GB2177698A - Improvements in the preparation of antibiotics - Google Patents

Abstract

Ethoxycarbonyloxyethyl esters of 6-APA or of a penicillin or of a cephalosporin are obtained by reacting the 6-APA, penicillin or cephalosporin or a salt thereof with alpha-bromodiethylcarbonate.

Description

SPECIFICATION Improvements in the preparation of antibiotics Field of the invention This invention relates to a novel method of manufacturing the I-ethoxy-carbonyloxyethyl ester of the 6-(D-( )-a-amino-os-phenylacetamido) penicillanic acid of formula I::

Furthermore, the invention relates to - the novel compound a-brnmodiethylcarbonate, which with great advantage is used in the said novel methodforpreparing bacampicillin oftheformula I, and which in a more general sense is also used with great advantage in the preparation of the ethoxycarbonyloxyethyl ester of 6-aminopenicillanic acid, penicillins and cefalosporins - novel methods for the preparation of a-bromodiethylcarbonate - novel intermediates in the preparation of a-bromodiethylcarbonate - the use of a-bromodiethylcarbonate in the preparation of the ethoxy-carbonyloxyethyl ester of 6aminopenicillanic acid, penicillins such as penicillin G, penicillin V and ampicillin, and cefalosporins - improvements in the process for preparing ethoxycarbonyloxyethyl esters of 6-aminopenicillanic acid, penicillins and cefalosporins.

The substance I concerned is an ampicillin ester which is extremely importantfrom the therapeutic point of view since it is well absorbed when administered orally and gives much higher blood levels ofampicillinthan ampicillin itself.This ester is isolated in the form of a hydrochloride and is known as bacampicillin hydrochloride.

Background of the invention On the basis of previous known processes (cf. Belgian patent No. 772723), bacampicillin hydrochloride can be synthesized bythetwofollowing methods: A) Reaction of potassium benzylpenicillin with a-chlorodiethylcarbonate in organic solvents or in an aqueous solution of 70% dioxane in the presence of sodium bicarbonate. The l-ethoxycarbonyloxyethyl ester of benzylpenicillin which is obtained is subjected to the reaction of removing the phenylacetic chain, via the iminochloride-iminoether, in orderto obtain the l-ethoxycarbonyloxyethyl ester ofthe 6-aminopenicillanic acid, which is isolated as the hydrochloride.

By subsequent condensation of the latter intermediate with D-(-)--phenylglycine, the compound according to formula I is obtained.

B) Esterification reaction of the 6-(D-(-)-a-azido-o-phenylacetamido)- pencillanic acid with os- chlorodiethylcarbonate in a polar solvent.

Subsequently, by catalytic hydrogenation of the l-ethoxycarbonyloxyethyl ester ofthe g-(D(-)-cr-azido-or- phenylacetamido) penicillanic acid the compound according to formula I is obtained.

As one can see, these methods are rather complex since they involve the use of numerous raw materials and lengthy processing times.

The invention A prime object of this invention is to provide a method of preparing the active substance concerned which is easierto carry out and industrially more advantageous. A more specific object ofthis invention isto provide a method of preparing bacampicillin using ampicillin as starting material,with considerable simplification of the said method and obtaining a high degree of purity of the desired product.

The invention also provides the novel compound a-bromodiethylcarbonate, novel methods forthe prepar- ation thereof; novel intermediates in the preparation of cw-bromodiethylcarbonate; the use of abromodiethylcarbonate in the preparation of the ethoxycarbonyloxethyl ester of 6-amino-penicillanic acid and penicillins such as penicillin 0, penicillin V and ampicillin; and improvements in the processforprepar- ing ethoxy-carbonyloxyethyl esters of 6-aminopenicillanic acid, pencillins and cefalosporins.

a-Bromodiethylcarbonate is used with great advantage as a reactant in these esterification processes. The use ofa-brnmodiethylcarbonate leads to particularly high yield and high purity ofthefinal products such as bacampicillin.

It is possible to achieve the said prime object of preparing the l-ethoxycarbonyloxyethyl ester ofthe 6-(D(-)- a-amino-a-phenylacetamido) pencillanic acid having the following formula:

characterized bythefollowing stages: a) reacting of ampicillin, preferably in the form of an alkaline salt, with a reactive derivative of acetoacetic acid to form the corresponding enamine having the following formula::

R1 represents an alkyl group containing 1 to 4 carbon atoms, a substituted or unsubstituted aryl group or an aralkyl group; R2 represents hydrogen, an alkyl group containing 1 to 4 carbon atoms, a substituted or unsubstituted aryl group oran arylkyl group; R3 represents an alkyl group containing 1 to 4 carbon atoms, a substituted or unsubstituted aryl group, an arylkyl group, an alkoxy group containing 1 to 4carbon atoms, an aryloxy group or an amino group,and Xrepresentsan alkali metal, an alkaline-earth metal or an organic base; b) reaction of the resulting intermediate with an a-bromo-diethylcarbonate having the following formula:

toformthecorresponding ester having thefollowing formula::

where R1, R2 and R3 have the same significance as above and c) hydrolysis in an acid medium, obtaining the compound according to formula (I).

The esterification reaction between the compounds II and Ill can be carried outwith our without an esterification catalyst present.

The addition of a catalyst at this stage considerably shortens the reaction times and provides higheryields ofthe productwith a greater degree of purity.

For this purpose the following substances can be used as catalysts: quaternary ammonium salts,forexampletetrabutylammonium bromide, the bromides or iodides of alkali metals and; cyclic ethers.

The catalyst may be used in an amount which varies from 0.005 to 0.10 moles per mole of compound Illto amounts which are equimolarwith the compound Ill. In a preferred embodimenttetrabutylammonium bromide is used in an amountoffrom 0.01 to 0.10 moles permoleofcompound Ill.

The invention also includes an embodiment of the process outlined above forthe preparation of bacampici- llinwhich comprises reacting a compound oftheformula II with a compound oftheformula

wherein Z is Cl or I, which embodiment is characterized inthatthe process is carried out in the presence of a catalyticamountofa catalyst as specified above. The catalyst is suitably used in an amount offrom 0.005 to 0.10 moles per mole of compound V.

