Complexes oxamate

The direct conversion of alcohols and amines into carbamate esters by oxidative carbonylation is also an attractive process from an industrial point of view, since carbamates are useful intermediates for the production of polyurethanes. Many efforts have, therefore, been devoted to the development of efficient catalysts able to operate under relatively mild conditions. The reaction, when applied to amino alcohols, allows a convenient synthesis of cyclic urethanes. Several transition metal complexes, based on Pd [218— 239], Cu [240-242], Au [243,244], Os [245], Rh [237,238,246,247], Co [248], Mn [249], Ru [224,250-252], Pt [238] are able to promote the process. The formation of ureas, oxamates, or oxamides as byproducts can in some cases lower the selectivity towards carbamates. 

Under appropriate conditions, alcohols and amines can undergo an oxidative double carbonylation process, with formation of oxalate esters (Eq. 34), oxamate esters (Eq. 35) or oxamides (Eq. 36). These reactions are usually catalyzed by Pd(II) species and take place trough the intermediate formation of bis(alkoxycarbonyl)palladium, (alkoxycarbonyl)(carbamoyl)palladium or bis(carbamoyl)palladium complexes, as shown in Scheme 29 (NuH, Nu H = alcohol or amine) [227,231,267,293-300]. 

The reaction protocol was further extended to the concise synthesis of poly-oxamic acid, the unique polyhydroxyamino acid side-chain moiety of the antifungal polyoxin antibiotics (63). Treatment of the template 205 under standard thermal cycloaddition conditions with (5)-glyceraldehyde acetonide led to the formation of a single diastereoisomer 208 in 53% yield. Subsequent template removal released polyoxamic acid 209 in essentially quantitative yield. This represents a matched system, with the mismatched system leading to more complex reaction mixtures (Scheme 3.70). 

A related approach is to study complexes formed with normal NAD+ but with an unreactive second substrate. An example is oxamate, which binds well to lactate dehydrogenase to form stable ternary complexes for which equilibrium isotope effects have been studied.39... 

Gawlita E, Anderson VE, Paneth P (1995) Equilibrium isotope effect on ternary complex formation of [l-lsO]oxamate with NADH and lactate dehydrogenase. Biochemistry 34 6050-6058... 

Complexes with the thio-oxamate ligands NH2(CO)CSO and NH2(CS)C02, which bind through O and S, exhibit trigonal bipyramidal geometry around the zinc. The zinc complex of the 0,S-donor ligand 2-pyridinethiolate 1-oxide, known as zinc pyrithione, is a fungicide andbacteriocide much used in hair shampoos. 

Cobalt(iii) diketonate complexes generate alkyl peroxo adducts that can oxidize alkenes to oxiranes <1999IC1603>. 0-Phenylenebis(oxamate)-Iigated square-planar cobalt(iii) complexes catalyze high-yield epoxida-tions of unfunctionalized tri- and disubstituted alkenes <1997TL2377>. Low yields are obtained with terminal alkenes. Terminal alkenes can be converted smoothly to aldehydes using an epoxidation-isomerization with ruthenium(ii) porphyrin catalysts <2004AGE4950>. 

Diels-Alder type [4 -1- 2] cycloadditions of nonactivated coupling partners have been effected by various transition-metal catalyses [1]. Interestingly, the cationic ruthenium complex 68 catalyzed the intramolecular [4 -1- 2] cycloaddition between alkyne and enone moieties of 97 leading to 98 (Scheme 4.38) [83]. Such a formal hetero Diels-Alder reaction might proceed via a mthenacyclopentene 99 and an oxam-thenacycloheptadiene 100, which is an mthenium enolate species. 

Figure 17. X-ray structure of the active site of PFL in complex with the substrate analogue oxamate.

This process is supported by the evidence that the yields of amino acids are significant at high temperatiu s such as 90 °C. The presence of Mg enhances this process through the stabilization of oxamic acid caused by the complex formation between oxamic add and Mg and hence the yields of amino adds from ammonium oxalate increase remarkably in the presence of 0.1 M MgCl2. Here, the thermal conversion of soUd ammonium oxalate to oxamide via oxamic acid is weU known. 

Using molecular orbital theory, a significant correlation was found between the LIM) and the biological activity In a series of oxamates, qulnaldlc, and benzopyran-2-carboxyllc aclds.35 This was rationalized In terms of charge transfer stabilization of the drug-receptor complex. 

