Zinc chelates

Another compound, the antimicrobial action of which is associated with chelation, is 2-pyridinethiol-A/-oxide [3811-73-2] (Omadine). Activity has been shown to depend on coordinating property. The iron chelate is active, but not the free pyridine compound (200). In the form of its zinc chelate it is found in shampoos to control seborrheic dermatitis (201). Other appHcations of this useful chemical include preservation of adhesives, plastics, latex paints, polyurethane foam, and metal working fluids (202). 

A/,A/ -3,5-Bis(trifluoromethyl)phenylboronic Acid Derivative, 594 A/-[Phenyl(pentacarbonylchromium- or-tungsten)]carbenyl, 595 A/-Copper or A/-Zinc Chelate, 595 18-Crown-6 Derivative, 596... 

In the search for new fluorometric reagents for trace metal determinations, ferroin-type compounds, namely 2-(2-pyridyl)-2//- and 2-(3-isoquinolyl)-3//-imidazo[4,5-/i]quinolines, and their silver, lead, and zinc chelates were tested for luminiscence in aqueous ethanol solutions at various pH values (80TAL1021). 

The tridentate ligands C, L and M are effective catalysts for the enantioselective addition of dialkylzincs to aromatic aldehydes16,17. In particular, ligands L and M qualify as members of the chemical enzyme (chemzyme) class of synthetic reagents17, since they function in a predictable, clear-cut mechanistic way. As demonstrated by X-ray diffraction, the actual catalyst is a monomeric zinc chelate 2 formed in toluene at 50 C by reaction of L or M with one equivalent of diethylzinc. 

After 19 hours, no reaction between the zinc chelate 2 and benzaldehyde can be detected at 20 °C. However, 10 mol % of the zinc chelate effectively catalyzes theenantioselective addition of diethylzinc to aromatic aldehydes. The predominant formation of the S-configurated products, effected by this conformationally unambiguous catalyst, can be explained by a six-mem-bered cyclic transition state assembly17. The fact that the zinc chelate formed from ligand M is an equally effective catalyst clearly demonstrates that activation of the aldehyde moiety does not occur as a consequence of hydrogen bond formation between the ammonium proton of the pyrrolidine unit and the aldehydic oxygen. 

Zinc bromide promoted addition of a functionalized benzyl Grignard reagent to the a,/ -unsat-urated y-lactone, 3-(4-methylphenylsulfinyl)-2(5//)-furanone, via a zinc-chelated intermediate, gave, after several synthetic steps, (-)-podorhizon with 95% ee18. 

The reaction of bromoalcohol with ethyl acetoacetate in the presence of metallic zinc has been used to make zinc chelates, the compound of cross-esterification formed according to ... 

The catalytic cycle for these reactions is believed to involve dinuclear complexes formed among the zinc chelate, the aldehyde, and the zinc atom that releases the nucleophile. 

The patent describes the formation of complex metal chelates by treatment of the ketoester simultaneously with an alcohol and a metal to effect trans-esterification and chelate formation by distilling out the by-product ethanol [1], This process was being applied to produce the zinc chelate of 2-tris(bromomethyl)ethyl acetoacetate, and when 80% of the ethanol had been distilled out (and the internal temperature had increased considerably), a violent decomposition occurred [2], This presumably involved interaction of a bromine substituent with excess zinc to form a Grignard-type reagent, and subsequent exothermic reaction of this with one or more of the bromo or ester functions present. 

Some peptide sensors were based on binding to the zinc chelate of ligands such as 79, with specific quenching of the fluorescence of nearby tryptophan residues in the peptide... 

Open-chain 1,3-dicarbonyl compounds did not lead to methylene cyclopropenes when reacted with the cation 75 by the DIPEA method. However, 4,4-diacyl triaful-venes 83 can be prepared very easily and in high yields (60—80%) from cations 80 and copper (or zinc) chelates 81 of 1,3-dicarbonyl compounds (known to be capable... 

The same laboratory has prepared three tridentate zinc chelates from chiral tertiary amino phenolic alcohols and used them for enantioselective addition of diethylzinc to aryl aldehydes in 70-87% ee. Results with the ligand 4 [from (1S,2S)-(+ )-pseudoephedrine] are typical. 

In several cases (including the present example) where diastereoiso-meric aldol products are possible, there is a preference for the formation of the tAreo-diastereoisomer. This stereochemical preference presumably arises because the six-membered cyclic zinc chelate of the ffereo-isomer can exist in a chair conformation with both substituents in equatorial positions. Table I summarizes the results obtained in several aldol condensations performed by the present procedure. 

P.Y.129 is not employed in plastics. The type of metal in the stabilizer defines how much of a color change is observed as the chelated metal in the pigment molecule is exchanged. Tin stabilizers produce red complexes, which, like corresponding lead or zinc chelates, respond very poorly to light and weather (see also Sec. 1.6.7). 

Fig. 10.36. Recognition of uronic and sialic acid salts by the cooperative action of boronic acid and zinc chelate (Yamamoto M. et al. (1996) Tetrahedron 54, 3125).

Castillo, M., Martin-Orue, S. M., Taylor-Pickard, J. A., Perez, J. F., and Gasa, J. (2008). Use of mannanoligosaccharides and zinc chelate as growth promoters and diarrhea preventative in weaning pigs Effects on microbiota and gut function. /. Anim. Sci. 86, 94-101. 

Cherny, R.A. (2001) Treatment with a copper-zinc chelator markedly and rapidly inhibits P-amyloid accumulation in Alzheimer s disease transgenic mice. Neuron, 30, 665-676. 

In an attempt to move away from the hydroxamic acid zinc binding group, which is known to causes poor pharmacokinetics as a result of hydrolysis or glucuronidation, the group at Abbott searched for alternative zinc chelators culminating in the development of a series of electrophilic ketone HDACis (Figure 9.12) [64—66]. Subsequently, a group at IRBM developed a related series of thiophene trifluor-omethyl ketones and showed these to be selective class II HDACis [12]. 

Replacing the electrophilic epoxy ketone moiety in TPX by a reversible zinc chelator such as a hydroxamic acid was carried out by Yoshida et al. (Fig. 6) [51]. This modification led to a low nanomolar reversible inhibitor of the HDACl enzyme. Several other cyclic tetrapeptides containing the epoxyketone feature, such as chlamydocin, were converted into their hydroxamic acid coimterparts as well [52]. Additionally, the introduction of reversed hydroxamic acids (-N(OH)COR, with R = H or Me) onto the structure of Cyl-1 was reported to give potent HDAC inhibitors as illustrated in Fig. 6 [53]. Generally, the most potent inhibitors were the examples with R = H and m = 2. Apicidin, a cychc peptide more remotely related to TPX, exhibits potent antiprotozoal activity via HDAC inhibition in parasites [54]. 

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