GB1589367A

GB1589367A - Clavulanic acid ethers

Info

Publication number: GB1589367A
Application number: GB51808/76A
Authority: GB
Original assignee: Beecham Group PLC
Current assignee: Beecham Group PLC
Priority date: 1976-12-11
Filing date: 1976-12-11
Publication date: 1981-05-13
Also published as: FI771145A; NO830484L; NO771105L; DK139477A; AU515479B2; FI65251B; ZA777383B; FI65251C; SE7704084L; AU3132777A; BE861716A

Description

(54) CLAWLANIC ACID ETHERS (71) We, BEECHAM GROUP LIMITED, a British Company, of Beecham House, Great West Road, Brentford, Middlesex, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by whicll it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a process for the preparation of clavulanic acid ether.

Belgian Patent No. 847045 and British Patent Specification No. 1565209 disclose that ethers of clavulanic acid and their salts and esters may be used to enhance the effectiveness of penicillins and cephalosporins. The process illustrated for the preparation of such compounds involved the reaction of a dial!, compound on an ester of clavulanic acid. A generally more convenient, safer and often higher yielding process has now been discovered.

Accordingly the present invention provides a process for the preparation of a compound of the formula (I):

or a salt or ester thereof, wherein R is a methyl or ethyl group, which process comprises the reaction of clavulanic acid or a salt or ester thereof with a compound of the formula (it): RsOe Xe (IT) wherein R is a methyl or ethyl group and xe is an anion, and thereafter if desired, performing one or more of the following reactions: (a) converting the initially formed ester into the acid or salt and (b) converting the thus formed acid or salt into an alternative ester or alternative salt.

When the etherification is performed on an ester of clavulanic acid, then usually at least 1 equidistant (for example 1-5 equivalents) of a compound of the formula (II) is employed per equivalent of ester of clavulanic acid. When the etherification is performed on a salt of clavulanic acid (or on the acid) then usually at least 2 equivalents (for example 2--5 equivalents) of a compound of the formula (II) is employed per equivalent of clavulanic acid or its salt.

Most suitably Xl= is BF4 or its equivalent such as PFds or 2,4,6-trinitrobenzenesulphonate.

The preceding reaction generally employs a salt or ester of clavulanic acid which is the compound of the formula (III):

When clavulanic acid or its salt is employed etherification and esterification take place so that methyl 9-O-methylclavulanate or ethyl 9-O-ethylclavulanate are produced. This reaction generally proceeds via the methyl or ethyl ester intermediate formed in-situ.

Most suitably the compound of the formula (II) is either trimethyloxonium tetrafluoroborate or triethyloxononium tetrafluoroborate.

Thus in one particularly suitable aspect this invention provides a process for the preparation of a compound of the formula (I) or a salt or ester thereof which process comprises the reaction of an ester of clavulanic acid with trimethyloxonium tetrafluoroborate or triethyloxononium tetrafluoroborate and thereafter, if desired, forming the free acid or salt thereof from the resulting ester.

The ester of clavulanic acid is preferably one which is hydrolysable or hydrogenolysable to the parent acid or its salt.

Suitable esters of clavulanic acid for use in the process of this invention include those of the formula (IV) and also those of the formula (V)

where Al is an alkyl group of 1-8 carbon atoms optionally substituted by halogen or a group of the formula OA4, OCOA4, SA4, SO2A4 wherein A4 is a hydrocarbon group of up to 6 carbon atoms; A2 is a hydrogen atom, an alkyl group of up to 4 carbon atoms or a phenyl group optionally substituted by halogen or by a group A5 or OAs where A5 is an alkyl group of up to 6 carbon atoms; and As is a phenyl group optionally substituted by halogen or a group At or OA5 where A6 is an alkyl group of up to 6 carbon atoms.

Other suitable values for As include nitrophenyl.

Most suitably A1 is an alkyl group of up to 4 carbon atoms, for example the methyl, ethyl, n-propyl or n-butyl group, or such a group substituted by a group of the formula OA4 or OCOA4 where A4 is an alkyl group of up to 4 carbon atoms.

Preferably A' is either a methyl group or an ethyl group.

Particularly suitable values for CHA2An include benzyl and mono-substituted benzvl such as bromobenzyl, nitrobenzyl and methoxybenzyl in which the substituent is preferablv in the para-position.

Other ester-forming groups which may be employed include in-vivo hydrolysable esterforming groups such as those described in Belgian Patent No. 827926 as being in-vivo hydrolysable when atttached to clavulanic acid.

Such ester-forming groups include acetoxymethyl, a-acetoxyethyl, pivaloyloxymethyl, phthalidyl, ethoxycarbonyloxymethyl and n- ethoxycarbonyloxyethyl.

When the process of this invention employs a salt of clavulanic acid as starting material the process offers the advantages of good overall yields of pure products and an advantageously low number of reaction steps.

Suitable salts of clavulanic acid used in the process of this invention may be any convenient salt of clavulanic acid such as an alkali metal or alkaline earth metal salt or a salt of a nitrogenous base. Thus suitable salts of clavulanic acid for use in this process include the lithium, sodium, notassium calcium, magnesium, barium or N,N,N',N'-tetramethyl guanidinium salt.

The reaction of the compound of the formula (II) with a salt or ester of clavulanic acid will take place in an inert dry organic solvent such as dichloromethane or chloroform, or other haloalkane or other non-hydroxylic solvent such as nitromethane. Most suitably the solvent system is strictly non-hydroxylic.

Preferably the etherification takes place in the presence of a base. Most suitably the base is one which is insoluble in the reaction medium such as an alkali metal carbonate or bicarbonate or an alkaline earth metal oxide or hydroxide. Thus, suitable bases include sodium carbonate, magnesium bicarbonate, sodium bicarbonate, potassium bicarbonate, calcium carbonate, lithium carbonate, potassium carbonate, calcium carbonate and magnesium carbonate. The base used should be anhydrous.

Such insoluble bases are preferably present in excess, for example from 1--5 equivalents of base per equivalent of oxonium salt may be used.

