Complex lactams, hydrolysis

We now examine the metabolic fate of lactam bonds located in rings containing an additional heteroatom, designated here as complex lactams. Our first example is DN-9893 (5.73) a platelet-aggregation inhibitor [177]. Its ring-opened metabolite 5.74 was detected in rat urine after intravenous administration of DN-9893. However, insufficient evidence exists to determine whether hydrolysis of the lactam ring was enzymatic or nonenzymatic. 

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... 

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]. 

Model studies for metallo- 3-lactamases have been performed using mononuclear zinc hydroxide complexes.99,129,130 The breadth of (3-lactam hydrolysis reactivity of hydro-tris(pyrazolyl)borate-ligated mononuclear zinc hydroxide complexes has been explored.129 Treatment of the mononuclear zinc hydroxide complex [(Tpph Mc)Zn OH] with simple 3-lactams ( 3-propiolactam, 4-phenyl-(3-propiolactam, Scheme 21) does not result in ring opening, but instead results in the formation of 3-lactamide complexes and water. Treatment of [(Tpph,Me)Zn-OH] with /V-alkyl or -aryl 3-lactam derivatives instead results in no reaction (Scheme 21). Use of natural derivatives of penicillin and cephalosporin (Scheme 22) did not yield 3-lactam hydrolysis, but instead coordination of the carboxylate moiety of the antibiotic derivatives to the mononuclear Zn(II) center and release of water. 

Two Cd(II) complexes will be examined for their ability to hydrolyze OPs and p-lactam substrates. The mechanism of action will be probed using Cd-NMR, 0-labeling, solution IR and UV-Vis studies. The relevance of Cd(II) as biological metal and the observation of a blue intermediate in p-lactam hydrolysis will be discussed (Chap. 5). 

The next chapter will present an extensive investigation of the mechanistic pathways employed by two Cd(ll) complexes in phosphoester and p-lactam hydrolysis. 

The nucleophile in p-lactam hydrolysis by biomimetic Zn(II) complexes has been reported previously to be either a bridging hydroxide or a terminal water molecule. A bridging hydroxide was ruled out as nucleophile [26]. It is unlikely that this is the nucleophile in the reported complex as the two Cd(II) ions are separated by 4.162 A and thus too far away to establish a single hydroxide bridge between the two metal centers. Crystal structures with pyrazolate based ligands reported by Meyer et al. support this proposal [47, 48]. There are a number of findings that led to the proposal that the metal bound alcohol is the nucleophile (Fig. 5.22) ... 

As already mentioned, Lippard and co-workers have also studied the -hydroxy dizinc unit found in metallohydrolases in a model system [Zn2L(/x-OI I)(/x-02PPh2)]2+, L = 2,7-bis [2-(2-pyridylethyl)-aminomethyl]-l,8-naphthyridine) 454 Hydrolysis of phosphodiesters and beta-lactams was studied and related to the PI nuclease and beta-lactamase enzymes. The dinuclear complex... 

Catalysis of hydrolysis of P-lactams by dinuclear zinc complexes... 

Transition metal ions cause a dramatic increase in the rate of hydrolysis of /Madam antibiotics [75][133][134], For example, copper(II) and zinc(II) ions increase the rate of alkaline hydrolysis ca. 108-fold and 104-fold, respectively [76], It has been suggested that the metal ion coordinates with both the carboxylate group and the /3-lactam N-atom of penicillins (A, Fig. 5.20). This complex stabilizes the tetrahedral intermediate and, thus, facilitates cleavage of the C-N bond catalyzed by the HO ion [74] [75], Such a model appears applicable also to clavulanic acid, imipenem, and monobactams, but it re-... 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Another chelate structure exists for fi-lactams bearing a basic side chain. In ampicillin and cephalexin, e.g., the metal ion is bound by the carbonyl and amino groups (D, Fig. 5.20) [137]. When so attached, the metal ion does not appear to stabilize the tetrahedral transition-state complex, and, indeed, Cu11 ions did not significantly affect the hydrolysis of cefaclor [125]. 

N. V. Kaminskaia, B. Spingler, S. J. Lippard, Hydrolysis of /3-Lactam Antibiotics Catalyzed by Dinuclear Zinc(II) Complexes Functional Mimics of Metallo-/3-Lactamases ,. /. Am. Chem. Soc. 2000, 122, 6411-6422. 

Some evidence234 for Zn—OH attack in anhydride hydrolysis has been obtained using the complex (65) (Section 61.4.11) but the evidence is not definitive, and other mechanisms could apply. Large rate enhancements occur in the Zn11- and Cu -promoted hydrolysis of the lactam (66) (Section 61.4.10). Rates increase commensurate with the ionization of a metal-bound water molecular and sigmoidal pH-rate profiles are observed. Rate enhancements of 9 x 105 and 1 x 103 occur with (66)—Cu—OH and (66)—Zn—OH compared with the free ligand. A number of other reactions which are believed to proceed via M—OH species, in kinetically labile systems, are considered in Section 61.4.3. 

Very rapid hydrolysis of the lactam (125) in the presence of copper(II) and zinc(II) occurs.417,235 The lactam gives 1 1 complexes with Cu11, Zn11 and Co11 which are believed to have the structure (126). For the copper(II) complex the pKa of the coordinated water molecule is ca. 7.6. Hydrolysis... 

Another concise route to 107 featured the facile construction of the cyclohexanone derivative 109 via the Michael addition of triply deprotonated methyl dioxohexanoate to the nitrostyrene (108 (Scheme 9) (115). Ketalization of 109 followed by hydrogenation of the nitro function and then cyclization of the resulting amino ester by thermolysis in refluxing xylene furnished the lactam 110, which was reduced LiAlH4 to the amine 111. All attempts to cyclize 111 via a Pictet-Spengler reaction led to complex mixtures of products. However, when the unstable enone 112, which was obtained by acid-catalyzed hydrolysis of 111,... 

The SM2/AM1 model was successful in studies on alkaline hydrolysis ofcla-vulanic acid - />-lactam antibiotic and / -lactamase inhibitor [86], The SM3/PM3 model was used to study the influence ofthe solvent on the basic hydrolysis of the />-lactame ring, and calculations for a supramolecular complex with 20 molecules of water in the solvation sphere around the solute yielded a potential barrier very close to the experimental value [87, 88],... 

---