Single-Molecule Observation of Intermediates in Bioorthogonal 2-Cyanobenzothiazole Chemistry

We report a single-molecule mechanistic investigation into 2-cyanobenzothiazole (CBT) chemistry within a protein nanoreactor. When simple thiols reacted reversibly with CBT, the thioimidate monoadduct was approximately 80-fold longer-lived than the tetrahedral bisadduct, with important implications for the design of molecular walkers. Irreversible condensation between CBT derivatives and N-terminal cysteine residues has been established as a biocompatible reaction for site-selective biomolecular labeling and imaging. During the reaction between CBT and aminothiols, we resolved two transient intermediates, the thioimidate and the cyclic precursor of the thiazoline product, and determined the rate constants associated with the stepwise condensation, thereby providing critical information for a variety of applications, including the covalent inhibition of protein targets and dynamic combinatorial chemistry.     

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.     

Oligomeric Hydrogels Self‐Assembled from Reduction‐Controlled Condensation

Polymer hydrogels and small‐molecule‐based (SMB) supramolecular hydrogels have been widely explored. But oligomeric hydrogels have remained a challenge because synthetic difficulties of the oligomers and control of their amphiphilicities. Reported herein is the rational design of two precursors Cys(SEt)‐Lys‐CBT ( 1 ) and (Cys‐Lys‐CBT)₂ ( 2 ) (CBT=2‐cyano‐6‐aminobenzothiazole) and the use of a biocompatible condensation to prepare oligomeric hydrogels. Glutathione reduction of 1 or 2 yields the same gelator Cys‐Lys‐CBT ( 3 ) which condenses with each other to yield amphiphilic cyclic oligomers. The oligomers instantly self‐assemble into nanofibers and form oligomeric hydrogels with similar mechanic properties. Chemical analyses indicated that the major condensation product in both two hydrogels is a cyclic dimer. Considering its biocompatibility, optimal mechanical strength, and biodegradability, we believe that our oligomeric hydrogel might be useful for long‐term drug delivery in the future.     

Intracellular Self‐Assembly of Taxol Nanoparticles for Overcoming Multidrug Resistance

Multidrug resistance (MDR) remains the biggest challenge in treating cancers. Herein we propose the intracellular self‐assembly of nanodrugs as a new strategy for overcoming MDR. By employing a biocompatible condensation reaction, we rationally designed a taxol derivative Ac‐Arg‐Val‐Arg‐Arg‐Cys(StBu)‐Lys(taxol)‐2‐cyanobenzothiazole (CBT‐Taxol) which could be subjected to furin‐controlled condensation and self‐assembly of taxol nanoparticles (Taxol‐NPs). In vitro and in vivo studies indicated that, compared with taxol, CBT‐Taxol showed a 4.5‐fold or 1.5‐fold increase in anti‐MDR effects, respectively, on taxol‐resistant HCT 116 cancer cells or tumors without being toxic to the cells or the mice. Our results demonstrate that structuring protease‐susceptible agents and assembling them intracellularly into nanodrugs could be a new optimal strategy for overcoming MDR.     

An E1‐Catalyzed Chemoenzymatic Strategy to Isopeptide‐N‐Ethylated Deubiquitylase‐Resistant Ubiquitin Probes

Triazole‐based deubiquitylase (DUB)‐resistant ubiquitin (Ub) probes have recently emerged as effective tools for the discovery of Ub chain‐specific interactors in proteomic studies, but their structural diversity is limited. A new family of DUB‐resistant Ub probes is reported based on isopeptide‐N‐ethylated dimeric or polymeric Ub chains, which can be efficiently prepared by a one‐pot, ubiquitin‐activating enzyme (E1)‐catalyzed condensation reaction of recombinant Ub precursors to give various homotypic and even branched Ub probes at multi‐milligram scale. Proteomic studies using label‐free quantitative (LFQ) MS indicated that the isopeptide‐N‐ethylated Ub probes may complement the triazole‐based probes in the study of Ub interactome. Our study highlights the utility of modern protein synthetic chemistry to develop structurally and new families of tool molecules needed for proteomic studies.     

