 10(a) (5RS) p-Nitrobenzyl (Z)-6-[(1-methyl-1,2,4-triazol-3-yl)methylene]penem-3-carboxylate The penem (107) (0.78 g) from Preparation 10(i) was treated with 1,8-diazabicyclo[5.4.0]undec-7-ene as for Example 1(a), but after 15 min at -40° the reaction was filtered. The product which had precipitated out was washed with cold dichloromethane and dried under vacuum giving the title penem (108) as a yellow solid 0.45 g, 66% yield. m.p. 1970 (decomp.) λ max (CHCl₃/EtOH) 272 (Em 21,426) and 311 nm (infl.) ν max (KBr) 1781 and 1716 cm⁻¹, δ (CDCl₃) 3.97 (3H, s), 5.29 and 5.46 (2H, ABq, J 13.6 Hz), 6.58 (1H, d, J 0.98Hz), 7.12 (1H, s), 7.38 (1H, s), 7.63 and 8.25 (4H, AA'BB', J 8.8 Hz) and 8.06 (1H, s). Example 10(b) (5RS) Sodium (Z)-6-[(1-methyl-1,2,4-triazol-3-yl)methylene]penem-3-carboxylate The penem ester (108) (0.1 g) from Example 10(a) was hydrogenated as for Example 1(b) to give the title sodium salt (109) as a yellow freeze-dried solid 27 mg, 32% yield. λ max (H₂O) 274 (Em 11,154) and 375 nm (Em 1,029) ν max (KBr) 1755 and 1600 cm⁻¹ , δ(D₂0) 3.95 (3H, s), 6.59 (1H, s), 7.06 (1H, s), 7.1 (1H, s) and 8.39 (1H, s). Preparation 11(a) (3RS, 4SR) 3-[Acetoxy(1-methylpyrazol-4-yl)methyl]-4-(t-butyldiphenylsilyloxyacetylthio)-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidin -2-one . The silver salt (18) (0.400 g) from Preparation 2(e) was suspended in dry dichloromethane (25 ml) and cooled in an ice-bath, with stirring. Pyridine (114 µl) was added, followed by a solution of t -butyldiphenylsilyloxyacetyl chloride (0.509 g) (EP 0 120 613 A, Beecham) in dry dichloromethane (5 ml). Stirring was continued at ice-bath temperature for 20 min. The resulting suspension was filtered through Kieselguhr, washing well with ethyl acetate. The filtrate was washed with IN hydrochloric acid, brine, saturated sodium hydrogen carbonate solution, brine and dried over MgS0₄. The solvent was evaporated at reduced pressure and the residue chromatographed over silica gel (10 g). Elution with a gradient of 50-100% ethyl acetate/hexane gave the title compound (110) as a colourless oil (0.228 g); ν max (CH₂Cl₂) 1760, 1695, 1625, 1608 cm⁻¹. Preparation 11(b) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy (1-methylpyrazol-4-yl) methyl]-2-(t-butyldiphenylsilyloxymethyl)penem-3-carboxylate The phosphorane (110) (0.228 g) from Preparation 11(a) was dissolved in dry toluene (100 ml) and refluxed for 2.5 h, under an atmosphere of argon. After cooling, the solvent was evaporated at reduced pressure and the residue chromatographed over silica gel (6 g). Elution with 50% ethyl acetate/hexane afforded the title compound (III) (0.142 g) as a colourless oil, ν max (CH₂Cl₂) 1792, 1735, 1712, 1608 cm⁻¹. Preparation 11(c) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy(1-methylpyrazol-4-yl)methyl]-2-(hydroxymethyl)penem-3-carboxylate The t-butyldiphenylsilyl ether (111) (0.142 g) from Preparation 11(b) was dissolved in dry tetrahydrofuran (25 ml) and cooled in an ice bath, with stirring. Glacial acetic acid (35 mg) and tetra- n -butyl ammonium fluoride (255 µl of a 1M solution in tetrahydrofuran) were added and the solution stirred at ice bath temperature for 30 min. Ethyl acetate was added and the organic solution was washed with brine, saturated sodium hydrogen carbonate solution, brine and dried over MgS0₄. The solvent was removed by evaporation at reduced pressure and the residue applied to a column of silica gel (6 g). Elution with a gradient of 75-100% ethyl acetate/hexane afforded the title compound (112) as a white solid (0.074 g) after trituration with diethyl ether; λ max (CH₃CN) 318 nm (ε 8581), 265 nm (ε 10858), ν max (KBr) 1784, 1726, 1700, 1604, 1573, 1522 cm⁻¹, δH (d-DMF) 2.06 (3H, s, CO Me ), 3.88 (s, N Me ), 4.54 (1H, dd, J 1.9 and 5.9Hz, 6- H ), 4.77 (2H, broad s, C H ₂0H), 5.35 (1H, d, J 14.2Hz, C0₂C H a), 5.49 (1H, d, J 14.1 Hz, C0₂C H b), 5.74 (1H, d, J 1.7Hz, 5-H), 6.08 (1H, broad res., O H ), 6.21 (1H, d, J 5.9Hz, 8-H), 7.56 (1H, s, pyrazole- H ), 7.78 (2H, d, J 8.7Hz, aromatic H ), 7.86 (1H, s, pyrazole- H ), 8.29 (2H, d, J 8,8Hz, aromatic H ). Example 11(a) (5RS) p-Nitrobenzyl 2-hydroxymethyl-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylate. The penem (112) (0.069 gm) from Preparation 11(c) was dissolved in dry tetrahydrofuran (100 ml) and cooled to -40°C, under an atmosphere of argon, with stirring. A solution of 1,8-diazabicyclo[5.4.0]undec-7-ene (0.054 g) in dry tetrahydrofuran (5 ml) was added dropwise and stirring was continued at this temperature for 30 min. Ethyl acetate was then added and the solution washed with 5% citric acid solution, brine, saturated sodium hydrogen carbonate solution, brine and dried over MgS0₄. The solvent was evaporated at reduced pressure and the residue applied to a column of silica gel (4 g). Elution with a gradient of 0-50% ethyl acetate/dichloromethane yielded the title compound (113) as a pale yellow solid (0.037 g) after trituration with diethyl ether; λmax (CH₃CN) 300 nm (ε 33099), ν max (KBr) 3311, 1772, 1701, 1682, 1604, 1568, 1550, 1516 cm⁻¹, δH(d-DMF) 3.97 (3H, s, N Me ), 4.78 (2H, ABq, CH₂0H), 5.38 (1H, d, J 14.0Hz, C0₂C H a), 5.54 (1H, d, J 14.0Hz, C0₂C H b), 6.05 (1H, broad res., 0 H ), 6.67 (1H, d, J 0.8Hz, 5-H), 7.28 (1H, s, vinylic H ), 7.80 (1H, s, pyrazole- H ), 7.84 (2H, d, J 8.7Hz, aromatic H ), 8.17 (1H, s, pyrazole- H ), 8.30 (2H, d, J 8.7Hz, aromatic H ). (Found: M⁺ 428.0793. C₁₉H₁₆N₄0₆S requires 428.0791). Example 11(b) Sodium (5RS)-2-hydroxymethyl-6(Z)-[(1-methylpyrazol-4-yl) methylene]penem-3-carboxylate The ester (113) (0.020 g) from Example 11(a) was dissolved in 30% aqueous 1,4-dioxan (15 ml) and shaken for 1 h at atmospheric pressure and room temperature with hydrogen in the presence of 5% palladium on carbon catalyst (0.040 g). The suspension was then filtered through Kieselguhr, washing well with water. The filtrate was concentrated to small volume at reduced pressure and applied to a column of Biogel P-2, which was eluted with water. The fractions were monitored by u.v. and those containing the product were combined. Lyophilisation of the resulting solution afforded the title compound (114) as a pale yellow solid (0.011 g), λ max (H₂0) 295 nm (ε 21,563), ν max (KBr) 3217, 1746, 1677, 1603, 1580, 1551 cm⁻¹ δH (D₂0) 3.90 (3H, s, N Me ), 4.52 (1H, d, J 15.0HZ, C H a0H), 4.75 (1H, d, C H b0H), 6.41 (1H, s, 5- H ), 7.16 (1H, s, vinylic H ), 7.67 (1H, s, pyrazole- H ), 7.83 (1H, s, pyrazole - H ). Preparation 12(a) (3RS, 4SR) 3-[Acetoxy-(1-methylpyrazol-4-yl)methyl]-4-[(ethylthio)thiocarbonylthio]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidin-2-one The silver salt (18) (1.10 g) from Preparation 2(e) was suspended in dry dichloromethane (100 ml) and cooled in an ice bath. Pyridine (0.315 ml) was added to the stirred suspension, followed by a solution of ethyl dithiochloroformate (0.393 g) in dichloromethane (5 ml). The resulting solution was stirred at room temperature for 1 h and then filtered through Kieselguhr. The filtrate was washed with 5% citric acid solution, saturated sodium hydrogen carbonate solution, brine, and dried over MgS0₄. The solvent was evaporated at reduced pressure and the residue chromatographed over silica gel (30 g). Elution with a gradient of 50-100% ethyl acetate/hexane afforded the title compound (115) as a pale yellow foam (0.810 g); ν max (CH₂Cl₂) 1762, 1738 (shoulder), 1623, 1608, 1520 cm⁻¹. Preparation 12(b) (5RS, 6SR) and (5RS, 6RS) p-Nitrobenzyl 6-[acetoxy(1-methylpyrazol-4-yl)methyl]-2-ethylthiopenem-3-carboxylate The phosphorane (115) (0.810 g) from Preparation 12(a) was dissolved in dry xylene (800 ml) and heated to reflux for 11 h, under an atmosphere of argon, in the presence of hydroquinone (0.060 g). After cooling, the solvent was evaporated at reduced pressure and the crude product chromatographed over silica gel (30 g). Elution with 75% ethyl acetate/hexane provided the slightly impure product as a pale yellow oil (0.310 g). This was re-chromatographed over silica gel (15 g). Elution with a gradient of 0-20% ethyl acetate/dichloromethane afforded the title compound (116) (0.261 g) as a mixture of (5RS, 6SR) and (5RS, 6RS) diastereoisomers in the approximate ratio of 3:1 (by nmr); λ max (Et0H) 337, 260 nm, ν max (CH₂Cl₂) 1792, 1740, 1695, 1605, 1521 cm⁻¹, δH (CDCl₃) inter alia 1.39 (3H, t, CH₂ CH ₃), 2.05 (minor) and 2.08 (major isomer) (3H, s, C0C H ₃), 3.88 (major isomer) and 3.91 (minor isomer) (3H, s, NC H ₃), 4.23 (dd, J 1.5 and 6.6 Hz, 6-H of major isomer) and 4.51 (dd, J 4.0 and 11.1 Hz, 6-H of minor isomer) (1H), 5.24 (1H, d, C0₂C H a), 5.44 (1H, d, CO₂C H b), 5.65 (d, J 1.4Hz, 5-H of major isomer) and 5.82 (d, J 3.9Hz, 5-H of minor isomer) (1H), 6.18-6.24 (1H, two d, 8-H of majorand minor isomers), 7.4-7.7 (4H, m, pyrazole and aromatic H), 8.22 (2H, d, J 8.8Hz, aromatic H). Example 12(a) (5RS) p-Nitrobenzyl 2-ethylthio-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylate The penem (116) (0.060 g) from Preparation 12(b) was converted to the title compound (117) by the procedure described in Example 11(a). The product was obtained as a yellow solid (0.026 g) after trituration with ethyl acetate; λ max (EtOH) 299 nm (ε 28253), ν max (KBr) 1772, 1669, 1604, 1548, 1508 cm⁻¹, δ H (CDCl₃) 1.38 (3H, t, J 7.4Hz, CH₂C H ₃), 2.85-3.12 (2H, m, SC H ₂), 3.97 (3H, s, NC H ₃), 5.26 (1H, d, J 13.8Hz, C0₂C H a), 5.51 (1H,d , J 13.8 Hz, C0₂C H b ), 6.35 (1H, d, J 1.0Hz, 5-H), 7.07 (1H, s, vinylic H ), 7.51 (2H, s, pyrazole H ), 7.67 (2H, d, J 8.9Hz, aromatic H ), 8.23 (2H, d, J 8.9 Hz, aromatic H ). Example 12(b) Sodium (5RS)-2-ethylthio-6(Z)-[1-methylpyrazol-4-yl)methylene]penem-3-carboxylate Hydrogenolysis of the ester (117) (0.020 g) from Example 12(a) by the procedure described in Example 11(b) afforded the title compound (118) (0.009 g) as a pale yellow fluffy solid after Biogel P-2 column chromatography and lyophilisation; λ max (H₂0) 296 nm (ε 18,728), ν max (KBr) 1761, 1675 and 1550 cm⁻¹, δH (D₂0) 1.30 (3H, t, J 7.4Hz, SCH₂C H ₃), 2.75-3.05 (2H, m, SC H ₂CH₃), 3.89 (3H, s, N M e), 6.43 (1H, s, 5- H ), 7.13 (1H, s, vinylic- H ), 7.65 (1H, s, pyrazole- H ), 7.82 (1H, s, pyrazole- H ). Preparation 13(a) (5RS, 6SR) and (5RS, 6RS) p-Nitrobenzyl 6-[Acetoxy(1-methyl-pyrazol-4-yl)methyl]-2-ethylsulphinylpenem-3-carboxylate. The mixture of diastereoisomers from Preparation 12(b) (0.200 g) was dissolved in dichloromethane (20 ml) and layered with saturated sodium hydrogen carbonate solution (20 ml). The resulting two-phase solution was cooled in an ice bath. A solution of m -chloroperbenzoic acid (0.092 g) in dichloromethane (5 ml) was then added to the vigorously stirred solution. Stirring was continued at ice-bath temperature for 15 minutes. The organic layer was then separated, washed with brine and dried over MgS0₄. The solvent was evaporated at reduced pressure and the residue chromatographed over silica gel (2 g). Elution with a gradient of 0 - 50% ethyl acetate/dichloromethane afforded the isomeric mixture of sulphoxides (119) as a white foam (0.100 g); ν max (CH₂Cl₂) 1801, 1740, 1708, 1610 cm⁻¹. Preparation 13(b) (5RS, 6SR) and (5RS, 6RS) p-Nitrobenzyl 6-[acetoxy(1-methylpyrazol-4-yl)methyl]-2-(2-hydroxyethylthio)penem-3-carboxylate. The mixture of sulphoxides (119) (0.100 g) obtained from Preparation 13(a) and 2-mercaptoethanol (0.031 gm) were dissolved in dry acetonitrile (10 ml) and cooled to -40°C, under an atmosphere of argon. A solution of diisopropylethylamine (0.026 g) in acetonitrile (2 ml) was added to the stirred solution and stirring was continued at -40°C for 30 min. Ethyl acetate was then added and the organic solution washed with 5% citric acid solution, saturated sodium hydrogen carbonate solution, brine and dried over MgS0₄. The solvent was evaporated at reduced pressure and the residue chromatographed over silica gel (2 g). Elution with a gradient of 0-50% ethyl acetate/dichloromethane gave the title compound (120) (0.080 g) as a mixture of (5RS, 6SR) and (5RS, 6RS) isomers in the approximate ratio of 5:2 (by nmr); νmax (CH₂Cl₂) 3600, 1798, 1741, 1698, 1609, 1527 cm⁻¹, δH (CDCl₃) inter alia 2.05 (s, CO Me of minor isomer) and 2.07 (s, CO Me of major isomer ) (3H), 3.05-3.35 (2H, m, SC H ₂), 3.8-4.0 (5H, m, C H ₂0H + N Me ), 4.24 (dd, J 1.3 and 6.5 Hz, 6-H of major isomer) and 4.52 (dd, J 3.9 and 11.0 Hz, 6-H of minor isomer) (1H), 5.23 (1H. d, J 13.6 Hz, CO₂C H a), 5.44 (1H, d, J 13.8Hz, C0₂C H b ), 5.65 (d, J 1.3Hz, 5-H of major isomer) and 5.82 (d, J 3.6Hz, 5-H of minor isomer) (1H), 6.19 (d, J 6.4 Hz, 8-H of major isomer) and 6.20 (d, J 11.1 Hz, 8-H of minor isomer) (1H), 7.4-7.7 (4H, m, pyrazole- H + aromatic-H), 8.22 (2H, d, J 8.5 Hz, aromatic-H). Example 13(a) p-Nitrobenzyl (5RS)-2-(2-hydroxyethylthio)-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylate Reaction of the isomeric mixture (120) (0.072 g) from Preparation 13(b) with 