Unveiling the reactivity of 2H-(thio)pyran-2-(thi)ones in cycloaddition reactions with strained alkynes through density functional theory studies

Over the past two decades, click chemistry transformations have revolutionized chemical and biological sciences. Among the different strain-promoted cycloadditions, the inverse electron demand Diels-Alder reaction (IEDDA) has been established as a benchmark reaction. We have theoretically investigated the IEDDA reaction of endo-bicyclo[6.1.0]nonyne (endo-BCN) with 2H-pyran-2-one, 2H-thiopyran-2-one, 2H-pyran-2-thione and 2H-thiopyran-2-thione. These 2H-(thio)pyran-2-(thi)ones have displayed different reactivity towards endo-BCN. Density functional theory (DFT) calculations show, in agreement with experiments, that endo-BCN reacts significantly faster with 2H-thiopyran-2-one compared to other 2H-(thio)pyran-2-(thi)one derivatives because of the lower distortion energy. Experimentally determined second-order rate constants for the reaction of a 2H-pyran-2-thione with different strained derivatives, including a 1-methylcyclopropene derivative and several cycloalkynes (exo-BCN, (1R,8S)-bicyclo[6.1.0]non-4-yne-9,9-diyl)dimethanol, dibenzocycylooctyne and a light activatable silacycloheptyne, were used to validate the computational investigations and shed light on this reaction.

Modular Semisynthetic Approach to Generate T Cell-Dependent Bispecific Constructs from Recombinant IgG1 Antibodies

Redirecting T cells to tumor cells by bispecific antibodies is an effective approach to treat cancer, and T cell-dependent bispecific antibodies (TDBAs) are an emerging class of potent immunotherapeutic agents. By simultaneously targeting antigens on tumor cells and T cells, T cells are activated to kill tumor cells. Herein, we report a platform to generate a novel class of 2:1 structure of T cell-dependent bispecific antibody with bivalency for HER2 receptors on tumor cells and monovalency for CD3 receptors on T cells. For this, we use a biogenic inverse electron-demand Diels-Alder (IEDDA) click reaction on genetically encoded tyrosine residues to install one TCO handle on therapeutically approved antibody trastuzumab. Subsequent TCO-tetrazine click with a tetrazine-functionalized CD3-binding Fab yields a 2:1 HER2 × CD3 TDBA that exhibits a tumor-killing capability at picomolar concentrations. Monovalency toward the CD3 receptor on T cells can lower the chances of cytokine release syndrome, which is a common side effect of such agents. Our semisynthetic approach can generate highly potent TDBA constructs in a few chemoenzymatic and synthetic steps.

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

DNA nanocrane-based catalysts for region-specific protein modification

This paper describes the development and performance of catalytic DNA-based nanocranes for the controlled modification of wild-type proteins. We show that the position of the catalyst offers control over the region of modification, and that reversible interactions between the catalytic structure and thrombin enable trigger-responsive modification, even in cell lysate.

Click chemistry in the synthesis of antibody-drug conjugates

Antibody-Drug Conjugates (ADC) are a new class of anticancer therapeutics with immense potential. They have been rapidly advancing in the last two decades. This fast speed of development has become possible due to several new technologies and methods. One of them is Click Chemistry, an approach that was created only two decades ago, but already is actively utilized for bioconjugation, material science and drug discovery. In this review, we researched the impact of Click Chemistry reactions on the synthesis and development of ADCs. The information about the most frequently utilized reactions, such as Michael's addition, Copper-catalyzed azide-alkyne [3+2] cycloaddition (CuAAC), Strain-promoted azide-alkyne [3+2] cycloaddition (SPAAC), oxime bond formation, hydrazine-iso-Pictet-Spengler Ligation (HIPS), Diels-Alder reactions have been summarized. The implementation of thiol-maleimide Click Chemistry reaction in the synthesis of numerous FDA-approved Antibody-Drug Conjugates has been reported. The data amassed in the present review provides better understanding of the importance of Click Chemistry in the synthesis, development and improvement of the Antibody-Drug Conjugates and it will be helpful for further researches related to ADCs.

