Process for the preparation of ethers of clavulanic acid

Abstract

 This invention provides a process for the preparation of the ethyl and propyl ethers of clavulanic acid and salts and esters thereof, which process comprises the reduction of an ester of a compound of the formula (II):

wherein R₂ is a CH =CH₂ or CH =CHCH₃ group, and thereafter or simultaneously, if desired, converting the thus-formed ester of the ethyl or propyl ether of clavulanic acid to the corresponding acid or a salt thereof.
The compounds of the formula (II) and salts and esters thereof are novel and have β-lactamase inhibitory properties, and this invention further provides these compounds, a process for their preparation, and pharmaceutical compositions containing them.

Description

[0001] The present invention relates to a process for the preparation of certain ethers of clavulanic acid, to chemical intermediates in the process, to a process for the preparation of these intermediates, and to pharmaceutical compositions containing them.

[0002] Ethers of clavulanic acid are disclosed in Belgian Patent No: 847045, Japanese Patent Application No: 122725/76 and U.S. Patent Application Serial No: 786345. It has been found that ethyl and propyl ethers of clavulanic acid and derivatives thereof can conveniently.be prepared via the vinyl and propenyl ethers of clavulanic acid.

[0003] Accordingly, the present invention provides a process for the preparation of the compounds of the formula (I):

and salts and esters thereof wherein R₁ is a CH₂CH₃ or CH₂CH₂CH₃ group, which process comprises the reduction of an ester of a compound of the formula (II):

wherein R₂ is a CH=CH₂ or CH=CH-CH₃ group and thereafter or simultaneously if desired converting the thus-formed ester of the compound of the formula (I) to the corresponding acid or a salt thereof.

[0004] Most suitably, the reaction is performed on an ester of the compound of the formula (II) wherein R₂ is a CH=CH₂ group.

[0005] The reduction reaction is normally effected by hydrogenation in the presence of a catalyst such as a transition metal catalyst, such as platinium oxide, palladium or the like. An approximately atmospheric pressure of hydrogen is generally most convenient for laboratory use, but sub-atmospheric pressures may also be employed, so long as extreme conditions are avoided.

[0006] If a compound of the formula (I) per se or a salt thereof is required, this may be obtained by de-esterifying a suitable ester of the compound, for example, by the hydrolysis of a methoxymethyl or like ester or by the hydrogenation of a benzyl, p-methoxybenzyl or like ester optionally in the presence of a base, for example, as described in Belgian Patent No: 847045.

[0007] If desired, the hydrogenation of the vinyl or propenyl group and hydrogenolysis of the ester may occur at the same time, by selecting a suitable catalyst and solvent, for example, by using a tetrahydrofuran - containing solvent system and by using a palladium or mixed platinium oxide/palladium catalyst.

[0008] The present invention also provides the compounds of the formula (II) and salts and esters thereof.

[0009] Preferably, R₂ is a CH=CH₂ group.

[0010] Suitable esters of the compounds of the formula (II) include those wherein the esterifying moiety is a group A¹ or CHA²A³ wherein A¹ is an alkyl group of 1-8 carbon atoms optionally substituted by halogen or by a grou of the formula OA⁴, OCOA⁴, SA 4, SO₂A⁴ wherein A 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 OA⁵ where A⁵ is an alkyl group of up to 6 carbon atoms; and A3 is a phenyl group optionally substituted by halogen or nitro or by a group A⁵ or OA⁵ where A⁵ is an alkyl group.

[0011] Preferably, the ester of the compound of the formula (II) is such that the corresponding ester of the compound of the formula (I) is a readily hydrolysable or hydrogenolysable ester. Suitable esters of this type include the methoxymethyl, ethoxymethyl, benzyl, methoxybenzyl, nitrobenzyl and the like esters.

[0012] A particularly preferred ester of the compounds of the formula (II) is the p-nitrobenzyl ester.