Multistep Flow Synthesis of 5‐Amino‐2‐aryl‐2H‐[1,2,3]‐triazole‐4‐carbonitriles

1,2,3‐Triazole has become one of the most important heterocycles in contemporary medicinal chemistry. The development of the copper‐catalyzed Huisgen cycloaddition has allowed the efficient synthesis of 1‐substituted 1,2,3‐triazoles. However, only a few methods are available for the selective preparation of 2‐substituted 1,2,3‐triazole isomers. In this context, we decided to develop an efficient flow synthesis for the preparation of various 2‐aryl‐1,2,3‐triazoles. Our strategy involves a three‐step synthesis under continuous‐flow conditions that starts from the diazotization of anilines and subsequent reaction with malononitrile, followed by nucleophilic addition of amines, and finally employs a catalytic copper(II) cyclization. Potential safety hazards associated with the formation of reactive diazonium species have been addressed by inline quenching. The use of flow equipment allows reliable scale up processes with precise control of the reaction conditions. Synthesis of 2‐substituted 1,2,3‐triazoles has been achieved in good yields with excellent selectivities, thus providing a wide range of 1,2,3‐triazoles.     

One‐Pot/Sequential Native Chemical Ligation Using N‐Sulfanylethylanilide Peptide

N‐Sulfanylethylanilide (SEAlide) peptides were developed with the aim of achieving facile synthesis of peptide thioesters by 9‐fluorenylmethyloxycarbonyl (Fmoc)‐based solid‐phase peptide synthesis (Fmoc SPPS). Initially, SEAlide peptides were found to be converted to the corresponding peptide thioesters under acidic conditions. However, the SEAlide moiety was proved to function as a thioester in the presence of phosphate salts and to participate in native chemical ligation (NCL) with N‐terminal cysteinyl peptides, and this has served as a powerful protein synthesis methodology. The reactivity of a SEAlide peptide (anilide vs. thioester) can be easily tuned with or without the use of phosphate salts. This interesting property of SEAlide peptides allows sequential three‐fragment or unprecedented four‐fragment ligation for efficient one‐pot peptide/protein synthesis. Furthermore, dual‐kinetically controlled ligation, which enables three peptide fragments simultaneously present in the reaction to be ligated in the correct order, was first achieved using a SEAlide peptide. Beyond our initial expectations, SEAlide peptides have served in protein chemistry fields as very useful crypto‐peptide thioesters. DOI 10.1002/tcr.201200007     

Origins of the Enantio‐ and N/O Selectivity in the Primary‐Amine‐Catalyzed Hydroxyamination of 1,3‐Dicarbonyl Compounds with In‐Situ‐Formed Nitrosocarbonyl Compounds: A Theoretical Study

Chemoselective control over N/O selectivity is an intriguing issue in nitroso chemistry. Recently, we reported an unprecedented asymmetric α‐amination reaction of β‐ketocarbonyl compounds that proceeded through the catalytic coupling of enamine carbonyl groups with in‐situ‐generated carbonyl nitroso moieties. This process was facilitated by a simple chiral primary and tertiary diamine that was derived from tert‐leucine. This reaction featured high chemoselectivity and excellent enantioselectivity for a broad range of substrates. Herein, a computational study was performed to elucidate the origins of the enantioselectivity and N/O regioselectivity. We found that a bidentate hydrogen‐bonding interaction between the tertiary N⁺? H and nitrosocarbonyl groups accounted for the high N selectivity, whilst the enantioselectivity was determined by Si‐facial attack on the (E)‐ and (Z)‐enamines in a Curtin–Hammett‐type manner. The bidentate hydrogen‐bonding interaction with the nitrosocarbonyl moieties reinforced the facial selectivity in this process.     

Light‐Driven Enantioselective Organocatalytic β‐Benzylation of Enals

A photochemical organocatalytic strategy for the direct enantioselective β‐benzylation of α,β‐unsaturated aldehydes is reported. The chemistry capitalizes upon the light‐triggered enolization of 2‐alkyl‐benzophenones to afford hydroxy‐o ‐quinodinomethanes. These fleeting intermediates are stereoselectively intercepted by chiral iminium ions, transiently formed upon condensation of a secondary amine catalyst with enals. Density functional theory (DFT) studies provided an explanation for why the reaction proceeds through an unconventional Michael‐type addition manifold, instead of a classical cycloaddition mechanism and subsequent ring‐opening.     

Exploring the Condensation Reaction between Aromatic Nitriles and Amino Thiols To Optimize In Situ Nanoparticle Formation for the Imaging of Proteases and Glycosidases in Cells

The condensation reaction between 6‐hydroxy‐2‐cyanobenzothiazole (CBT) and cysteine has been shown for various applications such as site‐specific protein labelling and in vivo cancer imaging. This report further expands the substrate scope of this reaction by varying the substituents on aromatic nitriles and amino thiols and testing their reactivity and ability to form nanoparticles for cell imaging. The structure–activity relationship study leads to the identification of the minimum structural requirement for the macrocyclization and assembly process in forming nanoparticles. One of the scaffolds made of 2‐pyrimidinecarbonitrile and cysteine joined by a benzyl linker was applied to design fluorescent probes for imaging caspase‐3/7 and β‐galactosidase activity in live cells. These results demonstrate the generality of this system for imaging hydrolytic enzymes.     