Illustrative examples ofthe radicals R1, R2 and R3are: alkyl: CH?, C2H5, n-C3H7, i-C3H7, n-C4Hg

OCH3, OC2Hs, OCH2CH2CH3, OCH(CH3)2, O(CH2)3CH3 substituted aryl: phenyl substituted with halogen such as Cl and 8r

he he radical x is selected among groups which are well known in the art,forexample alkali metal: Na, K alkaline earth metals: Ca, Mg organic base: organic bases which are known in the synthesis of penicillins, e.g.tertiaryammonium groups, triethylamine, ethylpiperidine and methylmorpholine.

In the preferred embodiment of the invention, the group protecting the amino group of the ampicillin is a l-methoxy-carbonyl-propen-2-yl group or a l-ethoxy-carbonyl-propen-2-yl group for which the preferred intermediate is the sodium or potassium salt of the N-(-l-methoxy-carbonyl-propen-2-yl) penicillanic acid re spectively N-(l-ethoxy-carbonylpropen-2-yl penicillanic acid according to formula II (R1 + methyl; R2 + methyl; R3 + methoxyorethoxy and X = Na or K).

The intermediate IV is stable in a neutral or alkaline medium, whereas in an acid medium it is possibleto remove the group protecting the amino group simply, quickly and selectively.

The group protecting the amino group ofthe ampicillin can be selected e.g.from the groups mentioned in the British patent specification 991586, and from other groups which are known in the art.

The cu-bromodiethylcarbonate, compound Ill, which is a novel compound and as such included in the scope ofthe invention may be prepared by reacting the corresponding a-chlorodiethylcarbonatewith sodium bromide as is exemplified in Example 1 below.

More specifically, therefore, the process method according to a preferred embodiment ofthis invention, comprises the following stages: - tra nsformation of ampicil lin trihydrate in a polar solvent, for example N, N-dimethylformamide, into a salt thereof, for example potassium,and subsequent formation ofthe corresponding enamine (Il) by reaction with a derivative of acetoacetic acid, for example methyl acetoacetate.

- addition of an esterification catalyst, preferably tetrabutylammonium bromide -addition of a-bromdiethylcarbonate to the reaction mixture to form the l-ethoxycarbonyloxyethyl ester of the ampicillin in the form ofthe enamine (IV).

- hydrolysis of the protective group with HCI diluted in an organic solvent, for example n-butyl acetatel water.

-recovery ofthe bacampicillin hydrochloride by saturation in the aqueous phase, for example with sodium chloride and extraction with a suitable solvent, for example n-butyl acetate.

- concentration of the solution at low pressure in n-butyl acetate in orderto crystallize the productto a high level of purity, the productthen being isolated byfiltration.

Among the main advantages ofthe process according to the invention, the principal one is that, bythis process, it is possible to obtain bacampicillin hydrochloride practically in one operation and with a high degree of purity.

In fact the impurities which are present in the product obtained by the process according to the present invention are negligible as compared with the known processes of the previous state ofthe art.

Another equally important advantage is that ampicillin trihydrate is used as the starting material, this being a known antibiotic which is easily obtainable in pure form and at low cost.

The intermediate (II) can be easily prepared as described for example in British patent specification 991586 with a yield of over95% by reaction of ampicillin trihydrate with methyl or ethyl acetoacetate, 10 to 50% more than the stoichiometric ratio, in the presence of an organic base or an alkali metal carbonate, forexample potassium carbonate.

The intermediate (II) can be isolated and added to the esterification reaction in solid form. Or, without isolation of the intermediate (II), the esterification reaction can be effected in the same solvent in which the reactionfortheformation ofenamine (II) took place.

The reaction for the formation ofampicillin enamine (Il) is conducted in an aprotic polarsolvent, such as N, N-dimethylacetamide, N, N-di-methylformamide, dimethoxyethane, dimethylsulphoxide, tetrahydrofuran or dioxane.

To complete the reaction, it is sufficientto leave the components of the mixture in contact at a temperature between 0 C and 60"C, preferably between 20"C and 30 C,for2to 8 hours, preferably3 hours.

The compound II can be prepared via acylation of 6-aminopenicillanic acid with a corresponding enamine derivative of phenylglycine to the formation of the compound II which thereafter can be esterified directly and converted to bacampicillin with isolation ofthe compound II.

The esterification reaction after the addition of the a-bromdiethylcarbonate to the said mixture, takes place at a temperature between 1 5 and 80"C, preferably between 45'C and 55"C, for a period of time from 1 hourto 24 hours, preferably from Sto 10 hours.

The esterification reaction is suitably carried out in an organic solvent such as methylene chloride oracetone, dimethylacetamide, dimethylformamide and dimethylsulfoxide,or in a mixture of organic solvents. It is possible to use also organic solvent containing water. The use of esterification catalyst is desirable when acetone is used as solventforthe esterification reaction.

In the easiest and most suitable conditions for industrial purposes, the esterified enamine (IV) is isolated by dilution of the reaction mixture with water and subsequent extraction with a suitable solvent which is immiscible with water, for example n-butyl acetate.

The acetate phase is agitated with a dilute solution (0.2 - 0.3N) of HCl until the protective group is completely hydrolysed, which requires a contacttime of 2 to 8 hours, preferably 4-5 hours, atordinarytemperatures.

By addition of sodium chloride, compound (I) separates outfrom the aqueous phase in the form ofthe hydrochloride, which is extracted with a suitable solvent, for example n-butyl acetate.

By concentrating the organic phase at low pressure at a temperature of 40"C until a small volume remains, crystallization of the product according to formula (I) takes place.

The crystalline product is isolated by filtration, washing and vacuum drying.

Thefollowing examples illustrate the present aspects of the invention without limiting it in anyway.

Example 1 Preparation ofa-bromdiethylcarbonate acetone

Sodium bromide (102.9 g) dissolved in aceton (600 ml) was reacted for 2-3 hours at ambienttemperature (20-25"C) with e-chlorodiethylcarbonate (152.6 g) dissolved in 100 ml of acetone. The mixture was then con- centrated undervacuum at low temperature, max. 350C, until a semi-solid mass was obtained. The reaction mixture was then partitioned with H2O/ethyl ether. The aqueous phase was separated and was then extracted twice with 400 ml of ethyl ether.

The combined organic phases containing the a-bromdiethylcarbonate were washed with 800 ml of H20 1000 ml of 1% sodium metabisulphate aqueous solution 1000 ml of NaCI saturated solution The organic phase was dried over Mg sulphate, and then concentrated under vacuum at low temperature, max. 350Cto give the title product (60%) intheform ofa liquid which initially was colourless orslightlyyellow-brown.

