The Emergence of Thiol-Ene Coupling as a Click Process for Materials and Bioorganic Chemistry

Stabilized Carbon-Centered Radical-Mediated Carbosulfenylation of Styrenes: Modular Synthesis of Sulfur-Containing Glycine and Peptide Derivatives

Sulfur-containing amino acids and peptides play critical roles in organisms. Thiol-ene reactions between the thiol residues of L-cysteine and the alkenyl fragments in the designed coupling partners serve as primary tools for constructing C─S bonds in the synthesis of unnatural sulfur-containing amino acid derivatives. These reactions are favored due to the preference for hydrogen transfer from thiol to β-sulfanyl carbon radical intermediates. In this paper, the study proposes utilizing carbon-centered radicals stabilized by the capto-dative effect, generated under photocatalytic conditions from N-aryl glycine derivatives. The aim is to compete with the thiol hydrogen, enabling radical C─C bond formation with β-sulfanyl carbon radicals. This protocol is robust in the presence of air and water, offers significant potential as a modular and efficient platform for synthesizing sulfur-containing amino acids and modifying peptides, particularly with abundant disulfides and styrenes.

Photocatalytic C(sp3)-H thiolation by a double SH2 strategy using thiosulfonates

Site-selective C(sp3)-H thiolation using thiosulfonates has been achieved using the decatungstate anion as a photocatalyst. Using the protocol, a variety of thiolated compounds were synthesized in good yields. The transformation consists of a cascade of double SH2 reactions, HAT and ArS group transfer, and PCET (proton-coupled electron transfer) of the leaving arylsulfonyl radical to arylsulfinic acid thus allowing the catalyst, W10O324-, to be recovered.

Computational Study on Radical-Mediated Thiol-Epoxy Reactions

Radical-mediated thiol-epoxy reactions were elucidated for analyzing the overlap problem of the thiol-ene/thiol-epoxy systems using computational approaches. Nine epoxy model molecules were evaluated to mimic the chemical structures and reactivity of some industrial epoxy molecules. Modeling reaction mechanisms was conducted through density functional theory (DFT) calculations using the M06-2X/6-31+G(d,p) level at 1.0 atm and 298.15 K. An analog thiol-ene mechanism was proposed for radical-mediated thiol-epoxide reactions. Unlike the thiol-ene reactions, the addition reaction to epoxides is relatively slow (rate constants <10-4 M-1 s-1). However, the chain transfer, which paves the way for the overlapping of dual curing systems, is quite fast (rate constants >101 M-1 s-1). High stability of thiyl radicals, epoxy ring strain, and the instability of formed alkoxy radical from addition reaction were emphasized as the main driving forces for the reaction energetics and kinetics. Control of temperature and using certain thiols are strongly recommended to avoid curing step overlap based on the findings in this study.

Direct Optical Patterning of Quantum Dots: One Strategy, Different Chemical Processes

Patterning, stability, and dispersion of the semiconductor quantum dots (scQDs) are three issues strictly interconnected for successful device manufacturing. Recently, several authors adopted direct optical patterning (DOP) as a step forward in photolithography to position the scQDs in a selected area. However, the chemistry behind the stability, dispersion, and patterning has to be carefully integrated to obtain a functional commercial device. This review describes different chemical strategies suitable to stabilize the scQDs both at a single level and as an ensemble. Special attention is paid to those strategies compatible with direct optical patterning (DOP). With the same purpose, the scQDs' dispersion in a matrix was described in terms of the scQD surface ligands' interactions with the matrix itself. The chemical processes behind the DOP are illustrated and discussed for five different approaches, all together considering stability, dispersion, and the patterning itself of the scQDs.

