External reagents

Another class of photochemically relevant polyphosphazenes is formed by macromolecules having chromophores able to absorb light in a selective way and to transfer it to external species, thus inducing different reactions by energy transfer processes. In some cases electron transfer processes are also involved. These situations are described by Formula below and the corresponding polymers and external reagents are reported in Table 26. 

In the frame of photochemical research, phosphazene polymers have also been exploited in combination with external reagents able to selectively absorb impinging light and induce reactivity on these materials. The general reaction scheme is shown below. 

Table 28 Hydrogen abstraction reactions in phosphazene polymers and copolymers photosensitized by external reagents...

Greaney and coworkers have introduced the conjugate addition of thiols to Michael acceptors as an effective adaptive DCL strategy [46,47]. The reaction is well suited for biological DCL synthesis, taking place in water with no requirement for external reagents. As with disulfide bond formation, the reaction is subject to simple and effective pH control. Under mildly basic conditions, the thiolate anion adds rapidly to Michael acceptors under equilibrium conditions. Acidification effectively switches the reaction... 

One way in which the Z-a,P-unsaturated carbonyl functionality could be exploited would be via its incorporation into lactone 17. It could be predicted with some confidence that external reagents would attack the bicyclic lactonic system from its convex face. Such an a attack by osmium tetroxide would provide the correct 7,8-erythro diol stereochemistry required to reach NeuSAc. This anticipation turned out to be well founded. 

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

Control of reaction selectivities with external reagents has been quite difficult. Unsolved problems remaining in the held of nitrile oxide cycloadditions are (a) Nitrile oxide cycloadditions to 1,2-disubstituted alkenes are sluggish, the dipoles undergoing facile dimerization to furoxans in most cases (b) the reactions of nitrile oxides with 1,2-disubstituted alkenes nonregioselective (c) stereo- and regiocontrol of this reaction by use of external reagents are not yet well developed and (d) there are few examples of catalysis by Lewis acids known, as is true for catalyzed enantioselective reactions. 

Effect of External Reagents 11.2.2. Magnesium Ion-Mediated Reactions... 

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