Copper -free cycloaddition strain-promoted

POC-click is formed by thermo-cross-linking the mixture of pre-POC-N (azide-containing POC prepolymer) and pre-POC-Al (alkyne-containing POC prepolymer) the process applies synchronous binary cross-link mechanism, esterification, and thermal click reaction, and the residual azide groups on the surface of POC-click film or scaffold paved the way of surface bioconjugation through strain-promoted alkyne-azide cycloaddition (SPAAC), another copper-free click reaction. 

Likewise, Hirao et al. applied copper free chck chemistry for RNA functionalization via the 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds) and pyrrole-2-carbalde-hyde (Pa) unnatural base pair [152], For this, an azide-modified Pa nucleobase (N3-Pa) was incorporated as triphosphate in a T7 in vitro transcription reaction using Ds as corresponding nucleobase in the DNA template (Fig. 3b, 2). Post-transcriptional modification was achieved using fluorescent dibenzocyclooctyne (DIBO) derivatives in a strain-promoted azide-alkyne cycloaddition reaction. Transcription and efficient site-specific labeling of a 260mer RNA was demonstrated. 

An azide-modified nucleoside has also been used for copper-free DNA detection with fluorescent cyclooctynes via strain-promoted azide-alkyne cycloaddition. 5-(Azidomethyl)-2 -deoxyuridine (AmdU) is, in contrast to various other azide containing nucleosides, stable in solution and metabolically stable and could be incorporated in cellular DNA [35]. Recently, this method has been extended for the detection of cellular RNA by 5-azidopropyl-modified UTP analogs [168]. 

Shelbourne M, Chen X, Brown T, El-Sagheer AH (2011) Fast copper-free click DNA ligation by the ring-strain promoted alkyne-azide cycloaddition reaction. Chem Commun 47(22) 6257-6259. doi 10.1039/clccl0743g... 

Copper-free strain-promoted azide-alkyne cycloaddition... 

Figure 14.5 Reaction scheme of copper-free strain-promoted azide-alkyne cycloaddition. The reaction occurs because of the interaction between functionahzed polymer with azide (blue) and the triple bond (red) of the reactive cyclooctyne, in absence of any catalysts.

Click chemistry is commonly used in organic synthesis due to its high yield and its rapid kinetic. Although Cu-catalyzed allqme-azide cycload-dition, also called CuAAC, is the most utilized one, " " the intrinsic toxicity of the copper catalyst and its associated removal difficulties limit its application in biology. For instance, to improve dendrimer s water solubility, PEG groups are often incorporated via this method but a recent study conducted by Week and coworkers has observed substantial copper contamination in them. Copper-free strain promoted allq ne-azide cycloaddition (SPAAC) has been more and more employed to avoid this drawback.Nevertheless this last method introduces rigid cyclooctatriazole units that can modify the dendrimer scaffold and thus its properties. 

Currently, copper-catalyzed and copper-free alkyne-azide cycloadditions are the most widely applied cycloaddition reactions for the modification of biomolecules. We developed an approach based on the combination of CuAAC or strain-promoted copper-free variants with enzymatic tailing (and optional enzymatic hgation), which allows the convenient postsynthetic introduction of an azide into existing unmodified RNA strands (chemically or enzymatically synthesized or even isolated from biological sources) at a desired position, and the subsequent functionalization with a wide variety of commercially available or self-synthesized functional alkynes [16c]. 

Scheme 16.21 Cyclooctyne derivatives for use in copper-free, strain-promoted azide-alkyne [3+2] cycloadditions designed by Bertozzi (81,83," and Boons j- The second-generation difluorinated derivative 84 is easier to synthesize than 83...

Fig. 13 Scheme for the stepwise cycloaddition reaction. Shoda s activation protocol followed by a CuAAC reaction with a tetrazine-alkyne bi-functional linker, which allowed the following copper free strain promoted final glycoconjugation. ... 

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