Copper-Catalyzed Regioselective Remote C-H Bond Chalcogenation of Aromatic Amine Derivatives without Using Any Large Template

A mild and convenient strategy has been developed for the para-selective chalcogenation of anilide scaffolds via C-H bond functionalization. This methodology employs one of the most earth-abundant and inexpensive Cu(II) catalysts and a commercially available simple aryl chalcogen source without any complex directing template, exogenous ligand, acid/base, oxidant, or other additives. The key feature of this methodology is an impressive regioselectivity along with a wide range of functional group tolerance with good to excellent yields under aerobic conditions.

Fabrication of Superhydrophobic and Light-Absorbing Polyester Fabric Based on Caffeic Acid

Caffeic acid (CA) was treated on the surface of polyester fabric (PET), and Fe²⁺ was used as an intermediate to form chelates with CA to increase the roughness of the polyester surface. With the addition of n-octadecyl mercaptan (SH), the mercapto group reacted with the carbon-carbon double bond of CA on the PET surface through enol click chemical reaction. Meanwhile, CA was polymerized under UV radiation, and thus CA-Fe-SH-PET was prepared. The introduction of SH with a long carbon chain reduced the surface energy of the PET, in order to endow the polyester fabric with a superhydrophobic/lipophilic function. Combined with XPS and FTIR tests, the new carbon-carbon double bond's binding energy and vibration peak were found on the fabric surface, indicating that CA was adsorbed on the PET fabric's surface. After adding SH, the double bond disappeared, demonstrating that SH and CA occurred a click chemical reaction and were grafted onto the PET fabric's surface. The water contact angle (WCA) of CA-Fe-SH-PET was about 156 ± 0.6°, and the scrolling angle (SA) was about 3.298°. The results showed that the modified polyester had a robust superhydrophobic stability in washing, mechanical friction, sun aging, seawater immersion, organic reagent, and acid-base erosion derived from the good adhesion of polymerized CA (PCA). At the same time, the modified polyester fabric had good self-cleaning, antifouling, and oil-water separation performance. It was found that the CA-Fe-SH-PET fabric had unique photothermal conversion characteristics, which can convert the absorbed ultraviolet light into thermal energy, providing a local warming effect due to rapid heating and improving the transmission speed of heavy oil (engine oil and diesel). The CA-Fe-SH-PET fabric can further prevent the transmission of ultraviolet rays, and the UV resistance of CA-Fe-SH-PET fabric is far higher than the UV resistance standard. The preparation method is simple, fast, efficient, and environmentally friendly, and it has better a potential application value in the oil-water separation field.

Extending the in vivo persistence of synthetic glycoconjugates using a serum-protein binder

Synthetic glycoconjugates are used in the development of vaccines and the design of inhibitors for glycan-protein interactions. The in vivo persistence of synthetic glycoconjugates is an important factor in their efficacy, especially when prolonged interactions with specific cell types may be required. In this study, we applied a strategy for non-covalent association of an active compound with serum proteins for extension of glycoconjugate half-life in serum. The small molecule, AG10, has previously been used to extend the half-life of small molecules through its high affinity for transthyretin (TTR), a serum protein. Using a tetravalent polyethylene glycol (PEG)-based scaffold we developed a synthetic strategy for glycoconjugates that allowed for controlled addition of multiple tags, such as a TTR affinity tag or fluorophore. We designed a version of AG10 modified at the pyrazole core, named GD10, amenable to our conjugation strategy and introduced to glycoconjugates using a tri-functional linker. This approach allowed for attachment of GD10 and fluorophore tags, as well as carbohydrate antigens. We then tested the influence of the GD10 tag on glycoconjugate half-life in vivo using a mouse model. Our results suggest that the combination of the GD10 tag and the PEG scaffold extended the half-life of glycoconjugates by as much as 10-fold when compared to proteins of similar molecular weight. The GD10 tag was able to extend the half-life of similar glycoconjugates by as much as 2-fold. We observed a role for the terminal saccharide residue of the carbohydrate antigen and confirmed that conjugates were able to penetrate multiple compartments in vivo including bone marrow, lymph nodes, and other organs. The introduction of the GD10 tag did not obstruct the ability of conjugates to interact with lectin receptors. We conclude that serum protein binders can be used to extend the persistence of glycoconjugates in vivo.

