Fluorogenic Strain‐Promoted Alkyne–Diazo Cycloadditions

Fluorogenic reactions, in which non‐ or weakly fluorescent reagents produce highly fluorescent products, are attractive for detecting a broad range of compounds in the fields of bioconjugation and material sciences. Herein, we report that a dibenzocyclooctyne derivative modified with a cyclopropenone moiety (Fl‐DIBO) can undergo fast strain‐promoted cycloaddition reactions under catalyst‐free conditions with azides, nitrones, nitrile oxides, as well as mono‐ and disubstituted diazo‐derivatives. Although the reaction with nitrile oxides, nitrones, and disubstituted diazo compounds gave cycloadducts with low quantum yield, monosubstituted diazo reagents produced 1H‐pyrazole derivatives that exhibited an approximately 160‐fold fluorescence enhancement over Fl‐DIBO combined with a greater than 10 000‐fold increase in brightness. Concluding from quantum chemical calculations, fluorescence quenching of 3H‐pyrazoles, which are formed by reaction with disubstituted diazo‐derivatives, is likely due to the presence of energetically low‐lying (n,π*) states. The fluorogenic probe Fl‐DIBO was successfully employed for the labeling of diazo‐tagged proteins without detectable background signal. Diazo‐derivatives are emerging as attractive reporters for the labeling of biomolecules, and the studies presented herein demonstrate that Fl‐DIBO can be employed for visualizing such biomolecules without the need for probe washout.     

Double Strain‐Promoted Macrocyclization for the Rapid Selection of Cell‐Active Stapled Peptides

Peptide stapling is a method for designing macrocyclic alpha‐helical inhibitors of protein–protein interactions. However, obtaining a cell‐active inhibitor can require significant optimization. We report a novel stapling technique based on a double strain‐promoted azide–alkyne reaction, and exploit its biocompatibility to accelerate the discovery of cell‐active stapled peptides. As a proof of concept, MDM2‐binding peptides were stapled in parallel, directly in cell culture medium in 96‐well plates, and simultaneously evaluated in a p53 reporter assay. This in situ stapling/screening process gave an optimal candidate that showed improved proteolytic stability and nanomolar binding to MDM2 in subsequent biophysical assays. α‐Helicity was confirmed by a crystal structure of the MDM2‐peptide complex. This work introduces in situ stapling as a versatile biocompatible technique with many other potential high‐throughput biological applications.     

Synthesis and Antimicrobial Evaluations of Some Novel Mono‐, Bis‐, and Tris‐Nitro‐1,2,4‐Triazines

Substituted‐1,2,4‐triazines were conveniently synthesized in one pot by the cyclization of arylnitroformaldehyde hydrazone derivatives 1 and 5 with different primary amines in ~37% formaldehyde solution. The synthesized compounds were arranged into novel mono‐, bis‐, and tris‐nitro‐1,2,4‐triazine derivatives 2 , 3 , 4 , 6 , and 7 . The antibacterial and antifungal activity of the synthesized compounds were screened against bacterial strains Escherichia coli (as Gram − ve) and Staphylococcus aureus (as Gram + ve), and fungal strains Aspergillus flavus and Candida albicans . All the synthesized compounds exhibit various patterns of inhibitory activity on the two pathogenic bacterial strains. However, the same compounds showed no activity against the tested fungal strains.     

A Free‐Radical‐Promoted Stereospecific Decarboxylative Silylation of α,β‐Unsaturated Acids with Silanes

A stereospecific decarboxylative silylation of acrylic and propiolic acids with silanes was developed. This reaction represents the first example of decarboxylative C? Si bond formation and provides an efficient and convenient approach to various synthetically useful alkenyl and alkynyl organosilicon compounds through the reaction of α,β‐unsaturated acids with silanes. Spin‐trapping and EPR experiments support a radical addition/elimination process.     

Rapid and Specific Post‐Synthesis Modification of DNA through a Biocompatible Condensation of 1,2‐Aminothiols with 2‐Cyanobenzothiazole

Post‐synthesis modification of DNA is an important way of functionalizing DNA molecules. Herein, we describe a method that first enzymatically incorporates a cyanobenzothiazole (CBT)‐modified thymidine. The side‐chain handle CBT can undergo a rapid and site‐specific cyclization reaction with 1,2‐aminothiols to afford DNA functionalization in aqueous solution. Another key advantage of this method is the formation of a single stereo/regioisomer in the process, which allows for precise control of DNA modification to yield a single component for aptamer selection work and other applications.     

Strain‐Promoted Cycloaddition of Cyclopropenes with o‐Quinones: A Rapid Click Reaction

Novel click reactions are of continued interest in fields as diverse as bio‐conjugation, polymer science and surface chemistry. Qualification as a proper “click” reaction requires stringent criteria, including fast kinetics and high conversion, to be met. Herein, we report a novel strain‐promoted cycloaddition between cyclopropenes and o‐quinones in solution and on a surface. We demonstrate the “click character” of the reaction in solution and on surfaces for both monolayer and polymer brush functionalization.     

