Lactam Hydrolysis

2 ft-Lac tarn Hydrolysis. - Pitarch et al,167 have studied the hydrolysis of / -lactams by p-lactamase, with particular interest in the acylation step. This same enzyme has also been the subject of pKa calculations (see Continuum Methods section). The catalytic mechanism proceeds by nucleophilic attack by the hydroxyl group of Ser-70 on the keto-carbon atom, followed by acylation, and then hydrolysis, and re-protonation of Ser-70. The proton acceptor is unknown, as is the proton donor to the /1-lactam nitrogen atom. Three possible reaction mechanisms are present. The first involves Glu-166 as the general base whose role is to assist in deprotonating Ser-70. It is the favoured mechanism but suffers from the fact that Glu-166 is a considerable distance away and would be required to move a fair distance to take part in the reaction mechanism. The 

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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). 

The most thoroughly studied mode of /3-lactam hydrolysis is that catalyzed by the enzyme /3-lactamase (EC 3.5.2.6). The elaboration of this enzyme is one of the three principal ways in which a bacterium can obtain resistance to a /3-lactam antibiotic (see Section 5.11.5.1), and much of the chemical work carried out on the penicillin molecule has been related to attempts to deal with this problem. A discussion of the /3-lactamases is beyond the scope of this work. The reader is referred to (B-79MI51101) for a recent review. 

Raman spectra, 6,652 kinetic studies, 6,699 Rhus vemicifera spectra, 6,652 type 2-depleted, 6,699 (3-Lactam antibiotics bacterial resistance, 6,462 (3-Lactamases zinc, 6, 612 Lactams hydrolysis... 

This all seemed very reasonable at the time, but subsequent work was not consistent with it. A small but measurable amount of 180 exchange was reported for some amides in reasonably concentrated HC1 media,277,278 and for at least one amide the amount of exchange decreased with increasing acidity,277 which is the opposite of what would be expected with the Scheme 14 one-water-molecule mechanism taking over from the equation (74) three-water-molecule mechanism as the acidity increased. Also, the solvent deuterium isotope effect was found to be close to unity for at least one amide,278 a result that has since been confirmed,279 which is not what would be expected on the basis of either a three- or a one-water-molecule process.280 Because of this it was decided to reexamine the lactam hydrolysis data subsequent to the publication of the excess acidity analysis of the H NMR results for these,268 a new study appeared with rate constant data for four of these molecules in aqueous H2S04 media obtained by UV spectroscopy at several temperatures,281 and this was included too.282... 

C. Oefner, A. D Arcy, J. J. Daly, K. Gubernator, R. L. Charnas, I. Heinze, C. Hubsch-werlen, F. K. Winkler, Redefined Crystal Structure of /3-Lactamase from Citrobacter freudii Indicates a Mechanism for /3-Lactam Hydrolysis , Nature 1990, 343, 284- 288. 

J. R. Alvarez-Idaboy, R. Gonzalez-Jonte, A. Hernandez-Laguna, Y. G. Smeyers, Reaction Mechanism of the Acyl-Enzyme Formation in /3-Lactam Hydrolysis by Means of Quantum Chemical Modeling , J. Mol. Struct. 2000, 204, 13 - 28. 

A. Dubus, S. Normarj, M. Kania, M. G. P. Page, The Role of Tyrosine 150 in Catalysis of /3-Lactam Hydrolysis by AmpC /3-Lactamase from Escherichia coli Investigated by Site-Directed Mutagenesis , Biochemistry 1994, 33, 8577-8586. 

Zinc-dependent enzymes [EC 3.5.2.6], including penicillinase and cephalosporinase, with varying specificity in their catalysis of j8-lactam hydrolysis. Some act more readily on penicillins, whereas the catalysis of others is more efficient with cephalosporins. 