Multivalent γ-PGA-Exendin-4 conjugates to target pancreatic β-cells

Targeting of glucagon‐like peptide 1 receptor (GLP‐1R), expressed on the surface of pancreatic β‐cells, is of great interest for the development of advanced therapies for diabetes and diagnostics for insulinoma. We report the conjugation of exendin‐4 (Ex‐4), an approved drug to treat type 2 diabetes, to poly‐γ‐glutamic acid (γ‐PGA) to obtain more stable and effective GLP‐1R ligands. Exendin‐4 modified at Lysine‐27 with PEG4‐maleimide was conjugated to γ‐PGA functionalized with furan, in different molar ratios, exploiting a chemoselective Diels‐Alder cycloaddition. The γ‐PGA presenting the highest number of conjugated Ex‐4 molecules (average 120 per polymeric chain) showed a double affinity towards GLP‐1R with respect to exendin per se, paving the way to improved therapeutic and diagnostic applications.

Sulfur-Containing Silsesquioxane Derivatives Obtained by the Thiol-ene Reaction: Synthesis and Thermal Degradation

This article presents the synthesis of new derivatives of octa(3-thiopropyl)silsesquioxane (SSQ-8SH) via thiol-ene reaction with simple olefins bearing alkyl groups as well as methoxysilyl, substituted aryl or fluoroalkyl groups. All products were characterized by ¹ H NMR, ¹³ C NMR, ²⁹ Si NMR, FTIR, GPC, MALDI-TOF-MS, as well as thermal analysis (TGA) to confirm their structures, purity and thermal stability, and finally give some insight into their thermal degradation pathway. In some of the structures obtained, the T_5% values take place at high temperatures and are close to 230 °C. The thiol-ene reaction allowed to obtain 7 new compounds with high yield (>92 %), no by-products and in a relatively short time (24 h). All products are characterized by high conversion >99 %. New derivatives can find potential use as modifiers of plastics to improve their certain properties for example hydrophobicity or thermo-oxidative stability.

Synthetic modifications of histones and their functional evaluation

Post-transrational modifications (PTMs) of histones play a key role in epigenetic regulation. Unraveling the roles of each epigenetic mark can provide new insights into their biological mechanisms. On the other hand, it is generally difficult to prepare homogeneously-modified histones/nucleosomes to investigate their specific functions. Therefore, synthetic approaches to acquire precisely mimicked histones/nucleosomes are in great demand, and further development of this research field is anticipated. In this review, synthetic strategies to modify histones/nucleosomes, including cysteine modifications, transformations of dehydroalanine residues and lysine acylation using a catalyst system, are cited. In addition, the functional evaluation of synthetically modified histones/nucleosomes is described.

Reductionist Three-Dimensional Tumor Microenvironment Models in Synthetic Hydrogels

Simple Summary

Tumors exist in a complex, three-dimensional environment which helps them to survive, grow, metastasize, and resist drug treatment. Simple, reproducible, in vitro models of this environment are necessary in order to better understand tumor behavior. Naturally derived polymers are great 3D cell culture substrates, but they often lack the tunability and batch-to-batch consistency which can be found in synthetic polymer systems. In this review, we describe the current state of and future directions for tumor microenvironment models in synthetic hydrogels.

Abstract

The tumor microenvironment (TME) plays a determining role in everything from disease progression to drug resistance. As such, in vitro models which can recapitulate the cell–cell and cell–matrix interactions that occur in situ are key to the investigation of tumor behavior and selecting effective therapeutic drugs. While naturally derived matrices can retain the dimensionality of the native TME, they lack tunability and batch-to-batch consistency. As such, many synthetic polymer systems have been employed to create physiologically relevant TME cultures. In this review, we discussed the common semi-synthetic and synthetic polymers used as hydrogel matrices for tumor models. We reviewed studies in synthetic hydrogels which investigated tumor cell interactions with vasculature and immune cells. Finally, we reviewed the utility of these models as chemotherapeutic drug-screening platforms, as well as the future directions of the field.