It was used directly in the esterification step according to Example 2 below.

Example2 25.8g (0.181 m) offinelyground anhydrous potassium carbonate are suspended in 200 ml of N, Ndimethylacetamide and 32.4 ml (0.3 m) of methyl acetoacetate and 60.4 g (0.15 m) pf ampicillin trihydrate are added.

The mixture is maintained under fast agitation for 5 hours at 20"C - 25"C; afterthis time 46.1 g (0.234 m) of bromdiethylcarbonate, 6 g (0.02 m) oftetrabutyl ammonium bromide and 100 ml of N, N-dimethylacetamide are added.

It is heated under agitation for 10 hours at 400C - 42'C; the reaction mass is poured into a mixture consisting of 1200 ml of water and 400 ml of n-butyl acetate.

The aqueous phase is collected and extracted with another 100 ml of n-butyl acetate.

The reunited organic phases are washed twice with 100 ml of water each time. 150 ml N HCI and 370 ml of water are added to the organic phase which is subjectedto agitation, it is left under agitation at 220C - 23 Cfor 4 hours.

The aqueous phase is collected and the organic phase is extracted with 100 ml ofwater.

The reunited aqueous phases are broughtto pH 4with a 10% aqueous solution of Na2CO3, then bleaching carbon is addedto them and they are filtered.

300 ml of n-butyl acetate and 80 g of sodium chloride are added to the aqueous filtrate.

The organic phase is separated and the aqueous phase is extracted with 200 ml of n-butyl acetate.

The reunited phase in n-butyl acetate are concentrated at low pressure at 40'C to a volume of approximately 300 ml. The product is leftto crystallize for 15 hours at +50C.

It is filtered, washed with n-butyl acetate (100 ml) and ethyl acetate (100 ml). It is vacuum dried at 400C for 24 hours.

Yield: 54.2 g (72%) ofthe l-ethoxycarbonyloxyethyl ester ofthe 6-(D(-)-a-amino-a-phenylacetamido) plenci Ilanic acid with m.p.160-2 C. (d) and characteristics conforming to the authentic hydrochloride sample.

Example 3 36.4 g (0.075 m) of potassium N-(l-methoxycarbonyl-propen-2-yl)-6-[D(-)-a-amino-a-phenylacetamido) penicillate are added to a solution of 17.8 g (0.116 m) of a-chlorodiethylcarbonate and 3 g (0.01 m) oftetra butylammonium bromide in 150 ml of N, N-dimethylformamide. Under agitation the temperature is raised to 45 C and maintained at45 C - 500for 5 hours.

When heating is completed, the reaction mixture is poured into a mixture comprising 300 ml of a 14% aqueous sodium chloride solution and 600 ml of n-butyl acetate. The mixture is agitated for 10 minutes,then the organic phase is separated and the aqueous phase is extracted with 100 ml of n-butyl acetate. The reunited organic phases, aftertwo washings with 75 ml of 14% sodium chloride aqueous solution, areconcentrated at low pressure until an oil is obtained.

The oil is mixed with 200 ml I ml oftetrahydrofuran and 100 ml of water; the solution obtained (pH 4.8) is brought under agitation to pH 1.5 by adding, in all, 12 ml of 6N HCI in 1 hour.

AFter leaving the solution to stand for another houratordinarytemperature, thetetrahydrofuran is removed at low pressure at 400C, 150 ml of n-butyl acetate are added to the remaining aqueous phase (150 ml) and then 15g of sodium chloride are added.

The organic phase is separated and the aqueous phase is extracted with 100 ml of n-butyl acetate.

The reunited organic phases are concentrated undervacuum at 400C to a volume of 120 ml.

The product is left to crystallize for 15 hours at50C.

It is then filtered, washed with n-butyl acetate (50 ml) and ethyl acetate (50 ml).

It is vacuum dried at 40"C.

The following is obtained: 25.2 g (66.9%) of the l-ethoxycarbonyloxyethyl ester of the 6-(D-(-)-a-amino-a- phenylacetamido) penicillanicacid hydrochloride with m.p. 160-2"C.

Analytical determinations: Titre: 97.82% Rotatory power: + 166.30 (c =1, EtOH950) pH: 4.05(2% aqueous solution) Moisture content: 0.82% Residual solvents: ethyl acetate 0.45; n-butyl acetate 0.98% IR and NMR spectra are standard Residual ampicillin: 0.06% Example 4 16.2 ml (0.15 ml) of methyl acetoacetate and 30.2 g (0.075 m) of ampicillin trihydrate are added to a suspension of 12.54 g (0.0907 m) offinely pulverized anhydrous potassium carbonate in 100 ml of N, Ndimethylformamide.

It is maintained with agitation at 22"C-23"C for 3 hours and after this time considerable fluidization ofthe mass can be observed.

17.8 g (0.117 m) ofa-chloro-diethylcarbonate,3g (0.01 m) oftetrabutylammoniumbromide and 50 ml of N, N-dimethylformamide are now added in that order.

The mixture is heated underagitationfor 5 hours at450C - 500C,then leftto stand at + 50for 15hours.

The reaction mass is poured into a mixture consisting of 600 ml of water and 200 ml of n-butyl acetate and it is agitated until a complete solution is obtained, the aqueous phase is collected and extracted with another 50 ml of n-butyl acetate.

The reunited organic phases are washed twice with 50 ml ofwatereach time. 75 ml of N HCI and 185 ml of water are added to the organic phase subjected to agitation; it is leftunderagitation at220C-230Cfor4hours.

The aqueous phase is collected and the organic phase is extracted with 50 ml of water. The reunited aqueous phases are broughtto pH4with a 10% aqueous solution of Na2CO3, then bleaching carbon is added to them and theyare filtered.

150 ml of n-butyl acetate and 40 g sodium chloride are added to the aqueous filtrate.

The organic phase is separated and the aqueous phase is extracted with 100 ml of n-butyl acetate.

The reunited phases in butyl acetate are concentrated at low pressure at40"Cto a volume of approximately 150 ml.

The product is left to crystallize for 15 hours at +50C.

It is filtrated, washed with n-butyl acetate (50 ml) and ethyl acetate (50 ml).

It is dried under avacuum of 10 mm Hg in the presence of moisture at 25 C for 24 hours.

