GB1569427A - Manufacture of semi-synthetic penicillin antibiotics - Google Patents

Abstract

In the synthesis of penicillin antibiotics by silylation of the carboxyl and, where appropriate, of the amino group of 6-aminopenicillanic acid, acylation with an acid chloride or protected acid chloride and subsequent elimination of the silyl groups, the silylation stage is carried out in an inert solvent using a compound of the formula: R<1>R<2>N-COR<3> (I) or R<1>N=C(OR<2>)R<3> (II> and in this way improved. A compound of the formula: <IMAGE> is formed and is, without isolation, reacted with the acid chloride or protected acid chloride. The meaning of the symbols is given in Claim 1.

Description

(54) IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF SEMI-SYNTHETIC PENICILLIN ANTIBIOTICS (71) We, GLAXO LABORATORIES LIMITED, a British Company, of Greenford, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to improvements in or relating to the manufacture of semisynthetic penicillin antibiotics.

Numerous processes have been proposed for preparing the semi-synthetic range of penicillin antibiotics, and particularly ampicillin. These proposals have generally involved the acylation of 6-aminopenicillanic acid (6-APA) or a protected derivative thereof using, for example, an acid halide serving to introduce the desired acyl group. Particularly favoured protected derivatives have been those which may readily be cleaved in situ once the acylation has been effected.

One of the more favoured types of protected derivative of 6-APA has been the mono- or bis- silylated intermediate. Silylated intermediates have been widely used for many years since they have the advantages not only that the silyl groups assist in solubilising the penicillin compounds so that reaction may be performed in aprotic solvents, thus preventing a certain amount of t3-lactam hydrolysis, but also that they may readily be cleaved in situ following acylation. British Patent Specifications Nos. 959853; 964449 and 1008468 for example, describe both the preparation and subsequent use of silylated 6-APA intermediates in penicillin antibiotic production.

The actual acylation step using the silylated intermediate is frequently carried out using an acid halide of the carboxylic acid whose acyl group it is proposed to introduce in the presence of an organic base as hydrogen halide acceptor. The great majority of bases previously employed or proposed to be employed in this role have been amines, particularly secondary or tertiary amines such as trialkylamines, dialkylanilines, piperidines or pyridines. Furthermore such acylations have occasionally been proposed to be carried out in tertiary amide solvents, as in for example, British Patent Specification No. 959853.

It has been a disadvantage of such known processes that the acylated antibiotic products obtained tend to be contaminated, even after purification, with minor amounts of the base or solvent employed in their preparation which are extremely difficult to remove from the antibiotic and which frequently possess significant toxicity. It is, of course, desirable that any pharmaceutical product be prepared and sold in the highest degree of purity possible and it is a fact that even the very small amounts of base with which the antibiotic product is contaminated may produce unpleasant side effects when the antibiotics are administered.

These include headaches, nausea or drowsiness and it is accordingly highly important either that the amounts of such contaminants in the antibiotic be minimised, or that the contaminants possess lower toxicity.

The disadvantages outlined above have applied particularly to the dialkylanilines when these have been used as bases. The dialkylanilines and especially dimethylaniline, have been extensively used commercially as hydrogen halide acceptors in the acylation reaction since their pKb values fall within the critical narrow range for the maximisation of yield of antibiotic product and avoidance of side-reactions. Unfortunately they have been found to occlude to a small extent into the antibiotic product and they also possess marked toxicity.

The small amounts of, for example, dimethylaniline which occlude into the antibiotic product are sufficient to produce unpleasant side-effects, including those outlined above.

We have now found that if the silylation of the 6-aminopenicillanic acid is carried out using silylating agents which generate primary or secondary carboxamide bases after transfer of the silyl group or groups to the 6-APA molecule, the acylation of the silylated intermediate may be carried out in situ in the presence of the primary or secondary carboxamide bases as hydrogen halide acceptors. Thus, it is a feature of our discovery that no further base needs to be added to the reaction mixture after the silylation and the overall process involves fewer steps and is generally more convenient.

By this method, the good yields of product which have previously been obtained by this type of acylation may be maintained even on a commerical scale, and also the antibiotic product obtained is significantly less contaminated with base than has previously been the case, especially where dimethylaniline has been employed. Our process has the further advantage that the basic contaminants which may be present in the antibiotic products are significantly less toxic than those previously employed, and so the incidence of side effects on administration of the antibiotic may be reduced, if not substantially eliminated.