Metal-free bioorthogonal click chemistry in cancer theranostics

Bioorthogonal chemistry is a powerful tool to site-specifically activate drugs in living systems. Bioorthogonal reactions between a pair of biologically reactive groups can rapidly and specifically take place in a mild physiological milieu without perturbing inherent biochemical processes. Attributed to their high selectivity and efficiency, bioorthogonal reactions can significantly decrease background signals in bioimaging. Compared with metal-catalyzed bioorthogonal click reactions, metal-free click reactions are more biocompatible without the metal catalyst-induced cytotoxicity. Although a great number of bioorthogonal chemistry-based strategies have been reported for cancer theranostics, a comprehensive review is scarce to highlight the advantages of these strategies. In this review, recent progress in cancer theranostics guided by metal-free bioorthogonal click chemistry will be depicted in detail. The elaborate design as well as the advantages of bioorthogonal chemistry in tumor theranostics are summarized and future prospects in this emerging field are emphasized.

Thiolate-assisted copper(i) catalyzed C–S cross coupling of thiols with aryl iodides: scope, kinetics and mechanism

The scope and mechanism of the C–S cross coupling of thiophenols with aryl iodides using a Cu(i) catalyst in a ligand-free environment is disclosed.

The journey of C–S bond formation from metal catalysis to electrocatalysis

This perspective describes the journey of C–S bond constructions starting from transition metal catalysis through oxidant catalysis, photocatalysis and very recently employed electrocatalysis by using various sulfur surrogates.

Thiol-Ene Click-Inspired Late-Stage Modification of Long-Chain Polyurethane Dendrimers

The construction of well-defined polyurethane dendrimers is challenging due to the high reactivity of externally added or in situ formed isocyanates leading to the formation of side products. With a primary focus of dendrimer research being the interaction of the periphery and the core, we report the synthesis of a common polyurethane dendron, which allows for the late-stage variation of both the periphery and the core. The periphery can be varied simply by installing a clickable unit in the dendron and then attaching to the core and vice-versa. Thus, a common dendron allows for varying periphery and core in the final two steps. To accomplish this, a protecting group-free, one-pot multicomponent Curtius reaction was utilized to afford a robust and versatile AB2 type polyurethane dendron employing commercially available simple molecules: 5-hydroxyisophthalic acid, 11-bromoundecanol, and 4-penten-1-ol. Subsequent late-stage modifications of either dendrons or dendrimers via a thiol-ene click reaction gave surface-functionalized alternating aromatic-aliphatic polyurethane homodendrimers to generation-three (G3). The dendrons and the dendrimers were characterized by NMR, mass spectrometry, and FT-IR analysis. A bifunctional AB2 type dendritic monomer demonstrated this approach’s versatility that can either undergo a thiol-ene click or attachment to the core. This approach enables the incorporation of functionalities at the periphery and the core that may not withstand the dendrimer growth for the synthesis of polyurethane dendrimers and other dendritic macromolecules.

Metal- and Solvent-Free Cascade Reaction for the Synthesis of Amino Pyrazole Thioether Derivatives

We developed an iodine-mediated cascade strategy to synthesize amino pyrazole thioether derivatives (11) in the absence of metals as well as solvents. The present approach provides amino pyrazole thioethers in a highly selective manner without the formation of diaryl sulfide and sulfenyl-enaminonitrile with broad substrate scope. The reactivity of nine sulfenylation sources and synthetic applications of the synthesized compounds have been demonstrated. Thus, the overall iodine-mediated multicomponent reaction (MCR) is more economically feasible, efficient, and environmentally benign.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Photoinduced Porcine Gelatin Cross-Linking by Homobi- and Homotrifunctional Tetrazoles

