The use of trimethyl and dimethyl chlorosilanes and silazanes as silylating agents for organic chemicals is cited. Physical and chemical properties of commercially available silanes are given, and the chemistry of silylation is briefly described. Examples taken from the published literature are given where solubility and stability of organic materials are modified with these materials. In addition, silylating agents are a means of temporarily blocking organic functional groups containing active hydrogen. Several examples are given of the use of silylation as a process aid in the pharmaceutical industry, primarily in the synthesis of penicillins.
The syntheses and chemical properties of some 54 compounds are described, all having structural units including ==SiCCCN= with either methyl, phenyl, siloxy, alkoxy, or amino groups on silicon and H, alkyl, or phenyl groups on nitrogen. Thirteen of the compounds are novel substituted heterocyclic l-aza-2-silacyclopentanes and one is a l-aza-2-silacyclohexane.
The design and Performance of """Tc chelates that are used for brain perfusion SPECT imaging relates to their extraction efficiency by brain and their degree of cerebral retention. The influence of chelate structure on BBB penetration and intracellular trapping mechanisms are discussed.
It is important to note that in selecting the alternative relating to medical treatments and equipment, the respondents not only evinced an unfounded belief that advances in medical technology are the primary source of escalating medical costs, but also implicitly accepted the concept that new medical treatments and equipment are necessarily more expensive. This implication warrants examination since there are instances in which advances in research and technology actually lead to reduced costs. In performing our examination, we will first draw on the wisdom of Lewis Thomas, who in The Lives ofa Cell discussed the cost-effectiveness of three levels of "technology of medicine."7 We will then consider a number of advances in the treatment of heart diseases in the context of economist Burton Weisbrod's graphic representation of the historical relationship between health expenditures and the state of medical technology (figure 2).
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