Site-Selective Tyrosine Reaction for Antibody-Cell Conjugation and Targeted Immunotherapy

Targeted immunotherapies capitalize on the exceptional binding capabilities of antibodies to stimulate a host response that effectuates long-lived tumor destruction. One example is the conjugation of immunoglobulins (IgGs) to immune effector cells, which equips the cells with the ability to recognize and accurately kill malignant cells through a process called antibody-dependent cellular cytotoxicity (ADCC). In this study, a chemoenzymatic reaction is developed that specifically functionalizes a single tyrosine (Tyr, Y) residue, Y296, in the Fc domain of therapeutic IgGs. A one-pot reaction that combines the tyrosinase-catalyzed oxidation of tyrosine to o-quinone with a subsequent [3+2] photoaddition with vinyl ether is employed. This reaction installs fluorescent molecules or bioorthogonal groups at Y296 of IgGs or the C-terminal Y-tag of an engineered nanobody. The Tyr-specific reaction is utilized in constructing monofunctionalized antibody-drug conjugates (ADCs) and antibody/nanobody-conjugated effector cells, such as natural killer cells or macrophages. These results demonstrate the potential of site-selective antibody reactions for enhancing targeted cancer immunotherapy.

Site-specific dual encoding and labeling of proteins via genetic code expansion

The ability to selectively modify proteins at two or more defined locations opens new avenues for manipulating, engineering, and studying living systems. As a chemical biology tool for the site-specific encoding of non-canonical amino acids into proteins in vivo, genetic code expansion (GCE) represents a powerful tool to achieve such modifications with minimal disruption to structure and function through a two-step "dual encoding and labeling" (DEAL) process. In this review, we summarize the state of the field of DEAL using GCE. In doing so, we describe the basic principles of GCE-based DEAL, catalog compatible encoding systems and reactions, explore demonstrated and potential applications, highlight emerging paradigms in DEAL methodologies, and propose novel solutions to current limitations.

Ligand-Directed Photocatalysts and Far-Red Light Enable Catalytic Bioorthogonal Uncaging inside Live Cells

Described are ligand-directed catalysts for live-cell, photocatalytic activation of bioorthogonal chemistry. Catalytic groups are localized via a tethered ligand either to DNA or to tubulin, and red light (660 nm) photocatalysis is used to initiate a cascade of DHTz oxidation, intramolecular Diels-Alder reaction, and elimination to release phenolic compounds. Silarhodamine (SiR) dyes, more conventionally used as biological fluorophores, serve as photocatalysts that have high cytocompatibility and produce minimal singlet oxygen. Commercially available conjugates of Hoechst dye (SiR-H) and docetaxel (SiR-T) are used to localize SiR to the nucleus and microtubules, respectively. Computation was used to assist the design of a new class of redox-activated photocage to release either phenol or n-CA4, a microtubule-destabilizing agent. In model studies, uncaging is complete within 5 min using only 2 μM SiR and 40 μM photocage. In situ spectroscopic studies support a mechanism involving rapid intramolecular Diels-Alder reaction and a rate-determining elimination step. In cellular studies, this uncaging process is successful at low concentrations of both the photocage (25 nM) and the SiR-H dye (500 nM). Uncaging n-CA4 causes microtubule depolymerization and an accompanying reduction in cell area. Control studies demonstrate that SiR-H catalyzes uncaging inside the cell, and not in the extracellular environment. With SiR-T, the same dye serves as a photocatalyst and the fluorescent reporter for microtubule depolymerization, and with confocal microscopy, it was possible to visualize microtubule depolymerization in real time as the result of photocatalytic uncaging in live cells.