Gelatin is a costless polypeptide material of natural origin, able to form hydrogels that are potentially useful in biomaterial scaffold design for drug delivery, cell cultures, and tissue engineering. However, gelatin hydrogels are unstable at physiological conditions, losing their features only after a few minutes at 37 °C. Accordingly, treatments to address this issue are of great interest. In the present work, we propose for the first time the use of bi- and trifunctional tetrazoles, most of them unknown to date, for photoinduced gelatin cross-linking towards the production of physiologically stable hydrogels. Indeed, after UV-B irradiation, aryl tetrazoles generate a nitrilimine intermediate that is reactive towards different functionalities, some of them constitutively present in the amino acid side chains of gelatin. The efficacy of the treatment strictly depends on the structure of the cross-linking agent used, and substantial improved stability was observed by switching from bifunctional to trifunctional cross-linkers.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Photoactive Metal-Organic Frameworks for the Selective Synthesis of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical Generation

Metal-organic framework (MOF) materials are intriguing photocatalysts to trigger radical-mediated chemical transformations. We report herein the synthesis and characterization of a series of isomorphic MOFs which show a novel structure, wide visible-light absorption, high chemical stability, and specific redox potential. The prepared MOFs were explored for the photoinduced single-electron oxidation of thiol compounds, generating reactive thiyl radicals to afford thioethers via a convenient thiol-olefin reaction. Importantly, we provide a widely applicable strategy by combing a photoactive MOF with phosphine to modulate the generation of thiyl radical in the reaction, thereby producing a single product of the thioether without the formation of a disulfide byproduct due to the dimerization of thiyl radicals. The photocatalytic reaction takes advantage of this strategy, showing great generality where tens of thiols and olefins have been examined as coupling partners. In addition, the strategy has also been demonstrated to be effective for the reactions catalyzed by other MOFs. Mechanism studies reveal that the selective synthesis of C-S products relies on a synergy between the photoinduced generation of a thiyl radical over the MOF and the in situ cleavage of S-S bond into a S-H bond by phosphine. It is notable that the synthesized MOFs show advanced performance in comparison with classical MOFs. The work not only provides a series of novel MOF photocatalysts that are capable of photoinduced thiol-olefin coupling but also indicates the great potential of MOFs for photochemical transformations mediated by reactive radicals.

Carbon-Sulfur Bond Constructions: From Transition-Metal Catalysis to Sustainable Catalysis

The recent decade evidenced a significant development in the construction of the C-S bond. The journey began with transitional-metal catalysis and reached sustainable catalysis via oxidant, photo, and electro catalyzed methods. A variety of catalytic systems have been explored for the C-S bond formation using a variety of sulfur precursors. This personal account provides an inclusive discussion of these developed methods in terms of reactivity, sustainability and productivity.

Plant oil-based polymers

Polymer materials derived from natural resources have gained increasing attention in recent years because of the uncertainties concerning petroleum supply and prices in the future as well as their environmental pollution problems. As one of the most abundant renewable resources, plant oils are suitable starting materials for polymers because of their low cost, the rich chemistry that their triglyceride structure provides, and their potential biodegradability. This chapter covers the structure, modification of triglycerides and their derivatives as well as synthesis of polymers therefrom. The remarkable advances during the last two decades in organic synthesis using plant oils and the basic oleochemicals derived from them are selectively reported and updated. Various methods, such as condensation, radical/cationic polymerization, metathesis procedure, and living polymerization, have also been applied in constructing oil-based polymers. Based on the advance of these changes, traditional polymers such as polyamides, polyesters, and epoxy resins have been renewed. Partial oil-based polymers have already been applied in some industrial areas and recent developments in this field offer promising new opportunities.