The first step in the theoretical study of this problem is a molecular dynamics computation on the human proteins. Our methodology is described in detail elsewhere [51], but, in brief the starting point for computations were crystal structures solved by Read et al. [52] for homo-tetrameric human heart, h-H4LDH, and muscle, h-M4LDH, isozymes in a ternary complex with NADH and oxamate at 2.1 A and 2.3 A resolution respectively. Numerical analysis of molecular dynamics computations followed our previously published approach [53]. 

Holbrook and coworkers [51] first studied the binding kinetics of oxamate with LDH/NADH using a LDH with Tyr-237 nitrated. Putting their results together (cf, Ref [46] with our current results yields the following preliminary kinetic scheme for the binding of substrate to LDH to form the Michaelis complex (at 20 °C) ... 

The enzyme is a homodimeric protein of A/r 170,000 and contains no known organic or metal ion cofactors. The enzyme is readily inactivated by oxygen and interconverts between active and inactive forms in vivo (173, 174). The activation process occurs under conditions of anaerobiosis and is catalyzed by an Fe(ll)-dependent activating enzyme (Mr 30,000) (775). Elegant studies on the in vitro activation of PFL by Knappe and co-workers (176, 177) have revealed that a complex activation cocktail is required, which includes the activating enzyme, pyruvate, or oxamate as allosteric effectors, S-adenosylmethionine (SAM), and flavodoxin (775) or photoreduced 5-deazariboflavin (178). A possible role for a B12 derivative in the activation or catalytic reaction for PFL is not likely in light of the observation that E. coli 113-3, a methionine/B auxotroph, pos-... 

Similar studies of the enzyme from pig skeletal muscle have been reported 175,183). In the earlier work, a fast burst of NADH formation in the dead-time of the apparatus was observed, equal in amplitude to the active center concentration at pH 8.0, but smaller at lower pH values. The suggestion that slow isomerization of the ternary product complex before pyruvate release may be the step responsible for the low steady-state maximum rate of lactate oxidation seems to be inconsistent with the full burst observed at pH 8.0, since it might be expected to result in partial equilibration of the reactant and product ternary complexes. Direct studies of the oxidation of E-NADH by pyruvate at pH 9.0 did indicate that reverse hydride transfer from NADH to pyruvate is indeed fast, but the absence of a deuterium isotope effect suggested that the observed rate constant of 246 sec, equal to the maximum steady-state rate of pyruvate reduction, may reflect an isomerization of the ternary complex preceding even faster hydride transfer. More recent studies 183) with improved techniques, however, appear to indicate no burst of enzyme-bound NADH formation preceding the steady-state phase of lactate oxidation at pH 8.0. On the basis of stopped-flow studies of lactate oxidation in the presence of oxamate, which forms a dead-end complex with E-NADH and can serve as an indicator of the rate of formation... 

LDH NAD-pyruvate ternary complex. This complex is isomorphous with respect to the LDH NAD oxalate and LDH NADH oxamate ternary complexes. All three compounds show clearly the position of the substrate site between the nicotinamide and histidine-195. The same position is occupied by a sulfate ion in the apo-structure. It can be replaced by various anions (172) but not oxamate. Although the position of the substrate site (close to the subunit center) is thus well characterized, the orientation of the studied inhibitors in the electron density can only be deduced indirectly by considering the interactions with the protein. The covalent nature of the adduct NAD-pyruvate adds another constraint to the orientation of the substratelike part of this inhibitor. Difference electron density maps among the ternary complexes show peaks in this region which have yet to be interpreted. 

Steady-state kinetics studies of the inhibition of the enzyme by oxR-mate and oxalate 203) hinted at the existence of enzyme-oxamate complexes. No binding of 0.15 milf oxamate was detected in the ultracentrifuge. However, Siidi 283) reported that oxalate (15 mM) and oxamate (60 mM) protect M4 enzyme from heat denaturation. Oxaloacetate and fructose 1,6-diphosphate are effective at protecting against thermal denaturation at 1 mM concentration 286). It is possible that these anions bind at sites similar to those detected in the dogfish M4 LDH molecule 139). 

Fig. 29. The apparent dissociation constants of nitrophenylpyruvate (A), oxamate (A), and lactate (O) from the E-NADH complex of H4 LDH and of oxamate from NADH...

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