It has been found that the presence of a crown-ether in the reaction medium can increase the yield of the desired compound from the clavulanate salt. Thus crown-ethers such as "18 crown 6", "15 crown 5", "dicyclohexo 18 crown 6", or their equivalents may be used.

A preferred aspect of this invention provides a process for the preparation of methyl 9-0-methylclavulanate which comprises the reaction of a salt of clavulanic acid with trimethyloxonium fluoroborate. A further preferred aspect of this invention provides a process for the preparation of ethyl 9-0-ethylclavulanate which comprises the reaction of a salt of clavulanic acid with triethyloxonium tetrafluoroborate.

Once the etherification reaction is substantially complete (for example as shown by thin layer chrcmatcgraphy-identification by per maoganate spray) the desired compound may be obtained from the mixture by washing the organic phase with water to remove ionic materials, drying the organic phase and evaporating the solvent and thereafter, if desired, further purifying the ester/ether chromatographically. Suitable chromatographic systems employ stationary phases such as silica gel or cellulose and solvents such as ester-hydrocarbon mixtures such as ethyl acetate/cyclohexane mixtures.

Esters of the compounds of the formula (I) may be converted to the free acid or its salts by the methods described in Belgian Patent No. 847045. Such methods include the hydrogenation of hydrogenolysable esters such as the benzyl or p-methoxybenzyl or equivalent esters (such as the p-nitrobenzyl or p-bromobenzyl esters) optionally in the presence of a base such as a lithium, sodium, potassium, calcium or magnesium carbonate, bicarbonate or hydroxide, and in the presence of a transition metal catalyst such as 10% palladium on charcoal. Suitably a base such as lithium, sodium or potassium carbonate or sodium bicarbonate is used. Also suitably a base such as lithium hydroxide is used. If a base is not present in the hydrogenation medium, then the compound of the formula (I) will be formed; this may be converted to its salts by conventional methods of neutralisation, for example using the same bases that are apt for use in the hydrogenation medium. Such methods also include mild base hydrolysis, wherein the hydrolysis is effected by the addition of an alkali metal base or alkaline earth metal base.

The alkali metal base is suitably a hydroxide.

Suitably a lithium,, sodium or potassium base is used, for example hydrolysis of the methyl ester by the controlled addition of LiOH or NaOH added at a rate to maintain the pH (as recorded on a pH meter) of the solution in the region 7.5-10, for example maintained between such ranges as 7.5--9, 8-10 or preferably 9-9.5. This may be conveniently effected using a pH-stat so that the base is normally used in an aqueous medium. Bases which may be employed include LiOH, NaOH, KOH, Li2CO3, NaCOg, KHCO3, K2COs) Mg(OH), and Ca(OH)2.

In a particularly favoured aspect this invention provides a process adapted to the preparation of a salt of a compound of the formula (I) which process comprises forming the methyl ester of the methyl ether of clavulanic acid or the ethyl ester of the ethyl ether of clavulanic acid as hereinbefore described and thereafter hydrolysing the ester group to yield a salt of the methyl ether of clavulanic acid or a salt of the ethyl ether of clavulanic acid.

Generally the hydrolysis is effected in an aqueous solvent system such as aqueous tetrahydrofuran using a base such as one of those described above.

A preferred salt of clavulanic acid for use in the process of this invention is the sodium salt. A further preferred salt of clavulanic acid for use in the process of this invention is the potassium salt. Yet another preferred salt for use in the process of this invention is the lithium salt.

It appears that the use of a finely divided form of the salt leads to improved yields.

Such finely divided forms include those prepared by freeze-drying a solution or by dehydrating a hydrated salt such as sodium clavulanate tetrahydrate.

The etherification reaction is normally carried out at a temDerature of --800C (or more usually -6O0C) to +600C (or up to boiling point of the solvent, although temperatures of not more than +400C are more conventional) and more usually at from -40 C to 30"C; suitably at from -300C to 20"C. Often it is convenient to start the reaction at a low temperature such as -300C to 0 C and to allow the reaction mixture to gradually increase in temperature until ambient or a slightly depressed temnerature is reached such as about 10 C to 200C.

The hydrolysis is conveniently effected at roughly ambient temperature, for example at from about 10 C to about 30"C, for example at 15"C to 25"C.

When the reaction is complete (for example, no further base is taken up without degradation or as judged by tlc) the pH of the medium may be adjusted to pH 7 by, for example, the addition of a small quantity of an acid such as acetic acid.

In order to obtain the desired salt the solvent may be removed, for example by evaporation, and the dried salt obtained in crystalline form by the addition of an appropriate solvent such as acetone, acetonitrile or tetrahydrofuran. It can be favourable if such solvents contain moisture, but large proportions of water should be avoided owing to the solubility of the ethers.

A particularly suitable form of this part of the invention comprises hydrolysis of the ester to yield the lithium salt, as this salt can be produced in highly pure form in good yield.

If other salts of the methyl or ethyl ether are required these may conveniently be prepared from the lithium salt, for example by dissolving the lithium salt in water, applying this solution to a cation exchange resin in the form of an alternative salt (for example, in the sodium, potassium, calcium or magnesium form) and eluting the alternative salt therefrom.

Suitable cation exchange resins include cross-linked polystyrene-divinylbenzene copolymers substituted by sulphonic acid moieties; for example Amberlite IR-20, IR118 or IR-122, Dowex 50X8, Zerolit 225, BioRad AG 50W-X8, or Ionac C250, C255 or C258. ("Amberlite", "Dowex", "Zerolit" and "Bio-Rad" are Registered Trade Marks).

Normally the elution solvent is water or water in admixture with an organic solvent such as methanol, ethanol or acetone. Most suitably the elution solvent is water. The cation exchange resin is preferably present in a large excess, for example at least a 3-fold excess, most suitably at least an 8-fold excess and preferably at least a 10-fold excess. In the simplest and most convenient form of the process a solution of the lithium salt is simply percolated through a bed of resin from wihch it emerges in the form of the alternative salt.

The desired salt may then be obtained from solution by conventional methods such as freeze-drying, evaporation or precipitation using an organic solvent.

The acids of the formula (I) may be prepared from the lithium or other salt by acidification, for example by using an acid such as a mineral acid or a strong acid cation exchange resin (which acts as a convenient insoluble acid).