According to one embodiment of the invention, therefore, there is provided a process for the manufacture of a 6-acylaminopenicillianic acid antibiotic product in which 6aminopenicillanic acid is reacted in an inert solvent with a silylating agent to form a silylated compound of formula (III)

wherein R4 represents a hydrogen atom or a tri(C1.6) alkylsilyl group or triarylsilyl group and R5 represents a tri(C1.6) alkylsilyl group or triarylsilyl group, and the compound of formula (III) is thereafter contacted with an acid chloride or protected acid chloride corresponding to the desired 6-acylamino group, the silyl groups are cleaved and the desired antibiotic product is recovered, wherein silylation is effected using a compound of formula (I) or formula (II) R1R2N.CoR3 (I) or R1N=C(oR2).R3 (II) wherein R1 is hydrogen or a C1 6 alkyl or aralkyl group in formula (I), or a C1 6 alkyl or aralkyl or a tri(C1.6) alkylsilyl or triarylsilyl group in formula (II), R2 is a tri(C1.6) alkylsilyl or triarylsilyl group and R3 is a C1 6 alkyl or aralkyl group or an amino group substituted by one or two C1 6 alkyl groups, and the compound of formula (III) produced is reacted without intermediate isolation with the acid chloride or protected acid chloride.

The acid chloride employed is chosen according to the nature of the desired 6-acylamino group. Where the latter contains sensitive groups it may be necessary to protect these during the process of the invention. Thus, in manufacturing ampicillin, the a-amino group of the D(-)-a-phenylglycyl chloride may be protected by, for example, hydrochloric acid.

The process according to the invention is generally applicable to the manufacture of known semi-synthetic penicillin antibiotics e.g. ampicillin, cloxacillin and dicloxacillin, the nature of which is well defined in existing literature.

It will be appreciated that once the silylating agent of formula (I) or formula (II) has lost its silyl group or groups, a compound of formula (IV) R3Co.NHR1 (IV) will be formed wherein R3 is as defined above for formulae (I) and (II) and R1 is a hydrogen atom, C1 6 alkyl or aralkyl group. R1 in formula (IV) will, of course represent a hydrogen atom if a silylating agent of formula (II) wherein R1 represents a silyl group is employed.

In view of the nature of the silylating agent used to prepare the compound of formula (III), the compound of formula (IV) as defined above, which is the residue of the silylating agent once silylation has occurred, functions as hydrogen chloride acceptor in the acylation step. It is thus an advantageous feature of the process of the present invention that no further base as hydrogen halide acceptor needs to be added to the reaction solution prior to acylation. Silylating agents which yield compounds of formula (IV) wherein R3 is a alkyl (e.g. C1-4) group are preferred, and those which yield compounds wherein R3 is a methyl group are especially preferred, for example N-trimethylsilylacetamide, N,O-bistrimethylsilyl acetamide and N-methyl-N-trimethyl-silylacetamide.The silylating agents which yield a compound of formula (IV) wherein R3 is methyl group and R1 is a hydrogen atom i.e. acetamide are particularly preferred in view of the non-toxicity of acetamide.

Where an aryl group is present in the silylating agent this will desirably be a monocyclic aryl group, for example containing 5 or 6 carbon atoms, e.g. a phenyl group.

The silylation reaction will desirably be carried out by allowing the 6-APA and silylating agent to react, desirably at elevated temperature and in an inert solvent, for sufficient time to allow complete consumption of the silylating agent. This will generally have occurred within about two hours.

The quantitity of silylating agent employed will, of course, depend on the number of silylating groups the various silylating agents possess, but it is generally preferred to use from 1 to 2 equivalents of the silylating agent per mole of 6-APA to be silylated.

The silylation is conducted in an inert organic solvent, e.g. a halogenated hydrocarbon, aromatic hydrocarbon or an ether, such as benzene, toluene, methylene chloride, ethylene chloride, chloroform or tetrahydrofuran. Methylene chloride is the solvent of choice.

Once silylaton has been effected, the solution will, preferably, be cooled to from +100C to -30"C for example +5 to 250, e.g. +5 to -50C prior to addition of the acyl halide.

Addition of the halide may be effected portionwise, the temperature being maintained around or below 0 C.

Once all the acyl halide has been added, temperature control is desirably maintained throughout the acylation reaction, which proceeds comparatively rapidly and should normally be complete within 30 minutes to 3 hours, e.g. about two hours. The extent of the acylation may be monitored by, for example, determining the proportion of residual starting material by thin-layer chromatography.