Chemically Orthogonal Three‐Patch Microparticles

Compared to two‐dimensional substrates, only a few methodologies exist for the spatially controlled decoration of three‐dimensional objects, such as microparticles. Combining electrohydrodynamic co‐jetting with synthetic polymer chemistry, we were able to create two‐ and three‐patch microparticles displaying chemically orthogonal anchor groups on three distinct surface patches of the same particle. This approach takes advantage of a combination of novel chemically orthogonal polylactide‐based polymers and their processing by electrohydrodynamic co‐jetting to yield unprecedented multifunctional microparticles. Several micropatterned particles were fabricated displaying orthogonal click functionalities. Specifically, we demonstrate novel two‐ and three‐patch particles. Multi‐patch particles are highly sought after for their potential to present multiple distinct ligands in a directional manner. This work clearly establishes a viable route towards orthogonal reaction strategies on multivalent micropatterned particles.     

2‐(1 H‐1,2,3‐Triazol‐4‐yl)‐Pyridine Ligands as Alternatives to 2,2′‐Bipyridines in Ruthenium(II) Complexes

The synthesis of a variety of 2‐(1H‐1,2,3‐triazol‐4‐yl)‐pyridines by click chemistry is demonstrated to provide straightforward access to mono‐functionalized ligands. The ring‐opening polymerization of ε‐caprolactone initiated by such a mono‐functionalized ligand highlights the synthetic potential of this class of bidentate ligands with respect to polymer chemistry or the attachment onto surfaces and nanoparticles. The coordination to Ru^II ions results in homoleptic and heteroleptic complexes with the resultant photophysical and electrochemical properties strongly dependent on the number of these ligands attached to the Ru^II core.     

Access to Pyrazolidin‐3,5‐diones through Anodic N–N Bond Formation

Pyrazolidin‐3,5‐diones are important motifs in heterocyclic chemistry and are of high interest for pharmaceutical applications. In classic organic synthesis, the hydrazinic moiety is installed through condensation using the corresponding hydrazine building blocks. However, most N,N′‐diaryl hydrazines are toxic and require upstream preparation owing to their low commercial availability. We present an alternative and sustainable synthetic approach to pyrazolidin‐3,5‐diones that employs readily accessible dianilides as precursors, which are anodically converted to furnish the N?N bond. The electroconversion is conducted in a simple undivided cell under constant‐current conditions.     

The [2+2] Cycloaddition‐Retroelectrocyclization (CA‐RE) Click Reaction: Facile Access to Molecular and Polymeric Push‐Pull Chromophores

The [2+2] cycloaddition‐retroelectrocyclization (CA‐RE) reaction between electron‐rich alkynes and electron‐deficient alkenes is an efficient procedure to create nonplanar donor–acceptor (D‐A) chromophores in both molecular and polymeric platforms. They feature attractive properties including intramolecular charge‐transfer (ICT) bands, nonlinear optical properties, and redox activities for use in next‐generation electronic and optoelectronic devices. This Review summarizes the development of the CA‐RE reaction, starting from the initial reports with organometallic compounds to the extension to purely organic systems. The structural requirements for rapid, high‐yielding transformations with true click chemistry character are illustrated by examples that include the broad alkyne and alkene substitution modes. The CA‐RE click reaction has been successfully applied to polymer synthesis, with the resulting polymeric push‐pull chromophores finding many interesting applications.     

In Situ Formation of Liquid Metals via Galvanic Replacement Reaction to Build Dendrite‐Free Alkali‐Metal‐Ion Batteries

Galvanic replacement reactions have been studied as a versatile route to synthesize nanostructured alloys. However, the galvanic replacement chemistry of alkali metals has rarely been explored. A protective interphase layer will be formed outside templates when the redox potential exceeds the potential windows of nonaqueous solutions, and the complex interfacial chemistry remains elusive. Here, we demonstrate the formation of room‐temperature liquid metal alloys of Na and K via galvanic replacement reaction. The fundamentals of the reaction at such low potentials are investigated via a combined experimental and computational method, which uncovers the critical role of solid‐electrolyte interphase in regulating the migration of Na ions and thus the alloying reaction kinetics. With in situ formed NaK liquid alloys as an anode, the dendritic growth of alkali metals can be eliminated thanks to the deformable and self‐healing features of liquid metals. The proof‐of‐concept battery delivers reasonable electrochemical performance, confirming the generality of this in situ approach and design principle for next‐generation dendrite‐free batteries.     