The following Examples illustrate the invention: EXAMPLE 1.

9-O-ethylclavulate.

To a vigorously stirred suspension of potassium clavulanate (1.52 g) and anhydrous sodium carbonate (4 g) in dry dichloromethane (70 ml) cooled to --20C, was added dropwise a solution of triethyloxonium tetrafluoroborate (4.86 g) in dry dichloromethane (40 ml). The reaction mixture was stirred for 3 hours at circa -200C (very slow reaction) and then for 1 hour at circa 50C (ice-bath). At this time tlc showed a moderately strong ester zone and a strong esterether zone. Water (90 ml) was added, the phases separated and the organic phase dried over sodium sulphate. The drying agent was filtered off and th efiltrate evaporated to an orange oil.

This was subjected to gradient elution chromatography on silica gel using ethyl acetate and cyclohexane, graded from 1:1 ratio to pure ethyl acetate. The ether-ester eluted before the ester. Fractions containing these (by tlc) were respectively combined and evaporated, to yield 44 mg of crude ethyl clavulanate and 520 mg of ester-ether. These were re-chromatographed separately. (The ester was re-chromatographed using the original solvent system to yield 15 mg pure ester).

The ester-ether was re-chromatographed using ethyl acetate and cyclohexane graded from 3:2 to 2:3 ratio, to yield 375 mg of pure ethyl 9-O-ethylclavulanate as a yellow oil.

I.R. v.,,.x (film) 1802, 1744 and 1699 cm-1; 8 (CDCl): 1.14 (3H, t, J 7Hz, ether CH, 1.26 (3H t, j 7 Hz ester CH,), 2.97 (1H, d, 7 17Hz, 6-B-CH), 3.46 (1H dd, J 17 and 3Hz 6-n-CH), 3.39 (2H, a. 1 7Hz, 9-OCH.), 4.01 (2H, d, J 1Hz 9-CH > ), 4.17 (2H, q, J 7Hz, CO. CH2), 4.78 (1H, t, I 7Hz, 8-CH), 4.99 (1H, s, 3-CH), 5.63 (1H, d, I 3Hz, 5-CH).

Tetramethylgtianodinium clavulanate can replace potassium clavulanate in this reaction, but without advantage, in spite of the solubility of the salt in dichloromethane.

EXAMPLE 2.

Ethyl 9-O-ethylclavulanate.

The process of Example 1 can be improved by the addition of a catalytic amount (0.17 g in this case) of crown ether ('18 crown 6') to the dichloromethane solution of the reagents before adding the oxonium salt. In this case 1.1 g (75%) of substantially pure ethyl 0ethylclavulanate was obtained after the first column (based on 89% pure potassium salt starting material).

EXAMPLE 3.

Lithium 9-O-ethylclavulanate.

A solution of ethyl 9-O-ethylclavulanate (1.1 g) in tetrahydrofuran/water (1:2, 60 ml) was maintained at pH 9.4 (pH-Stat) by the addition of 1M LiOH solution until 4.0 ml had been used (about 90 min) at 22"C with stirring. One small drop of acetic acid was added to bring the pH down to 7.0, and the solution then evaporated to an orange gum on the rotary evaporator at ambient temperature. The gum was dissolved in acetone (about 20 ml) and chilled at 2-30C for 1 hour, when the lithium salt crystallized. It was filtered off, washed with acetone (20 ml) and with ether (20 ml) and dried in vacuo, to yield highly pure lithium 9-O-ethylclavulanate as a pale yellow crystalline solid (0.73 g).

(Overall yield from potassium clavulanate via Example 2 = 55%) 20 using Cu K a radiation = 12.6, 13.3, 14.7, 17.2, 17.8, 18.7, 19.9, 20.8, 21.6, 22.8, 24.6, 26.8, 27.4, 28.2 and 28.9").

EXAMPLE 4.

Sodium 9-O-ethylclavulanate.

A part of the product of Example 3 (0.25 g) in water (2 ml) was passed through a bed of Amberlite IR-120 (Na+ form, 8 ml standard grade wet resin). The eluate was collected and evaporated under reduced pressure at ambient temperature. The residue was triturated under acetone-ether, filtered off, washed with ether and dried to yield sodium 9-0ethylclavulanate (0.2 g).

EXAMPLE 5.

Ethyl 9-O-ethylclavulanate.

Crystalline hydrated sodium clavulanate was dehydrated under vacuum over phosphorus pentoxide to constant weight. A susnension of the dry salt (1.11 g) and anhydrous sodium carbonate (2.65 g) in dry (treated with 3A molecular sieves), methanol free methylene chloride (50 ml) was treated with a crown ether (20 mg of 18 crown 6) and stirred and cooled to -200C, protected from atmospheric moisture. A solution of triethyloxonium tetrafluoroborate (3.8 g) in dry methylene chloride (50 ml) was added over 20 minutes and the mixture stirred vigorously at -200C for 3 hours. Samples were taken at intervals and examined by tlc to follow the reaction. The stirred reaction was then kent at about 5"C (ice/water bath) until the quantity of the desired product was at a maximum (as judged by tlc against a standard sample). Water (50 ml) was then added and the phases stirred and then separated. The methylene chloride solution was washed with more water (50 ml), dried over anhydrous sodium sulphate and the drying agent removed by filtration. The solvent was distilled at reduced pressure at < 200C to give crude title product as a light orange oil (1.10 g). The crude ether/ester was dissolved in a 1:1 mixture of cyclohexane:ethyl acetate (25 ml) and run onto a column of silica gel (30 g) prepared in the same solvent mixture. The column was eluted with 1:1 cyclohexane/ ethyl acetate and the eluent examined by tic at 10 ml intervals. Those fractions which contained the title compound were combined and the solvent distilled at reduced pressure and ; < 20 C. This gave ethyl 9-O-ethylclavulanate as a colourless oil (640 mg, 50%).

It showed the same spectrographic characteristics as the product from Example 1. Those fractions which were shown by tlc (against a standard sample) to contain ethyl clavulanate were combined and the solvent removed. The ester was obtained as a colourless oil, 350 rug, (31%).