[0013] Suitable salts of the compounds of the formula (II) include alkali metal, alkaline earth metal and ammonium and substituted ammonium salts.

[0014] Preferably, the salt is a pharmaceutically acceptable salt.

[0015] The novel compounds of this invention possess useful β-lactamase inhibitory properties which allow them to be utilised in antibacterial compositions. Accordingly, this invention provides a pharmaceutical composition which comprises a compound of the formula (II) or a pharmaceutically acceptable salt or ester thereof and a pharmaceutically acceptable carrier.

[0016] The compositions of this invention may be utilised as described in the aforementioned patent specifications (whose disclosure is incorporated herein by reference) and may advantageously contain a penicillin or cephalosporin.

[0017] This invention also provides a process for the preparation of the compounds of the formula (II) and salts and esters thereof which process comprises the reaction of an ester of clavulanic acid with a compound of the formula (III) or (IV):



wherein R₃ is an alkyl group of up to 6 carbon atoms, in the presence of mercuric ions as catalyst, and thereafter, if desired, de-esterifying the resulting ester of the compound of the formula (II).

[0018] Most suitably, R₃ in formulae (III) and (IV) is an ethyl group.

[0019] Most suitably, the catalyst is mercuric acetate or mercuric trifluoroacetate.

[0020] The reaction may take place in an inert organic solvent, or if a large excess of the compound of the formula (III) or (IV) is used, this may act as solvent.

[0021] The reaction may be effected at a non-extreme temperature such as about -10 to 60°C, for example from about 0 to 30°C.Low temperatures in this range are preferred when using a compound of the fromula (IV).

[0022] Compounds of the formula (II) and salts thereof may be formed by cleavage of the corresponding p-nitrobenzyl ester. This cleavage may be effected using a reducing agent which reduces the nitro group to an amino group, for example, as described in Dutch Patent Application No: 7702027. An example of a suitable reducing agent is iron powder/ammonium chloride solution.

[0023] The following Examples illustrate this invention:

Example 1

[0024] Benzyl 9-0-vinylclavulanate

[0025] Mercuric acetate (50 mg) was added to a solution of benzyl clavulanate (1g) in ethyl vinyl ether (10 ml). The solution was refluxed for 6 hours, then allowed to stand at room temperature for 72 hours. The solvent was removed under vacuum and the product purified by column chromatography (Kieselgel G, cyclohexane:ethyl acetate 3:1). Yield 0.676g. I.r:ν _max (film) 1805, 1750, 1700, 1638, 1620 cm^-1. N.m.r.: δ (CDC1₃) 3.00 (1H, d, J 17Hz), 3.43 (1H, dd, J 3Hz and 17 Hz), 3.95 (1H, dd, J 7Hz and 3Hz), 4.13 (1H, dd, J 3 and 14Hz), 4.26 (2H, d, J 7Hz), 4.81 (1H, broad t, J 7Hz), 5.05 (1H, s), 5.13 (2H, s), 5.64 (1H, d, J 3Hz), 6.33 (1H, dd, J 14 and 7Hz), 7.27 (5H, s).

Example 2

Benzyl 9-0-ethylclavulanate

[0026] 

[0027] A solution of benzyl 9-0-vinylclavulanate (81 mg) in ethyl acetate (10 ml) was hydrogenated over Adams catalyst (10 mg) for 2 hours at room temperature and pressure. The solution was filtered and the filtrate evaporated, and the product separated from starting material by column chromatography (Kieselgel, cyclohexane:ethyl acetate 3:1). Yield 54.8 mg.

[0028] I:rν_max(film) 1805, 1750, 1700cm^-1. N.m.r.δ(CDCl₃) 1.16 (3H, t, J 7Hz), 2.98 (1H, d, J 17Hz), 3.37 (2H, q, J 7Hz), 3.41 (1H, dd, J 3 and 17Hz), 4.00 (2H, d, J 7Hz), 4.79 (1H, t, J 7Hz), 5.04 (1H, s), 5.14 (2H, s) 5.62 (1H, d, J 3Hz), 7.29 (5H, s).