Polyaddition of Azide‐Containing Norbornene‐Based Monomer through Strain‐Promoted 1,3‐Dipolar Cycloaddition Reaction

The azide–alkyne “click” reaction has been well known in the past decade, however, another kind of 1,3‐dipolar cycloaddition, the azide–alkene reaction is not fully explored in polymer science to date. This contribution reports, for the first time, the discovery of a polyaddition of norbornene based monomer (NC11N₃) containing both strained double bond and azide moieties. The reaction product is characterized by Fourier transform infrared spectroscopy (FTIR), NMR, gel permeation chromatography (GPC), and mass spectrometry (MS), which confirmed the mechanism that is through cycloaddition of azide to strained double bond on norbornene ring to form triazoline linkage. The reaction can proceed at room temperature as indicated by the increase of molecular weight and viscosity during storage. Monomer, dimer, trimer, tetramer, etc., and species with loss of N₂ due to lability of triazoline moiety are identified in the mixture of reaction product. As a unique feature, elimination of N₂ in the five‐membered ring of triazoline affords a chance to form highly reactive materials, such as with aziridine, which can be a very powerful tool in chemical functionalizations, and find promising applications in reactive polymer resin industries.

     

Invoking Pairwise Interactions in Water‐Promoted Diels–Alder Reactions by using Ionic Liquids as Cosolvents

Rate constants and derived activation parameters of organic reactions in aqueous media, in particular Diels–Alder reactions, are sensitive to the presence of cosolvents in water. To enhance the solubility window of water, we introduced ionic liquids as cosolvents in the aqueous Diels–Alder reaction between anthracene‐9‐carbinol and N‐ethylmaleimide. The reactive potentials of the organic compounds are parameterized by using semi‐empirical quantum chemical methods. The principle of Savage–Wood additivity of group interactions is used to quantify the pairwise group interactions among chemically inert ionic liquids and organic reactants, both at initial and transition states of the reaction. The present approach shows promise, as the use of simple calculations from easily available kinetic data can help researchers to understand the versatility of green ionic‐liquid alternatives to volatile organic solvents.     

SLAP: Small Labeling Pair for Single‐Molecule Super‐Resolution Imaging

Protein labeling with synthetic fluorescent probes is a key technology in chemical biology and biomedical research. A sensitive and efficient modular labeling approach (SLAP) was developed on the basis of a synthetic small‐molecule recognition unit (Ni‐trisNTA) and the genetically encoded minimal protein His_6‐10‐tag. High‐density protein tracing by SLAP was demonstrated. This technique allows super‐resolution fluorescence imaging and fulfills the necessary sampling criteria for single‐molecule localization‐based imaging techniques. It avoids masking by large probes, for example, antibodies, and supplies sensitive, precise, and robust size analysis of protein clusters (nanodomains).     

Efficient Preparation of Pyridinyl‐1,2,4‐Triazines via Telescoped Condensation with Diversely Functionalized 1,2‐Dicarbonyls

The development of materials for efficient chemoselective extraction of minor actinides remains at the forefront of research efforts in the area of separation science. Lewis basic complexants derived from nitrogen‐donor scaffolds are often employed in this area due to favorable complexation with the transuranic element americium. In the present work an efficient procedure for the preparation of eight useful 3‐pyridin‐2‐yl‐1,2,4‐triazines (2 novel) is demonstrated via telescoped condensation with the requisite 1,2‐dicarbonyl in two‐pots without additives, differentially extractive work‐up procedures, or recrystallization. Additional efforts in this area have demonstrated the utility of polar aprotic solvents for the preparation of nine functionalized pyridinyl‐2,6‐bis‐1,2,4‐triazines (4 novel) directly from the requisite 2,6‐pyridine dicarbonitrile in 49–99% yield over four total steps. The streamlined preparation of these important materials and detailed synthetic procedures is reported herein.     

Metal‐Free Synthetic Approach to 3‐Monosubstituted Unsymmetrical 1,2,4,5‐Tetrazines Useful for Bioorthogonal Reactions

A facile, efficient and metal‐free synthetic approach to 3‐monosubstituted unsymmetrical 1,2,4,5‐tetrazines is presented. Dichloromethane (DCM) is for the first time recognized as a novel reagent in the synthetic chemistry of tetrazines. Using this novel approach 11 3‐aryl/alkyl 1,2,4,5‐tetrazines were prepared in excellent yields (up to 75 %). The mechanism of this new reaction, including the role of DCM in the tetrazine ring formation, has been investigated by ¹³C labeling of DCM, and is also presented and discussed as well as the photophysical and electrochemical properties.