EXAMPLE 6.

Ethyl 9-O-ethylclavulanate.

Potassium clavulanate (1.19 g), anhydrous sodium carbonate (2.65 g) and a trace of 'crown ether' (1 drop of 15-crown-5) were susnended in nitromethane (50 ml) and the mixture stirred and cooled to -200C and protected from atmosphere moisture. A solution of triethyloxonium tetrafluoroborate (3.8 g) in nitromethane (50 ml) was added over 20 minutes and the mixture stirred vigorously at 20OC for 3 hours.

The stirred reaction was then kept at about 50C (ice/water bath) for 5 hours, after which an analysis of the reaction mixture by tlc showed the presence of a large zone typical of ethyl 9-O-ethylclavulanate (as compared with a standard sample).

Example 7.

Ethyl 9-O-ethylclavulanate.

Lithium clavulanate (1.03 g), anhydrous sodium carbonate (2.65 g) and a trace of 'crown ether' (1 drop of 15-crown-S) were suspended in dry methylene chloride (50 ml) and the mixture stirred and cooled to -200C and protected from atmospheric moisture. A solution of triethyloxonium tetrafluoroborate (3.8 g) in dry methylene chloride (50 ml) was added over 20 minutes and the mixture stirred vigorously at -200C for 1 hour.

The stirred reaction mixture was then kept at about 5"C (ice/water bath) for 5 hours, after which it was treated with water (50 ml) and the crude ester/ether (250 mg) isolated as described in Example 5. This product was purified on a silica gel column to give the pure title compound (130 mg, 10%) and ethyl clavulanate (50 mg, 5%).

EXAMPLE 8.

Ethyl 9-O-ethylclavulanate.

Magnesium clavulanate (0.60 g), anhydrous sodium carbonate (1.5 g) and a trace of 'crown ether' (10 mg of 18-crown-6) were suspended in dry methylene chloride (30 ml) and the mixture stirred and cooled to -200C and protected from atmospheric moisture. A solution of triethyloxonium tetrafluoroborate (2.15 g) in dry methylene chloride (30 ml) was added over 15 minutes and the mixture stirred vigorously at -200C for 1 hour.

The stirred reaction mixture was then kept at 5"C (ice/water bath) for 2 hours, after which an analysis of the reaction mixture by tic showed the presence of a larger zone typical of ethyl 9-O-ethylclavulanate and only a small zone for ethyl clavulanate.

EXAMPLE 9.

Methyl 9-O-methyl clavulanate.

Potassium clavulanate (95% pure, 1.52 g) and anhydrous sodium carbonate (4.0 g) were suspended in dry methylene chloride (70 ml) in a vessel protected from moisture by a calcium chloride drying tube. 18-Crown-6 (0.17 g) was dissolved in the methylene chloride. The suspension was stirred and cooled to about -200C and a suspension of trimethyloxonium tetrafluoroborate (4.22 g) in dry methylene chloride (90 ml) added slowly. The reaction mixture was stirred at --200 for three hours and at about OOC for 1 hour. Water (90 ml) was then added and the organic phase separated and dried (with anhydrous sodium sulphate). The solvent was removed under vacuum and the product purified by column chromatography on silica gel, eluting with cyclohexane/ethyl acetate (1:1).

Evaporation of appropriate eluent fractions yielded 0.54 g methyl 9-0-me thylclavulanate (41%) and a further 0.37 g methyl clavulanate (29%).

The sample of methyl 9-O-methylclavulanate was hydrolysed in aqueous tetrahydrofuran solution with molar lithium hydroxide on a pH-stat at pH 9.5.

Crystalline lithium 9-O-methylclavulanate (.43 g) was isolated by evaporation and addition of acetone.

X-ray powder diffractorgram-refiections at following angles 2S, (Copper Ka radiation) 11.5, 12.9, 14.2, 15.3, 17.9, 19.1, 21.0, 21.3, 22.1, 23.5, 24.1, 24.6, 25.4, 28.6, 29.4.

EXAMPLE 10.

Methyl 9-O-methylclavulanate.

Potassium clavulanate (95 /O pure, 4.56 g), anhydrous sodium carbonate (12 g), crown ether (18-crown-6, 0.5 g) and trimethyloxonium tetrafluoroborate (12.7 g) were cooled at -700C and stirred while dried methylene chloride (200 ml) was added slowly. After addition of solvent the temperature was allowed to rise slowly to 200C. After three hours stirring at room temperature tic examination showed two zones (rf 0.35 and 0.12) with an area ratio of approximately 10:1.

Water (250 ml) was added to the stirring reaction mixture and the organic phase separated, dried (anhydrous sodium sulphate), evaporated, and purified by column chromatography as in Example 9. The eluent fractions containing the desired product were evaporated to give 2.6 g (62%) methyl 9-O-methyl- clavulanate (pure by tlc).

1.13 g of the above product was dissolved in aqueous tetrahydrofuran and hydrolysed on a pH-stat at pH 9.5 to give potassium 9-0 methylclavulanate.

EXAMPLE 11.

Methyl 9-0-methylcl avulanate.

Sodium clavulanate (1.6 g, vacuum dehy drated tetrahydrate, 92% pure), anhydrous sodium carbonate (4.0 g) and trimethyl oxonium tetrafluoroborate (4.9 g) were cooled to -700C and stirred while dried methylene chloride (100 ml) containing crown ether (about 50 mg 15-crown-5) was added gradu ally. Stirring was continued while the tem perature was allowed to increase to room temperature. Progress of the reaction was monitored by thin layer chromatography and after three hours at room temperature the reaction mixture was worked up as in Example 10. The yield of methyl 9-O-methylclavulanate was 1.03 g (70%).

(Methyl clavulanate (0.22 g) was also isolated).

8 (CDCI3) 2.99 1(H,d, J=16Hz, 6--PCH), 3.24 (3H, s, ether Cm3), 344 (1H, dd, J= 16Hz and 3 Hz, 6-aCH), 3.72 (3H, a, ester, C)?.) 3.96 (2H, d, J=7Hz, 9-CH2O), 4.79 (1H, t, J=7Hz, 8-CH) 5.00 (1H, bs. 3- CH), 5.63 (1H, d, J=3 (Hz, 5-C)?).