A number of interesting studies of lactam hydrolysis have been published. The metal(II)-catalyzed hydrolysis of some penicillin and cephalosporin derivatives displays saturation kinetics.410"412 A 1 1 complex is formed between the metal ion and penicillin which undergoes base hydrolysis up to 10s times faster than the free ligand. The catalytic activity follows the order Cu,>ZnII>NiII = Co11. Coordination of penicillin to copper(II) is believed to occur via the /3-lactam nitrogen and the carboxylate group (121)4l0-4n but other sites have been proposed.413... 

Scheme3.8. Rates of lactam hydrolysis with aqueous base [25]. For comparison, the rate of hydrolysis of N-methylacetamide is shown.

The presence of the double bond, which differentiates penems from penams, allows enamine resonance which renders the bridgehead nitrogen atom less basic and weakens the C(7)-N(4) bond, and hence facilitates /3-lactam hydrolysis. As a consequence, they are more labile than the penams, and such instability issues narrow the range of chemical reactivity of the penem skeleton. 

Lactam hydrolysis under acidic conditions only seems to occur with compounds in which Nb is neutral thus, strychninolones-a and -b are converted into the amino acids, isolated as the methyl esters, by warming with concentrated hydrochloric acid at 100° (50, 52). Strychninolone-c cannot be hydrolyzed under these conditions, which is in keeping with it being the thermodynamically most stable of the three isomers (52). 

Lactams are cyclic amides and are analogous to lactones, which are cyclic esters. In contrast to their lactone counterparts, P-lactams (i.e., four-member cyclic amides) are relatively stable. The antibiotics (penicillin G and cephalexin) are examples of P-lactams. P-Lactam hydrolysis (in presence of water or a nucleophile) is known for antibiotics [9]. P-Lactams are known to form polymers under the right conditions. (An example is ampicillin, in which the primary amine (acting as a nucleophile) attacks the lactam of another ampicillin molecule to form a dimer. A third molecule of ampicillin may attack the lactam of the dimer to form a trimer, and so on). 

The angular 7a-phenyl hicyclic lactam can he prepared hy the cyclocondensation of 3-henzoylpropionic acid and (S)-valinol in 85% yield. Dialkylation of this lactam also affords cleanly the a,a-disubstituted compound. Lactam hydrolysis releases chiral, nonracemic a,a-disuhstimted y-keto carboxylic esters (or acids) (eq 5) and 3,3-disuhstituted dihydronaphthalenes may he obtained via cyclization. ... 

An a-(4-bromobutyl) group can be used as a latent organo-lithium species by means of bromine-lithium exchange. Intramolecular addition of the organometallic tether to the carbonyl group, followed by lactam hydrolysis and aldol cyclization, affords enantiomerically pure hydrinden-2-ones (eq 7). ... 

Many clinically important yff-lactamases are serine proteases that catalyse y5-lactam hydrolysis by a double displacement mechanism involving a covalent acyl-enzyme intermediate. Inhibitors of these enzymes exert their effect by the formation of a stable acyl-enzyme complex. In most cases, this is as a result of changes that take place in the acyl residue after interaction with the enzyme, that is, the inhibitors are mechanism-based. In other cases, the inhibition of yS-lactamases may merely be due to the formation of a relatively stable covalent acyl-enzyme complex without additional alteration [31]. 

EtsN in CH2CI2 in the presence of 4-dimethylaminopyridine (Scheme 13.68). Dihydroxylations of the trimethylsilyl ethers of 217 and 218 generate the 4-amino-4-deoxy-heptono-1,4-lactam derivatives 219 and 220, respectively [121]. Lactam hydrolysis of 219 with LiOH, followed by the Malaprade diol cleavage with NaI04 and further oxidation and deprotection, allows the preparation of 4- p/-polyoxamic acid [122]. Lactam 217 and its enantiomer derived from (5 )-24 have been converted into all four stereomers of cw-l,2-dihydroxypyrrolizidine [123]. Compounds 217 and 218 have been used also to prepare the rm/i5 -2,3-c/5 -3,4-dihydroxyprolines [124,125]. 

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