Synthetic equivalents definition

If this protocol is applied to disconnect fragments 20 - 23, either carbon at the point of disconnection can be assigned as a donor (d) or an acceptor (a), giving four different possibilities. Disconnect products 20 and 21, for example, become 25 or 26 and 27 or 28, respectively. Fragments 25 - 28 are still not real molecules, and another step is required before the best donor-acceptor pair can be chosen each fragment must be correlated with a synthetic equivalent. Table 1.2 provides a list of common synthetic equivalents, leading to a modified definition of synthetic equivalent, which is a molecular fragment that is equivalent to a real molecule... 

Recently, Denmark and coworkers have developed a new strategy for the construction of complex molecules using tandem [4+2]/[3+2]cycloaddition of nitroalkenes.149 In the review by Denmark, the definition of tandem reaction is described and tandem cascade cycloadditions, tandem consecutive cycloadditions, and tandem sequential cycloadditions are also defined. The use of nitroalkenes as heterodienes leads to the development of a general, high-yielding, and stereoselective method for the synthesis of cyclic nitronates (see Section 5.2). These dipoles undergo 1,3-dipolar cycloadditions. However, synthetic applications of this process are rare in contrast to the functionally equivalent cycloadditions of nitrile oxides. This is due to the lack of general methods for the preparation of nitronates and their instability. Thus, as illustrated in Scheme 8.29, the potential for a tandem process is formulated in the combination of [4+2] cycloaddition of a donor dienophile with [3+2]cycload-... 

Before discussing the flow behavior of polymeric nanocomposites (PNCs), the nature of these materials should be outlined. As the name indicates, PNCs must contain at least two components, a polymeric matrix with dispersed nanoparticles [Utracki, 2004]. PNCs with thermoplastics, thermosets, and elastomers have been produced. The nanoparticles, by lUPAC s definition, must have at least one dimension that is not larger than 2 nm. They can be of any shape, but the most common for structural PNCs are sheets about 1 nm thick with the aspect ratio p=D/t= 20 to 6000, where D is the inscribed (or equivalent) diameter and t is the thickness of the sheet. These inorganic lamellar solids might be either natural or synthetic [Utracki et al., 2007]. 

The term environmentally friendly has more recently been superseded by its modern equivalents renewable and sustainable . As already discussed, it is sometimes assumed that polymers from renewable resources are by definition sustainable . One definition of sustainable suggests that the development of new products for the benefit of society should not have an unacceptable effect on resource depletion and environmental pollution. However, acceptable is a relative term and invites comparison of one material with another by life-cycle assessment (LCA) [4]. Companies engaged in the development degradable polymers from renewable resources have initiated life-cycle assessment comparisons of their products with the commodity synthetic polymers notably polyethylene. As will be seen in the following Sections, these have not so far shown unambiguously that bio-based polymers are more environmentally sustainable than the present range of commodity polymers [36]. This results from the same reason that led to the Green Report namely lack of consistency and uniformity of the assumptions made. In some cases they actually contradict one another. 

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