EXAMPLE 12.

Methyl 9-O-methylclavulanate.

Dry methylene chioride (110 ml) containing crown ether (about 50 mg. 15-crown-S) was added slowly to a stirred cold ( - 70"C) mixture of crystalline lithium clavulanate (1.0 g), anhydrous sodium carbonate (4.0 g) and trimethyloxonium tetrafluoroborate (4.4 g). The mixture was allowed to reach room temperature and then stirred for a further 4 hours. After work-up and chromatography as in Example 10 pure methyl 9-O-methylclavulanate was isolated (0.65 g, 57% yield).

NMR identical to Example 11 product.

EXAMPLE 13.

Methyl 9-O-methylclavulanate.

Dry methylene chloride (80 ml) containing crown ether (50 mg 18-crown-6) was added slowly to a stirred cold ( - 700C) mixture of magnesium clavulanate (0.6 g), magnesium oxide (3.0 g) and trimethyloxonium tetra fluoroborate. The reaction mixture was allowed to reach room temperature and the progress of the reaction was followed by tlc. After several hours at room temperature zones corresponding to methyl clavulanate and methyl 9-O-methylclavulanate could be seen on the developed tic plates.

EXAMPLE 14.

Methyl 9-O-methylclavulanate.

Dry nitromethane (110 ml) containing crown ether (about 20 mg 15-crown-5) was added slowly to a cooled, stirred mixture of potassium clavulanate (1.52 g), anhydrous sodium carbonate (4.0 g) and trimethyloxonium tettrafluoroborate (4.5 g). The reactio

The mixture was stirred at about - 100C' for 6 hr, then allowed to warm to ambient temperature during + hr. Water (100 ml) was added cautiously with stirring, the organic phase separated, dried over anhydrous sodium sulphate, and evaporated to a syrup. This was subjected to column chromatography on silica gel, eluting initially with 1:1, then with 2:1 ethyl acetate-cyclohexane mixtures. The first eluted product was the ethyl ether (4.9 g after evaporation of solvents), followed by recovered p-methoxybenzyl clavulanate (4 g).

The title compound was a pale yellow oil with the following properties.

I.r. (liquid film) 1805 (p-lactam C=O) 1750 (ester G=O) 1700 cm-1 (C=C); nmr (CDCL3) 1.17 (3H, t, J 7Hz, CH3CH2) 2.96 (1H, d, J 17Hz, 6-P-CH) 3.41 (2H, q, J 7Hz, CBBCH,-) 3.47 (1H, dd, J 17 and 3Hz, 6 a-CH) 3.79 (3H, s, OCH,) 4.03 (2H, d, J 7Hz, -CH20) 4.82 (1H, t, J 7Hz, CH=) 5.04 (1H, s, 3-CH) 5.11 (2H, s, PhCH2), 5.65 (1H, d, J 3H3, 5 - CH) 6.9, 7.3 (4H, A2B2q, J, 10H,, C6H4).

EXAMPLE 17.

Lithium and sodium 9-O-ethylclavulanate.

p-Methoxybenzyl 9-O-ethylclavulanate (2.5 g) in tetrahydrofuran (25 ml) containing water (0.1 ml) was hydrogenated over 10% palladised charcoal (0.8 g). After 2 hr. the absence of starting material was demonstrated by tlc. The catalyst was removed by filtration through a bed of finely divided silica, and the filtrate diluted with an equal volume of water to yield a solution of 9-O-ethylclavulanic acid.

This solution was titrated to pH 7.0 with 1M lithium hydroxide solution. Evaporation of the solvents and trituration with acetone yielded the lithium salt as a pale cream crystalline solid (1.05 g).

The sodium salt was prepared in an identical manner using 1M NaOH solution); yield 0.85 g.

Ir. (Nujol mull) 1785 (,B-lactam C=Oj 1685 (C=C) 1615 cm-1 (-CO2-). (Both salts).

(The starting material for this Example s produced as described in Example 16,.

("Nujol" is a Registered Trade Mark).

EXAMPLE 18.

Ethyl 9-O-ethylclavulanate.

Dehydrated sodium clavulanate (1.1 g), anhydrous sodium carbonate (2.65 g) and a trace of 'crown ether' (10 mg of 18-crown-6) were suspended in dry methanol-free methylene chloride (50 ml) and the mixture stirred and cooled to -200C. A solution of triethyloxonium hexafluorophosphate (5.0 g) in dry methylene chloride (50 ml) was added over 20 minutes and the mixture stirred vigorously at -200C for 3 hours. The stirred reaction mixture was then kept at about 5 C for 7 hours, after which it was treated with water (50 ml) and the ester/ether (250 mg) isolated as described in Example 5. This product was purified on a silica gel column to give the desired ethyl 9-O-ethylclavulanate.

EXAMPLE 19.

9-O-Methylclavulanic -acid.

A solution of lithium 9-O-methylclavulanate (0.9 g) in water (40 ml) was covered with a layer of ethyl acetate (150 ml) and stirred vigorously at room temperature. Strong acid ion exchange resin (Amberlite IR 120 (H+)) (10 ml wet resin) was added. After 5 mins the resin was removed by decantation, and the layers separated. The aqueous layer was extracted with a further 100 ml of ethyl acetate; the solvent layers were combined, washed with water (5 ml), dried over anhydrous calcium sulphate and filtered. The solution was evaporated to crystallization under reduced pressure, then the remainder of the solvent removed in vacuo, to leave the frets 9-O-methylclavulanic acid as a colourless crystalline solid (0.85 g).

EXAMPLE 20.

Nitrobenzyl 9-O-methylclavulanate.

Nitrobenzylclavulanate (3.51 g), trimethyloxonium tetrafluoroborate (3.15 g) and anhydrous sodium carbonate (4.0 g) were stirred and cooled to --70"C. To the stirred mixture was slowly added methylene chloride (150 ml) containing approximately 100 mg of 18 "crown" 6 crown ether. After the addition the reaction mixture was allowed to warm up to room temperature and then stirred for a further three hours. The product was isolated as described in Example 10 to yield p-nitrobenzyl 9-O-methylclavulanate (2.91 g) as a white crystalline solid.