Synthesis of cholera toxin B subunit glycoconjugates using site-specific orthogonal oxime and sortase ligation reactions

The chemoenzymatic synthesis of a series of dual N- and C-terminal-functionalized cholera toxin B subunit (CTB) glycoconjugates is described. Mucin 1 peptides bearing different levels of Tn antigen glycosylation [MUC1(Tn)] were prepared via solid-phase peptide synthesis. Using sortase-mediated ligation, the MUC1(Tn) epitopes were conjugated to the C-terminus of CTB in a well-defined manner allowing for high-density display of the MUC1(Tn) epitopes. This work explores the challenges of using sortase-mediated ligation in combination with glycopeptides and the practical considerations to obtain high levels of conjugation. Furthermore, we describe methods to combine two orthogonal labeling methodologies, oxime- and sortase-mediated ligation, to expand the biochemical toolkit and produce dual N- and C-terminal-labeled conjugates.

Catalytic Activation of Bioorthogonal Chemistry with Light (CABL) Enables Rapid, Spatiotemporally Controlled Labeling and No-Wash, Subcellular 3D-Patterning in Live Cells Using Long Wavelength Light

Described is the spatiotemporally controlled labeling and patterning of biomolecules in live cells through the catalytic activation of bioorthogonal chemistry with light, referred to as "CABL". Here, an unreactive dihydrotetrazine (DHTz) is photocatalytically oxidized in the intracellular environment by ambient O2 to produce a tetrazine that immediately reacts with a trans-cyclooctene (TCO) dienophile. 6-(2-Pyridyl)dihydrotetrazine-3-carboxamides were developed as stable, cell permeable DHTz reagents that upon oxidation produce the most reactive tetrazines ever used in live cells with Diels-Alder kinetics exceeding k2 of 106 M-1 s-1. CABL photocatalysts are based on fluorescein or silarhodamine dyes with activation at 470 or 660 nm. Strategies for limiting extracellular production of singlet oxygen are described that increase the cytocompatibility of photocatalysis. The HaloTag self-labeling platform was used to introduce DHTz tags to proteins localized in the nucleus, mitochondria, actin, or cytoplasm, and high-yielding subcellular activation and labeling with a TCO-fluorophore were demonstrated. CABL is light-dose dependent, and two-photon excitation promotes CABL at the suborganelle level to selectively pattern live cells under no-wash conditions. CABL was also applied to spatially resolved live-cell labeling of an endogenous protein target by using TIRF microscopy to selectively activate intracellular monoacylglycerol lipase tagged with DHTz-labeled small molecule covalent inhibitor. Beyond spatiotemporally controlled labeling, CABL also improves the efficiency of "ordinary" tetrazine ligations by rescuing the reactivity of commonly used 3-aryl-6-methyltetrazine reporters that become partially reduced to DHTzs inside cells. The spatiotemporal control and fast rates of photoactivation and labeling of CABL should enable a range of biomolecular labeling applications in living systems.

DNA-assisted site-selective protein modification

Protein modification is important for various types of biomedical research, including proteomics and therapeutics. Many methodologies for protein modification exist, but not all possess the required level of efficiency and site selectivity. This review focuses on the use of DNA to achieve the desired conversions and levels of accuracy in protein modification by using DNA (i) as a template to help concentrate dilute reactants, (ii) as a guidance system to achieve selectivity by binding specific proteins, and (iii) even as catalytic entity or construct to enhance protein modification reactions.

Site-selective and inducible acylation of thrombin using aptamer-catalyst conjugates

Two acyl-transfer catalysts were conjugated to thrombin-binding DNA aptamers to acylate thrombin. Modification occurred site-selectively on Lys (≫Ser) residues proximal to the respective aptamer-thrombin interface, was selective for thrombin in the presence of other proteins, and the activity of both DNA-catalysts could be controlled by an external trigger.

Non-Genetic Generation of Antibody Conjugates Based on Chemoenzymatic Tyrosine Click Chemistry

The availability of tools to generate homogeneous and stable antibody conjugates without recombinant DNA technology is a valuable asset in fields spanning from in vitro diagnostics to in vivo imaging and therapeutics. We present here a general approach for the conjugation to human IgG1 antibodies, by employing a straightforward two-stage protocol based on antibody deglycosylation followed by tyrosinase-mediated ortho-quinone strain-promoted click chemistry. The technology is validated by the efficient and clean generation of highly potent DAR2 and DAR4 antibody-drug conjugates (ADCs) with cytotoxic payloads MMAE or PBD dimer, and their in vitro evaluation.

