Penicillin selectivity

The penicillins are antibiotics that interfere with bacterial cell wall synthesis. The human body has no structures similar to the bacterial cell wall, so treatment with penicillins selectively destroys the bacteria, causing no harm to the patient. In practice, however, it must always be remembered that some individuals may develop an allergy to penidllins. 

Penicillin selection technique an aid to the isolation of chosen auxotrophic mutations in a bacterial population. It is based on the fact that penicillin kills growing bacterial cells, but does not affect nongrowing cells. 

Plasmid Vectors for Facile Introduction of Passenger DNA and Selection of Recombinants. The map of a commonly used plasmid vector, pUC19 (7), is shown in Figure 2. Three parts of the vector are key to its utility. The origin sequence, oh, allows the repHcation of plasmid DNA in high copy number relative to the chromosome. A gene, amp, encoding the enzyme beta-lactamase, which hydrolyzes penicillin compounds, allows... 

In a similar way, several cephalosporins have been hydrolyzed to 7-aminodeacetoxycephalosporanic acid (72), and nocardicin C to 6-aminonocardicinic acid (73). Penicillin G amidase from Pscherichia coli has been used in an efficient resolution of a racemic cis intermediate required for a preparation of the synthon required for synthesis of the antibiotic Loracarbef (74). The racemic intermediate (21) underwent selective acylation to yield the cis derivative (22) in 44% yield the product displayed a 97% enantiomeric excess (ee). 

Derivatiziag an organic compound for analysis may require only a few drops of reagent selected from silylatiag kits suppHed by laboratory supply houses. Commercial syathesis of penicillins requires silylatiag ageats purchased ia tank cars from the manufacturer (see Antibiotics, P-LACTAMS-penicillins AND others). 

Chemical Modification. The chemistry and synthetic strategies used in the commercial synthesis of cephalosporins have been reviewed (87) and can be broadly divided into ( /) Selection of starting material penicillin precursors must be rearranged to the cephalosporin nucleus (2) cleavage of the acyl side chain of the precursor (2) synthesis of the C-7 and C-3 side-chain precursors (4) acylation of the C-7 amino function to introduce the desked acylamino side chain (5) kitroduction of the C-3 substituent and 6) protection and/or activation of functional groups that may be requked. 

The antibacterial effectiveness of penicillins cephalospotins and other P-lactam antibiotics depends upon selective acylation and consequentiy, iaactivation, of transpeptidases involved ia bacterial ceU wall synthesis. This acylating ability is a result of the reactivity of the P-lactam ring (1). Bacteria that are resistant to P-lactam antibiotics often produce enzymes called P-lactamases that inactivate the antibiotics by cataly2ing the hydrolytic opening of the P-lactam ring to give products (2) devoid of antibacterial activity. 

After formation of the acylimine (12), methanol adds to the less sterically hindered a-face of the molecule with high selectivity to provide (13). A further direct incorporation of a 6a-methoxy group (41) and subsequendy a 6a-formamido group into penicillin has been achieved using ttiduoromethanesulfonamides of type (14) (42). 

The IPNS enzyme has also been shown to recognize modified tripeptides. The synthesis of a range of tripeptides, other than aminoadipoyl cysteinyl valine (ACV) (Table 6), has given rise to a selection of modified penicillins using IPNS as a means of cyclizing the tripeptide (58). 

These studies trace the evolution of penicillin investigations from microbiological curiosity through the development of increasing therapeutic utility to increasingly sophisticated chemical manipulations. This chapter will focus primarily on the chemical aspects of this area. Because of the vast amount of relevant literature, it has been possible to discuss only selected studies which hopefully are representative of the different investigative directions. The reader is urged to consult the cited references for more detailed discussion and for references to related studies. 

Application of the Curtius reaction to the 3-carboxyl of a penicillin has provided intermediates which have been used for the construction of cephem derivatives. As can be seen in Scheme 23, this route allows the selective cleavage of the C(3)—N(4) bond of the thiazolidine ring, thereby allowing a reconstruction of that ring in a different form (72HCA388 and the following three papers). The preparation of a related intermediate is shown in Scheme 24 (76HCA2298). 

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. 

In the post-World War II years, synthesis attained a different level of sophistication partly as a result of the confluence of five stimuli (1) the formulation of detailed electronic mechanisms for the fundamental organic reactions, (2) the introduction of conformational analysis of organic structures and transition states based on stereochemical principles, (3) the development of spectroscopic and other physical methods for structural analysis, (4) the use of chromatographic methods of analysis and separation, and (5) the discovery and application of new selective chemical reagents. As a result, the period 1945 to 1960 encompassed the synthesis of such complex molecules as vitamin A (O. Isler, 1949), cortisone (R. Woodward, R. Robinson, 1951), strychnine (R. Woodward, 1954), cedrol (G. Stork, 1955), morphine (M. Gates, 1956), reserpine (R. Woodward, 1956), penicillin V (J. Sheehan, 1957), colchicine (A. Eschenmoser, 1959), and chlorophyll (R. Woodward, 1960) (page 5). ... 

Sheehan s concentrated attack upon the penicillin synthesis problem began in 1948 and was conducted on a broad front. It was anticipated at the outset that the formidable penicillin V molecule would succumb to organic synthesis only in the event that new powerful and selective methods of organic synthesis are brought to bear on the problem. But, in addition, and perhaps more importantly, these new synthetic methods must be mild enough to contend with... 

For summaries of Sheehan s penicillin synthesis and related work, see (a) Fleming, I. Selected Organic Syntheses A Guidebook for Organic Chemists, John Wiley Sons New York, 1973, p. 80 (b) Johnson, F. In The Total Synthesis of Natural Products, Vol. 1, ApSimon, J., Ed., Wiley-Interscience New York, 1973, p. 331 (c) Holden, K.G. In Chemistry and Biology of f-Lactam Antibiotics, Morin, R.B. Gorman, M., Eds., Academic Press New York, 1982, Ch. 2, p. 99. 

So far we have shown how, by manipulating the formulation of media, improvements in product yield and product diversification were achieved in the early years of penicillin production. We have deliberately selected the high points of these development activities. We will now turn our attention to another aspect of the development of penicillin production the switch from surface to deep culture. 

As can be seen in Figure 6.12, penicillin G contains two amide functionalities, of which the (Madam linkage is extremely susceptible to basic and nucleophilic attack. Therefore, deavage of the phenylacetyl side chain could not be performed using classical base hydrolysis. The problem of selectivity was resolved by taking advantage of the fad that the amide bond to be hydrolysed is secondary rather than tertiary. 

The phenomenon of bacterial resistance to antibiotics was already known by the pioneers of the era of antibiotics, like Paul Ehrlich, who coined the term selective toxicity as the basic principle of antimicrobial therapeutics, as well as Gerhard Domagk, the inventor of the sulfonamide drugs, and Sir Alexander Fleming, the discoverer of the penicillins. When penicillin G was introduced into clinical practice in 1944, as many as 5% of the isolates of Staphylococcus aureus were resistant to penicillin, while 5 years later the percentage was 50%. 

---