1,8-diazabicyclo[5.4.0]undec-7-ene, by the procedure described in Example 11(a), afforded the title compound (121) as a yellow solid (0.019 g) after trituration with ethyl acetate; λ max (Et0H) 298 nm (ε 24,293), ν max (KBr) 3360, 1790, 1770, 1674, 1604, 1549, 1509 cm⁻¹, δH (CDCl₃) 1.95 (1H, t, 0 H ), 3.05-3.28 (2H, overlapping tq, SC H ₂), 3.87 (2H, q, C H ₂0H), 3.97 (3H, s, N Me ), 5.27 (1H, d, J 13.8Hz, C0₂C H a), 5.51 (1H, d, J 13.9Hz, C0₂C H b ), 6.35 (1H, d, J 1.0Hz, 5-H), 7.08 (1H, s, vinylic- H ), 7.50 (2H, two s, pyrazole protons), 7.67 (2H, d, J 8.9Hz, aromatic H ), 8.23 (2H, d, J 8.7Hz, aromatic H ). Example 13(b) Sodium (5RS)-2-(2-hydroxyethylthio)-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylate. Hydrogenolysis of the ester (121) (0.014 g) from Example 13(a) by the procedure described in Example 11(b) afforded the title compound (122) (0.008 gm) as a pale yellow fluffy solid after Biogel P-2 column chromatography and lyophilisation; λmax (H₂0) 298 nm (ε 18,635), ν max (KBr) 1761, 1676 and 1552 cm⁻¹, δH (D₂0) 3.04 (2H, overlapping tq, SC H ₂), 3.79 (2H, t, CH₂C H ₂0H), 3.88 (3H, s, N Me ), 6.43 (1H, s, 5- H ), 7.13 (1H, s, vinylic-H), 7.64 (1H, s, pyrazole- H ), 7.81 (1H, s, pyrazole- H ). Preparation (14a) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-(N-methyltetrazolecarbonyl)-4-tritylthioazetidin-2-one 1- t -Butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (9.18 g, 20 mmol) was reacted with an approximately 1:1 mixture of ethyl 1- and 2-methyltetrazole-5-carboxylate (3.94 g, 24.8 mmol) (prepared by treating ethyl tetrazole-5-carboxylate [D. Moderhack, Chem. Ber ., 1975, 108 , 887] with excess diazomethane [ cf 0. Gryszkiewicz-Trochimourski, Compt. Rendu , 1958, 246 , 2627]) using the method described in Preparation 4(a). The crude reaction mixture was chromatographed on silica eluting with ethyl acetate/hexane mixtures to give two fractions. The less polar fraction contained a regioisomer of the title azetidinone (123) (3.48 g, 30%) ν max (CH₂Cl₂) 1750, 1690 cm⁻¹ ; δ(CDCl₃) 0.34 (6H, s), 1.00 (9H, s), 4.10 (3H, s), 4.90 (2H, s), 6.77-7.50 (15H, m); The more polar fraction contained the other regioisomer of the title compound (124) (3.5g, 30%) (mp. 158-161°, needles ex ether/hexane) ν max (CH₂Cl₂) 1750, 1700 cm⁻¹; δ(CDCl₃) 0.37 (6H, s), 1.00 (9H, s), 4.37 (3H, s), 4.57 (1H, d, J 2Hz), 4.97 (1H, d, J 2Hz), 6.80-7.60 (15H, m). (Found C, 65.3; H; 6.3; N, 12.2; S, 5.6. C₃₁H₃₅N₅0₂SSi requires C, 65.4; H, 6.2; N, 12.3; S, 5.6%). Preparation 14(b) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(N-methyltetrazol-5-yl)methyl]-4-tritylthioazetidin-2-one. The ketone (124) (4.13 g, 7.26 mmol) from Preparation 14(a) in dimethoxyethane (50 ml) at -30° under argon was treated with sodium borohydride (103 mg, 2.72 mmol). After 15 minutes a further amount of sodium borohydride (17 mg) was added. After a further 15 minutes the reaction was worked up as in Preparation 4(b). Chromatography on silica eluting with dichloromethane and dichloromethane/ethyl acetate mixtures gave two fractions. The less polar fraction contained a trans isomer (isomer A) of the title azetidinone (125) (2.10 g, 51%) ν max (CH₂Cl₂) 3600-3400, 1750 cm⁻¹; δ(CDCl₃) 0.25 (3H, s), 0.28 (3H, s), 0.93 (9H, s), 2.47 (1H, d, J 6.3Hz, exch), 3.70-3.82 (2H, m, sharpens on exch.). 4.29 (3H, s), 4.60 (1H, d, J 1.8Hz), 7.14-7.53 (15H, m). (Found MH⁺ 572). The more polar fraction contained the other trans isomer (isomer B) of the title azetidinone (125), (1.75,g, 42%); mp 159-160° (plates from ether/hexane); ν max (CH₂Cl₂) 3600-3400, 1745 cm⁻¹; (Found C, 65.3; H, 6.7; N, 12.2: S; 5.5. C₃₁H₃₇N₅0₂SSi requires C, 65.1; H, 6.5; N, 12.3; S, 5.6%). Preparation 14(c) (3RS, 4SR) 3-[Hydroxy(N-methyltetrazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (125) (Isomer B) (1.89 g, 3.31 mmol) from Preparation 14(b) was treated as for Preparation 2(b) to give the title azetidinone (126) (1.39 g, 92%); ν max (CH₂Cl₂) 3600-3300, 1765 cm⁻¹; δ(CDCl₃) 3.25 (1H, d, J 5.1Hz), 3.72 (1H, dd, J 3.1, 3.1Hz), 4.24 (1H, bs), 4.47 (3H, s), 4.72 (1H, d, J 2.7Hz), 5.45 (1H, dd, J 3.7, 5.0Hz), 7.20-7.41 (15H, m); (Found MH⁺458). Preparation 14(d) (3RS, 4SR) 3-[Acetoxy(N-methyltetrazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (126) (1.4 g) from Preparation 14(c) was treated as for Preparation 1(c) to give the title acetate (127) (1.46g, 95%); ν max (CH₂Cl₂) 3400, 1775, 1760 (sh) cm⁻¹; δ(CDCl₃) 2.12 (3H, s), 3.78 (1H, dd, J 2.8, 5.3Hz), 4.17 (1H, bs), 4.43 (3H, s), 4.71 (1H, d, J 2.8Hz), 6.39 (1H, d, J 5.3Hz), 7.15-7.48 (15H, m). (Found MH⁺ 500). Preparation 14(e) (3RS, 4SR) 3-[Acetoxy(N-methyltetrazol-5-yl)methyl[-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidene-methyl)-4-tritylthioazetidin-2-one. The acetate (127) (1.36 g) from Preparation 14(d) was converted via the hydroxyesters (128) ν max (CH₂Cl₂) 3500, 1780, 1750 cm⁻¹; and the chloroesters (129) ν max (CH₂Cl₂) 1790, 1760 cm⁻¹ to the phosphorane (130) (2.00 g, 77%) ν max (CH₂Cl₂) 1755, 1620 cm⁻¹, using the methods described in Preparation 1(d). Preparation 14(f) (3RS, 4SR) Silver 3-[Acetoxy(N-methyltetrazol-5-yl)methyl] -1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphoshoranylidenemethyl)azetidin-2-one-4-thiolate. The phosphorane (130) (1.8 g) from Preparation 14(e) was treated with silver nitrate/pyridine as for Preparation 1(e) to give the title silver salt (131) (1.35 g, 87%) as an off-white solid. Preparation 14(g) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy(N-methyltetrazol-5-yl) methyl]penem-3-carboxylate The silver salt (131) (1.35 g) from Preparation 14(f) was treated as for preparation 1(f) to give the title penem (132) (631 mg, 83%); νmax (CH₂Cl₂) 1795, 1750, 1720 cm⁻¹; δ(CDCl₃) 2.16 (3H, s), 4.38 (3H, s), 4.53 (1H, ddd, J 1.1, 2.1, 3.8Hz), 5.28 and 5.43 (2H, ABq, J 13.6Hz), 6.06 (1H, d, J 2.2Hz), 6.55 (1H, d, J 3.8Hz), 7.34 (1H, d, J 1.1Hz), 7.59 (2H, d, J 8.8Hz), 8.24 (2H, d, J 8.7Hz). Example 14(a) (5RS) p-Nitrobenzyl (Z)-6-(N-methyltetrazol-5-ylmethylene) penem-3-carboxylate. The penem (132) (70 mg) from Preparation 14(g) was treated as for Example 1(a) to give the title penem (133) (51 mg, 84%); ν max (CH₂Cl₂) 1785, 1615 cm⁻¹; δ(CDCl₃) 4.41 (3H, s), 5.30 and 5.46 (2H, ABq, J 13.5Hz), 6.60 (1H, d, J 1.1Hz), 7.26 (s-obscurred by solvent peak), 7.41 (1H, s), 7.62 (2H, d, J 8.8Hz), 8.25 (2H, d, J 8.8Hz). Example 14 (b) (5RS) Sodium (Z)-6-(N-methyltetrazol-5-ylmethylene)penem-3-carboxylate