We generally prefer in the acylation step to employ an approximately stoichiometric amount of acyl halide, e.g. around 1 equivalent, advantageously 1.0 equivalant, relative to the quantity of silylated 6-APA. The residual compound that remains from the silylation reaction is preferably present in aounts of 1 to 2 equivalents per mole of silylated 6-APA.

After completion of the acylation reaction, for example as evidenced by consumption of all the starting material present, the resulting solution may be treated with a compound containing active hydrogen, e.g. water, acidified or basified water, an alcohol or a phenol, to remove any silyl groups present in the penicillin reaction product. Water is the preferred desilylating agent for this purpose.

The penicillin antibiotic may then be precipitated, e.g. in the case of ampicillin, by adjusting the pH of the diluted reaction mixture to the isoelectric point with a base, or, in the case of cloxacillin and dicloxacillin by formation of a salt thereof in an organic solvent and the precipitate may be recovered and dried by conventional means. Where it is desired to form a salt of the penicillin antibiotic, this may be achieved by addition of a suitable base for example an alkali metal alkanoate, e.g. sodium 2-ethylhexanoate.

The process according to the invention is particularly applicable to the manufacture of ampicillin, 6-(D-2-amino-2-phenylacetamido)-2,2-dimethylpenam-3-carboxylic acid, either as a hydrate (e.g. the trihydrate) or in anhydrous form. Other semi-synthetic penicillin antibiotics which may be manufactured by the process according to the invention include amoxycillin, cloxacillin, dicloxacillin, a-carboxybenzylpenicillin esters, oxacillin, fluorcloxacillin and metacillin.

It should be noted that the invention of the present application falls within the scope of that of our co-pending application No. 1540/77 (Serial No. 1569426) of even date herewith.

The invention will now be more particularly described in the following Examples which should not be construed as limiting the invention.

In the Examples, the nature and purity of the end products were determined by standard techniques, including polarimetry, spectrophotometry, acidimetry and bioassay. A description of the spectrophotometric method used to assay the ampicillin may be found in British Pharmacopoeia, (1973, H.M.S.O.) on page 30; a description of the bioassay technique employed is given in British Pharmacopoeia (1973 H.M.S.O.) on pages 102-104 of the Appendix; and an account of the acidimetric assay used to assay the cloxacillin and discloxacillin is given in British Pharmaceopoeia (1973, H.M.S.O.) on page 81.

The water content was determined by a Karl Fisher analysis for cloxacillin and dicloxacillin, and by this method or by measuring the weight loss on heating to form the anhydrous compound in the case of ampicillin trihydrate.

The purity of the ampicillin products is given after allowance has been made for the water (hydrate) content of the ampicillin product obtained.

In the Examples, the specific rotation of ampicillin trihydrate is determined at c = 0.25% solutions in water, and the specific rotations of dicloxacillin and cloxacillin are determined at c = 1.0% in water, the determinations being carried out at 20"C.

The specific rotation of anhydrous ampicillin is given as [a]20/D = +280 to +300 ( = 0.25 in water) and that of cloxacillin as sodium salt monohydrate is given as [a]20/D = +156 to +164 ( c = 1 in water) in the British Pharmacopoeia (1973, H.M.S.O.). That of dicloxacillin is given as [aJ 24/D = +134 ( c = 0.4 in water) in the Merck Index (8th Edition).

Example 1 Preparation of 6 (D-2-amino-2-phenylacetamido) -2,2-dimethylpenam-3-carboxylic acid trihydrate using N-trimethylsilylacetamide as silylating agent 52.5 g (0.400 mols) of N-trimethylsilylacetamide were added to a suspension of 43.2 g (0.200 mols) of APA in 350 ml of methylene chloride under agitation.The mixture was heated to +400C for 120 minutes, and then cooled to -25"C; during the cooling there was an abundant precipitation of acetamide. 43.3 g (0.200 mols) (purity 95.0%) of D(-)-aphenylglycyl-chloride hydrochloride were added at -250C and the temperature was allowed to increase to -50C where it was held for a total period of 90 minutes starting from the addition of the acid chloride hydrochloride. 450 ml of water were added, and then the ampicillin trihydrate was precipitated by adjusting the pH to 4.5 with dilute NH40H. After sixty minutes agitation at + 100C/+ 150C, the mixture ws filtered and then washed with 2 x 75 ml of water and 3 x 125 ml of acetone; the solid was dried at +35 C/+408C.