Alkyne–Azide Cycloadditions with Copper Powder in a High‐Pressure Continuous‐Flow Reactor: High‐Temperature Conditions versus the Role of Additives

A safe and efficient flow‐chemistry‐based procedure is presented for 1,3‐dipolar cycloaddition reactions between organic azides and acetylenes. This simple and inexpensive technique eliminates the need for costly special apparatus and utilizes Cu powder as a plausible Cu^I source. To maximize the reaction rates, high‐pressure/high‐temperature conditions are utilized; alternatively, the harsh reaction conditions can be moderated at room temperature by the joint application of basic and acidic additives. A comparison of the performance of these two approaches in a series of model reactions has resulted in the formation of useful 1,4‐disubstituted 1,2,3‐triazoles in excellent yields. The risks that are associated with the handling of azides are lowered, thanks to the benefits of flow processing, and gram‐scale production has been safely implemented. The synthetic capability of this continuous‐flow technique is demonstrated by the efficient syntheses of some highly functionalized derivatives of the antifungal cispentacin.     

Continuous Flow Organic Synthesis under High‐Temperature/Pressure Conditions

Microreactor technology and continuous flow processing in general are key features in making organic synthesis both more economical and environmentally friendly. When preformed under a high‐temperature/pressure process intensification regime many transformations originally not considered suitable for flow synthesis owing to long reaction times can be converted into high‐speed flow chemistry protocols that can operate at production‐scale quantities. This Focus Review summarizes the state of the art in high‐temperature/pressure microreactor technology and provides a survey of successful applications of this technique from the recent synthetic organic chemistry literature.     

One‐Pot Organocatalytic Tandem Aldol/Polycyclization Reactions between 1,3‐Dicarbonyl Compounds and α,β,γ,δ‐Unsaturated Aldehydes for the Straightforward Assembly of Cyclopenta[b]furan‐Type Derivatives: New Insight into the Knoevenagel Reaction

A new cascade pathway viable for Knoevenagel chemistry that involves the coupling between 1,3‐dicarbonyl systems and α,β,γ,δ‐unsaturated aldehydes has been developed. The process comprises the combination of a classic aldol‐type condensation and a rare spontaneous metal‐free cycloisomerization, representing a convergent and innovative approach for the stereoselective synthesis of cyclopenta[b]furan‐type derivatives. The scope and limitations with respect to both reaction partners and mechanistic features were investigated. Meaningfully, our study provides valuable guidance concerning the structural and electronic effects controlling the reactivity of conjugated polyene carbonyl systems.     

Heterogeneous Rhodium‐Catalyzed Aerobic Oxidative Dehydrogenative Cross‐Coupling: Nonsymmetrical Biaryl Amines

The first heterogeneously catalyzed oxidative dehydrogenative cross‐coupling of aryl amines is reported herein. 2‐Naphthylamine analogues were reacted with various electron‐rich arenes using a heterogeneous Rh/C catalyst under mild aerobic conditions, thus affording nonsymmetrical biaryl amines in excellent yields with high selectivities. This reaction provides a mild, operationally simple, and efficient approach for the synthesis of biaryls which are important to pharmaceutical and materials chemistry.     

Flash Chemistry Extensively Optimized: High‐Temperature Swern–Moffatt Oxidation in an Automated Microreactor Platform

The generally accepted benefits of small lateral dimensions of microreactors (1 μm to 1 mm) enable a different way of performing synthetic chemistry: Extremely short contact times in the millisecond range can circumvent the need for performing highly exothermic and fast reactions at very low temperatures. In order to fully exploit this technology, such fast processes need to be redesigned and investigated for optimal reaction conditions, which can differ drastically from the ones traditionally applied. In a comprehensive study, we optimized the selective Swern–Moffatt oxidation of benzyl alcohol to benzaldehyde by varying five experimental parameters, including reaction time and temperature. Employing an ultrashort mixing and reaction time of only 32 ms, the optimal temperature was determined to be 70 °C, approximately 150 °C higher than in the conventional batch conditions. This remarkable difference shows both the potency of continuous‐flow chemistry as well as the urgency of a paradigm shift in reaction design for continuous‐flow conditions.