PEG-polyaminoacid based micelles for controlled release of doxorubicin: Rational design, safety and efficacy study

A library of amphiphilic monomethoxypolyethylene glycol (mPEG) terminating polyaminoacid co-polymers able to self-assemble into colloidal systems was screened for the delivery and controlled release of doxorubicin (Doxo). mPEG-Glu/Leu random co-polymers were generated by Ring Opening Polymerization from 5 kDa mPEG-NH₂ macroinitiator using 16:0:1, 8:8:1, 6:10:1, 4:12:1 γ-benzyl glutamic acid carboxy anhydride monomer/leucine N-carboxy anhydride monomer/PEG molar ratios. Glutamic acid was selected for chemical conjugation of Doxo, while leucine units were introduced in the composition of the polyaminoacid block as spacer between adjacent glutamic repeating units to minimize the steric hindrance that could impede the Doxo conjugation and to promote the polymer self-assembly by virtue of the aminoacid hydrophobicity. The benzyl ester protecting the γ-carboxyl group of glutamic acid was quantitatively displaced with hydrazine to yield mPEG_5kDa-b-(hydGlu_m-r-Leu_n). Doxo was conjugated to the diblock co-polymers through pH-sensitive hydrazone bond. The Doxo derivatized co-polymers obtained with a 16:0:1, 8:8:1, 6:10:1 Glu/Leu/PEG ratios self-assembled into 30-40 nm spherical nanoparticles with neutral zeta-potential and CMC in the range of 4-7 μM. At pH 5.5, mimicking endosome environment, the carriers containing leucine showed a faster Doxo release than at pH 7.4, mimicking the blood conditions. Doxo-loaded colloidal formulations showed a dose dependent cytotoxicity on two cancer cell lines, CT26 murine colorectal carcinoma and 4T1 murine mammary carcinoma with IC₅₀ slightly higher than those of free Doxo. The carrier assembled with the polymer containing 6:10:1 hydGlu/Leu/PEG molar ratio {mPEG_5kDa-b-[(Doxo-hydGlu)₆-r-Leu₁₀]} was selected for subsequent in vitro and in vivo investigations. Confocal imaging on CT26 cell line showed that intracellular fate of the carrier involves a lysosomal trafficking pathway. The intratumor or intravenous injection to CT26 and 4T1 subcutaneous tumor bearing mice yielded higher antitumor activity compared to free Doxo. Furthermore, mPEG_5kDa-b-[(Doxo-hydGlu)₆-r-Leu₁₀] displayed a better safety profile when compared to commercially available Caelyx®.

Regioselective C-H Thioarylation of Electron-Rich Arenes by Iron(III) Triflimide Catalysis

A mild and regioselective method for the preparation of unsymmetrical biaryl sulfides using iron(III) catalysis is described. Activation of N-(arylthio)succinimides using the powerful Lewis acid iron(III) triflimide allowed the efficient thiolation of a range of arenes, including anisoles, phenols, acetanilides, and N-heterocycles. The method was applicable for the late-stage thiolation of tyrosine and tryptophan derivatives and was used as the key step for the synthesis of pharmaceutically relevant biaryl sulfur-containing compounds such as the antibiotic dapsone and the antidepressant vortioxetine. Kinetic studies revealed that while N-(arylthio)succinimides bearing electron-deficient arenes underwent thioarylation catalyzed entirely by iron(III) triflimide, N-(arylthio)succinimides with electron-rich arenes displayed an autocatalytic mechanism promoted by the Lewis basic product.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

Precisely Alternating Copolymerization of Episulfides and Isothiocyanates: A Practical Route to Construct Sulfur-Rich Polymers

The development of a controlled and reliable method to construct well-defined sulfur-containing polymers has sparked great interest in polymer science. Herein, we present the trial on the copolymerization of isothiocyanates with episulfides in the presence of organic onium salts, which provides direct access to a class of sulfur-rich polymers. This methodology has combined advantages of simple operation, no metals, mild conditions (25-100 °C), controlled polymerization performance (M_n > 10⁵ g mol^-1, Đ < 1.3), and high reactivity (turnover frequency over 1000 h^-1). The metal-free feature and versatility of the easily accessible monomers, along with fine adjustment of the final properties enable this strategy to be a feasible approach to produce sulfur-rich polymers (16 examples).