Yield: 20.8 g (55%) ofthe l-ethoxycarbonyloxyethyl ester of the 6-(D(-)-a-amino-a-phenylacetamido) penicillanic acid hydrochloridewith m.p. 159-161"C and characteristics conforming to an authentic sample.

Example 5 A mixture of 160 ml acetone, 22.6 g (0.075 mol) of the potassium salt of D(-)-N-methoxycarbonylpropen-2yl-aminophenylacetic acid, 6.9 ml (0.088 mol) ethyl chloroformate and 3 drops of N-methylmorpholine, is stirred for 15 minutes ata temperature of -20--30"C. Tothis reaction mixture a solution of 16.2 g 6aminopenicillanic acid, dissolved in 35 ml waterthrough gentle addition of 7.6 g (0.075 mol) triethylamine with agitation, is added in one portion, after which the mixture is diluted with 90 ml acetone and chilled to -20 C.

After stirring for45 minutes, without any additional cooling, 23.4g (0.117 mol) ofa- bromodiethylcarbonate, 3 g (0.01 mol) ortetrabutyl-ammonium bromide and 250 ml of N,Ndimethylformamide are added in that order. The mixture is stirred for 18 hours at 25"C. After that time the reaction mass is poured into a mixture consisting of 600 ml of water and 200 ml of n-butyl acetate and it is agitated until a complete solution is obtained. The aqueous phase is collected and extracted with another 50 ml of n-butyl acetate.

The reunited organic phases are washed twice with 50 ml ofwater each ti m e. 1 85 m I of waters added to the organic phase and I N HCI is added dropwise with agitation to a pH of 1.9. The mixture is left under agitation at 22-23"Cfor4 hours.

The aqueous phase is collected and the organic phase is extracted with 50 ml of water. The reunited aqueous phases are brought to pH 4 with a 10% aqueous solution of Na2CO3, active carbon is added to them and they are filtered. 150 ml of n-butyl acetate and 40 g of sodium chloride are added to the aqueous filtrate.

The organic phase is separated and the aqueous phase is extracted with 100 ml of n-butyl acetate. The reunited phases in butyl acetate are concentrated at low pressure at400C to a volume of approximately 150 ml. The product is left to crystallize for 15 hours at +5 C.

It is filtered, washed with n-butyl acetate (25 ml) and ethyl acetate (25 ml). It is dried under a vacuum of 10 mm Hg at 25 Cfor 24 hours.

Yield:1.17 g ofthe l-ethoxycarbonyloxyethyl ester of6-D(-)-a-amino-a-phenylacetamidopenicillanic acid hydrochloride with m.p. 159-161 C and characteristics (NMR, TLC) conforming to an authentic sample.

Example 5a The procedure of example was repeated with the differencethatthe 6-aminopenicillanic acid was dissolved in 20 ml water instead of in 35.

Yield:1.05 g of the ethoxycarbonyloxyethyl ester of 6-(D(-)-a-amino-a-phenylacetamidopenicillanic acid hydrochloride as a white crystalline powder with m.p. 148-151 C, with decomposition, and characteristics (TLC, IR) conforming to an authentic sample.

Example 6 6.25 g (0.045 m) offinely ground anhydrous potassium carbonate are suspended in 50 ml of dimethyl sulphoxide and 8.1 (0.075 m) of methyl acetoacetate and 15.1 g (0.0375 m) of ampicillin trihydrate areadded.

The mixture is maintained underfastagitation for 5 hours at20 C -25 C; afterthistime 11.5 g (0.059 m)of bromodiethylcarbonate and 25 ml of dimethyl sulphoxide are added.

It is heated under agitation for 17 hours at 35-37C; the reaction mass is poured into a mixture consisting of 300 ml of water and 100 ml of n-butyl acetate.

The aqueous phase is collected and extracted with another 100 ml of n-butyl acetate.

The reunited organic phases are washed twice with 25 ml of water each time.

92.5 ml ofwaterand NHCl (7.0 ml) to a pH of 1.9 are added to the organic phase which is subjected to agitation; it is left under agitation at220C - 23"C for 2.5 hours.

The aqueous phase is collected and the organic phase is extracted with 25 ml ofwater.

The reunited aqueous phases are brought to pH 4with 10% aqueous solution of Na2CO3, then active carbon is added to them and they are filtered.

75 ml of n-butyl acetate and 37 g of sodium chloride are added to the aqueous filtrate.

The organic phase is separated and the aqueous phase is extracted with 50 ml of n-butyl acetate.

The reunited phases in n-butyl acetate are concentrated at low pressure at 400C to a volume of approximately 75 ml. The product is leftto crystallize for 15 hours at +5 C.

It is filtered, washed with n-butyl acetate (25 ml) and ethyl acetate (25 ml). It is vacuum dried at 400C for3 hours.

Yield: 1.9 g (10%) of the l-ethoxycarbonyloxyethyl ester of the 6-D(-)-a-amino-a-phenylacetamido) penicillanic acid with m.p. 1 60-1 62"C and characteristics conforming to an authentic sample ofthe hydrochloride (e.g. IR :V 1790 cm~', t3-lactamcarbonyl).

The novel compound a-bromdiethylcarbonate ofthe invention, novel and inventive, novel and inventive processes for the preparation thereof and its use in the preparation of the ethoxycarbonyloxyethyl esters of 6-aminopenicillanic acid, penicillins such as penicillin G, penicillin V and ampicillin, and cefalosporins,will now be described and exemplified in more detail.

This aspect of the invention is concerned with improvements in and relating to the preparation of a- bromodiethylcarbonate ofthe formula:

The alpha-bromo diethylcarbonate oftheformula (III) may according to afurther aspect ofthe invention, which is further dealt witch later, be used in the synthesis of alpha(ethoxycarbonyloxy)-ethyl esters of 6 amino-penicillanic acid, penicillins and cefalosporins,forexamplethe antibiotic becampicillin. Alpha- bromodiethylcarbonate may thus advantageously be used in the preparation ofthe ethoxycarbonyloxyethyl esters of 6-aminopenicillanic acid, penicillin G, penicillin V and ampicillin.

According to the invention two novel and inventive processes, herebelow denoted process A and process B, are provided forthe preparation of alpha-bromodiethylcarbonate oftheformula Ill.

