Penicilloates

The behavior of penicillins under hydrolysis conditions has been extensively studied e.g. B-49MI51102,71JPS503), and Some of the early work has been summarized in Section 5.11.3.2. Generally, the rate of /3-lactam hydrolysis for various penicillins reaches a minimum between pH 6 and 7, and increases sharply above and below that minimum (74CPB1186). Under basic conditions, hydrolysis rates are relatively insensitive to the 6-substituent, and the product is the relatively stable penicilloate and, at least in some cases, its 5-epimer (Scheme 51) (77JHC503). 

Penicillin V—see Penicillin, phenoxymethyl-, 7, 300 Penicilloate, benzyl-, 7, 303 Penicilloate, D-a-benzyl-a-methyl ester, 7, 303 Penillamine, benzyl-, 7, 303 Penillic acid, benzyl-, 7, 303 Penilloaldehyde, benzyl-, 7, 303 Penilloic acid, benzyl-, 7, 303 Penillonic acid, benzyl-methyl ester, 7, 303 1,2,3,4,6-Pentaazaindene nomenclature, 1, 18 Pentadeca-5,10-dienols synthesis, 1, 428 Pentadienol, tetrachloro-2H-pyran synthesis from, 3, 740 Pentadienonitrile, 5-(l,2-benzoselenazol-3-yl)-X-ray diffraction, 6, 334 Penta-2,4-dienonitrile, 5-(dimethylamino)-2-(2-thienyl)-... 

Penicilloic acid 5, the substrate for the projected lactamization reaction, could be derived from the suitably protected intermediate 6. Retrosynthetic disassembly of 6, in the manner illustrated, provides D-penicillamine hydrochloride (7) and tert-butyl phthalimido-malonaldehydate (8) as potential building blocks. In the synthetic direction, it is conceivable that the thiol and amino groupings in 7 could be induced to converge upon the electrophilic aldehyde carbonyl in 8 to give thiazolidine 6 after loss of a molecule of water. 

In subsequent studies,22 Sheehan et al. demonstrated that the action of diisopropylcarbodiimide on penicilloate 24, prepared by protection of the free primary amino group in 23 with trityl chloride (see Scheme 6b), results in the formation of the desired -lactam 25 in a very respectable yield of 67 %. In this most successful transformation, the competing azlactonization reaction is prevented by the use of a trityl group (Ph3C) to protect the C-6 amino function. Hydrogenolysis of the benzyl ester function in 25, followed by removal of the trityl protecting group with dilute aqueous HC1, furnishes 6-aminopenicillanic acid (26), a versatile intermediate for the synthesis of natural and unnatural penicillins. 

The sodium and potassium salts are veiy soluble in water but they are hydrolysed in solution, at a temperature-dependent rate, to the corresponding penicilloic acid (Fig. 5.3 A see also Fig. 9.3), which is not antibacterial. Penicilloic acid is produced at alkaline pH or (via penieillenic acid Fig. 5.3B) at neutral pH, but at acid pH a molecular rearrangement oeeurs, giving penillic acid (Fig. 5.3C). Instability in acid medium logically precludes oral administration, since the antibiotic may be destroyed in the stomach for example at pH 1.3 and 35°C methicillin has a half-life of only 2-3 minutes and is therefore not administered orally, whereas ampicillin, with a half-life of 600 minutes, is obviously suitable for oral use. 

Fig. 5.3 Degradation products ofbenzylpenicillin in solution A, penicilloic acid B, penicillenic acid C, penillic acid.

To reach their target enzymes, (3-lactams must enter the bacterial cell and cross the periplasm. A bacterium that contains the gene for a (3-lactamase can contain thousands of copies of this enzyme within the periplasmic space. Hence the antibiotics are effectively neutralized by [3-lactamase-catalyzed hydrolysis to inert species. For example, penicillin is hydrolyzed to the inactive species penicilloic acid (Figure 8.13). 

As a simple model for the enzyme penicillinase, Tutt and Schwartz (1970, 1971) investigated the effect of cycloheptaamylose on the hydrolysis of a series of penicillins. As illustrated in Scheme III, the alkaline hydrolysis of penicillins is first-order in both substrate and hydroxide ion and proceeds with cleavage of the /3-lactam ring to produce penicilloic acid. In the presence of an excess of cycloheptaamylose, the rate of disappearance of penicillin follows saturation kinetics as the cycloheptaamylose concentration is varied. By analogy to the hydrolysis of the phenyl acetates, this saturation behavior may be explained by inclusion of the penicillin side chain (the R group) within the cycloheptaamylose cavity prior to nucleophilic attack by a cycloheptaamylose alkoxide ion at the /3-lactam carbonyl. The presence of a covalent intermediate on the reaction pathway, although not isolated, was implicated by the observation that the rate of disappearance of penicillin is always greater than the rate of appearance of free penicilloic acid. 

The answer is e. (Hardman, pp 1074—1076.) Penicillinase hydrolyzes the p-lactam ring of penicillin G to form inactive penicilloic acid. Consequently, the antibiotic is ineffective in the therapy of infections caused by penicillinase-producing microorganisms such as staphylococci, bacilli, E, call, P aeruginosa, and M tuberculosis,... 