WHAT WE CLAIM IS:- 1. A process for the preparation of a compound of the formula (I):

or a salt or ester thereof, wherein R is a methyl or ethyl group, which process comprises the reaction of clavulanic acid or a salt c ester thereof with an oxonium salt of the formula (II): R30d3 X9 (Il) wherein R is a methyl or ethyl group and X0 is an anion, and thereafter, if desired,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The mixture was stirred at about - 100C' for 6 hr, then allowed to warm to ambient temperature during + hr. Water (100 ml) was added cautiously with stirring, the organic phase separated, dried over anhydrous sodium sulphate, and evaporated to a syrup. This was subjected to column chromatography on silica gel, eluting initially with 1:1, then with 2:1 ethyl acetate-cyclohexane mixtures. The first eluted product was the ethyl ether (4.9 g after evaporation of solvents), followed by recovered p-methoxybenzyl clavulanate (4 g).

The title compound was a pale yellow oil with the following properties.

I.r. (liquid film) 1805 (p-lactam C=O) 1750 (ester G=O) 1700 cm-1 (C=C); nmr (CDCL3) 1.17 (3H, t, J 7Hz, CH3CH2) 2.96 (1H, d, J 17Hz, 6-P-CH) 3.41 (2H, q, J 7Hz, CBBCH,-) 3.47 (1H, dd, J 17 and 3Hz, 6 a-CH) 3.79 (3H, s, OCH,) 4.03 (2H, d, J 7Hz, -CH20) 4.82 (1H, t, J 7Hz, CH=) 5.04 (1H, s, 3-CH) 5.11 (2H, s, PhCH2), 5.65 (1H, d, J 3H3, 5 - CH) 6.9, 7.3 (4H, A2B2q, J, 10H,, C6H4).

EXAMPLE 17.

Lithium and sodium 9-O-ethylclavulanate.

p-Methoxybenzyl 9-O-ethylclavulanate (2.5 g) in tetrahydrofuran (25 ml) containing water (0.1 ml) was hydrogenated over 10% palladised charcoal (0.8 g). After 2 hr. the absence of starting material was demonstrated by tlc. The catalyst was removed by filtration through a bed of finely divided silica, and the filtrate diluted with an equal volume of water to yield a solution of 9-O-ethylclavulanic acid.

This solution was titrated to pH 7.0 with 1M lithium hydroxide solution. Evaporation of the solvents and trituration with acetone yielded the lithium salt as a pale cream crystalline solid (1.05 g).

The sodium salt was prepared in an identical manner using 1M NaOH solution); yield 0.85 g.

Ir. (Nujol mull) 1785 (,B-lactam C=Oj 1685 (C=C) 1615 cm-1 (-CO2-). (Both salts).

(The starting material for this Example s produced as described in Example 16,.

("Nujol" is a Registered Trade Mark).

EXAMPLE 18.

Ethyl 9-O-ethylclavulanate.

Dehydrated sodium clavulanate (1.1 g), anhydrous sodium carbonate (2.65 g) and a trace of 'crown ether' (10 mg of 18-crown-6) were suspended in dry methanol-free methylene chloride (50 ml) and the mixture stirred and cooled to -200C. A solution of triethyloxonium hexafluorophosphate (5.0 g) in dry methylene chloride (50 ml) was added over 20 minutes and the mixture stirred vigorously at -200C for 3 hours. The stirred reaction mixture was then kept at about 5 C for 7 hours, after which it was treated with water (50 ml) and the ester/ether (250 mg) isolated as described in Example 5. This product was purified on a silica gel column to give the desired ethyl 9-O-ethylclavulanate.

EXAMPLE 19.

9-O-Methylclavulanic -acid.

A solution of lithium 9-O-methylclavulanate (0.9 g) in water (40 ml) was covered with a layer of ethyl acetate (150 ml) and stirred vigorously at room temperature. Strong acid ion exchange resin (Amberlite IR 120 (H+)) (10 ml wet resin) was added. After 5 mins the resin was removed by decantation, and the layers separated. The aqueous layer was extracted with a further 100 ml of ethyl acetate; the solvent layers were combined, washed with water (5 ml), dried over anhydrous calcium sulphate and filtered. The solution was evaporated to crystallization under reduced pressure, then the remainder of the solvent removed in vacuo, to leave the frets 9-O-methylclavulanic acid as a colourless crystalline solid (0.85 g).

EXAMPLE 20.

Nitrobenzyl 9-O-methylclavulanate.

Nitrobenzylclavulanate (3.51 g), trimethyloxonium tetrafluoroborate (3.15 g) and anhydrous sodium carbonate (4.0 g) were stirred and cooled to --70"C. To the stirred mixture was slowly added methylene chloride (150 ml) containing approximately 100 mg of 18 "crown" 6 crown ether. After the addition the reaction mixture was allowed to warm up to room temperature and then stirred for a further three hours. The product was isolated as described in Example 10 to yield p-nitrobenzyl 9-O-methylclavulanate (2.91 g) as a white crystalline solid.

WHAT WE CLAIM IS:- 1. A process for the preparation of a compound of the formula (I):

or a salt or ester thereof, wherein R is a methyl or ethyl group, which process comprises the reaction of clavulanic acid or a salt c ester thereof with an oxonium salt of the formula (II): R30d3 X9 (Il) wherein R is a methyl or ethyl group and X0 is an anion, and thereafter, if desired,

performing one or both of the following reactions: (a) converting the thus formed ester into the acid or salt and (b) converting the thus formed acid or salt into an alternative ester or alternative salt.
2. A process as claimed in Claim 1 wherein Xe is BF4 .
3. A process as claimed in Claim 1 wherein X19 is PFc.
4. A process as claimed in Claim 1 wherein XO is 2,4,6-trinitrobenzenesulphonate.
5. A process as claimed in Claim 1 wherein the oxonium salt of the formula (II) is trimethyloxonium tetrafluoroborate.
6. A process as claimed in Claim 1 wherein the oxonium salt of the formula (II) is triethyloxonium tetrafluoroborate.
7. A process as claimed in any of Claims 1--5 which employs a hydrolysable ester of clavulanic acid.
8. A process as claimed in any of Claims 1-5 which employs a hydrogenolysable ester of clavulanic acid.
9. A process as claimed in any of Claims 1-6 which employs an ester of clavulanic acid of the formula (IV):