A. The first of these processes, process A, comprises the steps of: (a) reacting an aldehyde oftheformula CH3CHO VI with carbonyl bromide COBr2 VII to give an alpha-bromo-bromoformate oftheformula:

and; (b) reacting the alpha-bromo-bromoformate offormula VIII wfth an alcohol oftheformula C2H5-OH to yield the desired alpha-bromo-diethylcarbonateoftheformula Ill.

Thus, the process A in accordance with the invention may be summarised by the reaction scheme:

The alpha-bromo-bromoformate oftheformula VIII is in itselfa new compound and is provided as afurther feature ofthe invention.

The reaction between the aldehyde, CH3CHO, and carbonyl bromide is most suitably carried out in the presence of a catalyst which may be,forexample, atertiary amine (for example a tertiaryaliphatic amine,a tertiary mixed alkyl/aryl amine ora tertiary aromatic amine), tertiary phosphine, amide, substituted urea or thiurea, phosphoric acid amide, tertiary oxonium orsulphonium salt, our a quaternary ammonium orphosph- onium salt. Preferred examples of catalysts for use in the process A according to the invention include pyridine, dimethylformamide, tetra-n-butyl urea, hexamethyl-phosphoric-tri-amide and benzyltrimethyl ammonium bromide.

The catalyst is suitably used in an amount of from 0.05 to 0.5, preferably from 0.05 to 0.15, moles of catalyst per mole of aldehyde.

The reaction between the aldehyde and the carbonyl bromide is suitably carried out in the presence of a solventwhich may be,forexample, an aromatic hydrocarbon such astoluene or a halogenated hydrocarbon such as dichloromethane, carbon tetrachloride orchlorobenzene. The reaction between the aldehyde and the carbonyl bromide is suitably carried at a temperature offrom -40to 120"C, preferably 0 - 40"C. The carbonyl bromide will usually be used in molarexcesswith respectto the aldehyde, suitably in a molarexcessoffrom 10 to 100%, preferably from 20 to 50%.

The intermediate alpha-brnmo-brnmoforrnate offormula VIII produced instep (a) of the process A ofthe invention need not be isolated prior to reaction with the alcohol CPHSOH and, indeed, it is generally preferred not to do so. Thus, in accordance with a preferred embodiment of the invention,the reaction mixture obtained from step (a) is freed of excess carbonyl bromide, for example by warming under reduced pressure or by purging with nitrogen. The crude alpha-bromo-bromoformate-containing reaction mixture is then reacted with an excess of the alcohol. The reaction may conveniently be effected by heating the mixture under reflux until the evolution of hydrogen bromide ceases or by adding a tertiary baseto the mixture and, if necessary, warming it.Any residual catalystfrom step (a) or its complex with carbonyl bromide does notappearto interfere with the subsequent reaction and, in some cases, appears beneficial.

The resultant crude alpha-bromocarbonate may conveniently be isolated from the reaction mixture by fractional distillation under reduced pressure.

ProcessAis illustrated to Examples7 and 8, which are given byway of illustration only.

B. The second process, process B, ofthe invention forthe preparation of a-bromodiethylcarbonatewill now be described. Method B is exemplified in Example 9, which is given byway of illustration only.

Process B of the invention is concerned with improvements in and relating to the preparation of a bromodiethylcarbonate by a modification of the Finkelstein reaction, that is by reactionof an alkyl chloride or arylalkyl chloride (or a compound containing such a group) with an alkali metal bromide or alkali metal iodide to replace the chlorine substituent by a bromine or iodine substituent respectively; or by the reaction of an alkyl bromide orarylalkyl bromide (or a compound containing such a groupl) with an alkali metal iodideto replacethe bromine substituent by an iodinesubstituent The Finkelstein reaction is useful since the resulting iodides are generally more reactive than the bromides which in turn are more reactivethan the chlorides. In some cases only catalytic amounts ofthe alkali metal bromide or iodide are necessary and the resulting more reactive species is allowed to react with the desired substrate regenerating the alkali metal bromide or iodide, thus continuing the reaction.

Not all optionally substituted alkyl chlorides or arylalkyl chlorides undergo the reaction and, in particular, it has been found difficult to carry outthe reaction with alpha-chloro esters and alpha-chloro-carbonates,that is compounds in which the chlorine atom is attached to a carbon atom which is, in turn, attached to either end of a groupl -C(O)-O-. An example of such an alpha-chlorocarbonate is a-chlorodiethyicarbonate, which is a known intermediate in the preparation of ethoxycarbonyloxyethyl esters of 6-aminopenicillanic acid and of penicillins as described above.

It has now been found, in accordance with the present invention, that this problem may be overcome by carrying outthe reaction using a two-phase solvent system, one phase of which is water and the other is a water-immiscible organic solvent, in the presence of a phase transfer catalyst.

According to process B ofthe invention, therefore, there is provided a process for the preparation of abromodiethylcarbonate by reaction of os-chlorodiethylcarbonatewith an alkali metal bromide,which process is characterized in that the reaction is carried out in a two-phase solvent system comprising water and a water-immiscible organic solvent in the presence of a phase transfer catalyst.

Suitable water-immiscible organic solvents for use in accordance with the invention include halogenated hydrocarbons, for example halogenated paraffins such as dichloromethane; and aromatic hydrocarbons such astoluene. Suitable phasetransfercatalysts include quaternary ammonium salts, for exampletetraalkyl ammonium salts such as cetyltrimethyl ammonium bromide and tetra-n-butyl ammonium hydrogen sul phate.The alkali metal bromide may,forexample, be sodium, potassium, or lithium bromide, lithium bromide being preferred.

Thus, in process B ofthe invention, a-chlorodiethylcarbonate of the formula:

is reacted in a two-phase solvent system, one phase of which is water and the other is water-immiscible organic solvent, with an alkali metal bromideoftheformula R-Br X in which formula R is an alkali metal such as Na, Kand Li, to the formation ofthecompound oftheformula:

As noted above, the preferred alkali metal R is Liso sothatLiBrisa preferred reagent of the formula X.

In connection with process B it has been found that lithium bromide may be used with advantage in a conventional Finkelstein reaction (i.e. one employing a single phase organic solvent system), for exampleto halogenate an alpha-chloro-carbonate. This method is exemplified in Example 10.

Accordingly, the present invention also provides, in accordance with a further embodiment thereof, a process forthe preparation ofa-bromodiethylcarbonate which comprises reacting a-chlorodiethylcarbonate with lithium bromide.