Step 1 Benzylpenicillin is first converted to the corresponding penicilloic acid (a dicarboxylic acid) by carrying out the hydrolysis with sodium hydroxide solution, as follows ... 

Penicilloic acid on treatment with acid yields D-penicillamine and benzylpenilic acid, as shown under ... 

Fig. 5.7. The acid-catalyzed hydrolysis of penicillins involves first the formation of an acylium ion (5.22), which, by reacting with H20, yields penicil-loic acids 5.24 (Pathway b). The participation of a neighboring 6-acylamido group increases the rate of hydrolysis. During this intramolecular reaction (Pathway a), oxazolylthiazolidines (5.23) are formed and then give rise to the degradation products penicilloic acids 5.24, penicillenic acids 5.25,...

Penicillins hydrolyze to penicilloic acids, which retain the same L5/f,6/f(-configuration (Fig. 5.10). In the subsequent step, however, penicilloic acids slowly epimerize in aqueous solution to their (5S,6f )-isomers [110]. The mechanism of C(5)-epimerization involves opening of the thiazol-idine ring by C-S bond fission, followed by reclosure with inversion of configuration at C(5) (Fig. 5.10) [111]. 

In cephalosporins, the C(6) position corresponds to C(5) in penicillins or penicilloic acids. During the degradation of cephalosporins, epimerization at C(6) is generally not observed. However, there are exceptions to this rule. An investigation of the degradation kinetics of cefdinir (5.39a, Fig. 5.11) and its C(7)-epimer (5.39b) in aqueous solution showed that, after /3-lactam ring... 

Fig. 5.10. Base-catalyzed epimerization at C(5) in penicilloic acids involves opening the thiazolidine ring by CS bond fission, followed by reclosure with inversion of configuration at...

In 1H- and 19F-NMR spectroscopy studies of the metabolites of fluclox-acillin (5.52) in rat urine, the presence of both (5R)- and (5.S )-flucloxacillin penicilloic acids was demonstrated [154], During the first collection period, the concentration of the (R)-epimer largely exceeded that of the f.S )-epimer. The (R)/(S) ratio then decreased progressively until an excess of f.S )-epimer was reached. These findings are in agreement with the hypothesis that the f.S )-cpimer is formed by epimerization in urine (see above) [152],... 

Examples of such an approach are found in the synthesis of strychnine [22] and morphine [23], labours which have been said to bear resemblance to Sysiphus s torment [24], since they involve linear sequences of more than 25 steps. However, the most illustrative example is found, perhaps, in the synthesis of penicillin (10 ). in the course of which the penicilloic acid derivative JJL was synthesised, though through a laborious and lenghty route. Because this intermediate was easily available from natural penicillin, it was convenient to resort to such a method of degradation in order to make it available in sufficient quantities for studying the last step -that requires the formation of a P-lactam- and thus accomplishing successfully the total synthesis [25]. 

As a result, the penicillin occupies the active site of the enzyme, and becomes bound via the active-site serine residue. This binding causes irreversible enzyme inhibition, and stops cell-wall biosynthesis. Growing cells are killed due to rupture of the cell membrane and loss of cellular contents. The binding reaction between penicillinbinding proteins and penicillins is chemically analogous to the action of P-lactamases (see Boxes 7.20 and 13.5) however, in the latter case, penicilloic acid is subsequently released from the P-lactamase, and the enzyme can continue to function. Inhibitors of acetylcholinesterase (see Box 7.26) also bind irreversibly to the enzyme through a serine hydroxyl. 

Tolerances for these compounds are generally O-.Ol ppm except for penicillin G in cattle (.05 ppm) and cephapirin in edible tissue (0.1 ppm) and milk (.02 ppm) (70). Many chromatographic methods have been described for determination of these compounds in clinical applications, but these methods are not sufficiently sensitive for residue analysis. The summary of methods in Table II includes one GLC (71), five TLC (72-76), and five HPLC methods (77-81). Four of the TLC methods use detection by bioautography. Three HPLC methods have been described for milk (77-79) and two for tissue (80,81). The HPLC methods described by Moats C7 ,80) and by Munns et al (77) are satisfactory for any penicillin with a neutral side-chain and this may be true with the procedure of Terada, et al. (81). The procedure of Terada and Sakabe (79) is also satisfactory for the aminopenicillin, amplclllin. The method of Munns et al (77) can also be used to detect the corresponding penicilloic acid metabolites. 

The penicillins are a large group of bactericidal compounds. They can be subdivided and classified by their chemical structure and spectrum of activity. The structure common to all penicillins is a (3-lactam ring fused with a thiazolidine nucleus (Fig. 45.1).The antimicrobial activity of penicillin resides in the (3-lactam ring. Splitting of the (3-lactam ring by either acid hydrolysis or (3-lactamases results in the formation of penicilloic acid, a product without antibiotic activity. Addition of various side chains (R) to the basic penicillin molecule... 

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