wherein Al is an alkyl group of 1-8 carbon atoms optionally substituted by halogen or a group of the formula OA4, OCOA4, SA4 or SOYA" wherein A4 is a hydrocarbon group of up to 6 carbon atoms.
10. A process as claimed in any of Claims 1-6 which employs an ester of clavulanic acid of the formula (IV) as shown in claim 9 wherein A1 is an alkenyl or alkynyl group of up to 4 carbon atoms.
11. A process as claimed in any of Claims 1-6 which employs an ester of clavulanic acid of the formula (IV) as shown in Claim 9 wherein A' is a group such that the com pound is an acetoxymethyl, a-acetoxyethyl, pivaloyloxymethyl, phthalidyl, ethoxycarbonyl oxymethyl or er-ethoxycarbonyloxyethyl ester.
12. A process as claimed in any of Claims 1-6 which employs an ester of clavulanic acid of the formula (V):

wherein A2 is a hydrogen atom, an alkyl group of up to 4 carbon atoms or a phenyl group optionally substituted by halogen or by a group A or OA above A is an alkyl group of up to 6 carbon atoms; and A3 is a phenyl group optionally substituted by halogen or by a group A5 or OA" where As is an alkyl group of up to 6 carbon atoms.
13. A process as claimed in Claim 9 wherein Al is an alkyl group of up to 4 carbon atoms optionally substituted by a group of the formula OA4 or OCOA4 where A4 is an alkyl group of up to 4 carbon atoms.
14. A process as claimed in Claim 9 wherein Al is a methyl group.
15. A process as claimed in Claim 9 wherein A' is an ethyl group.
16. A process as claimed in any of Claims 1-6 which employs an ester of the formula (V) as shown in Claim 12 wherein A2 is as defined in Claim 12 and A3 is a nitrophenyl group.
17. A process as claimed in Claims 12 or 16 wherein A2 is hydrogen.
18. A process as claimed in Claim 12 wherein CHA2Aa is benzyl.
19. A process as claimed in Claim 12 wherein CHA2A3 is p-methoxybenzyl.
20. A process as claimed in Claim 12 wherein CHA2A3 is p-bromobenzyl.
21. A process as claimed in Claim 16 wherein CHA2A3 is p-nitrobenzyl.
22. A process as claimed in any of Claims 1-6 which employs a salt of clavulanic acid.
23. A process as claimed in Claim 22 wherein the salt is an alkali metal salt.
24. A process as claimed in Claim 22 wherein the salt is an alkaline earth metal salt.
25. A process as claimed in claim 22 wherein the salt is of a nitrogenous base.
26. A process as claimed in Claim 23 wherein the salt is the lithium salt.
27. A process as claimed in Claim 23 wherein the salt is the sodium salt.
28. A process as claimed in Claim 23 wherein the salt is the potassium salt.
29. A process as claimed in Claim 25 wherein the base is N,N,N',N'-tetramethyl- guanidine.
30. A process as claimed in any of Claims 22-29 wherein the salt is employed in finely divided form.
31. A process as claimed in any of Claims 22-30 wherein the salt employed has been prepared by freeze-drying a solution.
32. A process as claimed in Claim 27 wherein the sodium salt employed has been prepared by dehydrating sodium clavulanate tetrahydrate.
33. A process as claimed in any of Claims 1-32 carried out at a temperature of from --60" to 600C.
34. A process as claimed in Claim 33 wherein the temperature is from --40" to 30"C.
35. A process as claimed in Claim 33 wherein the temperature is from - 300 to 20"C.
36. A process as claimed in Claim 33 wherei nthe initial temperature is from 300 to 0 C.
37. A process as claimed in Claim 33 wherein the final temperature is 10 to 20"C.
38. A process as claimed in any of Claims 1-37 wherein the solvent is a haloalkane.
39. A process as in Claim 38 wherein the haloalkane is dichloromethane.
40. A process as claimed in Claim 38 wherein the solvent is chloroform.
41. A process as claimed in any of Claims 1-37 wherein the solvent is nitromethane.
42. A process as claimed in any of Claims 1-41 wherein the reaction is performed in the presence of a base.
43. A process as claimed in Claim 42 wherein the base is an insoluble base present in an excess of from 1-5 equivalents cf base per equivalent of oxonium salt.
44. A process as claimed in Claims 42 or 43 wherein the base is an alkali metal carbonate or bicarbonate.
45. A process as claimed in Claims 41 or 42 wherein the base is an alkaline earth metal carbonate or bicarbonate.
46. A process as claimed in Claims 41 or 42 wherein the base is an alkaline earth metal oxide or hydroxide.
47. A process as claimed in Claim 44 wherein the base is sodium carbonate.
48. A process as claimed in Claim 44 wherein the base is sodium bicarbonate.
49. A process as claimed in Claim 44 wherein the base is lithium carbonate.
50. A process as claimed in Claim 44 wherein the base is potassium carbonate.
51. A process as claimed in Claim 44 wherein the base is potassium bicarbonate.
52. A process as claimed in Claim 44 wherein the base is calcium carbonate.
53. A process as claimed in Claim 44 wherein the base is calcium bicarbonate.
54. A process as claimed in Claim 44 wherein the base is magnesium carbonate or magnesium bicarbonate.
55. A process as claimed in any of Claims 1-34 wherein an ester of the ether is obtained from the reaction mixture by washing with water to remove ionic materials and thereafter evaporating the organic phase.
56. A process as claimed in any of Claims 1-55 wherein the initially produced ester of the compound of formula (I) is converted to a salt by hydrolysis.
57. A process as claimed in Claim 56 wherein the hydrolysis is effected by the addition of an alkali metal base.
58. A process as claimed in Claim 57 wherein the base is a hydroxide.
59. A process as claimed in any of Claims 56-58 wherein the base is a lithium base.
60. A process as claimed in any of Claims 56-58 wherein the base is a sodium base.
61. A process as claimed in any of Claims 56--58 wherein the base is a potassium base.
62. A process as claimed in Claim 59 wherein the base is lithium hydroxide.
63. A process as claimed in Claim 60 wherein the base is sodium hydroxide.