Suitable solvents for such a process include lower aliphatic alcohols, lower alphatic ketones, lower aliphatic ethers and lower aliphatic amides of formic acid.

The aspect ofthe invention which relates to the use of a novel compound a-bromodiethylcarbonate in the preparation of ethoxycarbonyloxy-ethyl estes of6-aminopenicillanic acid, penicillins and cefalosporins, will now be described.

In summary, this aspect ofthe invention comprises 1.the use of a-bromodiethylcarbonateinthe in the preparation of the ethoxycarbonyloxyethyl esters of 6- aminopenicillanic acid, penicillins such as penicillin G, penicillin V and ampicillin, and cefalosporins.

2. a process for the preparation ofthe ethoxycarbonyloxyethyl ester of6-aminopenicillanic acid, penicillins and cefalosporins, characterized by reacting 6-aminopenicillanic acid, the penicillin orthe cefalosporin, or a saltthereof, with (x-bromodiethylcarbonate to the formation of the ethoxycarbonyloxyethyl ester of the 6 aminopenicillanic acid, the penicillin and thecefalosporin, respectively.

3.The improvement in the esterification reaction between an a-halogerdiethylcarbonate and 6-apa, a penicillin era cefalosporin, which improvement comprises the use of a quaternary ammonium compound atthe esterification step, whereby the said quaternary ammonium compound is present in an amount of 1 -25, preferably 1 -10% ofthe equimolaramountwith respectto the amount of 6-apa, penicillin orcefalosporin.

The ethoxycarbonyloxyethyl ester in particular of 6-apa and of penicillin G are used as in known in the art in the preparation of any desired such semisynthetic penicillin ester by acylating the 6-NH2 group afterremov- ing the side chain in e.g. the penicillin G ester obtained.

This aspect of the invention is concerned with improvements in and relating to the preparation of esters by the reaction of salts of carboxylic acids with a-bromodiethylcarbonate.

The reaction of metal salts ofcarboxylicacidswith alkyl halides or arylalkyl halides to form esters iswell known. However, yields are not particularly high and the reaction generally requires forcing conditions such as high temperatures and/or extended reaction times. These forcing conditions limitthe synthetic utility of the reaction and its commercial applicability to heat sensitive and labile substances such as pyrethroids, prostaglandins, peptides, penicillins and cephalosporins.

The British patent specification 1443738 discloses the use of a quaternary ammonium salt of penicillins and cefalosporins in place of a metal salt thereof in the preparation of esters of penicillins and cefalosporins.

The preparation ofthe quaternary ammonium salt ofthe acid may be time-consuming and expensive.

However, as is also disclosed in the British patent specification 1443738, it is not necessary to first preparethe quaternary ammonium salt of a penicillin orcefalosporin, butthe reaction may be carried out by reacting a metal saltofthe carboxylicacid,that is the 6-apa, penicillin orcefalosporin with the alkyl orarylalkyl halide in the presence of a quaternary ammonium salt, otherthanthe saltofthe carboxylicacid.

It is now been found, according to the present invention, that it is not necessary to employ the said quater naryammonium salt in a stoichiometric amountwith respecttothe carboxylic acid, that is 6-apa, the pen icillin orthe cefalosporin, butthata less than stoichiometricamountwith respect to the carboxylic acid, e.g.

the 6-apa, penicillin or cefal osporin, wi 11 be sufficient.

According to the invention, therefore, there is provided a process for the preparation of an ethoxycarbonyloxyethyl ester of 6-a pa, a penicillin or a cefalosporin by reaction of a metal salt or the 6-apa, penicillin orcefalosporin with a-halogendiethylcarbonate in the presence of a quaternaryammonium salt (otherthan a salt of the said carboxylic acidl whereby the quaternary ammonium compound is present in a lessthan stoichiometric amountwith respectto the 6-apa, penicillin orcefalosporin.

In accordance with the invention, between 1% and 25%fan equivalent ofthe quaternary ammonium salt is used for each equivalent ofthe metal salt ofthe carboxylic acid, and more preferably between 1 % and 10% of an equivalent ofthe quaternary ammonium salt is used.

The quaternary ammonium salt of the carboxylic acid is suitably prepared by reaction of a metal salt ofthe carboxylic acid with a quaternary ammonium salt of an acid otherthan said carboxylic acid, typically a mineral acid such as hydrochloric, hydrobromic or sulphuric acid.

Suitable metal salts of carboxylic acids four use in accordance with the present aspect of the invention (either as precursorsforthe carboxylic acid quaternaryammonium saltoras such) are alkali metal or alkaline earth salts such as sodium, potassium, lithium, magnesium and calcium salts. Suitable quaternary ammonium salts of acids otherthan the carboxylic acid (for use either as precursors forthe carboxylic acids quaternary ammonium salts or as such) include for example tetra-alkyl ammonium salts such astetra-n-butyl ammonium bromide and cetyltrimethyl ammonium bromide and quaternary pyridinium salts such as cetylpyridinium bromide.Suitable halides include fluorides, chlorides, bromides and iodides, preferablyactivated fluorides or activated chlorides or bromides or iodides.

The esterification reaction in accordance with this aspect ofthe invention may be carried out in the presence or absence of a solvent. Suitable solvents include lower aliphatic alcohols, lower aliphatic ketones, lower aliphatic amides of formic acid and dimethyl sulphoxide. Alternatively, when no solvent is used, an excess ofthe esterforming halide may be used, particularly if this is a liquid atthetemperature ofthe reaction.

In the previously described aspect ofthe invention which relates to the use of a-bromodiethylcarbonate in the preparation of ethoxycarbonyloxy ethyl esters of 6-apa, penicillins and cefalosporins, the use of catalyst is optional. Approximately equimolar amounts ofthe quaternaryammonium saltofthe carboxylic acid andthe esterforming halide may be used in the reaction. Preferably between 5% and 100% excess of the esterfor- ming halide is used for each equivalent ofthe salt of the carboxylic acid used and more preferably an excess of between 20% and 60% ofthe ester forming halide is used.