Example 3

Methoxymethyl 9-0-vinylclavulanate

[0029] 

[0030] A mixture of methoxymethyl clavulanate (972 mg), ethyl vinyl ether (10 ml) and mercuric acetate (50 mg) was heated at reflux, with stirring, for 3½ hours. The mixture was allowed to stand at room temperature overnight, a further portion of methyl vinyl ether (3ml) was added, and the mixture was refluxed for a further hour. The solvent was removed under vacuum, and the product was isolated using column chromatography (Kieselgel 60, cyclohexane:ethyl acetate 3:1) Yield 550 mg as a pale yellow oil.

[0031] I.r:ν_max (film) 1810, 1755, 1700, 1635, 1620 cm^-1. N.m.r. δ (CDC13) 3.05 (1H, d, J 17Hz), 3.44 (3H, s), 3.48 (1H, dd, J 3 and 17Hz), 4.97 (1H, dd, J 2 and 7Hz), 4.16 (1H, dd, J 2 and 14Hz), 4.31 (2H, d, J 7Hz), 4.90 (1H, broad t, J 7Hz), 5.06 (1H, broad s), 5.22 (1H, d, J 5Hz), 5.33 (1H, d, J 5Hz), 5.68 (1H, d, J 3Hz), 6.36 (1H, dd, J 7 and 14 Hz).

Example 4

Benzyl 9-0-propenylclavulanate

[0032] 

[0033] A stirred solution of benzyl clavulanate (578 mg) in ethyl propenyl ether (5 ml) was cooled to 0°C and treated with mercuric trifluoroacetate (20 mg). After 2 hours at 0°C, the mixture was allowed to warm to room temperature for 16 hours. The solvent was removed under reduced pressure and the product was isolated by column chromatography (Kieselgel 60, cyclohexane: ethyl acetate 4:1) Yield 246 mg as a pale yellow oil.

[0034] I.r:ν_max (film) 1800, 1750, 1695, 16₇₀ cm^-1.

[0035] N.m.r.: δ (CDCl₃) 1.45 - 1.65 (3H, m), 2.99 (1H, d, J 17Hz), 3.44 (1H, dd, J 3 and 17Hz), 4.1 - 4.5 and 4.6 - 4.95 (4H, m), 5.05 (1H, s), 5.13 (2H, s), 5.62 (1H, d, J 3Hz), 5.8 - 6.2 (1H, m), 7.28 (5H, s).

Example 5

Benzyl 9-0-propyl clavulanate

[0036] 

[0037] A solution of benzyl 9-0-propenylclavulanate (42.2 mg) in ethyl acetate (3ml) was hydrogenated over Adams catalyst (10 mg) for 3 hours at room temperature and pressure. The solution was filtered and evaporated to yield the title compound (40.9 mg).

[0038] 1 spot by t.l.c (cyclohexane:ethyl acetate 3:1)

[0039] I.R. _max(film) 1805, 1755 and 1700cm^-1.

[0040] N.m.r. (CDC1₃) 0.88 (3H, t, J 7Hz) 1.55 (2H, sextet J 7Hz). 2.99 (1H, d, J 17Hz), 3.27 (2H, t, J 7Hz), 3.42 (1H, dd, J 3 and 17Hz), 3.99 (2H, d, J 7Hz), 4.78 (1H, broad t, 7Hz), 5.04 (1H, s), 5.14 (2H, s), 5.62 (1H, d, J 3Hz), 7.78 (5H, s).