The (Z)-penem ester (133) (50 mg) from Example 14(a) was hydrogenated as for Example 1(b) to give the title sodium salt (134) (13 mg, 36%) as a yellow freeze-dried solid; λ max (H₂0) 254 nm (εm 9495); δ(D₂0) 4.41 (3H, s), 6.62 (1H, d, J0.9Hz), 7.10 (1H, s), 7.32 (1H, bs). Preparation 15(a) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(N-methyltetrazol-5-yl)methyl]-4-tritylthioazetidin-2-one The ketone (123) (5.15 g) from Preparation 14(a) was treated as for Preparation 14(b) to give two fractions. The less polar fraction contained a trans isomer (isomer A) of the title azetidinone (135) (0.97 g, 19%); ν max (CH₂Cl₂) 3600-3300, 1745 cm⁻¹. The more polar fraction contained the other trans isomer (isomer B) of the title azetidinone (2.23g, 43%); ν max (CH₂Cl₂) 3700-3300, 1740 cm⁻¹. Preparation 15(b) (3RS, 4SR) 3-[Hydroxy(N-methyltetrazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (135) (isomer B) (2.78 g) from Preparation 15(a) was treated as for Preparation 2(b) to give the title azetidinone (136) (2.1 g, 97%); ν max (CH₂Cl₂) 3600-3300, 1755 cm⁻¹. Preparation 15(c) (3RS, 4SR) 3-[Acetoxy(N-methyltetrazol-5-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (136) (2.03 g) from Preparation 15(b) was treated as for Preparation 1(c) to give the title acetate (137) (2.11 g, 95%); ν max (CH₂Cl₂) 3380, 1775 cm⁻¹, δ(CDCl₃) 2.09 (3H, s), 3.60-3.80 (1H, m), 4.07 (3H, s), 4,30 (1H, bs), 4.83 (1H, d, J 2.5Hz), 6.11 (1H, d, J 6.0Hz), 7.22-7.73 (15H, m). Preparation 15(d) (3RS, 4SR) 3-[Acetoxy(N methyltetrazol-5-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (137) (2.00g) from Preparation 15(c) was converted via the hydroxyesters (138) ν max (CH₂Cl₂) 3600-3400, 1775, 1760 (sh) cm⁻¹, and the chloroesters (139), ν max (CH₂Cl₂) 1785, 1760 (sh) cm⁻¹; to the title phosphorane (140) (2.55 g, 67%), ν max (CH₂Cl₂) 1755, 1620 cm⁻¹, using the methods described in Preparation 1(d). Preparation 15(e) (3RS, 4SR) Silver 3-[acetoxyl(N methyltetrazol-5-yl)methyl] -1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidene methyl)-4-tritylthioazetidin-2-one-4-thiolate The phosphorane (140) (1.53g) from Preparation 15(d) was treated as for Preparation 1(e) to give the title silver salt (141) (1.11g, 85%). Preparation 15(f) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy(N-methyltetrazol-5-yl)methyl]penem-3-carboxylate The silver salt (141) (1.11 g) from Preparation 15(e) was treated as for Prepration 1(f) to give the title penem (142) (451 mg, 72%); ν max (CH₂Cl₂) 1795, 1750, 1720 cm⁻¹. Example 15(a) (5RS) p-Nitrobenzyl (Z)-6-(N-methyltetrazol-5-ylmethylene) penem-3-carboxylate The penem (142) (115 mg) from Preparation 15(f) was treated as for Example 1(a) to give the title penem (143) (85 mg, 85%), ν max (CH₂Cl₂) 1785, 1720 cm⁻¹; δ(CDCl₃) 4.17 (3H, s), 5.31 and 5.46 (2H, ABq, J 13.5Hz), 6.63 (1H, d, J 1.1Hz), 7.04 (1H, d, J 1.1Hz), 7.45 (1H, s), 7.62 (2H, d, J 8.9Hz), 8.25 (2H, d, J 8.8Hz). Example 15(b) (5RS) Sodium (Z)-6-(N-methyltetrazol-5-ylmethylene)penem -3-carboxylate The (Z) penem ester (143) (80 mg) from Example 15(a) was hydrogenated as for Example 1(b) to give the title sodium salt (144) (31 mg, 54%) as an orange amorphous solid; λmax (H₂0) 253 nm (εm 13,910), 370 (940); δ(D₂0) 4.16(3H, s), 6.61 (1H, s), 7.10 (1H, s), 7.32 (1H, s). Preparation 16(a) Ethyl 1-methyl-1,2,3-triazole-4-carboxylate A solution of ethyl propiolate (16.2 ml) in hexane (50 ml) was added, dropwise over 30 minutes with stirring, to ice-salt bath cooled methyl azide [prepared by addition of dimethyl sulphate (18.9 ml) to a stirred solution of sodium azide (13 g) in 1N sodium hydroxide (100 ml) at 70°C according to the method of O. Dimroth, Chem. Ber ., 1905, 38 , 1573]. The stirred mixture was allowed to attain room temperature during 2 hours and stood for a further 20 hours. The hexane liquors were decanted from the crystalline product which was washed with hexane (3 x 10 ml) and dried under vacuum to give the title ester (145) (3.39 g), m.p. 90-91°C (colourless needles), ν max (CHCl₃) 1720 cm⁻¹; 4ppm (CDCl₃) 1.40 (3H, t, J7Hz), 4.20 (3H, s), 4.44 (2H, q, J7Hz), 8.18 (1H, s). (Found: C, 46.4; H, 5.8; N, 27.0. C₆H₉N₃0₂ requires C, 46.4; H, 5.8; N, 27.1%). The combined hexane liquors and washings were kept at room temperature for a further 9 days when a second crop of the title ester (145) (6.98 g), mp 90-91°C was obtained after decanting, washing with hexane (3 x 20 ml) and drying under vacuum. Preparation 16(b) (3S, 4R) 3-Bromo-1-t-butyldimethylsilyl-4-tritylthioazetidin-2-one. A solution of triethylamine (116 mg) in dry dimethylformamide (1 ml) was added, dropwise over 1½ minute, to a stirred solution of (3S, 4R) 3-bromo-4-tritylthioazetidin-2-one (424 mg) (A. Martel et al ., Can. J. Chem ., 1983, 61 , 1899) and t-butyldimethylchorosilane (188 mg) in dry dimethylformamide (4 ml) at ice-bath temperature. The mixture was stirred at room temperature overnight, diluted with ethyl acetate (30 ml), and washed with 5% citric acid (2 ml), brine (2 ml), saturated sodium bicarbonate solution (2 ml), and brine (3 x 2 ml). The dried (MgS0₄) organic layer evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give the title azetidinone (146) (471 mg), a solid, m.p. 115-116°C (rods ex ethyl acetate/hexane; [α]D²²-31.8° (c 1 in CHCl₃); ν max (CHCl₃) 1755 cm⁻¹; δppm (CDCl₃) 0.90 (9H, s). 4.25 (1H, s), 4.30 (1H, s), 7.10-7.50 (15H, m), both Me₃Si signals were obscured by TMS. (Found: C, 62.6; H, 5.9; N, 2.6; Br, 14.7. C₂₈H₃₂NBr0SSi requires C, 62.4; H, 6.0; N, 2.6; S, 6.0; Br, 14.8%). Preparation 16(c) (3S, 4R) 1-t-Butyldimethysilyl-3-(1-methyl-1,2,3-triazol-4-ylcarbonyl)-4-tritylthioazetidin-2-one A solution of n-butyl lithium (1.68M in hexane, 0.595 ml) was added to a stirred solution of the azetidinone (146) (538 mg) from Preparation 16(b) in dry THF (10 ml) at -76°C under dry argon. After 10 minutes at -76°C the stirred mixture was treated, in one portion, with solution of the triazole ester (145) (155 mg) from Preparation 16(a) in dry THF (3 ml). After 10 minutes at -76°C the stirred mixture was treated with saturated ammonium chloride solution (3 ml) and allowed to attain room temperature. The mixture was diluted with ethyl acetate (30 ml) and was washed wih brine (3 x 5 ml). The dried (MgS0₄) organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give the title ketone (147) (387 mg) as an amorphous solid, [α]D²² + 58.6° (C 1 in CHCl₃); νmax (CHCl₃) 1750, 1680 cm⁻¹; δppm (CDCl₃) 0.92 (9H, s), 4.06 (3H, s), 4.70 (1H, d, J2Hz), 4.95 (1H, d, J2Hz), 6.9-7.5 (15H, m), 7.83 (1H, s), both Me₃Si signals were obscured by TMS. Preparation 16(d) (3S,4R) 1-t-Butyldimethylsilyl-3-[hydroxy(1-methyl-1,2,3-triazol-4-yl)methyl]-4-tritylthioazetidin-2-one The ketone (147) from Preparation 16(c) was treated with sodium borohydride (40 mg) as for Preparation 4(b) to give two fractions. The less polar fraction contained an isomer (Isomer A) of the title trans -azetinone (148) (67 mg) m.p. 195-197°C (needles ex ethylacetate/hexane); [α]D²² + 69.00 (c 1 in CHCl₃); ν max (CHCl₃) 3600-3100, 1735 cm⁻¹; δppm (CDCl₃) 0.93 (9H, s), 1.84 (1H, brs, exch. D₂0), 3.29-3.38 (1H, m), 3.88 (1H, a, J2Hz), 3.95 (3H, s), 4.51 (1H, d, J2Hz), 7.10-7.50 (15H, m), 7.82 (1H, s), both Me₃Si signals were obscured by TMS. (Found: C, 67.8; H, 6.7; N, 9.6; S, 5.6. C₃₂H₃₈N₄0₂SSi requires C, 67.3; H, 6.7; N, 9.8; S, 5.6%). The more polar fraction contained the other isomer (Isomer B) of the trans azetidinone (148) (167 mg) m.p. 128-130°C (plates ex ethyl acetate/hexane); [α]D²²-2.5° (c 1 in CHCl₃); ν max (CHCl₃) 3600-3100, 1730 cm⁻¹; δppm (CDCl₃) 0.78 (9H, s), 2.31 (1H, brs, exch. D₂0), 3.60 (1H, ad, J6 and 2Hz), 3.92 (3H, s), 4.11 (1H, d, J6Hz), 4.43 (1H, d, J2Hz), 7.10-7.60 (16H, m), both Me₃Si signals were obscured by TMS. (Found: C, 66.8; H, 6.9; N, 9.8; S, 5.4. C₃₂H₃₈N₄0₂SSi requires C, 67.3; H, 6.7; N, 9.8; S, 5.6%). Preparation 16(e) (3S, 4R) 3-[Hydroxy(1-methyl-1,2,3-triazol-4-yl)methyl]-4-tritylthioazetidin-2-one The alcohol (148) (Isomer B) (3.03 g) from Preparation 16(d) was treated with potassium fluoride (339 mg) as for Preparation 2(b) to give the title azetidinone (149) (2.15 g) as a solid, m.p. 196-197°C (nuggets ex ethyl acetate/hexane); [α]D²² - 124.7° (c 1 in CHCl₃); ν max (CHCl₃) 3600-3100, 1760 cm⁻¹; δppm (CDCl₃) 3.64 (1H, ad, J5 and 2Hz), 3.65-3.95 (1H, br. signal, exch. D₂0), 4.16 (3H, s), 4.33 (1H, s), 4.60 (1H, d, J2Hz), 5.26 (1H, slightly broadened d, J5Hz, sharpens on exch. D₂0), 7.19-7.42 (15H, m), 7.63 (1H, s). (Found: C, 68.3; H, 5.3; N, 12.4; S, 7.0. C₂₆H₂₄N₄0₂S requires C, 68.4; H, 5.3; N, 12.3; S, 7.0%). Preparation 16(f) (3S, 4R) 3-[Acetoxy (1-methyl-1,2,3-triazol-4-yl)methyl]-4-tritylthioazetidin-2-one The azetidinone (149) (2.10 g) from Preparation 16(e) was treated with triethylamine (0.77 ml) 4-dimethylaminopyridine (56 mg) and acetic anhydride (0.52 ml) as for Preparation 1(c) to give the title acetate (150) (2.29 g) as a solid, m.p. 111-113° (nuggets ex benzene/hexane); [α]D²¹ - 97.0 (c 1 in CHCl₃); ν max (CHCl₃) 3390, 1770 cm⁻¹; δppm (CDCl₃) 2.11 (3H, s), 3.84 (1H, ddd, J6.5, 2.6 and 1.1Hz); 4.11 (3H, s), 4.17 (1H, brs), 4.70 (1H, d, J2.6Hz), 6.17 (1H, a, J6.5Hz), 7.22-7.58 (16H + approximately 1 mole benzene, m). (Found: C, 70.8; H, 5.7; N, 9.6; S, 5.5. C₂₈H₂₆N₄S.C₆H₆ requires C, 70.8; H, 5.6; N, 9.7; S, 5.6%). Preparation 16(g) (3S, 4R) 3-[Acetoxy (1-methyl-1,2,3-triazol-4-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-4-tritylthioazetidin-2-one The acetate (150) (2.22g) from Preparation 16(f) was converted via the hydroxyester (151), ν max (CHCl₃) 3600-3100, 1770 br. cm⁻¹; and the chloroester (152), ν max (CHCl₃) 1780 cm⁻¹; to the title phosphorane (153) (3.66 g), [α]D²⁰ - 33.0° (c 1 in CHCl₃); ν max (CHCl₃) 1750, 1620, 1605 sh. cm⁻¹, using the methods described in Preparation 1(d). Preparation 16(h) (3S, 4R) Silver 3-[Acetoxy (1-methyl-1,2,3-triazol-4-yl) methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)-azetidin-2-one-4-thiolate. Silver nitrate (8.7 ml of a 0.15M solution in methanol) was added to a stirred mixture of the phosphorane (153) (951 mg) from Preparation 16(g) and pyridine (0.015 ml) in methanol (10 ml) and dichloromethane (1 ml). After stirring at room temperature for 30 minutes the mixture was evaporated. The residual gum was re-evaporated from dry dichloromethane (5 ml) and dried under vacuum to give the crude title silver salt (154) as an amorphous solid. The material was of sufficient purity for further synthetic elaboration. Preparation 16(i) (5R, 6S) p-Nitrobenzyl 6-[acetoxy(1-methyl-1,2,3-triazol-4-yl)methyl]penem-3-carboxylate. The crude silver salt (154) from Preparation 16(h) was treated 4-dimethylaminopyridine (122 mg), acetic formic anhydride (0.80 ml) and triethylammonium chloride (1.37g) as for Preparation 1(f) to give, after chromatography on silica gel eluting with dichloromethane/ethyl acetate mixtures, the penem (155) (378 mg) as a solid, mp. 151-153°C (hexagonal plates ex ethyl acetate/hexane); [α]D²⁰ + 24.0° (c 1 in CHCl₃); λ max (2% CHCl₃ in Et0H) 262 (εm 13380) and 316 nm (8889); ν max (CHCl₃) 1795, 1740 sh, 1720 cm⁻¹; δppm (CDCl₃) 2.09 (3H, s), 4.09 (3H, s), 4.56 (1H, ddd, J3.4, 2.2 and 1.0Hz), 5.25 and 5.43 (2H, ABq, J13.6Hz), 7.20 (1H, d, J3.4Hz), 7.23 (1H, d, J2.2Hz), 7.36 (1H, d, J1.0Hz), 7.60 (2H, d, J8.7Hz), 7.67 (1H, s), 8.23 (2H, d, J8.7Hz). (Found: C, 49.7; H, 3.7; N, 15.2; S, 6.9. C₁₉H₁₇N₅0₇S requires 49.7; H, 3.7; N, 15.3; S, 7.0%). Example 16(a) (5R) p-Nitrobenzyl 6-(1-methyl-1,2,3-triazol-4-ylmethylene) penem-3-carboxylate. The penem (155) (315 mg) from Preparation 16(i) was treated with 1,8-diazabicyclo[5.4.0] undec-7-ene (157 mg) as for Example 1(a) to give two fractions. The less polar fraction contained the (Z)-isomer (156) (243 mg) of the title penem, a solid, m.p. 174-175°C (microcrystalline solid ex dichloromethane/hexane); [α]D²⁰ + 343.2° (c 1 in CHCl₃); λ max (2% CHCl₃ in Et0H) 282 nm (εm 27183); ν max (CHCl₃) 1780, 1715 cm⁻¹; δppm (CDCl₃) 4.16 (3H, s), 5.29 and 5.46 (2H, ABq, J13.6Hz), 6.68 (1H, d, J1.0Hz), 7.06 (1H, a, J1.0Hz), 7.39 (1H, s), 7.62 (2H, d, J8.9Hz), 7.73 (1H, s), 8.24 (2H, d, J8.9Hz), a singlet at 5.30 ppm indicated the presence of dichloromethane. (Found: C, 48.7; H, 3.2; N, 16.4; S, 7.5; Cl, 5.1%). The more polar fraction contained the (E)-isomer (157) (7 mg) of the title penem, a solid, ν max (Nujol Mull) 1750, 1710, 1685 cm⁻¹; δppm (CDCl₃) 4.17 (3H, s), 5.31 and 5.47 (2H, ABq, J13.6Hz), 6.49 (1H, s), 6.95 (1H, s), 7.46 (1H, s), 7.62 (2H, a, J8.7Hz), 8.26 (2H, d,J8.7Hz), 8.74 (1H, s). Example 16(b) (5R) Sodium (Z)-6-(1-methyl-1,2,3-triazol-4-ylmethylene) penem-3-carboxylate The (Z)-penem ester (156) (200 mg) from Example 16(a) was hydrogenated as for Example 1(b) to give the title penem sodium salt (158) (81 mg) as a yellow freeze-dried solid, [α]D¹⁸ + 400° (c 1 in H₂0); λmax (H₂0) 282 (εm 18459) and 354 nm (1589); ν max (KBr) 3700-2750, 1760, 1680, 1600, 1555 cm⁻¹; δppm (D₂0) 4.10 (3H, s), 6.57 (1H, s), 7.02 (1H, s), 7.17 (1H, s), 8.13 (1H, s). The penem sodium salt (158) (14mg) was partitioned between ethyl acetate (10 ml) and water (2 ml) and cooled in an ice bath. The pH of the vigorously stirred mixture was adjusted to 3.5 using 1% citric acid. The organic layer was separated and washed with water (3 x 1 ml). The dried (MgSO₄) organic layer was evaporated to give a crude solid. The crude solid was redissolved in ethyl acetate (20 ml) and filtered through Kieselguhr. The filtrate was evaporated to low volume (approximately 1 ml) and diluted with a little hexane to give the free acid of the title penem as yellow clusters of prisms, slow decomposition >150°C finally melting at approximately 230°C; ν max (Nujol) 3400-2800, 1775, 1720, 1700 br. cm⁻¹; δppm [(CD₃)₂SO) 4.10 (3H,s), 6.64 (1H, d, J 0.7 Hz), 7.32 (1H, a, J 0.7 Hz), 7.57 (1H,s), 8.39 (1H,s), 12.87 br (1H,s), other signals indicated the presence of 10-15% ethyl acetate. Preparation 17(a) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-(1-methylpyrazol-3-yl-carbonyl)-4-tritylthioazetidin-2-one The azetidinone (1) (9.18 g) was reacted with methyl 1-methylpyrazole-3-carboxylate (2.86g) (K.V. Auwers and Th. Breyhan, J. Prakt. Chem ., 1935, 143 , 259) using the conditions described in Preparation 3(a) to give the title ketone (159) (8.63 g) as a solid, m.p. 200-202°C (prisms ex ethyl acetate/hexane); ν max (CHCl₃) 1745, 1675 cm⁻¹; δppm (CDCl₃) 0.31 (6H, s), 0.95 (9H, s), 3.88 (3H, s), 4.74 (1H, d, J2Hz), 4.89 (1H, d, J2Hz), 6.57 (1H, d, J3Hz), 6.90-7.55 (16H, m). (Found: C, 69.6; H, 6.5; N, 7.3; S, 5.5. C₃₃H₃₇N₃0₂SSi requires C, 69.8; H, 6.6; N, 7.4; S, 5.6%). Preparation 17(b) (3RS, 4SR 1-t-Butyldimethylsilyl-3-[hydroxy(1-methylpyrazol -3-yl)-methyl]-4-tritylthioazetidin-2-one The ketone (159) (8.85 g) from Preparation 17(a) was treated with sodium borohydride (0.9 g) as for Preparation 4(b) to give two fractions. The less polar fraction contained an isomer (Isomer A) of the title trans-azetidinone (160) (3.13 g), mp. 167-169°C (prisms ex ethyl acetate/hexane; ν max (CHCl₃) 3700-3100, 1740 cm⁻¹; δppm (CDCl₃) 0.24 (6H, s), 0.92 (9H, s), 2.30-2.60 (1H, broads, exch. D₂0), 3.44 (1H, dd, J2 and 3Hz), 3.74 (3H, s), 3.78-3.95 (1H, broad m, collapses to 3.82, d, J3Hz on exch. D₂0), 4.49 (1H, d, J2Hz), 6.60 (1H, d, J3Hz), 7.05-7.65 (16H, m). (Found: C, 69.4; H, 6.8; N, 7.4; S, 5.6. C₃₃N₃₉N₃0₂SSi requires C, 69.6; H, 6.9; N, 7.4; S, 5.6%). The more polar fraction contained an isomer (Isomer B) of the title trans -azetidinone (160) (4.63 g), mp 113-115°C (plates ex ethyl acetate/hexane); ν max (CHCl₃) 3700-3200, 1735 cm⁻¹; δppm (CDCl₃) 0.79 (9H, s), 1.90-2.60 (1H, broad s, exch. D₂0), 3.69 (dd, J2 and 6Hz) and 3.74(s) together 4H, 4.09 (1H, broad a, J6Hz, sharpens on exch. D₂0) 4.22 (1H, d, J2Hz), 5.89 (1H, d, J3Hz), 7.05-7.60 (16H, m), both Me₃Si signals were obscured by TMS. (Found: C, 69.6; H, 7.0; N, 7.4; S, 5.6. C₃₃N₃₉N₃0₂SSi requires C, 69.6; H, 6.9; N, 7.4; S 5.6%). Preparation 17(c) (3RS, 4SR) 3-[Hydroxy(1-methylpyrazol-3-yl)methyl]-4-tritylthioazetidin-2-one The azetidinone (160) (Isomer B) (4.57 g) from Preparation 17(b) was treated with potassium fluoride (0.51 g) as for Preparation 2(b) to give title alcohol (161) 3.41 g) as a solid, m.p. 225-227°C (decomp) (needles ex ethyl acetate/hexane); ν max (CHCl₃) 3400, 1760 cm⁻¹; δppm (CDCl₃) 2.40-2.92 (1H, m), 3.54 (1H, dd, J3 and 5Hz), 3.91 (3H, s), 4.22 (1H, broad s), 4.54 (1H, d, J3Hz), 5.13 (1H, a, J5Hz), 6.24 (1H, d, J1Hz), 7.10-7.50 (16H, m). (Found: C, 71.4; H, 5.5; N, 9.2; S, 7.1. C₂₇N₂₅N₃0₂S requires C, 71.2; H, 5.5; N, 9.2; S, 7.0%). Preparation 17(d) (3RS, 4SR) 3-[Acetoxy(1-methylpyrazol-3-yl)methyl]-4-trityl thioazetidin-2-one The alcohol (161) (3.016 g) from Preparation 17(c) was treated with 4-dimethylaminopyridine (81 mg), triethylamine (1.106 ml) and acetic anhydride (0.75 ml) as for Preparation 1(c) to give, after chromatography on silica gel eluting with ethyl acetate/ethanol mixtures, the title acetate (162) (3.04 g) as a solid ν max (CHCl₃) 3350, 1770 cm⁻¹; δppm (CDCl₃) 2.05 (3H, s), 3.73 (1H, dd, J6 and 3Hz with further fine coupling J approximately 1Hz), 3.88 (3H, s), 4.16 (1H, broad s), 4.50 (1H, d, J3Hz), 6.10-6.20 (2H, m), 7.10-7.51 (16H, m). Preparation 17(e) (3RS, 4SR) 3-[Acetoxy(1-methylpyrazol-3-yl)methyl]-1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidenemethyl) -4-tritylthioazetidin-2-one The acetate (162) (2.77 g) from Preparation 17(d) was converted to the hydroxyester (163), ν max (CHCl₃) 3550-2800, 1770 1750 cm⁻¹; the chloroester (164), ν max (CHCl₃) 1780 cm⁻¹; and finally the title phosphorane (165) (4.37 g), ν max (CHCl₃) 1755, 1620, 1605 cm⁻¹; using the methods described in Preparation 1(d). Preparation 17(f) (3RS, 4SR) Silver 3-[acetoxy (1-methylpyrazol-3-yl)methyl] -1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidene -methyl)azetidin-2-one-4-thiolate The phosphorane (165) (4.37 g) from Preparation 17(e) was treated with silver nitrate/pyridine as for Preparation 1(e) to give the title silver thiolate (166) (2.5 g) as a yellow solid, ν max (CHCl₃) 1750, 1615, 1605 cm⁻¹. Preparation 17(g) (5RS, 6SR) p-Nitrobenzyl 6-[acetoxy(1-methylpyrazol-3-yl) methyl]penem-3-carboxylate. The silver thiolate (166) (2.42 g) from Preparation 17(f) was treated as for Preparation 1(f) to give the penem (167) (1.09 g) as a solid, m.p. 158-160°C (prisms ex ethyl acetate/hexane); λ max (2% CHCl₃ in Et0H) 261 (εm 12846) and 317 nm (8327); ν max (CHCl₃) 1795, 1740 sh, 1720 cm⁻¹; δppm (CDCl₃) 2.10 (3H, s), 3.86 (3H, s), 4.50 (1H, ddd, J3.8, 2.2 and 1.0Hz), 5.35 (2H, ABq, J13.6Hz), 6.05 (1H, d, J2.2Hz), 6.26 (1H, d, J3.8Hz), 6.29 (1H, d, J2.2Hz), 7.30 (1H, d, J2.2Hz); 7.32 (1H, d, J1Hz), 7.60 (2H, d, J8.5Hz), 0.23 (2H, d, J8.5Hz) (Found: C, 52.7; H, 4.0; N, 12.0; S, 6.8. C₂₀H₁₈N₄0₇S requires C, 52.4; H, 4.0; N, 12.2; S, 7.0%). Example 17(a) (5RS) p-Nitrobenzyl 6-(1-methylpyazol-3-ylmethylene)penem -3-carboxylate The penem (167) (458 mg) from Preparation 17(g) was treated with 1,8-diazabicyclo[5.4.0]undec-7-ene (228 mg) as for Example 1(a) to give two products. The less polar product, the (Z)-isomer (168) (373 mg) was obtained as a yellow solid, mp 175-180°C decomp. (prisms ex dichloromethane/hexane); λ max (2% CHCl₃/Et0H) 297 nm (εm 30426); ν max (CHCl₃) 1775, 1715, 1685 sh. cm⁻¹; δppm (CDCl₃) 3.96 (3H, s), 5.29 and 5.45 (2H, ABq, J13.6Hz), 6.40 (1H, d, J2.3Hz), 6.50 (1H, s), 7.04 (1H, s), 7.35 (1H, s), 7.39 (1H, d, J2.3Hz), 7.63 (2H, d, J8.7Hz), 8.24 (2H, d, J8.7Hz) (Found: C, 54.3; H, 3.5; N, 14.1; S, 8.0. C₁₈H₁₄N₄0₅S requires C, 54.4; H, 3.4; N, 13.9; S, 8.0%). The more polar product, the (E)-isomer (169) (10 mg) of the title penem, was obtained as a yellow solid, ν max (CHCl₃) 1775, 1720, 1680 cm⁻¹; δppm (CDCl₃) 3.94 (3H, s), 5.30 and 5.46 (2H, ABq, J13.7Hz), 6.45 (1H, s), 6.75 (1H,s), 7.39 (1H, a, J2.5Hz), 7.41 (1H, d, J2.5Hz), 7.43 (1H, s), 7.62 (2H, d, J8.7Hz), 8.25 (2H, d, J8.7Hz). Example 17(b) (5RS) Sodium (Z)-6-(1-methylpyrazol-3-ylmethylene)penem-3-carboxylate The (Z)-penem ester (168) (200 mg) from Example 17(a) was hydrogenated as for Example 1(b) to give the title sodium salt (169) (82 mg) as a yellow freeze-dried solid, λ max (H₂0) 291 (εm 23228) and approximately 360 nm (2036); ν max (KBr) 3700-2600, 1750, 1680, 1600, 1550 cm⁻¹; δppm (D₂0) 3.90 (3H, s), 6.42 (1H, d, J2.3Hz), 6.52 (1H, s), 7.00 (1H, s), 7.09 (1H, s), 7.62 (1H, d, J2.3Hz). Preparation 18(a) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-(1-methylimidazol-4-yl carbonyl)-4-tritylthioazetidin-2-one A solution of n-butyl lithium (1.68M in hexane, 0.71 ml) was added to a solution of diisopropylamine (0.17 ml) in dry THF (10 ml) at -30°C under dry argon. After 15 minutes the stirred mixture was cooled to -70°C and treated with a solution of the azetidinone (1) (459 mg) in dry THF (6 ml). After a further 15 minutes at -70°C the stirred mixture was treated with a solution of methyl 1-methylimidazole-4-carboxylate (0.14g) (P.K. Martin et al, J. Org. Chem ., 1968, 33 , 3758) in dry THF (3 ml). The mixture was stirred at -70°C for a further 30 minutes, treated with saturated ammonium chloride solution, and worked up as for Preparation 3(a) to give the title ketone (170) (0.37 g) as a solid, ν max (CHCl₃) 1745, 1655 cm⁻¹; δppm (CDCl₃) 0.91 (9H, s), 3.64 (3H, s), 4.53 (1H, d, J1Hz), 4.93 (1H, d, J1Hz), 6.90-7.50 (17H, m), both Me₃Si signals obscured by TMS. Preparation 18(b) (3RS, 4SR) 1-t-Butyldimethylsilyl-3-[hydroxy(1-methylimidazol-4-yl)methyl]-4-tritylthioazetidin-2-one The ketone (170) (1.60g) from Preparation 18(a) was treated with sodium borohydride as for Preparation 4(b) to give a 5:1 mixture of isomers of the title alcohol (171) (1.41g) as a solid, ν max (CHCl₃) 1735 cm⁻¹; δppm (CDCl₃) (major isomer) 0.01 and 0.02 (6H, each s), 0.79 (9H, s), 2.40-2.70 (1H, broad signal), 3.55 (3H, s), 3.68 (1H, dd, J1.7 and 6.4Hz), 4.02 (1H, d, J6.4Hz), 4.30 (1H, d, J1.7Hz), 6.60 (1H, d, J0.8Hz), 7.17-7.52 (16H, m). δppm (CDCl₃) (minor isomer) ( inter alia ) 0.26 and 0.27 (6H, each s), 0.94 (9H, s), 1.80-2.15 (1H, broad signal), 3.36-3.41 (1H, m), 3.58 (3H, s), 4.53 (1H, d, J2Hz), 6.95 (1H, broad s). Preparation 18(c) (3RS, 4SR) 3-[Hydroxy(1-methylimidazol-4-yl)methyl]-4-tritylthioazetidin-2-one The mixture of isomers of the alcohol (171) (1.204 g) from Preparation 18(b) was treated with potassium fluoride (135 mg) as for Preparation 2(b) to give a mixture of isomers of the title azetidinone (172) (958 mg) as a solid after trituration with ether, ν max (Nujol Mull) 3700-3100, 1765 cm⁻¹. Preparation 18(d) (3RS, 4SR) 3-[Acetoxy(1-methylimidazol-4-yl)methyl]-4-tritylthioazetidin-2-one The mixture of isomers of the azetidinone (172) (622 mg) from Preparation 18(c) was treated with 4-dimethylaminopyridine (16.7 mg), triethylamine (0.21 ml), and acetic anhydride (0.14 ml) as for Preparation 1(c) to give a mixture of isomers of the title acetate (173), ν max (CHCl₃) 3390, 1765 cm⁻¹. Preparation 18(e) (3RS, 4SR) 3-[Acetoxy(1-methylimidazol-4-yl)methyl]-1-(1-p-nitrobenzyloxcarbonyl-1-triphenylphosphoranylidenemethyl) -4-tritylthioazetidin-2-one The mixture of isomers of the acetate (173) (267 mg) from Preparation 18(d) was converted to the hydroxyesters (174), ν max (CHCl₃) 3600-3100, 1765 cm⁻¹; the chloroesters (175), ν max (CHCl₃) 1780 cm⁻¹; and finally a mixture of isomers of the title phosphoranes (176) (114 mg), ν max (CHCl₃) 1740, 1620, 1610 cm⁻¹ using the methods described in Preparation 1(d). Preparation 18(f) (3RS, 4SR) Silver 3-[acetoxy(1-methylimidazol-4-yl)methyl] -1-(1-p-nitrobenzyloxycarbonyl-1-triphenylphosphoranylidene -methyl)azetidin-2-one-4-thiolate. The mixture of isomers of the phosphorane (176) (0.11 g) from Preparation 18(e) was treated with silver nitrate/pyridine as for Preparation 16(h) to give a crude mixture of isomers of the title silver salt (177). Preparation 18(g) (5RS, 6SR) p-Nitrobenzyl 6-[Acetoxy(1-methylimidazol-4-yl) methyl]penem-3-carboxylate. The crude mixture of isomers of the silver salt (177) from Preparation 18(f) was treated as for Preparation 1(f) to give two products. The less polar product, an isomer (Isomer A) of the title penem (178) (50 mg), was obtained as a solid contaminated with triphenylphosphine oxide, ν max (CHCl₃) 1790, 1740 sh, 1720 cm⁻¹. The more polar product, an isomer (Isomer B) of the title penem (178) (20 mg) was obtained as a solid, ν max (CHCl₃) 1790, 1720 br cm⁻¹. Isomer A of the title penem (178) could be obtained free of triphenylphosphine oxide by careful chromatography but not without considerable column losses. Example 18 (5RS) p-Nitrobenzyl (Z)-6-(1-methylimidazol-4-ylmethylene) penem-3-carboxylate. The penem (178) (Isomer A - free from triphenylphosphine oxide) (12 mg) from Preparation 18(g) was treated with 1,8-diazabicyclo[5.4.0]undec-7-ene (5.9 mg) as for Example 1(a) to give the title penem (179) (6 mg) as a solid, ν max (CHCl₃) 1775, 1715, 1695 cm⁻¹; δppm (CDCl₃) 3.72 (3H, s), 5.28 and 5.46 (1H, ABq, J13.4Hz), 6.65 (1H, s), 6.98 (1H, s), 7.18 (1H, s), 7.35 (1H, s), 7.48 (1H, s), 7.62 (2H, d, J8.6Hz), 8.23 (2H, d, J8.6Hz). Biological data The following Table 1 summaries the antibacterial activity of selected compounds according to the invention (which are identified by compound numbers as given in the examples) against selected micro-organisms and, for comparison purposes, also gives the same data for amoxycillin. The data is given in the form of MIC values (minimum inhibitory concentration) in µg/ml. Table 2 summaries the β-lactamase activity of the compounds listed in Table 1 when used in conjunction with amoxycillin against the same micro-organisms. The data is given in the form of the minimum inhibitory amount of amoxycillin in µg/ml when used in conjunction with 5 µg/ml of the respective compound according to the invention.