64. A process as claimed in Claim 61 wherein the base is potassium hydroxide.
65. A process as claimed in Claim 56 wherein the hydrolysis is effected by the addition of an alkaline earth metal base.
66. A process as claimed in Claims 59 or 62 wherein the initially produced lithium salt is converted into a sodium, potassium, calcium or magnesium salt.
67. A process as claimed in any of Claims 1-55 wherein the initially produced ester of the compound of formula (I) is converted to the free acid by hydrogenolysis of a hydrogenolysable ester.
68. A process as claimed in any of Claims 1-55 wherein the initially produced hydrogenolysable ester is converted to the salt by hydrogenolysis in the presence of a base.
69. A process as in Claim 67 wherein the acid is converted to a salt by reaction with a base.
70. A process as claimed in Claims 68 or 69 wherein the base is a lithium, sodium, potassium, calcium or magnesium carbonate, bicarbonate or hydroxide.
71. A process as claimed in Claim 70 wherein the base is lithium, sodium or potassium carbonate or sodium bicarbonate.
72. A process as claimed in Claim 70 wherein the base is lithium hydroxide.
73. A process as claimed in any of Claims 1-21 wherein the ester of clavulanic acid is prepared in-situ.
74. A process as claimed in Claims 14 or 15 wherein the ester of clavulanic acid is produced by the reaction of a salt of clavulanic acid and an oxonium salt of the formula (II) as defined in Claim 1.
75. A process as claimed in Claim 74 wherein the compound of the formula (II) is trimethyloxonium tetrafluoroborate or triethyloxonium tetrafluoroborate.
76. A process as claimed in Claim 74 or 75 wherein the salt of clavulanic acid is the lithium salt.
77. A process as claimed in Claims 74 or 75 wherein the salt of clavulanic acid is the sodium salt.
78. A process as claimed in Claim 74 or 75 wherein the salt of clavulanic acid is the potassium salt.
79. A process for the preparation of methyl 9-O-methylclavulanate which process comprises the reaction of a salt of clavulanic acid with a trimethyloxonium salt.
80. A process for the preparation of ethyl 9-O-ethylclavulanate which process comprises the reaction of a salt of clavulanic acid with a triethyloxonium salt.
81. A process as claimed in Claims 79 or 80 wherein the oxonium salt is the tetrafluoroborate.
82. A process as claimed in any of Claims 79-81 carried out at a temperature as defined in any of Claims 33-37.
83. A process as claimed in any of Claims 79--81 carried out at a temperature of from --80"C to 600C.
84. A process as claimed in any of Claims 79-83 wherein the solvent is as defined in any of Claims 38--41.
85. A process for the preparation of a salt of 9-O-methylclavulanic acid which comprises the base hydrolysis of methyl 9-O-methylclavulanate prepared by a process as claimed in any of Claims 79 or 81-84.
86. A process for the preparation of a salt of 9-O-ethylclavulanic acid which comprises the base hydrolysis of ethyl 9-O-ethylclavulanate prepared by a process as claimed in any of Claims 80-84.
87. A process as claimed in Claims 85 or 86 wherein the hydrolysis employs a base as defined in any of Claims 57-65.
88. A process as claimed in Claims 85 or 86 wherein the hydrolysis is effected by lithium hydroxide.
89. A process as claimed in Claim 88 wherein the initially produced lithium salt is converted into a sodium, potassium, calcium or magnesium salt.
90. A process as claimed in Claim 88 wherein the conversion is effected by contacting a solution of the lithium salt with a cation exchange resin in the form of the sodium, potassium, calcium or magnesium salt and thereafter eluting the desired salt from the resin.
91. A process for the preparation of the sodium, potassium, calcium or magnesium salt of 9-O-methylclavulanic acid which process comprising contacting a solution of the lithium salt of 9-O-methylclavulanic acid as prepared by the process as claimed in any of Claims 59, 62, 71 or 72 with a cation exchange resin in the form of a sodium, potassium, calcium or magnesium salt and thereafter eluting the desired salt from the resin.
92. A process for the preparation of the sodium, potassium, calcium or magnesium salt of 9-O-ethylclavulanic acid which process comprises contacting a solution of the lithium salt of 9-O-ethylclavulanic acid as prepared by the process as claimed in any of Claims 59, 62, 71 or 72 with a cation exchange resin in the form of a sodium, potassium, calcium or magnesium salt and thereafter eluting the desired salt from the resin.
93. A process as claimed in Claims 91 or 92 wherein the elution solvent is water or water in admixture with a miscible organic solvent.
94. A process as claimed in Claim 93 wherein the solvent is water.
95. Lithium 9-O-ethylclavulanate.
96. Ethyl 9-O-ethylclavulanate.
97. A salt of 9-O-ethylcla'ulanate acid when prepared from ethyl 9-O-ethylclavulanate, prepared by a process as claimed in any of Claims 80-84, by a process as claimed in Claim 86.
98. The lithium salt as claimed in Claim 97.
99. The sodium salt as claimed in Claim 97.
100. The potassium salt as claimed in Claim 97.
101. An alkaline earth metal salt as claimed in Claim 97.
102. A process for the preparation of 9-0methylclavulanic acid or 9-O-ethyclavulanic acid which comprises the acidification of a salt of 9-O-methylclavulanic acid or of 9-0ethylclavulanic acid prepared as claimed in Claim 1.
103. A process as claimed in Claims 1 or 102 for the preparation of 9-O-methylclavulanic acid or 9-O-ethylclavulanic acd or a salt or ester thereof substantially as described in any one of the Examples herein.
104. A compound of the formula (I) as defined in Claim 1 or a salt or ester thereof whenever prepared by a process as claimed in any of Claims 1-94.
105. A compound as claimed in Claim 104 whenever prepared substantially as described in any one of the Examples herein.