Penicillin ester hydrolysis

Of interest is also that the rate of liberation of penicillin G from the prodrug was much faster than the breakdown of the former by opening of its /3-lactam ring. Furthermore, no breakdown of the pro-moiety A-(hydroxy-methyl)-A-[(ethoxycarbonyl)methyl]benzamide (8.58) to liberate formaldehyde was detected over the timescale of ester hydrolysis. 

In a penicillin synthesis, the carboxyl group was protected as a p-bromophenacyl ester that was cleaved by nucleophilic displacement (PhSK, DMF, 20°C, 30 min, 64% yield). Hydrogenolysis of a benzyl ester was difficult (perhaps because of catalyst poisoning by sulfur present in the penicillin) basic hydrolysis of methyl or ethyl esters led to attack at the P-lactam ring. The phenacyl ester may also be cleaved by photolysis in the presence of 9,10-dimethylanthracene. ... 

Ofloxacin, an antibiotic of the quinolone class (Section 25-7), is used in the treatment of infections of the respiratory and urinary tract, the eyes, the ears, and skin tissue. The quinolone antibiotics are yet another alternative in the ongoing battle against penicillin- (and other drug)-resistant bacteria. The final three steps in the synthesis of ofloxacin are shown below, the last of which is a simple ester hydrolysis (Section 20-4). Formulate mechanisms for the other two transformations from A to B and then to C. 

The bulky triphenylmethyl group has been used to protect a variety of amines such as amino acids, penicillins, and cephalosporins. Esters of N-trityl a-amino acids are shielded from hydrolysis and require forcing conditions for cleavage. The a-proton s also shielded from deprotonation, which means that esters elsewhere in the molecule can be selectively deprotonated. 

This ester is conveniently formed from a penicillinic acid with PhCH2C02CH2Cl and TEA. Cleavage is accomplished by enzymatic hydrolysis with penicillin G. acylase in 70-90% yield. 

In some cases enzymes can increase the rate of reaction by up to lO times. Carnell and Roberts (1997) have briefly discussed the scope of biotransformations that are used to make pharmaceuticals like penicillins, cephalosporines, erythromycin, lovastatin, cyclosporin, etc., and for food additives like citric acid, L-glutamate, and L-lysine. A very successful transformation by Zeneca has been that of benzene reduction, with Pseudomonase Putida, to dihydrocatechol and catechol the dihydro derivative is used to produce (+/-) pinitol. Fluorobenzene has been converted to fluorodihydrocatechol, an intermediate for pharmaceuticals. The highly stereo selective Bayer-Villeger reaction has been carried out with genetically engineered S-cerevisvae. Hydrolases have allowed enantioselective, and in some cases regioselective, hydrolysis of racemic esters. 

R. Moreira, T. Calheiros, J. Cabrita, E. Mendes, M. Pimentel, J. Iley, Acyloxymethyl as a Drug Protecting Group. Part 3. Tertiary O-Amidomethyl Esters of Penicillin G Chemical Hydrolysis and Anti-Bacterial Activity , Pharm. Res. 1996, 13, 70-75. 

Note that penicillins and structurally related antibiotics are frequently deactivated by the action of bacterial -lactamase enzymes. These enzymes also contain a serine residue in the active site, and this is the nucleophile that attacks and cleaves the P-lactam ring (see Box 7.20). The P-lactam (amide) linkage is hydrolysed, and then the inactivated penicillin derivative is released from the enzyme by further hydrolysis of the ester linkage, restoring the functional enzyme. The mode of action of these enzymes thus closely resembles that of the serine proteases there is further discussion in Box 7.20. 

The optimum yield of a condensation product is obtained at the pH where Ka has a maximum. For peptide synthesis with serine proteases this coincides with the pH where the enzyme kinetic properties have their maxima. For the synthesis of penicillins with penicillin amidase, or esters with serine proteases or esterases, the pH of maximum product yield is much lower than the pH optimum of the enzymes. For penicillin amidase the pH stability is also markedly reduced at pH 4-5. Thus, in these cases, thermodynamically controlled processes for the synthesis of the condensation products are not favorable. When these enzymes are used as catalysts in thermodynamically controlled hydrolysis reactions an increase in pH increases the product yield. Penicilhn hydrolysis is generally carried out at pH about 8.0, where the enzyme has its optimum. At this pH the equiUbrium yield of hydrolysis product is about 97%. It could be further increased by increasing the pH. Due to the limited stability of the enzyme and the product 6-aminopenicillanic acid at pH>8, a higher pH is not used in the biotechnological process. 

Before interest developed in the structure of penicillin, the imidazo[5,1-b ]thiazole ring system was unknown, and even today, almost all of the examples which have been described are those prepared in connection with penicillin studies. Very mild acidic hydrolysis converts benzylpenicillin methyl ester (293) into benzylpenicillic acid (294) by a mechanism suggested... 

---