Claims: Claims for the following contracting states: BE, CH, DE, FR, GB, IT, LI, LU, NL, SE A compound of the general formula I: or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof in which one of R¹ and R² denotes hydrogen, the other of R¹ and R² denotes an unsubstituted or substituted five-membered hetero-aromatic ring bonded through a carbon atom thereof and having one hetero-atom selected from nitrogen, oxygen and sulphur and additionally having from one to three nitrogen atoms, and R³ denotes hydrogen or a group of formula -R⁴ or -SR⁴, where R⁴ denotes an unsubstituted or substituted (C₁₋₁₀)hydrocarbon or heterocyclyl group. A compound of the general formula IA or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof in which R¹, R² and R³ are defined as in claim 1. A compound as claimed in claim 1 or claim 2, wherein R¹ denotes the hetero-aromatic group and R² denotes a hydrogen atom. A compound as claimed in any one of claims 1 to 3, wherein the hetero-aromatic group is an oxazolyl, isoxazolyl, pyrazolyl or triazolyl group. A compound selected from: (5RS) (Z)-6-(isothiazol-5-ylmethylene)penem-3-carboxylic acid; (5RS) (E)-6-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylpyrazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(2-methylthiazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-(isoxazol-3-ylmethylene)penem-3-carboxylic acid; (5RS) (Z)-6-[(2,4-dimethyloxazol-5-yl)methylene] penem-3-carboxylic acid; (5RS) (E)-6-[(4-methyl-1,2,3-thiadiazol-5-yl) methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(4-methyl-1,2,3-thiadiazol-5-yl) methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(2-methyloxazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (E)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5R) (Z)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(5-methyl-1,2,4-oxadiazol-3-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methyl-1,2,4-triazol-3-yl)methylene] penem-3-carboxylic acid; (5RS) 2-hydroxymethyl-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) 2-ethylthio-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) 2-(2-hydroxyethylthio)-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-(N-methyltetrazol-5-ylmethylene)penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylpyrazol-3-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylimidazol-4-yl)methylene] penem-3-carboxylic acid; or a pharmaceutically acceptable salt or in-vivo hydrolysable ester thereof. A process for the preparation of a compound of the general formula I as defined in claim 1, or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof, which comprises eliminating the elements of a compound of the general formula II: H-X II from a penem or penem intermediate of the general part-formula III: in which formulae R¹ and R² are defined as in claim 1, and X denotes a hydroxy group or a leaving group, to give a compound of the general part-formula IV: in which R¹ and R² are defined as in claim 1, and, if the resulting compound of the general formula IV is a penem intermediate, converting it into a penem of the general formula I or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof. A process for the preparation of a compound of the general formula I given in claim 1, in which R¹ and R² are defined as in claim 1 and R³ denotes a group of the formula -SR⁴ (in which R⁴ is defined as in claim 1), or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof, which comprises reacting a compound of the general formula XIII: in which R¹ and R² are defined as in claim 1, R¹⁵ denotes an organic radical different from the group R⁴, and R x denotes hydrogen or a carboxyl-blocking group, with a thiol of the general formula XI: R⁴-SH XI in which R⁴ is defined as in claim 1, or a reactive derivative thereof; and thereafter if necessary or desired: (a) removing any carboxyl-blocking group R x , and/or (b) converting the product into the free acid or into a pharmaceutically acceptable salt or in vivo hydrolysable ester. A compound of the general formula IIIC: in which R¹ and R² are defined as in claim 1, R¹⁷ denotes (C₁₋₆) alkyl, aryl, aryl(C₁₋₆)alkyl, (C₁₋₆)alkylthio, arylthio, hetero-aromatic-thio, acyl (for example, (C₁₋₆)alkylcarbonyl, especially acetyl), (C₂₋₆)alkenyl (especially vinyl), or aryl(C₂₋₆)alkenyl, all of which may optionally be substituted, R¹⁸ denotes hydrogen or an N-protecting group, and X denotes a hydroxy group or a leaving group. A compound of the general formula IVC: in which, R¹ and R² are defined as in claim 1 and R¹⁷ and R¹⁸ are defined as in claim 8. A compound of the general formula IIID: in which R¹ and R² are defined as in claim 1. R¹⁶ denotes hydrogen or an organic group, R x denotes hydrogen or a carboxyl-blocking group, and X denotes a hydroxy group or a leaving group. A compound of the general formula XIII. in which R¹ and R² are defined as in claim 1, R¹⁵ denotes an organic group, and R x denotes hydrogen or a carboxyl-blocking group. A pharmaceutical composition comprising a compound as claimed in any one of claims 1 to 5 and a pharmaceutically acceptable carrier. A pharmaceutical composition as claimed in claim 12, which additionally comprises a penicillin, cephalosporin or other β-lactam antibiotic. A compound as claimed in any one of claims 1 to 5 for use as a therapeutically active substance. A compound as claimed in any one of claims 1 to 5 for use in the treatment of a bacterial infection. A compound as claimed in any one of claims 1 to 5 for use in the treatment of a bacterial infection in admixture or conjunction with a penicillin, cephalosporin or other β-lactam antibiotic. A compound as claimed in any one of claims 1 to5 for use as a β-lactamase inhibitor. Claims for the following Contracting State: AT A process for the preparation of 6-methylene penem-3-carboxylic acids of the general formula I: or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof in which one of R¹ and R² denotes hydrogen, the other of R¹ and R² denotes an unsubstituted or substituted five-membered hetero-aromatic ring bonded through a carbon atom thereof and having one hetero-atom selected from nitrogen, oxygen and sulphur and additionally having from one to three nitrogen atoms, and R³ denotes hydrogen or a group of formula -R⁴ or -SR⁴, where R⁴ denotes an unsubstituted or substituted (C₁₋₁₀) hydrocarbon or heterocyclyl group; characterised in that the process comprises eliminating the elements of a compound of the general formula II: from a penem or penem intermediate of the general part-formula III: in which formulae R¹ and R² are defined as above, and X denotes a hydroxy group or a leaving group, to give a compound of the general part-formula IV: in which R¹ and R² are defined as above, the process being carried out at a temperature of from -70°C to +100°C in an inert aprotic solvent, and, if the resulting compound of the general formula IV is a penem intermediate, converting it into a penem of the general formula I or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof. A process for the preparation of a compound of the general formula I given in claim 1, in which R¹ and R² are defined as in claim 1 and R³ denotes a group of the formula -SR⁴, in which R⁴ is defined as in claim 1, a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof, which comprises reacting a compound of the general formula XIII: in which R¹ and R² are defined as in claim 1, R¹⁵ denotes an organic radical different from the group R⁴, and R x denotes hydrogen or a carboxyl-blocking group, with a thiol of the general formula XI: R⁴-SH XI in which R⁴ is defined as in claim 1, or a reactive derivative thereof; and thereafter if necessary or desired: (a) removing any carboxyl-blocking group R x , and/or (b) converting the product into the free acid or into a pharmaceutically acceptable salt or in vivo hydrolysable ester. A process as claimed in claim 1 or claim 2, wherein the product is of the general formula IA or a pharmaceutically acceptable salt or in vivo hydrolysable ester thereof in which R¹, R² and R³ are defined as in claim 1. A process as claimed in any one of claims 1 to 3, wherein, in general formula I or IA, R¹ denotes the hetero-aromatic group and R² denotes a hydrogen atom. A process as claimed in any one of claims 1 to 4, wherein the hetero-aromatic group R¹ or R² is an oxazolyl, isoxazolyl, pyrazolyl or triazolyl group. A process as claimed in any one of claims 1 to 5, wherein the product is a compound selected from: (5RS) (Z)-6-(isothiazol-5-ylmethylene)penem-3-carboxylic acid; (5RS)(E)-6-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylpyrazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-[(2-methylthiazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-(isoxazol-3-ylmethylene)penem-3-carboxylic acid; (5RS) (Z)-6-[(2,4-dimethyloxazol-5-yl)methylene] penem-3-carboxylic acid; (5RS) (E)-6-[(4-methyl-1,2,3-thiadiazol-5-yl) methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(4-methyl-1,2,3-thiadiazol-5-yl) methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(2-methyloxazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (Z)-6-((1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5RS) (E)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3-carboxylic acid; (5R)(Z)-6-[(1-methyl-1,2,3-triazol-4-yl)methylene] penem-3- carboxylic acid; (5RS)(Z)-6-[(5-methyl-1,2,4-oxadiazol-3-yl)methylene] penem-3-carboxylic acid; (5RS)(Z)-6-[(1-methyl-1,2,4-triazol-3-yl)methylene] penem-3-carboxylic acid; (5RS)2-hydroxymethyl-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) 2-ethylthio-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS)2-(2-hydroxyethylthio)-6(Z)-[(1-methylpyrazol-4-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-(N-methyltetrazol-5-ylmethylene)penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylpyrazol-3-yl)methylene]penem-3-carboxylic acid; (5RS) (Z)-6-[(1-methylimidazol-4-yl)methylene] penem-3-carboxylic acid; or a pharmaceutically acceptable salt or in-vivo hydrolysable ester thereof.

Inventors: OSBORNE, NEAL FREDERICK
Applicants: BEECHAM GROUP PLC
Application year: 1985
Main classification: C07D499
IPC codes: C07D249, C07D413, C07D499, C07D417, C07F9, C07D403, A61P31, A61K31, C07F7, C07D263