Enabling In Vivo Photocatalytic Activation of Rapid Bioorthogonal Chemistry by Repurposing Silicon-Rhodamine Fluorophores as Cytocompatible Far-Red Photocatalysts

Chromophores that absorb in the tissue-penetrant far-red/near-infrared window have long served as photocatalysts to generate singlet oxygen for photodynamic therapy. However, the cytotoxicity and side reactions associated with singlet oxygen sensitization have posed a problem for using long-wavelength photocatalysis to initiate other types of chemical reactions in biological environments. Herein, silicon-Rhodamine compounds (SiRs) are described as photocatalysts for inducing rapid bioorthogonal chemistry using 660 nm light through the oxidation of a dihydrotetrazine to a tetrazine in the presence of trans-cyclooctene dienophiles. SiRs have been commonly used as fluorophores for bioimaging but have not been applied to catalyze chemical reactions. A series of SiR derivatives were evaluated, and the Janelia Fluor-SiR dyes were found to be especially effective in catalyzing photooxidation (typically 3%). A dihydrotetrazine/tetrazine pair is described that displays high stability in both oxidation states. A protein that was site-selectively modified by trans-cyclooctene was quantitatively conjugated upon exposure to 660 nm light and a dihydrotetrazine. By contrast, a previously described methylene blue catalyst was found to rapidly degrade the protein. SiR-red light photocatalysis was used to cross-link hyaluronic acid derivatives functionalized by dihydrotetrazine and trans-cyclooctenes, enabling 3D culture of human prostate cancer cells. Photoinducible hydrogel formation could also be carried out in live mice through subcutaneous injection of a Cy7-labeled hydrogel precursor solution, followed by brief irradiation to produce a stable hydrogel. This cytocompatible method for using red light photocatalysis to activate bioorthogonal chemistry is anticipated to find broad applications where spatiotemporal control is needed in biological environments.

Click Chemistry with Cyclopentadiene

Cyclopentadiene is one of the most reactive dienes in normal electron-demand Diels-Alder reactions. The high reactivities and yields of cyclopentadiene cycloadditions make them ideal as click reactions. In this review, we discuss the history of the cyclopentadiene cycloaddition as well as applications of cyclopentadiene click reactions. Our emphasis is on experimental and theoretical studies on the reactivity and stability of cyclopentadiene and cyclopentadiene derivatives.

Oxidation-Induced "One-Pot" Click Chemistry

Click chemistry has been established rapidly as one of the most valuable methods for the chemical transformation of complex molecules. Due to the rapid rates, clean conversions to the products, and compatibility of the reagents and reaction conditions even in complex settings, it has found applications in many molecule-oriented disciplines. From the vast landscape of click reactions, approaches have emerged in the past decade centered around oxidative processes to generate in situ highly reactive synthons from dormant functionalities. These approaches have led to some of the fastest click reactions know to date. Here, we review the various methods that can be used for such oxidation-induced "one-pot" click chemistry for the transformation of small molecules, materials, and biomolecules. A comprehensive overview is provided of oxidation conditions that induce a click reaction, and oxidation conditions are orthogonal to other click reactions so that sequential "click-oxidation-click" derivatization of molecules can be performed in one pot. Our review of the relevant literature shows that this strategy is emerging as a powerful approach for the preparation of high-performance materials and the generation of complex biomolecules. As such, we expect that oxidation-induced "one-pot" click chemistry will widen in scope substantially in the forthcoming years.

nSite-Selective, Chemical Modification of Protein at Aromatic Side Chain and Their Emergent Applications

Site-selective chemical modification of protein side chain has probed enormous opportunities in the fundamental understanding of cellular biology and therapeutic applications. Primarily, in the field of biopharmaceutical where formulation of bioconjugates is found to be potential medicine than an individual constituent. In this regard, Lysine and Cysteine are the most widely used endogenous amino acid for these purposes. Recently, the aromatic side chain residues (Trp, Tyr, and His) that are low abundant in protein have gained more attention in therapeutic applications due to their advantages of chemical reactivity and specificity. This review discusses the site-selective bioconjugation methods for aromatic side chains (Trp, Tyr and His) and highlights the developed strategies in the last three years, along with their applications. Also, the review highlights the prevalent methods published earlier. We have examined that metal-catalyzed and photocatalytic reactions are gaining more attention for bioconjugation, though their practical operation is under development. The review has been summarized with the future perspective of protein and peptide conjugations contemplating therapeutic applications and challenges.

