Lead, calcium analog

Cannizzaro (1858) pointed out that ethylene, C2H4, and propylene, CaHg, are diatomic radicals analogous to the radicals of mercuric and cupric salts, and of salts of zinc, lead, calcium, magnesium, etc. and these radicals, like the... 

Electroless plating formulations are inherently unstable and the presence of active nuclei such as dust or metallic particles can lead over a period of time to a process analogous to homogeneous reduction. Stabilizers, which adsorb onto these active nuclei and shield them from the solution, are added in small concentrations to overcome this problem. Typical examples are lead, calcium or thallium ions, thiourea and mercaptobenzthiazole. However, if they are used in excess, deposition will be completely prevented even on the substrate itself. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

Calcium reacts with phosphine in an analogous manner as the alkali metals. In liquid ammonia, solid Ca(PH2)2 nNHs is formed with hydrogen evolution 128,280) -pjjg corresponding reaction with a solution of elemental strontium in liquid ammonia does not lead to a uniform product. ... 

Furoannelated analogs of uracil acyclic nucleosides were prepared as compounds with potential as anti-HIV agents (HIV = human immunodeficiency virus). Hence, 6-benzyluracil 320 is converted to the precursor 321 first by hydroxymethylation, silylation in situ with bis(trimethylsilyl)acetamide (BSA), then by alkylation (Scheme 28) <2001S559>. Ring closure to 322 was effected by treatment with lead tetraacetate and calcium carbonate <2004AP148>. 

Vitamin D3 is a precursor of the hormone 1,25-dihy-droxyvitamin D3. Vitamin D3 is essential for normal calcium and phosphorus metabolism. It is formed from 7-dehydrocholesterol by ultraviolet photolysis in the skin. Insufficient exposure to sunlight and absence of vitamin D3 in the diet leads to rickets, a condition characterized by weak, malformed bones. Vitamin D3 is inactive, but it is converted into an active compound by two hydroxylation reactions that occur in different organs. The first hydroxylation occurs in the liver, which produces 25-hydroxyvita-min D3, abbreviated 25(OH)D3 the second hydroxylation occurs in the kidney and gives rise to the active product 1,25-dihydroxy vitamin D3 24,25 (OH)2D3 (fig. 24.13). The hydroxylation at position 1 that occurs in the kidney is stimulated by parathyroid hormone (PTH), which is secreted from the parathyroid gland in response to low circulating levels of calcium. In the presence of adequate calcium, 25(OH)D3 is converted into an inactive metabolite, 24,25 (OH)2D3. The active derivative of vitamin D3 is considered a hormone because it is transported from the kidneys to target cells, where it binds to nuclear receptors that are analogous to those of typical steroid hormones. l,25(OH)2D3 stimulates calcium transport by intestinal cells and increases calcium uptake by osteoblasts (precursors of bone cells). 

Two analogs of calcitriol, doxercalciferol and paricalcitol, are approved for the treatment of secondary hyperparathyroidism of chronic renal failure. Their principal advantage is that they are less likely than calcitriol to induce hypercalcemia. Their biggest impact will be in patients in whom the use of calcitriol may lead to unacceptably high serum calcium levels. 

This section presents a brief overview of a few other compounds that have not been described in previous sections. Because it can function as a nonmetal, silicon forms sihcides with several metals. These materials are often considered as alloys in which the metal and silicon atoms surround each other in a pattern that may lead to unusual stoichiometry. Examples of this type are Mo3Si and TiSi2. In some sihcides, the Si-Si distance is about 235 pm, a distance that is quite close to the value of 234 pm found in the diamond-type structure of elemental silicon. This indicates that the structure contains Si22-, and CaSi2 is a compound of this type. This compound is analogous to calcium carbide, CaC2 (actually an acetylide that contains C22- ions (see Chapter 10)). 

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