The improvements in the esterification processes ofthe invention are particularly suitable for the preparation ofthe esters of 6-a pa, penicillins and cephalosporins and thus, in accordance with a preferred embodi mentofthe invention the carboxylic acid may be oftheformula:

in which R1 is a hydrogen atom or acyl groupl, particularly a substituted acetyl group such as a phenylacetyl; alpha-aminophenylacetyl; alpha-aminoparahydroxyphenylacetyl; phenoxyacetyl; alpha carboxyphenylacetyl oralpha-carboxy-3-thienylacetyl group or, when the carboxylic acid is of theformula Xll, a group:

in which R3 is a hydrogen atom or an amino protecting group such as a benzyloxycarboxyl; trimethylsilyl or t -butyloxycarboxyl group, and R2 is a hydrogen atom; an alkyl groulp (e.g. a methyl group), a substituted alkyl group, e.g. a hydroxymethylene; alkoxy or arylkoxy methylene oracetoxy methylene group) or an acetoxy or substituted ac etoxy group (e.g. an alkyl acetoxy, aryl acetoxy, or arylalkyl acetoxy group of the group C6H5.CHOH)CO-) In the preparation of esters of penicillins and cephalosporins according to the invention, the esterforming halide is an alpha-halodialkyl carbonate oftheformula CH3-CH(X)-O-CO-O-CH2-CH3, in which Xis a chlorine, bromine or iodine atom, preferably a bromine atom.

In accordancewith a preferred embodiment of the invention forthe preparation of esters of penicillins and cephalosporinsthe quaternary ammonium saltemployed istetra-n-butyl ammonium bromide.

In orderthatthe invention may be well understood the following examples are given byway ofillustration.

Example 7 A mixture of acetaldehyde (44 g, 1 mole), carbon tetrachloride (300 ml) and freshly distilled carbonyl bromide (235 g, 1.25 mole) was cooled to 0 C and maintained at this temperature by external cooling during the addition over a period of 1 hour of pyridine (11.9 g, 0.15 mole).

The mixture was allowed to warm up to ambient temperature and then heated to 50"C and maintained at this temperature for a period of hours during which time a precipitate formed.

Evaporation of the reaction mixture under reduced pressure at 50"C gave a semi solid oily mass which readily dissolved in ethanol (92 g, 2 mole) on warming and heating under reflux. After heating under refluxfor a further 2 hours, excess ethanol was removed in vacuo and the residue triturated with water (100 ml) and methylene dichloride (200 ml).

Separation ofthe organic layer and fractional distillation afforded pure ethyl alpha-bromo-ethyl-carbonate (130 g, 66% yield) having a boiling point of 90-92"C at 45 mms of mercury pressure and identical in all respects with an authentic specimen.

Example 8 A mixture of acetaldehyde (44 g, 1 mole), dichloromethane (300 ml) and hexamethylphosphoric-tri-amide (17.9 g 0.1 mole) was cooled to -10 C and freshly distilled carbonyl bromide) (207 g, 1.1 mole) was gradually added over a period of4 hours during which time the temperature was allowed to rise to 1 00C.

The mixture was then heated under gentle reflux (ca. 40'C) for4 hours. While still under reflux, ethanol (69 g, 1.5 mole) was carefully added over a period of 1 hour and heating under reflux continued for further 1 hour.

Fractional distillation ofthe resulting mixture afforded pure ethyl alpha-bromoethyl-carbonate directly (114 g, 58% yield).

The authenticity of the ethyl alpha-bronoethyl carbonate formed was confirmed by analysis and independent synthesis as follows.

Diethylcarbonate (118 g, 1.0 mole) was stirred and heated to between 1 10'C and 1200C and illuminated bya 150watttungsten filament lamp. Bromine (96 g, 0.6 mole) was added dropwise overa period of 3to 4 hours and at such a rate that the mixture did not deepen beyond a pale orange colour.

After addition of bromine was complete, the mixture was cooled to ambient temperature and sodium bicarbonate (20 9) added.

Distillation and fractionation of the resulting mixture gave authentic ethyl alpha-bromo-ethyl carbonate (84.2 g, 70% yield) having a boiling point of 87-880C at40 mms of mercury pressure.

Example 9 A mixture of lithium bromide (43 g, 0.5 m) ethyl alphachloroethyl carbonate (15.3 g, 0.1 m); water (100 ml), dichloromethane (100 ml) and cetyl trimethyl ammonium bromide (1.5 g) was stirred at ambienttemperature for 24 hours. The aqueous layer was removed and replaced bya fresh solution of lithium bromide (26 g, 0.3 m) in water (40 ml) containing cetyl trimethyl ammonium bromide (1 g). After stirring for a further 24 hours during which time the temperature was raised to 350the organic layer was separated, dried and vacuum distilled to afford after repeated fractionation the new compound, ethyl alpha-bromo-ethyl carbonate (15.0 g, 76%yield) having a boiling point of 90-92"C at 35 mms of mercury pressure.

Found: C30.7; H4.8 Br40.1% Calculated: C30.5: H4.6: Br40.6% The NMR spectrum exhibited peaks as follows: 1.2 - 1.6 (3H, triplet) -CH2. CH3 2.0 - 2.2 (3H, doublet) -.CH.CH3 4.1 -4.5(2H,Ouartet)-CH2. CH3 6.5 - 6.8(1 H, Quartet) - CH. CH3 Example 10 Lithium bromide (17.4 g, 0.2 m) was dissolved in dimethyl formamide (150 ml) and the mixture cooled to ambient temperature. Ethyl alpha-chloroethyl carbonate (30.5 g, 0.2 m) was added and the mixture stirred at ambient temperature for24 hours. The precipitated lithium chloride was filtered off and thefiltratevacuum distilled to afford after careful re-fractionation, ethyl alpha-bromoethyl carbonate in 76% yield based upon recovered ethyl alphachloroethyl carbonate.

Example 11 The authenticity of the foregoing new compound ethyl alpha-bromoethyl carbonate was confirmed by independent synthesis as follows:- Admixture of diethyl carbonate (35g, 0.3 m) in carbon tetrachloride (50 ml) and alpha-azo-isobutyronitrile (AIBN) (0.1 g) was heated to gentle reflux and dibromodimethyl hydantoin (28.6 g, 0.1 m) was added in small aliquots over a period of 8 hourstogetherwith further additions of AIBN (8x0.05 g): care being taken to ensure thatfree bromine did not accumulate in the reaction mixture.Atthe end ofthe reaction the mixturewas subjected to vacuum fractional distillation to afford pure ethyl alpha-bromoethyl carbonate (32.3 g, 82%yield) identical in all respects with the product of Examples 9 and 10.

