Zinc, role

Cagen SZ, Klaassen CD. 1979. Protection of carbon tetrachloride-induced hepatotoxicity by zinc Role of metallothionein. Toxicol AppI Pharmacol 51 107-116. 

Stabilization Mechanism. Zinc and cadmium salts react with defect sites on PVC to displace the labHe chloride atoms (32). This reaction ultimately leads to the formation of the respective chloride salts which can be very damaging to the polymer. The role of the calcium and/or barium carboxylate is to react with the newly formed zinc—chlorine or cadmium—chlorine bonds by exchanging ligands (33). In effect, this regenerates the active zinc or cadmium stabilizer and delays the formation of significant concentrations of strong Lewis acids. 

The role of activators in the mechanism of vulcanization is as follows. The soluble zinc salt forms a complex with the accelerator and sulfur. This complex then reacts with a diene elastomer to form a mbber—sulfur—accelerator cross-link cursor while also Hberating the zinc ion. The final step involves completion of the sulfur cross-link to another mbber diene segment (18). 

Insufficient zinc results in slowed growth, delayed wound healing, poor appetite, mental lethargy, and sexual immaturity and it interferes with the immune response. The main function of zinc in metaboHsm is enzymatic and there is evidence of other physiologic roles, eg, in stabilization of membrane stmcture (57). 

In addition to their endocrine disrupting properties, it must be appreciated that many of the chemicals in question possess more general toxic properties, which may be potentiated by metabolism by the organism. Several PAHs, PCBs and PCDDs are carcinogenic, while certain phthalate esters can enhance the excretion of zinc, potentially leading to zinc deficiency. Zinc, an essential element, plays a vital role in spermatogenesis and mature T-cell production. Deficiency may result in abnormalities of the male reproductive system, depletion of spermatogenesis and suppression of the immune system. 

The role of antimony oxide is not entirely understood. On its own it is a rather weak fire retardant although it appears to function by all of the mechanisms listed above. It is, however, synergistic with phosphorus and halogen compounds and consequently widely used. Other oxides are sometimes used as alternatives or partial replacements for antimony oxide. These include titanium dioxide, zinc oxide and molybdenic oxide. Zinc borate has also been used. 

Apparently the role of methanol is to intercept unstable species which otherwise tend to polymerize or rearrange. The methoxy peroxide (72) can be isolated in crystalline form if desired, but it is preferable to treat the methylene dichloride solution at 0° with zinc dust and acetic acid until the mixture shows a negative potassium iodide test. The resulting crude seco-aldehyde (73) is then cyclized to (74) by stirring with neutral alumina in benzene at room temperature for 3 hr. ° Wechter has recently reported a number of transformations of a 5yS-hydroxy-6yS-formyl-B-norpregnane prepared in 8% yield by photolysis and hydrolysis of a 5a-hydroxy-6 -azidopregnane. 

Tlie constrLiction of carbocydic cotnpoutidi by ring-annulation procedures frequently plays a prominent role in total syntliesis. Tlie tolerance of various functional groups in tlie zinc reagents employed in copper-catalyzed asymmetric 1,4-additions fornis tlie basis for tliree novd catalytic enantioselective annulation metliods discussed bete. 

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