Example 6

Sodium 9-0-propyl clavulanate

[0041] 

[0042] A solution of benzyl 9-0-propenylclavulanate (117 mg) in tetrahydrofuran (4 ml) was hydrogenated over 1096 palladium on charcoal (40 mg) at room temperature and presssure for 10 minutes. The solution was filtered through celite and the residue washed with tetrahydrofuran. The combined filtrates were treated with a solution of sodium bicarbonate (29.9 mg) in distilled water (4 ml). The tetrahydrofuran was then removed on a rotary evaporator. The residual aqueous solution was extracted twice with ethyl acetate and filtered through celite. The solution was evaporated on a rotary evaporator and the residue dried over phosporus pentoxide to yield the title compound (77.5 mg).

[0043] N.m.r. (D₂0) 0.84 (3H, t, J 7Hz), 1.53 (2H, sextet, J 7Hz), 3.05 (1H, d, J 17Hz), 3.41 (2H, t, J 7Hz), 3.54 (1H, dd, J 3 and 17Hz), 4.07 (2H, d, J 7Hz), 4.86 (1H, broad t, J 7Hz), 4.91 (1H, s), 5.69 (1H, d, J 3Hz).

Example 7

Pivaloyloxymethyl 9-0-vinylclavulanate

[0044] 

[0045] A mixture of pivaloyloxymethyl clavulanate (700 mg of crude material) ethyl vinyl ether (5 ml) and mercuric acetate (50 mg) was refluxed for 6 hours and left to stand at room temperature overnight. The solvent was removed under vacuum and the product isolated by column chromatography (Kieselgel 60, cyclohexane:ethyl acetate 3:1). Yield 19 mg.

[0046] I.r:ν_max (film) 1810, 1760, 1700, 1635, 162₀ cm^-1.

[0047] N.m.r.: δ (CDCl₃) 1.19 (9H, s), 3.02 (1H, d, J 17Hz), 3.46 (1H, dd, J 3 and 17Hz), 3.96 (1H, dd, J 2.5 and 7Hz), 4.05 - 4.35 (3H, m), 4.84 (1H, broad t, J 7Hz), 5.04 (1H, broad s), 5.66 (1H, d, J 3Hz), 5.71 (1H, d, J 5Hz), 5.78 (1H, d, J 5Hz), 6.35 (1H, dd, J 7 and 14Hz).

Example 8

Methyl 9-0-vinylclavulanate

[0048] 

[0049] A mixture of methyl clavulanate (426 mg), ethyl vinyl ether (5 ml) and mercuric acetate (25 m^g) was heated at reflux for 4 hours. The solvent was removed on a rotary evaporator and the product purified by column chromatography. (Kieselgel, cyclohexane:ethyl acetate 1:1) Yield 168 mg.

[0050] I.r.:ν _max (film) 1800, 1750, 1700, 1632, 1618 cm^-1.

[0051] N.m.r: δ (CDC1₃) 3.02 (1H, d, J 17Hz), 3.46 (1H, dd, J 3 and 17Hz), 3.74 (3H, s), 3.97 (1H, dd, J 3 and 7Hz), 4.16 (1H, dd, J 3 and 14Hz), 4.28 (2H, d, J 8Hz), 4.85 (1H, broad t, J 8Hz), 5.04 (1H, s), 5.66 (1H, d, J 3Hz), 6.36 (1H, dd, J 7 and 14Hz).

[0052] Analysis: Found C 55.19, H5.70, N 5.93% C₁₁H₁₃NO₅ requires C 55.23, H 5.48, N 5.86%.

Example 9

p-Nitrobenzyl 9-0-vinyl-clavulanate

[0053] p-Nitrobenzyl clavulanate (3.34g) was dissolved in dry tetrahydrofuran (30ml). Ethyl vinyl ether (50ml) and mercuric acetate (0.5g) were added to this solution and the resulting mixture was stirred and refluxed (bath temperature 40-50°) with exclusion of moisture for 24 hours. The mixture was filtered and the solvent was evaporated from the filtrate to yield a bright yellow gum. The gum was chromatographed on silica gel (25 g) using ethyl acetate/petroleum ether (b.p. 60-80⁰). The appropriate fractions, which were recognised using t.l.c., were combined and the solvent was evaporated to give the title compound as a pale yellow gum (270mg).