Yield: 84.0+1.0% Specific rotation: +296 + 1", (obtained from a value of +256 + 1" for the hydrated compound).

Spectrophotometric assay: 99.0 + 0.5% (obtained from an ampicillin purity of 85.7% + 0.5% for the hydrated compound).

Water content: between 13.4 and 14.0%.

Example 2 Preparation of 6-(D-2-amino-2-phenylacetamido)-2,2-dimethylpenam-3-carboxylic acid trihydrate using N,O-bis-trimethylsilylacetamide as silylating agent 48.9 ml (0.200 mols) of N,O-bis-trimethyl-silylacetamide were added to a suspension of 43.2 g (0.200 mols) of 6-aminopenicillanic acid in 350 ml of methylene chloride under agitation. The mixture was heated at +400C for 120 minutes, then cooled to -25"C and 43.3 g (0.200 mols); (purity 95.0%) of D(-)-a-phenyl-glycylchlon.de hydrochloride were added and thereafter the procedure followed ws as that described in Example 1.

Yield: 80.0%.

Specific rotation: +296 .

Spectrophotometric assay: 98.7% Water content: 13.9%.

Example 3 Preparation of 6-[3-(2-chlorophenyl)-5-methyl-4-isoxazolecarboxamidoj-2,2 dimethylpenam-3-carboxylate (cloxacillin)sodium monohydrate using N-trimethylsilylacetamide as silylating agent 52.5 g (0.400 mols) of N-trimethylacetamide were added to a suspension of 43.2 g (0.200 mols) of 6-amino-penicillanic acid in 350 ml of methylene chloride under agitation.The mixture was heated at +40"C for 120 minutes and then cooled to -25 C; during the cooling acetamide precipitated. 51.2 gl (0.200 mols) of 3-(2-chlorophenyl)-5-methylisoxazolyl-4 carbonyl chloride were added; the temperature was allowed to increase to 0 C and held there for a total period of 60 minutes starting from the addition of the acid chloride. 175 ml of methyl isobutyl ketone and 300 ml of water were then added; the phases were separated and the aqueous phase was discarded; the organic phase was again washed with 300 ml of water, and the aqueous phase discarded. The organic phase was treated for 30 minutes with anhydrous sodium sulphate; the drying agent was then filtered off and washed with 175 ml of methyl isobutyl ketone which was combined with the main organic phase. The sodium salt monohydrate of cloxacillin was then precipitated by adding to the combined organic phases 200 ml of a 1N solution of sodium 2-ethyl hexanoate in methyl isobutyl ketone. After 60 minutes agitation the crystalline white solid was filtered and then washed with 3 x 150 ml of acetone; drying was in vacuum oven at +35"C/+40"C.

Yield: 84.6% Acidimetric assay: 98.8% as sodium salt monohydrate.

Specific rotation: +163 .

Water content: 3.9%.

Example 4 Preparation of 6-[3-(2,6-dichlorphenyl)-5-methyl-4-isoxazolecarboxamido]-2,2- dimethylpenam-3-carboxylate (dicloxacillin) sodium monohydrate using N-trimethylsilylacetamide as silylating agent 52.5 g (0.400 mols) of N-trimethylsilylacetamide were added to a suspension of 43.2 g (0.200 mols) of 6-aminopenicillanic acid in 350 ml of methylene chloride under agitation.

The mixture was heated at +40"C for 120 minutes, and then cooled to -25"C; during the cooling, acetamide precipitated. 58.1 g (?.l/4?? MOLS( OF 3/s-(2,6-dichlorophenyl)-5methylisoxazolyl-4-carbonyl-chloride were added at -250C, and the temperature was allowed to increase to 0 C where it was held for a total period of 60 minutes starting from the addition of the acid chloride. 175 ml of methyl isobutyl ketone and 300 ml of water were added and the phases separated. The aqueous phase was discarded, the organic phase was again washed with 300 ml of water and the aqueous phase again discarded.The organic phase was treated for 30 minutes with anhydrous sodium sulphate; the drying agent was then filtered, and washed with 175 ml of methyl isobutyl ketone, which was combined with the main organic phase. The sodium salt monohydrate of dicloxacillin was precipitated by adding to the combined organic phases 200 ml of a 1N solution of sodium 2-ethyl-hexanoate in methyl isobutyl ketone. After 60 minutes' agitation the crystalline white solid was filtered, washed with 3 x 150 ml of acetone and dried in vacuum oven at +350C/+400C.

Yield: 81.8% Acidimetric assay: 98.6% as sodium salt monohydrate.