Overview of in situ gelling injectable hydrogels for diabetic wounds

Diabetes mellitus (DM) is an endocrine disorder that causes increased blood glucose than usual due to insulin impairment. In DM, several complications arise in which diabetic wound (DW) is the most devastating complication. About 25% of patients with DM expected to develop DWs in their lifetime and undergo limb amputations. Even though several treatments such as surgery, debridement, wound dressings, advanced therapies were available, the overall conclusion has been that with very few exceptions, patients still suffer from limitations like pain, frequent dress changing, high rates of failure, and cost involvement. Further, the treatments involving the delivery of therapeutic agents in treating DWs have limited success due to abnormal levels of proteases in the DW environment. In this backdrop, in situ gelling injectable hydrogels have gained special attention due to their easy encapsulation of therapeutic medications and prolonged release, filling the wound defect areas, ease of handling, and minimally invasive surgical procedures. Though the in situ gelling injectable hydrogels are developed a couple of decades ago, their use for treating DW has not yet been explored thoroughly. Thus, in this review, we have covered the sequential events of DW healing, pathophysiology, current treatments, and its limitations, along with a particular emphasis on the mechanism of action of these in situ gelling injectable hydrogels treating DWs.

Photo-induced radical thiol-ene chemistry: a versatile toolbox for peptide-based drug design

While the global market for peptide/protein-based therapeutics is witnessing significant growth, the development of peptide drugs remains challenging due to their low oral bioavailability, poor membrane permeability, and reduced metabolic stability. However, a toolbox of chemical approaches has been explored for peptide modification to overcome these obstacles. In recent years, there has been a revival of interest in photoinduced radical thiol-ene chemistry as a powerful tool for the construction of therapeutic peptides.

Role of thiol oxidation by air in the mechanism of the self-initiated thermal thiol–ene polymerization

We highlight the role of thiyl initiating radicals generated by air oxidation of thiols in the mechanism accounting for the self-initiation of thiol–ene polymerization. The driving force of this reaction is polarization of the S–H bond.

Functional Bio-inorganic Hybrids from Silicon Quantum Dots and Biological Molecules

Quantum dots (QDs) are semiconductor nanoparticles that exhibit photoluminescent properties useful for applications in the field of diagnostics and medicine. Successful implementation of these QDs for bio-imaging and bio/chemical sensing typically involves conjugation to biologically active molecules for recognition and signal generation. Unfortunately, traditional and widely studied QDs are based upon heavy metals and other toxic elements (e.g., Cd- and Pb-based QDs), which precludes their safe use in actual biological systems. Silicon quantum dots (SiQDs) offer the same advantages as these heavy-metal-based QDs with the added benefits of nontoxicity and abundance. The preparation of functional bio-inorganic hybrids from SiQDs and biomolecules has lagged significantly compared to their traditional toxic counterparts because of the challenges associated with the synthesis of water-soluble SiQDs and their relative instability in aqueous environments. Advances in SiQD synthesis and surface functionalization, however, have made possible the preparation of functional bio-inorganic hybrids from SiQDs and biological molecules through different bioconjugation reactions. In this contribution, we review the various bioconjugate reactions by which SiQDs have been linked to biomolecules and implemented as platforms for bio-imaging and bio/chemical sensing. We also highlight the challenges that need to be addressed and overcome for these materials to reach their full potential. Lastly, we give prospective applications where this unique class of nontoxic and biocompatible materials can be of great utility in the future.