Example 12 Benzylpenicillin eth oxycarb on yloxyeth yl ester A mixture of potassium penicillin G (7.4 g, 20 mmole), ethyl alpha-chloro-ethyl carbonate (4.6 g, 30 mmole), tetra-n-butyl ammonium bromide (0.8 g, 2.5 mmole) and acetone (80 ml) were stirred and heated under gentle reflux for 4 hours. Excess acetone was removed under partial vacuum and the residue triturated with ice-coidwaterand methyl isobutylketone.Evaporation ofthe dried methyl isobutylketone undervacuum gave a semi-crystalline oil (3.8 g) which on trituration with ethanol deposited white crystals (0.9 g) ofthe alpha-(ethoxycarbonyloxy)-ethyl ester of penicillin G having a purity of 98-99% by HPLC Found C43.0 H7.4 N7.7% Calculated: C43.4 H7.4 N8.0% Example 13 Benzylpenicillin ethoxycarbonbyloxyethyl ester The foregoing experiment of Example 12 was repeated using ethyl alpha-bromo-ethyl carbonate (5.9 9,30 m mole) instead of ethyl alpha-chloro-ethyl carbonate, whereon there was obtained, on evaporation ofthe methyl isobutyl ketone, 6.0 g of a semicrysta Iline oil. Trituration of this oil with warm ethanol and then cooling afforded white crystals (2.5 g, 35% yield) of the alpha-(ethoxycarbonyloxy)-ethyl ester of penicillin G.

Example 14 Benzylpenicillin ethoxycarbonyloxyethyl ester Potassium benzylpenicillinate (25.08 g, 66.7 mmol) sodium bicarbonate (0.50 g, 6.0 mmol), andtetrabutylammonium bromide (2.15 g, 6.67 mmol) were carefully stirred in methylene chloride (41 ml) and warmed to 40"C. When this temperature was reached a-bromodiethyl carbonate 17.16 g, 86.7 mmol) was added and the slurry was stirred for4.0 hours. Water (30 ml) was added, followed by a mineral acid to a pH of approx.5. The mixture was stirredforapprox. 4 hours, during which time sodium hydroxide (4%)wasadded in orderto maintain pH between 2.5-3.0.Methylene chloride (50 ml) was then added and the mixture was allowed to separatefora few minutes. The organic phasewaswashed with water (65 ml) and wasthen evaporated under reduced pressure. The oily product thus obtained was dissolved in methylene chloride (100 ml) andwasevaporated again.The remaining oil was dissolved in methylene chloride to a total volume of 100 ml.

HPLC-analysis of the methylene chloride solution showed a yield of benzylpenicillin eth- oxycarbonyloxyethyl ester of 96-97%.

Example 15 Benzylp enicillin eth oxycarb on yloxyeth yl ester Potassium benzylpenicillinate (5.02 g, 13.3 mmol) and potassium bicarbonate (2.99 g, 38.3 mmol) in dimethyl sulfoxide (13.5 ml)were carefully stirred in an ice-bath. a-bromodiethyl carbonate (3.70 g, 18.6 mmol) was added over a period of 30-40 min using a syringe pump. Stirring was continued while keeping the reac tion mixture in the ice-bath. HPLC-analyses showed that a yield of about70% of the benzylpenicillin eth- oxycarbonyloxyethyl ester was obtained within 5-10 minutes.

Example 16 Benzylpenicillin ethoxycarbonyloxyethyl ester Potassium benzylpenicillinate (47.03 g, 125 mmol) sodium bicarbonate (0.94g, 11 mmol), and tetra butylam moniu m bromide (2.01 g, 6.25 mmol) were carefully stirred in acetone (77 ml) and warmed to 40"C.

When this temperature was reached a-bromodiethyl carbonate (26.06 g, 131 mmol) was added and the slurry was stirred for4.5 hours. Water (56 ml) was added,followed bya mineral acid to a pH ofapprox. 5. The mixture was stirred for approx.3 3 hours, during which time sodium hydroxide (4%) was added in orderto maintain pH between 4.5-4.8. Butyl acetate (100 ml) was then added and the mixture was allowed to separate for a few minutes. The organic phase was washed with water (80 ml) and then evaporated under reduced pressure. The remaining oily productwas dissolved in methylene chlorideto a total volune of 250 ml.

HPLC-analysis ofthe methylene chloride solution showed a yield of benzylpenicillin ethoxycarbonyloxyethyl ester of 98-99%.

Example 17 The procedure of Example 4 was repeated but using ethyl acetoacetate and ampicillin dane salt. 16.1 g of a white crystalline product was obtained of m.p. 144-1480C (Tottoli apparatus). The IR andTLC analysis con- formed to that of a genuine sample, and the product had KF of 0.35%, a pH of 3.55 (2% aqueous solution),an assayTQ of 95.2% and 3.5% total residue solvents.

The procedure was repeated again but with 9 g of alpha-chlorodiethyl carbonate added immediately and the balance added after 2 hours. The reaction mixture was heated for 3 hours at 45"C. 1 3.7 g. of a crystalline beige product was obtained, m.p. 1 43-1 46"C (Tottoli apparatus). The IR and TLC analysis conformed to that of a genuine sample and the product had a KF of 0.2%, a pH of 3.43 (2% aqueous solution), an assayto 94.8%and 2.6% residual solvents.

Claims (7)

1. A process for the preparation ofthe ethoxycarbonyloxyethyl ester of 6-aminopenicillanic acid or of a penicillin ora cephalosporin in which comprises reacting 6-aminopenicillanic acid ora penicillin oracephal- osporin, or a salt thereof, with alpha-bromodiethyl carbonate.

2. A process according to claim 1 wherein the reaction is carried out in the presence of a catalytic amount of a quaternary ammonium salt.

3. A process according to ciaim 2 wherein the quaternary ammonium salt is used in an amount of from 1 % to 25% of an equivalent of the quaternary ammonium saltfor each equivalent of the 6-aminopenicillanicacid, the penicillin orcephalosporin.

4. Aprocess according to claim 3wherein the ammonium salt used is in an amountoffrom 1 to 10%.

5. A process according to any one of claims 2 to 4 wherein tetra-n-butyl ammonium bromide is the catalyst.

6. A process according to anyone ofthe preceding claimswhereinthe penicillin is benzylpenicillin.

7. A process according to claim 1 substantially as hereinbefore described with reference to any one of the Examples.