Specific rotation: +139 .

Water content: 3.8%; Example 5 Preparation of 6-(D-2-amino-2-phenylacetamido)-2,2-dimethylpenam-3-carboxylic acid trihydrate using N-trimethylsilyl acetamide as silylating agent A mixture of 43.2 g (0.20 mols) of 6-aminopenicillanic acid, 350 ml of methylene chloride and 52.5 g (0.40 mols) of N-trimethylsilylacetamide was heated under reflux for two hours.

The temperature of the mixture was then lowered to -5"C and 43.2 g (0.2 mols, purity 95%) of D(-)-a-phenylglycyl chloride hydrochloride were added, the temperature being maintained below 0 C. After stirring for one and a half hours, between 0 C and + 100C, the temperature was lowered to OOC and 450 ml of water added. Ampicillin trihydrate was precipitated by adjusting the pH to 4.5 with diluted NH40H and the crystalline product was filtered, washed with water and acetone and then dried.

Yield: 82.7%.

Spectrophotometric assay: 99.0%.

Bioassay: 97.9%.

Specific rotation: +296 .

Water content: 13.4%.

Example 6 Preparation of 6-(D-2-amino-2-phenylacetamido)-2,2-dimethylpenam-3-carboxylic acid trihydrate using N-methyl N-trimethylsilylacetamide as silylating agent 58.1 g (0.40 mols) of N-methyl-N-trimethylsilylacetamide were added, with stirring to a suspension of 43.2 g (0.20 mols) of 6-aminopenicillanic acid in 350 ml methylene chloride.

The mixture was heated at +400C for 120 minutes and then cooled to -250C. Thereafter, the procedure was as described in Example 1.

A yield of 79.3% was obtained.

Specific rotation: +292".

Spectrophotometric assay: 97.6%.

Water content: 13.7%.

WHAT WE CLAIM IS: 1. A process for the manufacture of a 6-acylamino-penicillanic acid antibiotic product in which 6-aminopenicillanic acid is reacted in an inert solvent with a silylating agent to form a silylated compound of formula (III)

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

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. Acidimetric assay: 98.8% as sodium salt monohydrate. Specific rotation: +163 . Water content: 3.9%. Example 4 Preparation of 6-[3-(2,6-dichlorphenyl)-5-methyl-4-isoxazolecarboxamido]-2,2- dimethylpenam-3-carboxylate (dicloxacillin) sodium monohydrate using N-trimethylsilylacetamide as silylating agent 52.5 g (0.400 mols) of N-trimethylsilylacetamide were added to a suspension of 43.2 g (0.200 mols) of 6-aminopenicillanic acid in 350 ml of methylene chloride under agitation. The mixture was heated at +40"C for 120 minutes, and then cooled to -25"C; during the cooling, acetamide precipitated. 58.1 g (?.l/4?? MOLS( OF 3/s-(2,6-dichlorophenyl)-5methylisoxazolyl-4-carbonyl-chloride were added at -250C, and the temperature was allowed to increase to 0 C where it was held for a total period of 60 minutes starting from the addition of the acid chloride. 175 ml of methyl isobutyl ketone and 300 ml of water were added and the phases separated. The aqueous phase was discarded, the organic phase was again washed with 300 ml of water and the aqueous phase again discarded.The organic phase was treated for 30 minutes with anhydrous sodium sulphate; the drying agent was then filtered, and washed with 175 ml of methyl isobutyl ketone, which was combined with the main organic phase. The sodium salt monohydrate of dicloxacillin was precipitated by adding to the combined organic phases 200 ml of a 1N solution of sodium 2-ethyl-hexanoate in methyl isobutyl ketone. After 60 minutes' agitation the crystalline white solid was filtered, washed with 3 x 150 ml of acetone and dried in vacuum oven at +350C/+400C. Yield: 81.8% Acidimetric assay: 98.6% as sodium salt monohydrate. Specific rotation: +139 . Water content: 3.8%; Example 5 Preparation of 6-(D-2-amino-2-phenylacetamido)-2,2-dimethylpenam-3-carboxylic acid trihydrate using N-trimethylsilyl acetamide as silylating agent A mixture of 43.2 g (0.20 mols) of 6-aminopenicillanic acid, 350 ml of methylene chloride and 52.5 g (0.40 mols) of N-trimethylsilylacetamide was heated under reflux for two hours. The temperature of the mixture was then lowered to -5"C and 43.2 g (0.2 mols, purity 95%) of D(-)-a-phenylglycyl chloride hydrochloride were added, the temperature being maintained below 0 C. After stirring for one and a half hours, between 0 C and + 100C, the temperature was lowered to OOC and 450 ml of water added. Ampicillin trihydrate was precipitated by adjusting the pH to 4.5 with diluted NH40H and the crystalline product was filtered, washed with water and acetone and then dried. Yield: 82.7%. Spectrophotometric assay: 99.0%. Bioassay: 97.9%. Specific rotation: +296 . Water content: 13.4%. Example 6 Preparation of 6-(D-2-amino-2-phenylacetamido)-2,2-dimethylpenam-3-carboxylic acid trihydrate using N-methyl N-trimethylsilylacetamide as silylating agent 58.1 g (0.40 mols) of N-methyl-N-trimethylsilylacetamide were added, with stirring to a suspension of 43.2 g (0.20 mols) of 6-aminopenicillanic acid in 350 ml methylene chloride. The mixture was heated at +400C for 120 minutes and then cooled to -250C. Thereafter, the procedure was as described in Example 1. A yield of 79.3% was obtained. Specific rotation: +292". Spectrophotometric assay: 97.6%. Water content: 13.7%. WHAT WE CLAIM IS:

1. A process for the manufacture of a 6-acylamino-penicillanic acid antibiotic product in which 6-aminopenicillanic acid is reacted in an inert solvent with a silylating agent to form a silylated compound of formula (III)

wherein R4 represents a hydrogen atom or a tri(C1 6) alkylsilyl group or triarylsilyl group and R5 represents a tri(Cl 6) alkylsilyl group or triarylsilyl group, and the compound of formula (III) is thereafter contacted with an acid chloride or protected acid chloride corresponding to the desired 6-acylamino group, the silyl groups are cleaved and the desired antibiotic product is recovered, wherein silylation is effected using a compound of formula (I) or formula (II) R1R2N.CoR3 (I) or R1N=C(OR2).R3 (II) wherein R1 is hydrogen, or a C16 alkyl or aralkyl group in formula (I) or a C1 6 alkyl or aralkyl or a tri(C1 6) alkylsilyl or triarylsilyl group in formula (II), R2 is a tri(Cl 6)alkylsilyl or triarylsilyl group and R3 is a C16 alkyl or aralkyl group or an amino group substituted by one or two C16 alkyl groups, and the compound of formula (III) produced is reacted without intermediate isolation with the acid chloride or protected acid chloride.

2. A process as claimed in claim 1 wherein the silylating agent provides, after silylation, a compound of formula (IV) R3.CONHR1 (IV) wherein R3 is as defined in claidm 1 and R1 is a hydrogen atom, a C16 alkyl or an aralkyl group.

3. A process as claimed in claim 1 or claim 2 wherein R3 represents a CiA alkyl group and Rl represents a hydrogen atom.

4. A process as claimed in claim 1 or claim 2 wherein R3 is a methyl group and R1 is a hydrogen atom.

5. A process as claimed in any of claims 1 to 4 wherein the silylating agent is N-trimethylsilylacetamide or N,O-bis-trimethylsilylacetamide.

6. A process as claimed in any of claims 1 to 5 wherein from 1 to 2 equivalents of silylating agent are employed per mole of 6-aminopenicillanic acid to be silylated.

7. A process as claimed in any of claims 1 to 6 wherein the inert solvent is methylene chloride.

8. A process as claimed in any of claims 1 to 7 wherein the acid chloride or protected acid chloride is added to the silylated 6-aminopenicillanic acid at from -300C to +10 C and acylation is effected at around or below 0 C.

9. A process as claimed in any of claims 1 to 8 wherein the protected acid chloride is D-phenylglycyl chloride hydrochloride and the antibiotic product recovered is ampicillin trihydrate.

10. A process as claimed in any of claims 1 to 8 wherein the acid chloride is 3-(2,6-dichlorophenyl)-5-methylisoxazolyl-4-carbonyl chloride and the antibiotic product recovered is dicloxacillin.

11. A process as claimed in any of claims 1 to 8 wherein the acid chloride is 3-(2-chlorophenyl)-5-methylisoxazolyl-4-carbonyl chloride and the antibiotic product recovered is cloxacillin.

12. A process as claimed in claim 1 substantially as hereinbefore described.

13. A process as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.

14. Ampicillin trihydrate whenever prepared by a process as claimed in any of claims 1 to 9, 12 and 13.