Recent developments in bioorthogonal chemistry and the orthogonality within

The emergence of bioorthogonal reactions has greatly advanced research in the fields of biology and medicine. They are not only valuable for labeling, tracking, and understanding biomolecules within living organisms, but also important for constructing advanced bioengineering and drug delivery systems. As the systems studied are increasingly complex, the simultaneous use of multiple bioorthogonal reactions is equally desirable. In this review, we take a look at the different bioorthogonal reactions that have recently been developed, the methods of cellular incorporation and the strategies to create orthogonality within the bioorthogonal landscape.

Site-Specific and Trigger-Activated Modification of Proteins by Means of Catalytic Hemin/G-quadruplex DNAzyme Nanostructures

Catalytic nanostructures have the potency to mimic enzymatic features. In this paper, we show that the complex between hemin and G-quadruplex DNA efficiently catalyzes the modification of proteins with N-methyl luminol derivatives. Final conversions are reached within 15–30 min, and LC-MS analysis of tryptic digests of the proteins shows that the reaction proceeds with chemoselectivity for electron-rich aromatic residues (Tyr ≫ Trp), and the site-specificity of the modification depends on the sequence and secondary structure folding of the G-quadruplex nanostructure. Furthermore, the modification can be applied on proteins with different biomedical functions, and the nanostructure can be designed to contain a regulatory element in order to regulate protein modification by an external stimulus.

Flow Photochemical Syntheses of trans-Cyclooctenes and trans-Cycloheptenes Driven by Metal Complexation

trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisom erization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silica gel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the trans-isomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.

Activatable Small-Molecule Hydrogen Sulfide Donors

Significance: Hydrogen sulfide (H2S) is an important biological signaling molecule involved in many physiological processes. These diverse roles have led researchers to develop contemporary methods to deliver H2S under physiologically relevant conditions and in response to various stimuli. Recent Advances: Different small-molecule donors have been developed that release H2S under various conditions. Key examples include donors activated in response to hydrolysis, to endogenous species, such as thiols, reactive oxygen species, and enzymes, and to external stimuli, such as photoactivation and bio-orthogonal chemistry. In addition, an alternative approach to release H2S has utilized the catalyzed hydrolysis of carbonyl sulfide (COS) by carbonic anhydrase to generate libraries of activatable COS-based H2S donors. Critical Issues: Small-molecule H2S donors provide important research and pharmacological tools to perturb H2S levels. Key needs, both in the development and in the use of such donors, include access to new donors that respond to specific stimuli as well as donors with well-defined control compounds that allow for clear delineation of the impact of H2S delivery from other donor byproducts. Future Directions: The abundance of reported small-molecule H2S donors provides biologists and physiologists with a chemical toolbox to ask key biological questions and to develop H2S-related therapeutic interventions. Further investigation into different releasing efficiencies in biological contexts and a clear understanding of biological responses to donors that release H2S gradually (e.g., hours to days) versus donors that generate H2S quickly (e.g., seconds to minutes) is needed.

Highly Efficient Mono-Functionalization of Knob-in-Hole Antibodies with Strain-Promoted Click Chemistry

Knob-in-hole antibodies can be utilized to introduce a single tag for chemo-enzymatic functionalization. By either introducing a single C-terminal sortase tag (sortase-tag expressed protein ligation) or tyrosine tag (G4Y), mono-functionalization of the monoclonal antibody trastuzumab was achieved rapidly and in high yields. This method was applied to selectively and efficiently introduce a single fluorescent tag, cytokine or single-chain variable fragment, as well as produce clean homo dimers of trastuzumab.

