Synthetic click chemistry

Besides short ELPS, longer ELPs have also been conjugated to synthetic polymers. In one approach, Cu(I)-catalyzed azide-alkyne cycloaddition click chemistry was applied. For this purpose, ELPs were functionalized with azides or alkynes via incorporation of azidohomoalanine and homopropargyl glycine, respectively, using residue-specific replacement of methionine in ELP via bacterial expression [133]. More recently, an alternative way to site-selectively introduce azides into ELPs was developed. Here, an aqueous diazotransfer reaction was performed directly onto ELP[V5L2G3-90] using imidazole-1-sulfonyl azide [134]. 

Allying click chemistry with the use of environmentally benign catalysts, such as zeolites, may constitute a valuable synthetic tool, representing a true green and sustainable approach. 

Sharpless et al. recently introduced the term click chemistry for the use of simple (but selective), high-yielding reactions forming mostly carbon-heteroatom bonds, preferably in aqueous medium, using only easily available compounds [72]. The authors urge the synthetic community to concentrate on the development of new properties rather than new compounds. 

Dendrimers Containing Ferrocene-Type Units at Their Periphery By exploiting efficient synthetic approaches (dendrimer growth strategy pioneered by Newkome18 and click chemistry), several dendrimers containing 27 (compound 1, Fig. 6.1), 54, 81, and 243 ferrocene units in their periphery have been... 

In summary, epoxides are produced not only as endproducts, but also as intermediates because they are valuable building blocks in synthetic organic chemistry [82-84] (Table 1.3). Until recently, epoxide intermediates were produced by direct oxygen transfer to olefins by a variety of stoichiometric methods. Recently, considerable efforts have been made to conduct the transformations selectively under catalytic conditions. Because epoxides are reactive substances, they can undergo diverse transformations by reactions with acids and bases, and their reactivity has been exploited to form a diverse range of products by so-called click chemistry [85,86], which combines the breadth of combinatorial methods with the precise synthesis of organic chemistry. 

The outstanding characteristics mentioned for both the imcatalyzed and the catalyzed reactions have led to the 1,3-dipolar cycloadditions of azide and alkynes being considered the most efficient reaction under the new concept of click chemistry [ 12], The term coined by Sharpless and coworkers refers to a modular synthetic approach based on a set of the most practical end rehable chemical reactions. These are wide in scope, highly efficient in terms of both conversion and selectivity under very mild and simple reaction conditions, and use simple product isolation procedures. An indicative parameter of the importance of the 1,3-dipolar cycloaddition of azide and alkyne as a prototype of a click chemistry reaction is the exponential number of applications based on this reaction that have appeared in the literature since the description in 2002 of the catalyzed version. Currently, it can be asserted that click chemistry and the Huisgen 1,3-dipolar cycloaddition of azides and alkynes are synonymous. 

S.2.2.4. Triazoles from Azides. Cycloadditions of this type constitute a valuable synthetic route to the triazole ring system. This is shown in Scheme 5.30. This combination dates back to the early work of Huis-gen, but in more recent times it was discovered to be subject to catalysis by Cu(I) compounds. The reactions are fast under mild conditions, have high regiospecificity, and occur in a variety of solvents including water. In addition, reaction products are easily isolated. Reactions with these characteristics have become known as comprising click chemistry this term was coined by K. B. Sharpless. The first and most commonly used reaction referred to by this name is indeed the azide-alkyne cycloaddition, and new interest has developed in triazole hemistry since the discoveiy of the copper catalysis. In addition to its use in organic... 

In addition to the design principles just mentioned, peptides, proteins, and peptide-based supramolecular structures can be cross-linked into 3D networks via conventional synthetic chemistry (e.g., active ester chemistry or short synthetic cross-linkers) or via bioorthogonal click chemistry (e.g., Michael-type addition), utilizing their inherent multitude of functional groups (—NH, —COOH, —SH) and/or other additionally introduced functionalities. 

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