Strain-promoted alkyne-azide cycloaddition

Beatty KE, Fisk JD, Smart BP et al (2010) Live-cell imaging of cellular proteins by a strain-promoted azide-alkyne cycloaddition. Chembiochem 11 2092-2095... 

Strain-promoted azide-alkyne cycloaddition (SPAAC)... 

The successful preparation of cycloalkynes also opened up the possibility to explore their unique chemical reactivity. In fact, the transient existence of the cycloalkyne species could initially only be indirectly corroborated by fast in situ trapping of the smaller-sized rings (seven carbons and below) before decomposition [31]. While not strictly applicable to cyclooctyne, which is the smallest cyclic alkyne that can be isolated and stored in pure form, Blomquist already noted that nevertheless careful exclusion of air was requisite to avoid rapid decomposition. More importantly, he was also the first to observe that cyclooctyne reacts explosively when treated with phenyl azide, forming a viscous liquid product [8]. This remark is in fact the first historic administration of a process that has now become known as strain-promoted azide-alkyne cycloaddition (SPAAC). 

Strain-promoted azide-alkyne cycloaddition (SPAAC), since its inception in 2004, has firmly established itself as a powerful click chemistry tool. The commercial access of starting materials, its ease of operation, the nowadays practical reaction... 

From the cycloaddition reaction standpoint, the photo-generated reactive species suitable for both 1,3-dipolar cycloaddition reactions and Diels-Alder reactions have been studied in the literature. In this review, we will focus our discussion on four cycloaddition reactions (1) 1,3-dipolar cycloaddition between aUcenes and photogenerated nitrile imines (2) 1,3-dipolar cycloaddition between aUcenes and photogenerated nitrile ylides (3) photoinduced hetero Diels-Alder reactions and (4) photoinduced strain promoted azide-alkyne cycloaddition (Scheme 3). It is... 

Fig. 1 Cycloaddition reactions employed in nucleic acid labeling with reporter groups (green star). A Cu -mediated azide-alkyne cycloaddition (CuAAC) of a terminal alkyne with an azide. B Strain-promoted azide-alkyne cycloaddition (SPAAC) of an azide with a cyclooctyne derivative. C Staudinger ligation of an azide with a phosphine derivative (not a cycloaddition reaction, see below). D Norbornene cycloaddition of a nitrile oxide as 1,3-dipole and a norbornene as dipolarophile. E Inverse electron-demand Diels- Alder cycloaddition reaction between a strained double bond (norbornene) and a tetrazine derivative. F Photo-cUck reaction of a push-pull-substituted diaiyltetrazole with an activated double bond (maleimide)...

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]. 

Figure 10.13 Classification of different types of chemical cross-linking (Kharkar et al., 2013). CuAAC, Cu-catalyzed azide-alkyne addition SPAAC, 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.

In the BONCAT method, the cellular system of interest (cultured cells, tissue slices, or live animals) is pulse-labeled with a non-canonical amino acid that carries a reactive side chain. In our initial experiments, we used azidohomoalanine (Aha) as the label because Aha-Iabeled proteins can be selectively tagged with dyes or affinity reagents through copper-catalyzed or strain-promoted azide-alkyne cycloaddition reactions [34-36]. Tagged proteins can then be separated from other... 

Generation of chemical patterns with photoreactive polymers photo-Diels-Alder surface anchoring followed by azide-alkyne click reaction to immobilize fluorescent dyes, (a) SPAAC strain-promoted azide-alkyne cycloaddition and (b) CuAAC Cu(l)-catalyzed azide-alkyne cycloaddition. (Source Adapted with permission from Reference [36d].)... 

The orthogonal modification of a-alkyne, m-azido heterotelechelic polystyrene via sequential nitrile oxide-alkyne and azide-alkyne Huisgen cycloadditions was reported by the group of Lutz [36]. Alternatively, Su et al. reported a sequential triple chck modification of polyhedral oligomeric silsesquioxanes (POSS), involving strain-promoted azide-alkyne cycloaddition (SPAAC), copper-catalyzed azide-alkyne cycloaddition (CuAAC), and thiol-ene click coupling (TECC) [37]. 

In Chap. 6, Click Triazoles for Bioconjugation , Zheng, Rouhanifard, Jalloh and Wu describe the application of copper catalysts for conjugating biomolecules, including proteins, nucleic acids, lipids and glycans, with biophysical probes for both in vitro and in vivo studies. Recent development of the strain-promoted azide-alkyne cycloaddition that eliminates the use of copper is also discussed. 

Strain-Promoted Azide-Alkyne Cycloaddition (SPAAC) Reactions... 

Sequential click strategies were utihzed by Zhang, Cheng, and coworkers to precisely construct new surfactants based on polyhedral oligomeric silsesquiox-ane (POSS), tethering cychc polymers [109]. The general synthetic approach involves different types of click chemistries sequentially performed strain-promoted azide-alkyne cycloaddition (SPAAC), CuAAC for POSS—polymer conjugation, and thiol-ene reactions for POSS surface functionalization. 

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