Site-switchable mono-O-allylation of polyols

Site-selective modification of complex molecules allows for rapid accesses to their analogues and derivatives, and, therefore, offers highly valuable opportunities to probe their functions. However, to selectively manipulate one out of many repeatedly occurring functional groups within a substrate represents a grand challenge in chemistry. Yet more demanding is to develop methods in which alterations to the reaction conditions lead to switching of the specific site of reaction. We report herein the development of a Pd/Lewis acid co-catalytic system that achieves not only site-selective, but site-switchable mono-O-allylation of polyols with readily available reagents and catalysts. Through exchanging the Lewis acid additives that recognize specific hydroxyls in a polyol substrate, our system managed to install a versatile allyl group to the target in a site-switchable manner. Our design demonstrates remarkable scope, and is amenable to the direct derivatization of various complex, bioactive natural products.

Selective manipulation of one functional group, out of many repeatedly occurring in a substrate, represents a grand challenge in chemistry. Here, the authors report a Pd/Lewis acid cocatalytic system that achieves not only site-selective, but also site-switchable mono-O-allylation of polyols.

Photopolymerizable Biomaterials and Light-Based 3D Printing Strategies for Biomedical Applications

Since the advent of additive manufacturing, known commonly as 3D printing, this technology has revolutionized the biofabrication landscape and driven numerous pivotal advancements in tissue engineering and regenerative medicine. Many 3D printing methods were developed in short course after Charles Hull first introduced the power of stereolithography to the world. However, materials development was not met with the same enthusiasm and remained the bottleneck in the field for some time. Only in the past decade has there been deliberate development to expand the materials toolbox for 3D printing applications to meet the true potential of 3D printing technologies. Herein, we review the development of biomaterials suited for light-based 3D printing modalities with an emphasis on bioprinting applications. We discuss the chemical mechanisms that govern photopolymerization and highlight the application of natural, synthetic, and composite biomaterials as 3D printed hydrogels. Because the quality of a 3D printed construct is highly dependent on both the material properties and processing technique, we included a final section on the theoretical and practical aspects behind light-based 3D printing as well as ways to employ that knowledge to troubleshoot and standardize the optimization of printing parameters.

Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Thioglycosides and C-glycosides represent pharmacologically useful classes of glycomimetics that possess a high degree of biological stability. One emerging tool for the stereoselective synthesis of thioglycosides is the photoinitiated addition of thiols to unsaturated sugars. Moreover, thiyl radical-mediated reactions of exo-glycals and 1-substituted endo-glycals offer facile routes to β-C-glycosidic structures. This Concept article summarizes the thiol-ene coupling strategies developed recently by our group and Somsák's group for the synthesis of several kinds of glycomimetics which are difficult to synthesize by conventional methods. One unusual characteristic of the thiol-ene reactions of endo-glycals is that heating inhibits, whereas cooling promotes the reaction. This unique temperature dependence as well as the effects of the enose structures and thiol configurations on the efficacy and stereoselectivity of the reactions are also discussed.

Regioselective Photooxidation of Citronellol: A Way to Monomers for Functionalized Bio-Polyesters

Dye-sensitized photooxygenation reaction of bio-based double bond-containing substrates is proposed as sustainable functionalization of terpenes and terpenoids to transform them into polyoxygenated compounds to be employed for the synthesis of new bio-based polyesters. As proof of concept, citronellol 1 has been regioselectively converted into diol 4 using singlet oxygen (¹O₂), a traceless reagent that can be generated from air, visible light and zeolite supported-photosensitizer (Thionine-NaY). With our synthetic approach, diol 4 has been obtained in two-steps, with good regioselectivity, using green reagents such as light and air, and finally a solvent-free oxidation step. From this compound, a citronellol-based copolyester of poly(butylene succinate) (PBS) has been synthesized and fully characterized. The results obtained evidence that the proposed copolymerization of PBS with the citronellol-based building blocks allows to obtain a more flexible and functionalizable material, by exploiting a largely available natural molecule modified through a green synthetic path.