= + 31.6° (c 1.0, CHC1₃). ν_max (CHC1₃): 1802, 1750, 1700, 1620, 1615, 1525, 1350 cm^-1. δ (CDC1₃): 3.07 (1H, d, J 16Hz), 3.51 (1H, dd, J 16 and 2Hz), 3.95-4.15 (2H, m) 4.32 (2H, d, J 8Hz), 4.88 (1H, t, J 8Hz), 5.15 (1H, br.s), 5.28 (2H, s), 5.70 (1H, d, J 2Hz), 6.39 (1H, dd, J 14 and 7Hz), 7.49 (2H, d, J 8.5Hz), 8.22 (2H, d, J 8.5Hz).

Example 10

Lithium 9-0-vinyl-clavulanate

[0054] 

[0055] p-Nitrobenzyl 9-0-vinyl-clavulanate (190mg) was dissolved in tetrahydrofuran (6ml) and to the resulting stirred solution 1M ammonium chloride solution (6ml) and iron powder (0.8g) were added. The mixture was stirred for 20 minutes and then more iron powder (0.8g) and 1M ammonium chloride solution (0.5ml) were added. The mixture was stirred for a further 15 minutes and was then diluted with ethyl acetate (100ml). The resulting mixture was stirred rapidly while hydrogen sulphide was bubbled through it for 5 minutes. The mixture was filtered and the solid was washed well with water. The aqueous layer of the filtrate was saturated with sodium chloride and the mixture was filtered again. The filtrate was treated with 1N HC1 (3m1), was shaken, and the layers were separated. The organic layer was dried (sodium sulphate) and filtered. The resulting solution was extracted with 1/15 M phosphate buffer (pH 7; 3 x 30ml). The combined extracts were overlayed with ethyl acetate (50ml) and were treated with 1N HC1 (5ml). The mixture was shaken and the layers were separated. The organic layer was dried (magnesium sulphate) and the solvent was evaporated under reduced pressure. The resulting residue was immediately dissolved in tetrahydrofuran (5ml). This solution was diluted with water (5ml) and the pH was adjusted to 7 by dropwise addition of 0.1M lithium hydroxide solution (ca 2ml). The neutralised solution was filtered and the solvent was evaporated from the filtrate under reduced pressure. The resulting residue was stirred with a mixture of acetone (3ml) and ether (6ml). The resulting solid was collected by filtration, washed with ether, and dried in vacuo.

[0056] The title compound was thus obtained as a very pale yellow powder which appeared to be 70-80% pure as judged by t.l.c. and n.m.r. ν_max (KBr): 1770, 1690, 1610 cm^-1. δ (D₂O); inter alia: 3.02 (1H, d, J 17Hz), 3.50 (1H, dd, J 17 and 2.5Hz), 4.00-4.25 (2H, m), 5.68 (1H, d, J 2.5Hz), 6.42 (1H, dd, J 14 and 7Hz).

Demonstration of β-Lactamase Inhibitory Activity

[0057] The minimum inhibitory concentrations (MIC) of certain compounds of this invention and of ampicillin, alone or in the presence of compounds of the invention, were determined in vitro for a range of micro-organisms.

[0058] The results are shown in Table 1.

Claims

1. A process for the preparation of a compound of the formula (I):

wherein R₁ is a CH₂CH₃ or CH₂CH₂CH₃ group or a salt or ester thereof, which process comprises the reduction of an ester of a compound of the formula (II):

wherein R₂ is a CH=CH₂ or CH=CH-CH₃ group and thereafter if desired converting the thus-formed ester of the compound of the formula (I) to the corresponding acid or a salt thereof.

2. A process as claimed in claim 1 wherein R₂ is a CH=CH₂ group.

3. A process as claimed in claim 1 or claim 2 wherein the reduction is effected by hydrogenation in the presence of a transition metal catalyst.