Site-Selective Antibody Functionalization via Orthogonally Reactive Arginine and Lysine Residues

Homogeneous antibody-drug conjugates (ADCs) that use a highly reactive buried lysine (Lys) residue embedded in a dual variable domain (DVD)-IgG1 format can be assembled with high precision and efficiency under mild conditions. Here we show that replacing the Lys with an arginine (Arg) residue affords an orthogonal ADC assembly that is site-selective and stable. X-ray crystallography confirmed the location of the reactive Arg residue at the bottom of a deep pocket. As the Lys-to-Arg mutation is confined to a single residue in the heavy chain of the DVD-IgG1, heterodimeric assemblies that combine a buried Lys in one arm, a buried Arg in the other arm, and identical light chains, are readily assembled. Furthermore, the orthogonal conjugation chemistry enables the loading of heterodimeric DVD-IgG1s with two different cargos in a one-pot reaction and thus affords a convenient platform for dual-warhead ADCs and other multifaceted antibody conjugates.

Secondary Orbital Interactions Enhance the Reactivity of Alkynes in Diels-Alder Cycloadditions

We have investigated the inverse electron-demand Diels–Alder reactions of trans-cyclooctene (TCO) and endo-bicyclo[6.1.0]nonyne (BCN) with a 1,2,4,5-tetrazine, a cyclopentadienone, and an ortho-benzoquinone. Tetrazines react significantly faster with TCO compared to BCN because the highest occupied molecular orbital (HOMO) of TCO is significantly higher in energy than the HOMO of BCN and there is less distortion of the tetrazine. Despite the different HOMO energies, TCO and BCN have similar reactivities toward cyclopentadienones, while BCN is significantly more reactive than TCO in the cycloaddition with ortho-benzoquinone. We find that the higher reactivity of BCN compared to TCO with ortho-benzoquinone is due to secondary orbital interactions of the BCN HOMO-1 with the diene LUMO.

Inducible, Selective Labeling of Proteins via Enzymatic Oxidation of Tyrosine

Proteins can be labeled site-specifically and in inducible fashion by exposing a small peptide tag (G₄Y) on any of its termini and activating the newly exposed tyrosine residue with the enzyme mushroom tyrosinase. The enzyme generates a quinone by oxidizing the tyrosine, which in turn can perform strain-promoted oxidation-controlled ortho-quinone cycloaddition (SPOCQ) with strained alkynes and alkenes, generating a stable conjugation product. Here, we describe a protocol to perform SPOCQ reaction on proteins, along with notes to optimize yield and reaction rates. Conjugation efficiencies of over 95% to antibodies have been reported using this protocol.

The one-pot nonhydrolysis Staudinger reaction and Staudinger or SPAAC ligation

The one-pot nonhydrolysis Staudinger reaction and Staudinger or SPAAC ligation were used for producing a FRET-based dyad in living cells as a proof-of-concept study.

Chemoselectivity of Tertiary Azides in Strain-Promoted Alkyne-Azide Cycloadditions

The strain‐promoted alkyne‐azide cycloaddition (SPAAC) is the most commonly employed bioorthogonal reaction with applications in a broad range of fields. Over the years, several different cyclooctyne derivatives have been developed and investigated in regard to their reactivity in SPAAC reactions with azides. However, only a few studies examined the influence of structurally diverse azides on reaction kinetics. Herein, we report our investigations of the reactivity of primary, secondary, and tertiary azides with the cyclooctynes BCN and ADIBO applying experimental and computational methods. All azides show similar reaction rates with the sterically non‐demanding cyclooctyne BCN. However, due to the increased steric demand of the dibenzocyclooctyne ADIBO, the reactivity of tertiary azides drops by several orders of magnitude in comparison to primary and secondary azides. We show that this chemoselective behavior of tertiary azides can be exploited to achieve semiorthogonal dual‐labeling without the need for any catalyst using SPAAC exclusively.