4. A process as claimed in claim 3 wherein'the catalyst is platinium oxide or palladium.

5. A process as claimed in any one of claims 1-4 for the preparation of a compound of the formula (I) or a salt thereof wherein the methoxymethyl ester of the compound of the formula (I) is de-esterified by hydrolysis.

6. A process as claimed in any one of claims 1-4 for the preparation of a compound of the formula (I) or a salt thereof wherein the benzyl or p-methoxybenzyl ester of the compound of the formula (I) is de-esterified by hydrogenolysis, optionally in the presence of a base.

7. A process as claimed in any one of claims 1-4 and 6 wherein the hydrogenation of the vinyl or propenyl group and hydrogenolysis of the ester occur at the same time.

8. A compound of the formula (II) as defined in claim 1 or a salt or ester thereof.

9. A compound as claimed in claim 8 wherein R₂ is a CH=CH₂ group.

10. An ester of a compound as claimed in claim 8 or claim 9 wherein the esterifying moiety is a group A or _CHA²_A³ wherein A¹ is an alkyl group of 1-8 carbon atoms optionally substituted by halo en or by 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 _OA⁵ where A⁵ is an alkyl group of up to 6 carbon atoms; and A3 is a phenyl group optionally subsituted by halogen or nitro or by a group A⁵ or OA⁵ where A⁵ is an alkyl group.

11. An ester as claimed in claim 10 such that the corresponding ester of a compound.of the formula (I) as defined in claim 1 is a readily hydrolysable or hydrogenolysable ester.

12. An ester as claimed in claim 11 which is a methoxymethyl, ethoxymethyl, benzyl, methoxybenzyl or nitrobenzyl ester.

13. An ester as claimed in claim 10 which is a p-nitrobenzyl ester.

14. A compound as claimed in claim 8 or claim 9 which is an alkali metal,alkaline earth metal, ammonium or substituted ammonium salt of a compound of the formula (II).

15. A compound as claimed in claim 14 which is a pharmaceutically acceptable salt of a compound of the formula (II).

16. A pharmaceutical composition comprising a compound of the formula (II) or a pharmaceutically acceptable salt or ester thereof and a pharmaceutically acceptable carrier.

17. A composition as claimed in claim 16 which also comprises a penicillin or cephalosporin.

18. A process for the preparation of a compound of the formula (II) as defined in claim 1 or a salt or ester thereof which process comprises the reaction of an ester of clavulanic acid with a compound of the formula (III) or (IV):



wherein R₃ is an alkyl group of up to 6 carbon atoms, in the presence of mercuric ions as catalyst, and thereafter, if desired, de-esterifying the resulting ester of the compound of the formula (II).

19. A process as claimed in claim 18 wherein R₃ is an ethyl group.

20. A process as claimed in claim 18 or claim 19 wherein the catalyst is mercuric acetate or mercuric trifluoroacetate.

21. A process as claimed in any one of claims 18-20 for the preparation of a compound of the formula (II) or a salt thereof comprising the cleavage of the corresponding p-nitrobenzyl ester.

22. A process as claimed in claim 21 wherein the cleavage of the ester is effected using a reducing agent which reduces the nitro group to an amino group.

23. A process as claimed in claim 22 wherein the reducing agent is iron powder/ammonium chloride solution.

24. A compound of the formula (I) or a salt or ester thereof when prepared by the process of any one of claims 1-7.

25. A process for the preparation of a compound of the formula (I) or a salt or ester thereof substantially as described with reference to any of Examples 2, 5 or 6 herein.

26. A compound of the formula (II) or a salt or ester thereof when prepared by the process of any one of claims 18-23.

27. A process for the preparation of a compound of the formula (II) or a salt or ester thereof substantially as described with reference to any of Examples 1, 3, 4, 7, 8, 9 and 10 herein.

28. A compound of the formula (II) or a salt or ester thereof as described in any of Examples 1, 3, 4, 7, 8, 9 and 10 herein.